AN OVERVIEW OF THE CARDIOVASCULAR SYSTEM
Blood flows through a network of blood vessels that extend between the heart and peripheral tissues. Those blood vessels can be subdivided into a pulmonary circuit, which carries blood to and from the gas exchange surfaces of the lungs, and a systemic circuit, which transports blood to and from the rest of the body. Each circuit begins and ends at the heart (Figure 20-1), and blood travels through these circuits in sequence. For example, blood returning to the heart from the systemic circuit must complete the pulmonary circuit before reentering the systemic circuit.
Arteries, or efferent vessels, carry blood away from the heart; veins, or afferent vessels, return blood to the heart. Capillaries are small, thin-walled vessels between the smallest arteries and veins. Capillaries are called exchange vessels, because their thin walls permit the exchange of nutrients, dissolved gases, and waste products between the blood and surrounding tissues.
Despite its impressive workload, the heart is a small organ, roughly the size of a clenched fist. The heart contains four muscular chambers, two associated with each circuit. The right atrium (plural, atria) receives blood from the systemic circuit and passes it to the right ventricle (little belly). The right ventricle discharges blood into the pulmonary circuit. The left atrium collects blood from the pulmonary circuit and empties it into the left ventricle. Contraction of the left ventricle ejects blood into the systemic circuit. When the heart beats, the atria contract first, followed by the ventricles. The two ventricles contract at the same time and eject equal volumes of blood into the pulmonary and systemic circuits.
ANATOMY OF THE HEART
The heart is located near the anterior chest wall, directly posterior to the sternum (Figure 20-2a ). A midsagittal section through the trunk would not divide the heart into two equal halves because the heart (1) lies slightly to the left of the midline, (2) sits at an angle to the longitudinal axis of the body, and (3) is rotated toward the left side. The heart is surrounded by the pericardial cavity, located in the anterior portion of the mediastinum. The mediastinum, which separates the two pleural cavities, also contains the thymus, esophagus, and trachea. Figure 20-2b is a sectional view that illustrates the position of the heart relative to other structures in the mediastinum.
The serous membrane lining the pericardial cavity is called the pericardium. To visualize the relationship between the heart and the pericardial cavity, imagine pushing your fist toward the center of a large balloon (Figure 20-2c ). The balloon represents the pericardium, and your fist is the heart. Your wrist, where the balloon folds back on itself, corresponds to the base of the heart, where the great vessels, the largest veins and arteries in the body, are attached to the heart. The space inside the balloon is the pericardial cavity.
The pericardium can be subdivided into the visceral pericardium and the parietal pericardium. The visceral pericardium, or epicardium, covers the outer surface of the heart; the parietal pericardium lines the inner surface of the pericardial sac, which surrounds the heart (Figure 20-2c ). The pericardial sac, which is reinforced by a dense network of collagen fibers, stabilizes the position of the heart and associated vessels within the mediastinum.
The space between the opposing parietal and visceral surfaces is the pericardial cavity. This cavity normally contains 10–20 ml of pericardial fluid secreted by the pericardial membranes. Pericardial fluid acts as a lubricant, reducing friction between the opposing surfaces as the heart beats.
Superficial Anatomy of the Heart
The four cardiac chambers can easily be identified in a superficial view of the heart (Figure 20-3a ). The two atria have relatively thin muscular walls, and they are highly expandable. When not filled with blood, the outer portion of each atrium deflates and becomes a lumpy, wrinkled flap. This expandable extension of an atrium is called an auricle (auris, ear), because it reminded early anatomists of the external ear, or an atrial appendage (Figure 20-3a ). The coronary sulcus, a deep groove, marks the border between the atria and the ventricles. The anterior interventricular sulcus and the posterior interventricular sulcus, shallower depressions, mark the boundary line between the left and right ventricles (Figure 20-3b ).
The connective tissue of the epicardium at the coronary and interventricular sulci generally contains substantial amounts of fat. In fresh or preserved hearts, this fat must be stripped away to expose the underlying grooves. These sulci also contain the arteries and veins that supply blood to the cardiac muscle of the heart.
The heart has an attached base and a free
apex. The great veins and arteries of the circulatory system are
connected to the superior end of the heart at the base. The base sits posterior
to the sternum at the level of the third costal cartilage, centered about
Internal Anatomy and Organization
The right atrium communicates with the right ventricle, and the left atrium with the left ventricle. The two atria are separated by the interatrial septum (septum, wall), and the two ventricles are separated by the much thicker interventricular septum (Figure 20-4a ). Each septum is a muscular partition. Atrioventricular (AV) valves, folds of fibrous tissue, extend into the openings between the atria and ventricles. These valves permit blood flow in one direction only: from the atria into the ventricles.
The Right Atrium
The right atrium receives blood from the systemic circuit through the two great veins, the superior vena cava (plural, venae cavae) and the inferior vena cava. The superior vena cava delivers blood to the right atrium from the head, neck, upper limbs, and chest. The superior vena cava opens into the posterior and superior portion of the right atrium. The inferior vena cava carries blood to the right atrium from the rest of the trunk, the viscera, and the lower limbs. The inferior vena cava opens into the posterior and inferior portion of the right atrium. The coronary veins of the heart return blood to the coronary sinus, which opens into the right atrium inferior to the connection with the inferior vena cava.
Prominent muscular ridges, the pectinate muscles (pectin, comb), or musculi pectinati, run along the inner surface of the auricle and across the adjacent anterior atrial wall (Figure 20-4a ). The interatrial septum separates the right atrium from the left atrium. From the fifth week of embryonic development until birth, the foramen ovale, an oval opening, penetrates the septum and connects the two atria. The foramen ovale permits blood flow from the right atrium to the left atrium while the lungs are developing. At birth, the foramen ovale closes; after 48 hours, the opening is permanently sealed. A small depression, the fossa ovalis, persists at this site in the adult heart (Figure 20-4a ). If the foramen ovale does not close, blood will flow from the left atrium into the right atrium rather than the opposite way, because after birth, blood pressure in the pulmonary circuit is lower than that in the systemic circuit. We will consider the physiological effects of this condition in Chapter 21.
Blood travels from the right atrium into the right ventricle through a broad opening bounded by three fibrous flaps. These flaps, or cusps, are part of the right atrioventricular (AV) valve, also known as the tricuspid (tri, three) valve. The free edge of each cusp is attached to tendinous connective tissue fibers called the chordae tendineae (tendinous cords). These fibers originate at the papillary muscles, conical muscular projections that arise from the inner surface of the right ventricle. The valve closes when the right ventricle contracts, preventing the backflow of blood into the right atrium. Without the chordae tendineae, the cusps would be like swinging doors that permitted blood flow in both directions.
The internal surface of the ventricle also contains a series of muscular ridges, the trabeculae carneae (carneus, fleshy). The moderator band is a muscular ridge that extends horizontally from the inferior portion of the interventricular septum and connects to the anterior papillary muscle. The moderator band is variable in size in humans. It is noteworthy because it contains a portion of the conducting system, an internal network that coordinates the contractions of cardiac muscle cells. The moderator band delivers the contraction stimulus to the papillary muscles so that they begin tensing the chordae tendineae before the rest of the ventricle contracts.
The superior end of the right ventricle tapers to a conical pouch, the conus arteriosus, which ends at the pulmonary semilunar valve. The pulmonary semilunar valve consists of three semilunar (half-moonРshaped) cusps of thick connective tissue. Blood flowing from the right ventricle passes through this valve to enter the pulmonary trunk, the start of the pulmonary circuit. The arrangement of cusps prevents backflow as the right ventricle relaxes. Once within the pulmonary trunk, blood flows into the left pulmonary arteries and the right pulmonary arteries. These vessels branch repeatedly within the lungs before supplying the capillaries where gas exchange occurs.
The Left Atrium
From the respiratory capillaries, blood collects into small veins that ultimately unite to form the four pulmonary veins. The posterior wall of the left atrium receives blood from two left and two right pulmonary veins. Like the right atrium, the left atrium has an auricle and a valve, the left atrioventricular (AV) valve, or bicuspid valve (Figure 20-4a ). As the name bicuspid implies, the left AV valve contains a pair, not a trio, of cusps. Clinicians often use the term mitral (mitre, a bishop's hat) when referring to this valve. The left AV valve permits the flow of blood from the left atrium into the left ventricle.
The Left Ventricle
The right and left ventricles contain equal
amounts of blood, but the left ventricle is much larger than the right because
it has thicker walls. The thick, muscular wall enables the left ventricle to
develop pressure sufficient to push blood through the large systemic circuit;
the right ventricle needs to pump blood, at lower pressure, only about
Blood leaves the left ventricle by passing through the aortic semilunar valve into the ascending aorta. The arrangement of cusps in the aortic semilunar valve is the same as that in the pulmonary semilunar valve. Saclike dilations of the base of the ascending aorta occur adjacent to each cusp. These sacs, called aortic sinuses, prevent the individual cusps from sticking to the wall of the aorta when the valve opens. Once the blood has been pumped out of the heart and into the systemic circuit, the aortic semilunar valve prevents backflow into the left ventricle. From the ascending aorta, blood flows on through the aortic arch and into the descending aorta (Figure 20-4a ). The pulmonary trunk is attached to the aortic arch by the ligamentum arteriosum, which marks the path of an important fetal blood vessel that linked the pulmonary and systemic circuits.
Structural Differences between the Left and Right Ventricles
The function of an atrium is to collect blood that is returning to the heart and deliver it to the attached ventricle. The functional demands on the right and left atria are very similar, and the two chambers look almost identical. The demands on the right and left ventricles, however, are very different, and there are significant structural differences between the two.
Anatomical differences between the left and right ventricles are best seen in a three-dimensional view (Figure 20-5a ). The lungs are close to the heart, and the pulmonary blood vessels are relatively short and wide. Thus the right ventricle normally does not need to push very hard to propel blood through the pulmonary circuit. The wall of the right ventricle is relatively thin, and in sectional view it resembles a pouch attached to the massive wall of the left ventricle. When it contracts, the right ventricle acts like a bellows pump, squeezing the blood against the mass of the left ventricle. This mechanism moves blood very efficiently with minimal effort, but it develops relatively low pressures.
A comparable pumping arrangement would not be suitable for the left ventricle, because six to seven times as much force must be exerted to push blood around the systemic circuit. The left ventricle has an extremely thick muscular wall, and it is round in cross section. When this ventricle contracts, two things happen: (1) The distance between the base and apex decreases, and (2) the diameter of the ventricular chamber decreases. If you imagine the effects of simultaneously squeezing and rolling up the end of a toothpaste tube, you will get the idea. The forces generated are quite powerful, more than enough to open the semilunar valve and eject blood into the ascending aorta. As the powerful left ventricle contracts, it also bulges into the right ventricular cavity (Figure 20-5c ). This dual action improves the efficiency of the right ventricle's efforts. Individuals whose right ventricular musculature has been severely damaged may survive because the contraction of the left ventricle helps push blood into the pulmonary circuit.
We will now detail the structure and function of the various heart valves.
The Atrioventricular Valves
The atrioventricular valves prevent the backflow of blood from the ventricles to the atria when the ventricles are contracting. The chordae tendineae and papillary muscles play an important role in the normal function of the AV valves. During the period known as ventricular diastole, the ventricles are relaxed. As each relaxed ventricle fills with blood, the chordae tendineae are loose and the AV valves offer no resistance to the flow of blood from the atria to the ventricles (Figure 20-6a ). The ventricles contract during the period of ventricular systole. As the ventricles begin to contract, blood moving back toward the atria swings the cusps together, closing the valves (Figure 20-6b ). At the same time, the contraction of the papillary muscles tenses the chordae tendineae and stops the cusps before they swing into the atria. If the chordae tendineae are cut or the papillary muscles damaged, the valves act like swinging doors, and there is backflow, or regurgitation, of blood into the atria each time the ventricles contract.
The pulmonary and aortic semilunar valves prevent the backflow of blood from the pulmonary trunk and aorta into the right and left ventricles. The semilunar valves do not require muscular braces because the arterial walls do not contract, and the relative positions of the cusps are stable. When these valves close, the three symmetrical cusps support one another like the legs of a tripod (Figure 20-6c ).
Cardiac Muscle Tissue
Recall from Chapter 4 that cardiac muscle cells are interconnected by intercalated discs. These discs convey the force of contraction from cell to cell and propagate action potentials. In Chapter 10, we briefly compared the properties of cardiac muscle tissue with the properties of other muscle types.
Connective Tissues and the Fibrous Skeleton
The connective tissues of the heart include large numbers of collagen and elastic fibers. Each cardiac muscle cell is wrapped in a strong but elastic sheath, and adjacent cells are tied together by fibrous cross-links, or "struts." These fibers are in turn interwoven into sheets that separate the superficial and deep muscle layers. These connective tissue fibers (1) provide physical support for the cardiac muscle fibers, blood vessels, and nerves of the myocardium; (2) help distribute the forces of contraction; (3) add strength and prevent overexpansion of the heart; and (4) provide elasticity that helps return the heart to its original size and shape after a contraction.
The fibrous skeleton of the heart consists of four dense bands of fibroelastic tissue that encircle the bases of the pulmonary trunk and aorta and the heart valves (Figure 20-6 ). These bands stabilize the positions of the heart valves and ventricular muscle cells and physically isolate the ventricular cells from the atrial cells.
The heart works continuously, and cardiac muscle cells require reliable supplies of oxygen and nutrients. The coronary circulation supplies blood to the muscles of the heart. During maximum exertion, the oxygen demand rises considerably, and the blood flow to the heart may increase to nine times that of resting levels. The coronary circulation includes an extensive network of coronary blood vessels.
The Coronary Arteries
The left and right coronary arteries originate at the base of the ascending aorta (Figure 20-8a ). Blood pressure here is the highest in the systemic circuit, and this pressure ensures a continuous flow of blood to meet the demands of active cardiac muscle tissue.
The Right Coronary
The right coronary artery, which follows the coronary sulcus around the heart, supplies blood to (1) the right atrium, (2) portions of both ventricles, and (3) portions of the conducting system of the heart, including the SA (sino-atrial) and AV nodes. The cells of the SA node and AV node are essential to establishing the normal heart rate. We will focus on their functions and their part in regulation of the heart rate in a later section. Inferior to the right atrium, the right coronary artery generally gives rise to one or more marginal branches, which extend across the ventricular surface (Figure 20-8c ). It then continues across the posterior surface of the heart, supplying the posterior interventricular branch, or posterior descending artery, which runs toward the apex within the posterior interventricular sulcus. The posterior interventricular branch supplies blood to the interventricular septum and adjacent portions of the ventricles.
The left coronary artery supplies blood to the left ventricle, left atrium, and the interventricular septum. As it reaches the anterior surface of the heart, it gives rise to a circumflex branch and an anterior interventricular branch. The circumflex branch curves to the left around the coronary sulcus, eventually meeting and fusing with small branches of the right coronary artery. The much larger anterior interventricular branch, or left anterior descending artery, swings around the pulmonary trunk and runs along the anterior surface within the anterior interventricular sulcus. This branch supplies small tributaries continuous with those of the posterior interventricular branch of the right coronary artery. Such interconnections between arteries are called anastomoses (anastomosis, outlet). Because the arteries are interconnected in this way, the blood supply to the cardiac muscle remains relatively constant despite pressure fluctuations in the left and right coronary arteries as the heart beats.
The Cardiac Veins
The great cardiac vein begins on the anterior surface of the ventricles, along the interventricular sulcus. This vein drains blood from the region supplied by the anterior interventricular branch of the left coronary artery. The great cardiac vein reaches the level of the atria and then curves around the left side of the heart within the coronary sulcus. The vein empties into the coronary sinus, a large, thin-walled vein that lies in the posterior portion of the coronary sulcus. The coronary sinus communicates with the right atrium near the base of the inferior vena cava. The other cardiac veins, which empty into the great cardiac vein or the coronary sinus, include (1) the posterior cardiac vein, draining the area served by the circumflex branch of the left coronary artery; (2) the middle cardiac vein, draining the area supplied by the posterior interventricular branch of the right coronary artery; and (3) the small cardiac vein and anterior cardiac veins, draining the other regions supplied by the right coronary artery and its tributaries (Figure 20-8d ).
THE VASCULAR system is divided for descriptive purposes into (a) the blood vascular system, which comprises the heart and bloodvessels for the circulation of the blood; and (b) the lymph vascular system, consisting of lymph glands and lymphatic vessels, through which a colorless fluid, the lymph, circulates. It must be noted, however, that the two systems communicate with each other and are intimately associated developmentally.
The heart is the central organ of the blood vascular system, and consists of a hollow muscle; by its contraction the blood is pumped to all parts of the body through a complicated series of tubes, termed arteries. The arteries undergo enormous ramification in their course throughout the body, and end in minute vessels, called arterioles, which in their turn open into a close-meshed network of microscopic vessels, termed capillaries. After the blood has passed through the capillaries it is collected into a series of larger vessels, called veins, by which it is returned to the heart. The passage of the blood through the heart and blood-vessels constitutes what is termed the circulation of the blood, of which the following is an outline.
The HEART is a hollow muscular organ, which is situated in thoracic cavity in middle mediastinum. It has a heart apex, which is directed down to the left and heart base. Heart has a sternocostal (anterior) surface, diaphragmatic (posterior) surface, right/left pulmonary surfaces. Coronal sulcus passes on diaphragmatic and partially on sternоcostal surfaces, which marks the border between ventricles and atriums. Anterior interventricular sulcus and posterior interventricular sulcus pass from coronal sulcus downward and project borders between right and left ventricles. On heart base right and left auricles are situated, which envelop the great vessels. On heart base at the anterior from right ventricle pulmonary trunk passes, which subdivides into two pulmonary arteries. Aorta
passes behind pulmonary trunk; behind from aorta from right side superior vena cava and inferior vena cava, and to the left four pulmonary veins.
Front view of heart and lungs.
Heart cavity subdivides on right and left atriums and right and left ventricles. Left chambers of heart are arterial and in adult do not communicate with right venous half of heart. Exist two blood circles.
Big circle or systemic circulation of the blood starts in left ventricle by aorta and terminates in right atrium by vena cava superior and inferior. Systemic circulation of the blood provides by arterial blood all of organs and tissues.
The small circle or pulmonary circulation of the blood begins by pulmonary trunk from right ventricle and terminates in left atrium by 4 pulmonary veins. Venous blood flows in arteries of pulmonary circulation of which and arterial (oxygenated) blood - in veins.
Right atrium consists of own atrium and right auricle.
Internal wall is smooth, but in auricle pectinate muscles are situated. Right atrium receives the superior and inferior venae cavae, which open by foramen of inferior vena cava and foramen of superior vena cava. Intervensus tubercle is situated between these foramens. Broadened posterior area, where two venae cavae fall is called as sinus venae cavae. Right atrium is separated from left by interatrial septum, where oval fossa is situated. It is limited by limbus of oval fossa. Atrium communicates by right ventricle through the right atrioventricular ostium. Foramen of coronal sinus situated between last and foramen of inferior vena cava. Alongside are contained foramens of venarum minimarum.
Right ventricle consists of own ventricle and conus arteriosus - superior part, which continues through the ostium of pulmonary trunk into pulmonary trunk. The right and left ventricles are separated by interventricular septum, which has muscular part (greater) and membranous part (lesser). On internal surface of right ventricle are situated the trabeculi carneae, which carry cone-shaped anterior, posterior and septal pappillar muscles. From top of these muscles chordae tendineae start and terminate at cusps of right atrioventricular valve.
Right atrioventricular ostium closes by right atrioventricular (tricuspidal) valve, which consists of anterior cusp, posterior cusp and septal cusp edges of which attach to chordae tendineae. During contraction of atria blood stream presses the cusps to the wall of ventricle. During contraction of ventricles free edges of cusps close up but do not pull out because they are kept by chordae tendineae from ventricle. Ostium of pulmonary trunk closes by valve of pulmonary trunk, which consists of right, left and anterior semilunar valvulae, which have on superior margin the nodules of semilunar valvulae. Nodules assist to compact closing up. Between each semilunar valvula and pulmonary trunk wall sinuses of pulmonary trunk are situated.
Base and diaphragmatic surface of heart.
Left atrium has an irregular cube shape; anterior wall forms a left auricle. Internal wall surfaces of left atrium is smooth and only in auricle area pectinate muscles are situated. The ostia of 4 pulmonary veins open into left atrium. Left atrium communicates with left ventricle by the means of left atrioventricular ostium. Oval fossa makes a mark poorly on interatrial septum.
Left ventricle is the largest heart chamber, its wall forms larger part of diaphragmatic surface. Internal surface containes the trabeculi carneae, which attach anterior papillary muscle and posterior papillary muscle. The tops of these muscles by means of cordae tendineae hold the cusps of mitral valve.
Left atrioventricular ostium closes by left atrioventricular (bicuspidal) valve [valve mitralis], which consists of anterior cusp and posterior cusp edges of which attach to chordae tendineae. From left ventricle aorta starts. Aortic ostium closes by aortic valve, which consists of right, left and posterior semilunar valvulae, which have on superior margin the nodules of semilunar valvulae. Between each semilunar valvula and aorta walls are situated aortic sinuses.
Base of ventricles exposed by removal of the atria.
Size.—The heart, in the adult, measures
Component Parts.—As has already been stated (page 497), the heart is subdivided by septa into right and left halves, and a constriction subdivides each half of the organ into two cavities, the upper cavity being called the atrium, the lower the ventricle. The heart therefore consists of four chambers, viz., right and left atria, and right and left ventricles.
The division of the heart into four cavities is indicated on its surface by grooves. The atria are separated from the ventricles by the coronary sulcus (auriculoventricular groove); this contains the trunks of the nutrient vessels of the heart, and is deficient in front, where it is crossed by the root of the pulmonary artery. The interatrial groove, separating the two atria, is scarcely marked on the posterior surface, while anteriorly it is hidden by the pulmonary artery and aorta. The ventricles are separated by two grooves, one of which, the anterior longitudinal sulcus, is situated on the sternocostal surface of the heart, close to its left margin, the other posterior longitudinal sulcus, on the diaphragmatic surface near the right margin; these grooves extend from the base of the ventricular portion to a notch, the incisura apicis cordis, on the acute margin of the heart just to the right of the apex.
The base (basis cordis) (491), directed upward, backward, and to the right, is separated from the fifth, sixth, seventh, and eighth thoracic vertebræ by the esophagus, aorta, and thoracic duct. It is formed mainly by the left atrium, and, to a small extent, by the back part of the right atrium. Somewhat quadrilateral in form, it is in relation above with the bifurcation of the pulmonary artery, and is bounded below by the posterior part of the coronary sulcus, containing the coronary sinus. On the right it is limited by the sulcus terminalis of the right atrium, and on the left by the ligament of the left vena cava and the oblique vein of the left atrium. The four pulmonary veins, two on either side, open into the left atrium, while the superior vena cava opens into the upper, and the anterior vena cava into the lower, part of the right atrium.
The Apex (apex cordis).—The
apex is directed downward, forward, and to the left, and is overlapped by the
left lung and pleura: it lies behind the fifth left intercostal space, 8 to
The sternocostal surface (492) is directed forward, upward, and to the left. Its lower part is convex, formed chiefly by the right ventricle, and traversed near its left margin by the anterior longitudinal sulcus. Its upper part is separated from the lower by the coronary sulcus, and is formed by the atria; it presents a deep concavity (494), occupied by the ascending aorta and the pulmonary artery.
The diaphragmatic surface (491), directed downward and slightly backward, is formed by the ventricles, and rests upon the central tendon and a small part of the left muscular portion of the diaphragm. It is separated from the base by the posterior part of the coronary sulcus, and is traversed obliquely by the posterior longitudinal sulcus.
The right margin of the heart is long, and is formed by the right
atrium above and the right ventricle below. The atrial portion is rounded and
almost vertical; it is situated behind the third, fourth, and fifth right
costal cartilages about
The left or obtuse margin is shorter, full, and rounded:
it is formed mainly by the left ventricle, but to a slight extent, above, by
the left atrium. It extends from a point in the second left intercostal space,
Right Atrium (atrium dextrum; right auricle).—The right atrium is larger than the left, but its walls are somewhat
thinner, measuring about
Sinus Venarum (sinus venosus).—The sinus venarum is the large quadrangular cavity placed between the two venæ cavæ. Its walls, which are extremely thin, are connected below with the right ventricle, and medially with the left atrium, but are free in the rest of their extent.
Auricula (auricula dextra; right auricular appendix).—The auricula is a small conical muscular pouch, the margins of which present a dentated edge. It projects from the upper and front part of the sinus forward and toward the left side, overlapping the root of the aorta.
Sternocostal surface of heart.
The separation of the auricula from the sinus venarum is indicated externally by a groove, the terminal sulcus, which extends from the front of the superior vena cava to the front of the inferior vena cava, and represents the line of union of the sinus venosus of the embryo with the primitive atrium. On the inner wall of the atrium the separation is marked by a vertical, smooth, muscular ridge, the terminal crest. Behind the crest the internal surface of the atrium is smooth, while in front of it the muscular fibers of the wall are raised into parallel ridges resembling the teeth of a comb, and hence named the musculi pectinati.
Its interior presents the following parts for examination:
Superior vena cava.
Inferior vena cava.
Valve of the inferior vena cava.
Foramina venarum minimarum.
Valve of the coronary sinus.
Limbus fossæ ovalis.
The superior vena cava returns the blood from the upper half of the body, and opens into the upper and back part of the atrium, the direction of its orifice being downward and forward. Its opening has no valve.
The inferior vena cava, larger than the superior, returns the blood from the lower half of the body, and opens into the lowest part of the atrium, near the atrial septum, its orifice being directed upward and backward, and guarded by a rudimentary valve, the valve of the inferior vena cava (Eustachian valve). The blood entering the atrium through the superior vena cava is directed downward and forward, i.e., toward the atrioventricular orifice, while that entering through the inferior vena cava is directed upward and backward, toward the atrial septum. This is the normal direction of the two currents in fetal life.
The coronary sinus opens into the atrium, between the orifice of the inferior vena cava and the atrioventricular opening. It returns blood from the substance of the heart and is protected by a semicircular valve, the valve of the coronary sinus (valve of Thebesius).
Interior of right side of heart.
The foramina venarum minimarum (foramina Thebesii) are the orifices of minute veins (venœ cordis minimœ), which return blood directly from the muscular substance of the heart.
The atrioventricular opening (tricuspid orifice) is the large oval aperture of communication between the atrium and the ventricle; it will be described with the right ventricle.
The valve of the inferior vena cava (valvula venœ cavœ inferioris [Eustachii]; Eustachian valve) is situated in front of the orifice of the inferior vena cava. It is semilunar in form, its convex margin being attached to the anterior margin of the orifice; its concave margin, which is free, ends in two cornua, of which the left is continuous with the anterior edge of the limbus fossæ ovalis while the right is lost on the wall of the atrium. The valve is formed by a duplicature of the lining membrane of the atrium, containing a few muscular fibers. In the fetus this valve is of large size, and serves to direct the blood from the inferior vena cava, through the foramen ovale, into the left atrium. In the adult it occasionally persists, and may assist in preventing the reflux of blood into the inferior vena cava; more commonly it is small, and may present a cribriform or filamentous appearance; sometimes it is altogether wanting.
The valve of the coronary sinus (valvula sinus coronarii [Thebesii]; Thebesian valve) is a semicircular fold of the lining membrane of the atrium, at the orifice of the coronary sinus. It prevents the regurgitation of blood into the sinus during the contraction of the atrium. This valve may be double or it may be cribriform.
The fossa ovalis is an oval depression on the septal wall of the atrium, and corresponds to the situation of the foramen ovale in the fetus. It is situated at the lower part of the septum, above and to the left of the orifice of the inferior vena cava.
The limbus fossæ ovalis (annulus ovalis) is the prominent oval margin of the fossa ovalis. It is most distinct above and at the sides of the fossa; below, it is deficient. A small slit-like valvular opening is occasionally found, at the upper margin of the fossa, leading upward beneath the limbus, into the left atrium; it is the remains of the fetal aperture between the two atria.
The intervenous tubercle (tuberculum intervenosum; tubercle of Lower) is a small projection on the posterior wall of the atrium, above the fossa ovalis. It is distinct in the hearts of quadrupeds, but in man is scarcely visible. It was supposed by Lower to direct the blood from the superior vena cava toward the atrioventricular opening.
Right Ventricle (ventriculus dexter).—The right ventricle is triangular in form, and extends from the right atrium to near the apex of the heart. Its anterosuperior surface is rounded and convex, and forms the larger part of the sternocostal surface of the heart. Its under surface is flattened, rests upon the diaphragm, and forms a small part of the diaphragmatic surface of the heart. Its posterior wall is formed by the ventricular septum, which bulges into the right ventricle, so that a transverse section of the cavity presents a semilunar outline. Its upper and left angle forms a conical pouch, the conus arteriosus, from which the pulmonary artery arises. A tendinous band, which may be named the tendon of the conus arteriosus, extends upward from the right atrioventricular fibrous ring and connects the posterior surface of the conus arteriosus to the aorta. The wall of the right ventricle is thinner than that of the left, the proportion between them being as 1 to 3; it is thickest at the base, and gradually becomes thinner toward the apex. The cavity equals in size that of the left ventricle, and is capable of containing about 85 c.c.
Its interior (493) presents the following parts for examination:
The right atrioventricular orifice is the large oval aperture of
communication between the right atrium and ventricle. Situated at the base of
the ventricle, it measures about
The opening of the pulmonary artery is circular in form, and situated at the summit of the conus arteriosus, close to the ventricular septum. It is placed above and to the left of the atrioventricular opening, and is guarded by the pulmonary semilunar valves.
The tricuspid valve (valvula tricuspidalis) (493, 495) consists of three somewhat triangular cusps or segments. The largest cusp is interposed between the atrioventricular orifice and the conus arteriosus and is termed the anterior or infundibular cusp. A second, the posterior or marginal cusp, is in relation to the right margin of the ventricle, and a third, the medial or septal cusp, to the ventricular septum. They are formed by duplicatures of the lining membrane of the heart, strengthened by intervening layers of fibrous tissue: their central parts are thick and strong, their marginal portions thin and translucent, and in the angles between the latter small intermediate segments are sometimes seen. Their bases are attached to a fibrous ring surrounding the atrioventricular orifice and are also joined to each other so as to form a continuous annular membrane, while their apices project into the ventricular cavity. Their atrial surfaces, directed toward the blood current from the atrium, are smooth; their ventricular surfaces, directed toward the wall of the ventricle, are rough and irregular, and, together with the apices and margins of the cusps, give attachment to a number of delicate tendinous cords, the chordæ tendineæ.
Heart seen from above.
The trabeculæ carneæ (columnœ carneœ) are rounded or irregular muscular columns which project from the whole of the inner surface of the ventricle, with the exception of the conus arteriosus. They are of three kinds: some are attached along their entire length on one side and merely form prominent ridges, others are fixed at their extremities but free in the middle, while a third set (musculi papillares) are continuous by their bases with the wall of the ventricle, while their apices give origin to the chordæ tendineæ which pass to be attached to the segments of the tricuspid valve. There are two papillary muscles, anterior and posterior: of these, the anterior is the larger, and its chordæ tendineæ are connected with the anterior and posterior cusps of the valve: the posterior papillary muscle sometimes consists of two or three parts; its chordæ tendineæ are connected with the posterior and medial cusps. In addition to these, some chordæ tendineæ spring directly from the ventricular septum, or from small papillary eminences on it, and pass to the anterior and medial cusps. A muscular band, well-marked in sheep and some other animals, frequently extends from the base of the anterior papillary muscle to the ventricular septum. From its attachments it may assist in preventing overdistension of the ventricle, and so has been named the moderator band.
The pulmonary semilunar valves (494) are three in number, two in front and one behind, formed by duplicatures of the lining membrane, strengthened by fibrous tissue. They are attached, by their convex margins, to the wall of the artery, at its junction with the ventricle, their free borders being directed upward into the lumen of the vessel. The free and attached margins of each are strengthened by tendinous fibers, and the former presents, at its middle, a thickened nodule (corpus Arantii). From this nodule tendinous fibers radiate through the segment to its attached margin, but are absent from two narrow crescentic portions, the lunulæ, placed one on either side of the nodule immediately adjoining the free margin. Between the semilunar valves and the wall of the pulmonary artery are three pouches or sinuses (sinuses of Valsalva).
Left Atrium (atrium sinistum; left auricle).—The left atrium is rather smaller than the right, but its walls are
thicker, measuring about
The principal cavity is cuboidal in form, and concealed, in front, by the pulmonary artery and aorta; in front and to the right it is separated from the right atrium by the atrial septum; opening into it on either side are the two pulmonary veins.
Auricula (auricula sinistra; left auricular appendix).—The auricula is somewhat constricted at its junction with the principal cavity; it is longer, narrower, and more curved than that of the right side, and its margins are more deeply indented. It is directed forward and toward the right and overlaps the root of the pulmonary artery.
Interior of left side of heart.
The interior of the left atrium (496) presents the following parts for examination:
Openings of the four pulmonary veins.
Left atrioventricular opening.
The pulmonary veins, four in number, open into the upper part of the posterior surface of the left atrium—two on either side of its middle line: they are not provided with valves. The two left veins frequently end by a common opening.
The left atrioventricular opening is the aperture between the left atrium and ventricle, and is rather smaller than the corresponding opening on the right side.
The musculi pectinati, fewer and smaller than in the right auricula, are confined to the inner surface of the auricula.
On the atrial septum may be seen a lunated impression, bounded below by a crescentic ridge, the concavity of which is turned upward. The depression is just above the fossa ovalis of the right atrium.
Left Ventricle (ventriculus sinister).—The left ventricle is longer and more conical in shape than the right, and on transverse section its concavity presents an oval or nearly circular outline. It forms a small part of the sternocostal surface and a considerable part of the diaphragmatic surface of the heart; it also forms the apex of the heart. Its walls are about three times as thick as those of the right ventricle.
Its interior (496) presents the following parts for examination:
Bicuspid or Mitral.
The left atrioventricular opening (mitral orifice) is placed below and to the left of the aortic orifice. It is a little smaller than the corresponding aperture of the opposite side, admitting only two fingers. It is surrounded by a dense fibrous ring, covered by the lining membrane of the heart, and is guarded by the bicuspid or mitral valve.
Aorta laid open to show the semilunar valves.
The aortic opening is a circular aperture, in front and to the right of the atrioventricular, from which it is separated by the anterior cusp of the bicuspid valve. Its orifice is guarded by the aortic semilunar valves. The portion of the ventricle immediately below the aortic orifice is termed the aortic vestibule, and possesses fibrous instead of muscular walls.
The bicuspid or mitral valve (valvula bicuspidalis [metralis]) (495, 496) is attached to the circumference of the left atrioventricular orifice in the same way that the tricuspid valve is on the opposite side. It consists of two triangular cusps, formed by duplicatures of the lining membrane, strengthened by fibrous tissue, and containing a few muscular fibers. The cusps are of unequal size, and are larger, thicker, and stronger than those of the tricuspid valve. The larger cusp is placed in front and to the right between the atrioventricular and aortic orifices, and is known as the anterior or aortic cusp; the smaller or posterior cusp is placed behind and to the left of the opening. Two smaller cusps are usually found at the angles of junction of the larger. The cusps of the bicuspid valve are furnished with chordæ tendineæ, which are attached in a manner similar to those on the right side; they are, however, thicker, stronger, and less numerous.
The aortic semilunar valves (494, 497) are three in number, and surround the orifice of the aorta; two are anterior (right and left) and one posterior. They are similar in structure, and in their mode of attachment, to the pulmonary semilunar valves, but are larger, thicker, and stronger; the lunulæ are more distinct, and the noduli or corpora Arantii thicker and more prominent. Opposite the valves the aorta presents slight dilatations, the aortic sinuses (sinuses of Valsalva), which are larger than those at the origin of the pulmonary artery.
The trabeculæ carneæ are of three kinds, like those upon the right side, but they are more numerous, and present a dense interlacement, especially at the apex, and upon the posterior wall of the ventricle. The musculi papillares are two in number, one being connected to the anterior, the other to the posterior wall; they are of large size, and end in rounded extremities from which the chordæ tendineæ arise. The chordæ tendineæ from each papillary muscle are connected to both cusps of the bicuspid valve.
The course of the blood from the left ventricle through the body generally to the right side of the heart constitutes the greater or systemic circulation, while its passage from the right ventricle through the lungs to the left side of the heart is termed the lesser or pulmonary circulation.
It is necessary, however, to state that the blood which circulates through the spleen, pancreas, stomach, small intestine, and the greater part of the large intestine is not returned directly from these organs to the heart, but is conveyed by the portal vein to the liver. In the liver this vein divides, like an artery, and ultimately ends in capillary-like vessels (sinusoids), from which the rootlets of a series of veins, called the hepatic veins, arise; these carry the blood into the inferior vena cava, whence it is conveyed to the right atrium. From this it will be seen that the blood contained in the portal vein passes through two sets of vessels: (1) the capillaries in the spleen, pancreas, stomach, etc., and (2) the sinusoids in the liver. The blood in the portal vein carries certain of the products of digestion: the carbohydrates, which are mostly taken up by the liver cells and stored as glycogen, and the protein products which remain in solution and are carried into the general circulation to the various tissues and organs of the body.
Speaking generally, the arteries may be said to contain pure and the veins impure blood. This is true of the systemic, but not of the pulmonary vessels, since it has been seen that the impure blood is conveyed from the heart to the lungs by the pulmonary arteries, and the pure blood returned from the lungs to the heart by the pulmonary veins. Arteries, therefore, must be defined as vessels which convey blood from the heart, and veins as vessels which return blood to the heart.
Section of the heart showing the ventricular septum.
Ventricular Septum (septum ventriculorum; interventricular septum) (498).
—The ventricular septum is directed obliquely backward and to the right, and is curved with the convexity toward the right ventricle: its margins correspond with the anterior and posterior longitudinal sulci. The greater portion of it is thick and muscular and constitutes the muscular ventricular septum, but its upper and posterior part, which separates the aortic vestibule from the lower part of the right atrium and upper part of the right ventricle, is thin and fibrous, and is termed the membranous ventricular septum. An abnormal communication may exist between the ventricles at this part owing to defective development of the membranous septum.
THE DISTRIBUTION of the systematic arteries is like a highly ramified tree, the common trunk of which, formed by the aorta, commences at the left ventricle, while the smallest ramifications extend to the peripheral parts of the body and the contained organs. Arteries are found in all parts of the body, except in the hairs, nails, epidermis, cartilages, and cornea; the larger trunks usually occupy the most protected situations, running, in the limbs, along the flexor surface, where they are less exposed to injury.
There is considerable variation in the mode of division of the arteries: occasionally a short trunk subdivides into several branches at the same point, as may be observed in the celiac artery and the thyrocervical trunk: the vessel may give off several branches in succession, and still continue as the main trunk, as is seen in the arteries of the limbs; or the division may be dichotomous, as, for instance, when the aorta divides into the two common iliacs.
A branch of an artery is smaller than the trunk from which it arises; but if an artery divides into two branches, the combined sectional area of the two vessels is, in nearly every instance, somewhat greater than that of the trunk; and the combined sectional area of all the arterial branches greatly exceeds that of the aorta; so that the arteries collectively may be regarded as a cone, the apex of which corresponds to the aorta, and the base to the capillary system.
The arteries, in their distribution, communicate with one another, forming what are called anastomoses, and these communications are very free between the large as well as between the smaller branches. The anastomosis between trunks of equal size is found where great activity of the circulation is requisite, as in the brain; here the two vertebral arteries unite to form the basilar, and the two anterior cerebral arteries are connected by a short communicating trunk; it is also found in the abdomen, where the intestinal arteries have very ample anastomoses between their larger branches. In the limbs the anastomoses are most numerous and of largest size around the joints, the branches of an artery above uniting with branches from the vessels below. These anastomoses are of considerable interest to the surgeon, as it is by their enlargement that a collateral circulation is established after the application of a ligature to an artery. The smaller branches of arteries anastomose more frequently than the larger; and between the smallest twigs these anastomoses become so numerous as to constitute a close network that pervades nearly every tissue of the body.
Throughout the body generally the larger arterial branches pursue a fairly straight course, but in certain situations they are tortuous. Thus the external maxillary artery in its course over the face, and the arteries of the lips, are extremely tortuous to accommodate themselves to the movements of the parts. The uterine arteries are also tortuous, to accommodate themselves to the increase of size which the uterus undergoes during pregnancy.
The pulmonary artery conveys the venous blood from the right ventricle of the heart to the lungs. It is a short, wide vessel, about 5 cm. in length and 3 cm. in diameter, arising from the conus arteriosus of the right ventricle. It extends obliquely upward and backward, passing at first in front and then to the left of the ascending aorta, as far as the under surface of the aortic arch, where it divides, about the level of the fibrocartilage between the fifth and sixth thoracic vertebræ, into right and left branches of nearly equal size.
Relations.—The whole of this vessel is contained within the pericardium. It is enclosed with the ascending aorta in a single tube of the visceral layer of the serous pericardium, which is continued upward upon them from the base of the heart. The fibrous layer of the pericardium is gradually lost upon the external coats of the two branches of the artery. In front, the pulmonary artery is separated from the anterior end of the second left intercostal space by the pleura and left lung, in addition to the pericardium; it rests at first upon the ascending aorta, and higher up lies in front of the left atrium on a plane posterior to the ascending aorta. On either side of its origin is the auricula of the corresponding atrium and a coronary artery, the left coronary artery passing, in the first part of its course, behind the vessel. The superficial part of the cardiac plexus lies above its bifurcation, between it and the arch of the aorta.
The right branch of the pulmonary artery (ramus dexter a. pulmonalis), longer and larger than the left, runs horizontally to the right, behind the ascending aorta and superior vena cava and in front of the right bronchus, to the root of the right lung, where it divides into two branches. The lower and larger of these goes to the middle and lower lobes of the lung; the upper and smaller is distributed to the upper lobe.
The left branch of the pulmonary artery (ramus sinister a. pulmonalis), shorter and somewhat smaller than the right, passes horizontally in front of the descending aorta and left bronchus to the root of the left lung, where it divides into two branches, one for each lobe of the lung. Above, it is connected to the concavity of the aortic arch by the ligamentum arteriosum, on the left of which is the left recurrent nerve, and on the right the superficial part of the cardiac plexus. Below, it is joined to the upper left pulmonary vein by the ligament of the left vena cava.
Myocardium (middle layer) is formed by muscular tissue, which consists of cardiomyocytes. Muscular fibbers of atria and ventricles start from fibrous tissue, which enters to composition of soft heart skeleton. Last includes right and left fibrous rings, that are situated around right and left atrioventricular ostia, right fibrous triangle and left fibrous triangle, that are situated around valve of aorta and valve of pulmonary trunk, and membranous part of interventricular septum.
Myocardium of atria consists of two layers: superficial, which is common for both of atria and consists of circulation fibres, and deep layer, which consists of longitudinal bundles and is separate each from other.
Myocardium of ventricles consists of three layers: external, middle and internal. External (oblique) layer origins from fibrous annuli, continues downward till apex cordis where forms vortex cordis and passes into internal layer of opposite side with longitudinal fibres. So, external and internal layers are common for both ventricles and middle (circular) layer separate for each ventricle.
External heart membrane epicardium is visceral sheet of serous pericardium. Epicardium covers a heart, initial departments of aorta and pulmonary trunk, and also terminal departments of venae cavae and pulmonary veins. Visceral sheet passes into parietal sheet of serous pericardium on these vessels.
Strucutre.—The heart consists of muscular fibers, and of fibrous rings which serve for their attachment. It is covered by the visceral layer of the serous pericardium (epicardium), and lined by the endocardium. Between these two membranes is the muscular wall or myocardium.
The endocardium is a thin, smooth membrane which lines and gives the glistening appearance to the inner surface of the heart; it assists in forming the valves by its reduplications, and is continuous with the lining membrane of the large bloodvessels. It consists of connective tissue and elastic fibers, and is attached to the muscular structure by loose elastic tissue which contains bloodvessels and nerves; its free surface is covered by endothelial cells.
The fibrous rings surround the atrioventricular and arterial orifices, and are stronger upon the left than on the right side of the heart. The atrioventricular rings serve for the attachment of the muscular fibers of the atria and ventricles, and for the attachment of the bicuspid and tricuspid valves. The left atrioventricular ring is closely connected, by its right margin, with the aortic arterial ring; between these and the right atrioventricular ring is a triangular mass of fibrous tissue, the trigonum fibrosum, which represents the os cordis seen in the heart of some of the larger animals, as the ox and elephant. Lastly, there is the tendinous band, already referred to, the posterior surface of the conus arteriosus.
The fibrous rings surrounding the arterial orifices serve for the attachment of the great vessels and semilunar valves. Each ring receives, by its ventricular margin, the attachment of some of the muscular fibers of the ventricles; its opposite margin presents three deep semicircular notches, to which the middle coat of the artery is firmly fixed. The attachment of the artery to its fibrous ring is strengthened by the external coat and serous membrane externally, and by the endocardium internally. From the margins of the semicircular notches the fibrous structure of the ring is continued into the segments of the valves. The middle coat of the artery in this situation is thin, and the vessel is dilated to form the sinuses of the aorta and pulmonary artery.
Cardiac Muscular Tissue.—The fibers of the heart differ very remarkably from those of other striped muscles. They are smaller by one-third, and their transverse striæ are by no means so well-marked. They show faint longitudinal striation. The fibers are made up of distinct quadrangular cells, joined end to end so as to form a syncytium (499). Each cell contains a clear oval nucleus, situated near its center. The extremities of the cells have a tendency to branch or divide, the subdivisions uniting with offsets from other cells, and thus producing an anastomosis of the fibers. The connective tissue between the bundles of fibers is much less than in ordinary striped muscle, and no sarcolemma has been proved to exist.
Purkinje Fibers (500).—Between the endocardium and the ordinary cardiac muscle are found, imbedded in a small amount of connective tissue, peculiar fibers known as Purkinje fibers. They are found in certain mammals and in birds, and can be best seen in the sheep’s heart, where they form a considerable portion of the moderator band and also appear as gelatinous-looking strands on the inner walls of the atria and ventricles. They also occur in the human heart associated with the terminal distributions of the bundle of His. The fibers are very much larger in size than the cardiac cells and differ from them in several ways. In longitudinal section they are quadrilateral in shape, being about twice as long as they are broad. The central portion of each fiber contains one or more nuclei and is made up of granular protoplasm, with no indication of striations, while the peripheral portion is clear and has distinct transverse striations. The fibers are intimately connected with each other, possess no definite sarcolemma, and do not branch.
The muscular structure of the heart consists of bands of fibers, which present an exceedingly intricate interlacement. They comprise (a) the fibers of the atria, (b) the fibers of the ventricles, and (c) the atrioventricular bundle of His.
The fibers of the atria are arranged in two layers—a superficial, common to both cavities, and a deep, proper to each. The superficial fibers are most distinct on the front of the atria, across the bases of which they run in a transverse direction, forming a thin and incomplete layer. Some of these fibers run into the atrial septum. The deep fibers consist of looped and annular fibers. The looped fibers pass upward over each atrium, being attached by their two extremities to the corresponding atrioventricular ring, in front and behind. The annular fibers surround the auriculæ, and form annular bands around the terminations of the veins and around the fossa ovalis.
The fibers of the ventricles are arranged in a complex manner, and various accounts have been given of their course and connections; the following description is based on the work of McCallum. 94 They consist of superficial and deep layers, all of which, with the exception of two, are inserted into the papillary muscles of the ventricles. The superficial layers consist of the following: (a) Fibers which spring from the tendon of the conus arteriosus and sweep downward and toward the left across the anterior longitudinal sulcus and around the apex of the heart, where they pass upward and inward to terminate in the papillary muscles of the left ventricle; those arising from the upper half of the tendon of the conus arteriosus pass to the anterior papillary muscle, those from the lower half to the posterior papillary muscle and the papillary muscles of the septum. (b) Fibers which arise from the right atrioventricular ring and run diagonally across the diaphragmatic surface of the right ventricle and around its right border on to its costosternal surface, where they dip beneath the fibers just described, and, crossing the anterior longitudinal sulcus, wind around the apex of the heart and end in the posterior papillary muscle of the left ventricle. (c) Fibers which spring from the left atrioventricular ring, and, crossing the posterior longitudinal sulcus, pass successively into the right ventricle and end in its papillary muscles. The deep layers are three in number; they arise in the papillary muscles of one ventricle and, curving in an S-shaped manner, turn in at the longitudinal sulcus and end in the papillary muscles of the other ventricle. The layer which is most superficial in the right ventricle lies next the lumen of the left, and vice versa. Those of the first layer almost encircle the right ventricle and, crossing in the septum to the left, unite with the superficial fibers from the right atrioventricular ring to form the posterior papillary muscle. Those of the second layer have a less extensive course in the wall of the right ventricle, and a correspondingly greater course in the left, where they join with the superficial fibers from the anterior half of the tendon of the conus arteriosus to form the papillary muscles of the septum. Those of the third layer pass almost entirely around the left ventricle and unite with the superficial fibers from the lower half of the tendon of the conus arteriosus to form the anterior papillary muscle. Besides the layers just described there are two bands which do not end in papillary muscles. One springs from the right atrioventricular ring and crosses in the atrioventricular septum; it then encircles the deep layers of the left ventricle and ends in the left atrioventricular ring. The second band is apparently confined to the left ventricle; it is attached to the left atrioventricular ring, and encircles the portion of the ventricle adjacent to the aortic orifice.
The atrioventricular bundle of His (501), is the only direct muscular connection known to exist between the atria and the ventricles. Its cells differ from ordinary cardiac muscle cells in being more spindle-shaped. They are, moreover, more loosely arranged and have a richer vascular supply than the rest of the heart muscle. It arises in connection with two small collections of spindle-shaped cells, the sinoatrial and atrioventricular nodes. The sinoatrial node is situated on the anterior border of the opening of the superior vena cava; from its strands of fusiform fibers run under the endocardium of the wall of the atrium to the atrioventricular node. The atrioventricular node lies near the orifice of the coronary sinus in the annular and septal fibers of the right atrium; from it the atrioventricular bundle passes forward in the lower part of the membranous septum, and divides into right and left fasciculi. These run down in the right and left ventricles, one on either side of the ventricular septum, covered by endocardium. In the lower parts of the ventricles they break up into numerous strands which end in the papillary muscles and in the ventricular muscle generally. The greater portion of the atrioventricular bundle consists of narrow, somewhat fusiform fibers, but its terminal strands are composed of Purkinje fibers.
Dr. A. Morison 95 has shown that in the sheep and pig the atrioventricular bundle “is a great avenue for the transmission of nerves from the auricular to the ventricular heart; large and numerous nerve trunks entering the bundle and coursing with it.” From these, branches pass off and form plexuses around groups of Purkinje cells, and from these plexuses fine fibrils go to innervate individual cells.
Schematic representation of the atrioventricular bundle of His. The bundle, represented in red originates near the orifice of the coronary sinus, undergoes slight enlargement to form a node, passes forward to the ventricular septum, and divides into two limbs. The ultimate distribution cannot be completely shown in this diagram.
Structure of Arteries—The arteries are composed of three coats: an internal or endothelial coat (tunica intima of Kölliker); a middle or muscular coat (tunica media); and an external or connective-tissue coat (tunica adventitia). The two inner coats together are very easily separated from the external, as by the ordinary operation of tying a ligature around an artery. If a fine string be tied forcibly upon an artery and then taken off, the external coat will be found undivided, but the two inner coats are divided in the track of the ligature and can easily be further dissected from the outer coat.
The arteries, in their distribution throughout the body, are included in thin fibro-areolar investments, which form their sheaths. The vessel is loosely connected with its sheath by delicate areolar tissue; and the sheath usually encloses the accompanying veins, and sometimes a nerve. Some arteries, as those in the cranium, are not included in sheaths.
All the larger arteries, like the other organs of the body, are supplied with bloodvessels. These nutrient vessels, called the vasa vasorum, arise from a branch of the artery, or from a neighboring vessel, at some considerable distance from the point at which they are distributed; they ramify in the loose areolar tissue connecting the artery with its sheath, and are distributed to the external coat, but do not, in man, penetrate the other coats; in some of the larger mammals a few vessels have been traced into the middle coat. Minute veins return the blood from these vessels; they empty themselves into the vein or veins accompanying the artery. Lymphatic vessels are also present in the outer coat.
Arteries are also supplied with nerves, which are derived from the sympathetic, but may pass through the cerebrospinal nerves. They form intricate plexuses upon the surfaces of the larger trunks, and run along the smaller arteries as single filaments, or bundles of filaments which twist around the vessel and unite with each other in a plexiform manner. The branches derived from these plexuses penetrate the external coat and are distributed principally to the muscular tissue of the middle coat, and thus regulate, by causing the contraction and relaxation of this tissue the amount of blood sent to any part.
The Capillaries.—The smaller arterial branches (excepting those of the cavernous structure of the sexual organs, of the splenic pulp, and of the placenta) terminate in net-works of vessels which pervade nearly every tissue of the body. These vessels, from their minute size, are termed capillaries. They are interposed between the smallest branches of the arteries and the commencing veins, constituting a net-work, the branches of which maintain the same diameter throughout; the meshes of the net-work are more uniform in shape and size than those formed by the anastomoses of the small arteries and veins.
The diameters of the capillaries vary in the different tissues of the body, the usual size being about 8μ. The smallest are those of the brain and the mucous membrane of the intestines; and the largest those of the skin and the marrow of bone, where they are stated to be as large as 20μ in diameter. The form of the capillary net varies in the different tissues, the meshes being generally rounded or elongated.
The rounded form of mesh is most common, and prevails where there is a dense network, as in the lungs, in most glands and mucous membranes, and in the cutis; the meshes are not of an absolutely circular outline, but more or less angular, sometimes nearly quadrangular, or polygonal, or more often irregular.
Elongated meshes are observed in the muscles and nerves, the meshes resembling parallelograms in form, the long axis of the mesh running parallel with the long axis of the nerve or muscle. Sometimes the capillaries have a looped arrangement; a single vessel projecting from the common net-work and returning after forming one or more loops, as in the papillæ of the tongue and skin.
The number of the capillaries and the size of the meshes determine the degree of vascularity of a part. The closest network and the smallest interspaces are found in the lungs and in the choroid coat of the eye. In these situations the interspaces are smaller than the capillary vessels themselves. In the intertubular plexus of the kidney, in the conjunctiva, and in the cutis, the interspaces are from three to four times as large as the capillaries which form them; and in the brain from eight to ten times as large as the capillaries in their long diameters, and from four to six times as large in their transverse diameters. In the adventitia of arteries the width of the meshes is ten times that of the capillary vessels. As a general rule, the more active the function of the organ, the closer is its capillary net and the larger its supply of blood; the meshes of the network are very narrow in all growing parts, in the glands, and in the mucous membranes, wider in bones and ligaments which are comparatively inactive; bloodvessels are nearly altogether absent in tendons, in which very little organic change occurs after their formation. In the liver the capillaries take a more or less radial course toward the intralobular vein, and their walls are incomplete, so that the blood comes into direct contact with the liver cells. These vessels in the liver are not true capillaries but “sinusoids;” they are developed by the growth of columns of liver cells into the blood spaces of the embryonic organ.
Structure.—The wall of a capillary consists of a fine transparent endothelial layer, composed of cells joined edge to edge by an interstitial cement substance, and continuous with the endothelial cells which line the arteries and veins. When stained with nitrate of silver the edges which bound the epithelial cells are brought into view. These cells are of large size and of an irregular polygonal or lanceolate shape, each containing an oval nucleus which may be displayed by carmine or hematoxylin. Between their edges, at various points of their meeting, roundish dark spots are sometimes seen, which have been described as stomata, though they are closed by intercellular substance. They have been believed to be the situations through which the colorless corpuscles of the blood, when migrating from the bloodvessels, emerge; but this view, though probable, is not universally accepted.
The nerves are derived from the cardiac plexus, which are formed partly from the vagi, and partly from the sympathetic trunks. They are freely distributed both on the surface and in the substance of the heart, the separate nerve filaments being furnished with small ganglia.
The Cardiac Cycle and the Actions of the Valves.—By the contractions of the heart the blood is pumped through the arteries to all parts of the body. These contractions occur regularly and at the rate of about seventy per minute. Each wave of contraction or period of activity is followed by a period of rest, the two periods constituting what is known as a cardiac cycle.
Each cardiac cycle consists of three phases, which succeed each other as follows: (1) a short simultaneous contraction of both atria, termed the atrial systole, followed, lowed, after a slight pause, by (2) a simultaneous, but more prolonged, contraction of both ventricles, named the ventricular systole, and (3) a period of rest, during which the whole heart is relaxed. The atrial contraction commences around the venous openings, and sweeping over the atria forces their contents through the atrioventricular openings into the ventricles, regurgitation into the veins being prevented by the contraction of their muscular coats. When the ventricles contract, the tricuspid and bicuspid valves are closed, and prevent the passage of the blood back into the atria; the musculi papillares at the same time are shortened, and, pulling on the chordæ tendineæ, prevent the inversion of the valves into the atria. As soon as the pressure in the ventricles exceeds that in the pulmonary artery and aorta, the valves guarding the orifices of these vessels are opened and the blood is driven from the right ventricle into the pulmonary artery and from the left into the aorta. The moment the systole of the ventricles ceases, the pressure of the blood in the pulmonary artery and aorta closes the pulmonary and aortic semilunar valves to prevent regurgitation of blood into the ventricles, the valves remaining shut until reopened by the next ventricular systole. During the period of rest the tension of the tricuspid and bicuspid valves is relaxed, and blood is flowing from the veins into the atria, being aspirated by negative intrathoracic pressure, and slightly also from the atria into the ventricles. The average duration of a cardiac cycle is about 8/10 of a second, made up as follows:
The rhythmical action of the heart is muscular in origin—that is to say, the heart muscle itself possesses the inherent property of contraction apart from any nervous stimulation. The more embryonic the muscle the better is it able to initiate and propagate the contraction wave; this explains why the normal systole of the heart starts at the entrance of the veins, for there the muscle is most embryonic in nature. At the atrioventricular junction there is a slight pause in the wave of muscular contraction. To obviate this so far as possible a peculiar band of marked embryonic type passes across the junction and so carries on the contraction wave to the ventricles. This band, composed of special fibers, is the atrioventricular bundle of His (p. 537). The nerves, although not concerned in originating the contractions of the heart muscle, play an important role in regulating their force and frequency in order to subserve the physiological needs of the organism.
Conducting heart system consists of atypical muscular fibres, which have ability to carry impulses from nerves of heart to myocardium of atria and ventricles. Centre of conducting heart system includes two ganglia:
Sinoatrial ganglion (Kiss-Fleck) disposed in wall of right atrium between foramen of superior vena cava and right auricle. This ganglion gives off the branches to myocardium of atria and directs a heart contraction rhythm.
Atrioventricular ganglion (Ashoff-Tavar) lies in thickness of inferior department of interatrial septum. This ganglion continues in atrioventricular fascicle (Giss) which communicates myocardium of atria and ventricles. Fascicle subdivides on right leg and left leg in muscular part of interventricular septum, the terminal branches of which (Purkinje fibres) terminate in ventricles myocardium.
Blood supplying of the heart realizes by means of right coronal artery and left coronal artery, which take its beginning from aorta bulb in suitable its sinuses. Right coronal artery passes to the right under right auricle, lies into coronal sulcus and passes on posterior interventricular sulcus, where anastomose with circumflex branch of left coronal artery. Branches of right coronal artery supply wall of right ventricle and atrium, back portion of interventricular septum, papillary muscles of right ventricle and ganglia of conducting heart system. Left coronal artery passes under left auricle where divides into two branches: anterior interventricular branch and circumflex branch. Last rounds a heart in coronal sulcus and passes on posterior surface where anastomoses with right coronal artery, forming circular arterial anastomose of arterial heart vessels. Anterior interventricular branch passes on same name heart sulcus to the apex, where anastomoses with terminal portion of right coronal artery, forming longitudinal arterial heart anastomose. Left coronal artery supplies wall of left ventricle, anterior wall of right ventricle, wall of left atrium and larger half of interventricular septum.
Innervation of the Heart. Sympathetic fibres pass from sympathetic trunk and form the superior, middle and inferior cervical cardiac nerves. Also thoracic department of sympathetic trunk gives off the thoracic cardiac sympathetic nerves. They hasten cardiac contractions and add their amplitude, broaden the coronal vessels. The parasympathetic fibres pass in composition of superior, inferior and thoracic cardiac branches of vagus nerve. They slow a rhythm of cardiac contractions, reduce their amplitude and narrow space of coronal arteries. The sensory fibres from heart wall receptor pass in composition of cardiac nerves and cardiac branches to spinal cord.
The heart nerves form superficial extraorgan cardiac plexus and deep extraorgan cardiac plexus. The branches of extraorgan cardiac plexus continue into one intraorgan cardiac plexus, which conventionally subdivides on subepicardial plexus, intramuscular plexus and subendocardial plexus. Subepicardial plexus is reach developed.
Topography of the heart. Heart is situated in thoracic cavity; two thirds are disposed to the left from middle line and one third on the right side. From sides a heart is covered by pleural sacs, and lesser its front surface adjoins to sternum and costal cartilages.
Superior heart border passes on line, which connects upper margins of third costal cartilages. Right heart boundary path passes from ІІІ right costal superior margin to V right costal cartilage. Inferior heart border passes on line, which passes from right V right costal cartilage to apex cordis. Apex cordis projects into left V intercostal space 1-1,5 cm medially from medioclavicular line. Left heart boundary path lies through superior margin of left ІІІ costal cartilage to apex cordis. Palpitation sound of bicuspidal valve is listened in apex cordis area. Aortic valve is listened into second intercostal space to the right from sternum. Valve of pulmonary trunk - into ІІ intercostal space to the left from sternum. Right atrioventricular (tricuspidal) valve is listened by base xyphoid process of sternum to the right (joint of ІV costal cartilage with sternum).
Heart is enveloped by pericardium, which consists of fibrous and serous portions. Fibrous pericardium near base of big vessels passes into their external membrane. Serous pericardium has parietal lamina that covers fibrous pericardium from within and visceral lamina that covers surface of the heart and is known as epicardium. Parietal lamina passes into visceral lamina closely the base of heart. There is space like fissure between two laminae – pericardial cavity with small amount of serous liquid that prevents friction during palpitation. There are two deepening in pericardial cavity: transverse sinus and oblique sinus. Transverse sinus is bordered in front by aorta and pulmonary trunk, behind by superior vena cava. Oblique pericardial sinus is situated on the diaphragmatic surface between pulmonary veins on the left and inferior vena cava on the right.
The sympathetic and parasympathetic divisions of the ANS provide innervation to the heart through the cardiac plexus (Figure 20-10 ). Postganglionic sympathetic neurons are located in the cervical and upper thoracic ganglia. The vagus nerve (N X) carries parasympathetic preganglionic fibers to small ganglia in the cardiac plexus. Both ANS divisions innervate the SA and AV nodes and the atrial muscle cells. Although the ventricular muscle cells are innervated by both divisions, sympathetic fibers far outnumber parasympathetic fibers.
The cardiac centers of the medulla oblongata contain the autonomic headquarters for cardiac control. (We introduced these centers in Chapter 14.) Stimulation of the cardioacceleratory center activates the necessary sympathetic neurons; the nearby cardioinhibitory center governs the activities of the parasympathetic neurons. The cardiac centers receive input from higher centers, especially from the parasympathetic and sympathetic headquarters in the hypothalamus. Information about the status of the cardiovascular system arrives over visceral sensory fibers accompanying the vagus nerve and the sympathetic nerves of the cardiac plexus.
The cardiac centers monitor baroreceptors and chemoreceptors innervated by the glossopharyngeal (N IX) and vagus nerves (N X). On the basis of this information, they adjust cardiac performance to maintain adequate circulation to vital organs, such as the brain. These centers respond to changes in blood pressure and in the arterial concentrations of dissolved oxygen and carbon dioxide. For example, a decline in blood pressure or oxygen concentrations or an increase in carbon dioxide levels generally indicates that the heart must work harder to meet the demands of peripheral tissues. The cardiac centers then call for an increase in cardiac activity