LESSON 16
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 ). The inferior, pointed tip of the heart is the
apex. A typical adult heart measures approximately
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 ). The trabeculae carneae are prominent, and a
pair of large papillary muscles tense the chordae tendineae that brace the
cusps of the AV valve and prevent backflow of blood into the left atrium.
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 Semilunar
Valves
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 ).
A section through the wall of the heart (Figure
20-7a ) reveals three distinct layers: (1) an outer epicardium,
(2) a middle myocardium, and (3) an inner endocardium.
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
Artery
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
about
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,
about
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:
Openings »
Superior vena cava.
Inferior vena cava.
Coronary sinus.
Valves »
Valve of the inferior vena cava.
Foramina venarum minimarum.
Valve of the coronary sinus.
Atrioventricular.
Fossa ovalis.
Limbus fossæ ovalis.
Intervenous tubercle.
Musculi pectinati.
Crista terminalis.
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:
Openings »
Right atrioventricular.
Valves »
Tricuspid.
Pulmonary artery.
Pulmonary.
Trabeculæ carneæ
Chordæ tendineæ
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.
Musculi pectinati.
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:
Openings »
Left atrioventricular.
Valves »
Bicuspid or Mitral.
Aortic.
Aortic.
Trabeculæ carneæ.
Chordæ tendineæ
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
terminal branches of the pulmonary arteries will be described with the anatomy
of the lungs.
Purkinje’s
fibers from the sheep’s heart. A. In longitudinal section. B. In
transverse section.
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.
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.
The
lymphatics end in the thoracic and right lymphatic ducts.
-
lesser
cardiac vein [vena cordis parva], which passes
in right part of coronal sulcus;
-
middle
cardiac vein [vena cordis media] passes in posterior interventricular sulcus;
-
posterior
vein of left ventricle;
-
oblique
vein of left atrium.
There
are venae minimae (Tebezia) and anterior venae, positioned in
myocardium of right atrium.
2.
What prevents the AV valves from opening back into
the atria?
3.
Why is the left ventricle more muscular than the
right ventricle?
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.