Cardiovascular System

ANATOMY AND PHYSIOLOGY REVIEW

Heart

Structure. The human heart is a cone-shaped, hollow, muscular organ lo­cated in the mediastinum between the lungs. It is approximately the size of an adult fist. The heart rests on the diaphragm, tilting forward and to the left in the client's chest. This small organ must pump continuously. Each beat of the heart pumps approximately 60 mL of blood, or approxi­mately 5 L/min. During strenuous physical activity, the heart can double the amount of blood pumped to meet the increased oxygen needs of the peripheral tissues.

Surface anatomy of the heart.

The heart is encapsulated by a protective covering called the pericardium. Cardiac muscle tissue is com­posed of three layers: epicardium, myocardium, and endo­cardium. The epicardium, the outer surface, is a thin, trans­parent tissue. The myocardium, the middle layer, is composed of striated muscle fibers interlaced into bundles. This layer is responsible for the contractile force of the heart. The inner­most layer, the endocardium, is composed of endothelial tis­sue. This tissue lines the inside of the chambers of the heart and covers the four heart valves.

CHAMBERS OF THE HEART

A muscular wall (septum), separates the heart into two halves: right and left. Each half has an upper chamber (atrium) and a lower chamber (ventricle).

RIGHT SIDE. The right atrium is a thin-walled struc­ture that receives deoxygenated venous blood (venous re­turn) from all peripheral tissues by way of the superior and inferior venae cavae and from the heart muscle by way of the coronary sinus.

Most of this venous return flows pas­sively from the right atrium, through the opened tricuspid valve, and to the right ventricle during ventricular diastole, or filling. The remaining venous return is actively propelled by the right atrium into the right ventricle during atrial sys­tole, or contraction.

The right ventricle is a flat muscular pump located behind the sternum. The right ventricle generates enough pressure (approximately 25 mm Hg) to close the tricuspid valve, open the pulmonic valve, and propel blood into the pulmonary ar­tery and the lungs. The workload of the right ventricle is light compared with that of the left ventricle because the pul­monary system is a low-pressure system, which imposes less resistance to flow.

LEFT SIDE. After blood is reoxygenated in the lungs, it flows freely from the four pulmonary veins into the left atrium. Blood then flows through an opened mitral valve into the left ventricle during ventricular diastole. When the left ventricle is almost full, the left atrium contracts, pumping the remaining blood volume into the left ventricle. With systolic contraction, the left ventricle generates enough pressure (ap­proximately 120 mm Hg) to close the mitral valve and open the aortic valve. Blood is propelled into the aorta and into thesystemic arterial circulation.

The left ventricle is ellipsoid in shape and is the largest and most muscular chamber of the heart. Its wall is two to three times the thickness of the right ventricular wall. The left ven­tricle must generate a higher pressure than the right ventricle because it must contract against a high-pressure systemic cir­culation, which imposes a greater resistance to flow.

Blood is propelled from the aorta throughout the systemic circulation to the various tissues of the body; blood returns to the right atrium because of pressure differences. The pressure of blood in the aorta of a young adult averages approximately 100 to 120 mm Hg, whereas the pressure of blood in the right atrium averages about 0 to 5 mm Hg. These differences in pressure produce a pressure gradient, with blood flowing from an area of higher pressure to an area of lower pressure. The heart and vascular structures are responsible for main­taining these pressures.

HEART VALVES

The four cardiac valves are responsible for maintaining the forward flow of blood through the chambers of the heart. These valves open and close passively in re­sponse to pressure and volume changes within the cardiac chambers. The cardiac valves are classified into two types: atrioventricular (AV) valves and semilunar valves. Both AV valves are supported by chordae tendineae, which keep them from everting into the atria during systole.

ATRIOVENTRICULAR VALVES. The AV valves sepa­rate the atria from the ventricles. The tricuspid valve is com­posed of three leaflets and separates the right atrium from the right ventricle. The mitral (bicuspid) valve is composed of two leaflets and separates the left atrium from the left ventricle.

During ventricular diastole, the valves act as funnels and facilitate the flow of blood from the atria to the ventricles. During systole, the valves close to prevent the backflow (re-gurgitation) of blood into the atria.

SEMILUNAR VALVES. There are two semilunar valves: the pulmonic valve and the aortic valve. The pul­monic valve separates the right ventricle from the pulmonary artery. The aortic valve separates the left ventricle from the aorta. Each semilunar valve consists of three cuplike cusps, or pockets, around the inside wall of the artery. These cusps prevent blood from flowing back into the ventricles during ventricular diastole. During ventricular systole, these valves are open to permit blood flow into the pulmonary artery and the aorta.

CORONARY ARTERIES

The heart muscle receives blood to meet its metabolic needs through the coronary arterial system . The coro­nary arteries originate from an area on the aorta just beyond the aortic valve. There are two main coronary arteries: the left coronary artery (LCA) and the right coronary artery (RCA). Coronary artery blood flow to the myocardium occurs pri­marily during diastole, when coronary vascular resistance is minimized. To maintain adequate blood flow through the coronary arteries, diastolic blood pressure must be at least 60 mm Hg.

LEFT CORONARY ARTERY. The LCA divides into two branches: the left anterior descending (LAD) and the cir­cumflex coronary artery (LCX). The LAD branch descends to­ward the anterior wall and the apex of the left ventricle. It sup­plies blood to portions of the left ventricle, ventricular septum, chordae tendineae, papillary muscle, and right ventricle.

The LCX descends toward the lateral wall of the left ventri­cle and apex. It supplies blood to the left atrium, the lateral and posterior surfaces of the left ventricle, and sometimes portions of the interventricular septum. In 45% of people, the LCX sup­plies the sinoatrial (SA) node, and in 10% of people it supplies the AV node. Peripheral branches (diagonal and obtuse mar­ginal) arise from the LAD and LCX and form an abundant net­work of vessels throughout the entire myocardium.

 

RIGHT CORONARY ARTERY. The RCA originates from the right sinus of Valsalva, encircles the heart, and de­scends toward the apex of the right ventricle.

The RCA sup­plies the right atrium, right ventricle, and inferior portion of the left ventricle. In most people (more than 50%), the RCA supplies the SA node and the AV node. Considerable variation in the branching pattern of the coronary arteries exists among individuals.

■   Function

■  ELECTROPHYSIOLOGIC PROPERTIES OF THE HEART

The electrophysiologic properties of heart muscle are responsi­ble for regulating heart rate and rhythm. Cardiac muscle cells are unique and possess the special characteristics of automaticity, excitability, conductivity, contractility, and refractoriness.

Automaticity refers to the ability of all cardiac cells to ini­tiate an impulse spontaneously and repetitively. Excitability is the ability of the cells to respond to a stimulus by initiating an impulse (depolarization). Conductivity means that cardiac cells transmit the electrical impulses they receive.

 Because the cells possess the property of contractility, they also con­tract in response to an impulse. Refractoriness means that car­diac cells are unable to respond to a stimulus until they have recovered (repolarized) from the previous stimulus. .

 CONDUCTION SYSTEM OF THE HEART

The cardiac conduction system is composed of specialized tissue capable of rhythmic electrical impulse formation . It can conduct impulses much more rapidly than other cells located in the myocardium. The SA node, located at the junction of the right atrium and the superior vena cava, is considered the main regulator of heart rate. The SA node is composed of pacemaker cells, which spontaneously initiate impulses at a rate of 60 to 100 times per minute and myocardial working cells, which transmit the impulses to the sur­rounding atrial muscle. An impulse from the SA node initiates the process of de­polarization and hence the activation of all myocardial cells. The impulse travels through both atria to the atrioventricular (AV) node located in the junctional area. After the impulse reaches the AV node, conduction of the impulse is delayed briefly. This delay allows the atria to contract completely be­fore the ventricles are stimulated to contract. The intrinsic rate of the AV node is 40 to 60 beats/min.

The Bundle of His is a continuation of the AV node and is located in the interventricular septum. It divides into the right and left bundle branches. The bundle branches extend down­ward through the ventricular septum and fuse with the Pur-kinje fiber system. The Purkinje fibers are the terminal branches of the conduction system and are responsible for carrying the wave of depolarization to both ventricular walls. Purkinje fibers can act as an intrinsic pacemaker, but their discharge rate is only 20 to 40 beats/min. Thus these intrinsic pacemakers seldom initiate an electrical impulse.

 SEQUENCE OF EVENTS DURING THE CARDIAC CYCLE

The phases of the cardiac cycle are generally described in re­lation to changes in pressure and volume in the left ventricle during filling (diastole) and ventricular contraction (systole). Diastole, normally about two thirds of the car­diac cycle, consists of relaxation and filling of the atria and ventricles, whereas systole consists of the contraction and emptying of the atria and ventricles.

Cardiac muscle contraction results from the release of large numbers of calcium ions from the sarcoplasmic reticulum. These ions diffuse into the myofibril sarcomere (the basic contractile unit of the myocardial cell). Calcium ions promote the interaction of actin and myosin protein filaments, causing these filaments to link and overlap. Cross-bridges, or linkages, are formed as the protein filaments slide over or overlap each other. These cross-bridges act as force-generating sites. The sliding of these protein filaments of multiple myofibril sarcomeres short­ens the sarcomeres, producing myocardial contraction. Cardiac muscle relaxes when calcium ions are pumped back into the sarcoplasmic reticulum, causing a decrease in the number of calcium ions around the myofibrils. This re­duced number of ions causes the protein filaments to disen­gage or dissociate, the sarcomere to lengthen, and the muscle to relax. 

MECHANICAL PROPERTIES OF THE HEART

The electrical and mechanical properties of cardiac muscle determine the function of the cardiovascular system. The heart is able to adapt to various pathophysiologic conditions (e.g., stress, infections, and hemorrhage) to maintain adequate blood flow to the various body tissues.

Blood flow from the heart into the systemic arterial circulation is measured clini­cally as cardiac output (CO), the amount of blood pumped from the left ventricle each minute. CO depends on the rela­tionship between heart rate (HR) and stroke volume (SV); it is the product of these two variables:

Cardiac output = Heart rate x Stroke volume

CARDIAC OUTPUT AND CARDIAC INDEX. Car­diac output (CO) is the volume of blood (in liters) ejected by the heart each minute. In adults, the CO ranges from 4 to 7 L/min. Because cardiac output requirements vary ac­cording to body size, the cardiac index is calculated to ad­just for differences in body size.

The cardiac index can be determined by dividing the CO by the body surface area. The normal range is 2.7 to 3.2 L/min/m² of body surface area.

HEART RATE. Heart rate refers to the number of times the ventricles contract each minute. The normal resting heart rate for an adult is between 60 and 100 beats/min.

Increases in heart rate increase myocardial oxygen demand. Heart rate is extrinsically controlled by the autonomic nervous system, which adjusts rapidly when necessary to regulate cardiac out­put. The parasympathetic system slows the heart rate, whereas sympathetic stimulation has an excitatory effect. An increase in circulating endogenous catecholamine (e.g., epinephrine and norepinephrine) usually causes an increase in heart rate, and vice versa.

Other factors, such as the central nervous system (CNS) and baroreceptor (pressoreceptor) reflexes, influence the ef­fects of the autonomic nervous system on heart rate. Pain, fear, and anxiety can increase heart rate. The baroreceptor re­flex acts as a negative-feedback system. If a client experi­ences hypotension, the baroreceptors in the aortic arch sense a lessened pressure in the blood vessels. A signal is relayed to the parasympathetic system to have less of an inhibitory effect on the sinoatrial (SA) node; this results in a reflex increase in heart rate.

STROKE VOLUME. Stroke volume is the amount of blood ejected by the left ventricle during each systole. Severalvariables influence stroke volume and, ultimately, CO. These variables include heart rate, preload, afterload, and contractility.

PRELOAD. Preload refers to the degree of myocardial fiber stretch at the end of diastole and just before contraction. The stretch imposed on the muscle fibers results from the vol­ume contained within the ventricle at the end of diastole. Pre­load is determined by left ventricular end-diastolic (LVED) volume.

An increase in ventricular volume increases muscle fiber length and tension, thereby enhancing contraction and improv­ing stroke volume. This statement is derived from Starling's law of the heart: the more the heart is filled during diastole (within limits), the more forcefully it contracts. However, ex­cessive filling of the ventricles results in excessive LVED vol­ume and pressure and a decreased cardiac output

AFTERLOAD. Another determinant of stroke volume is afterload. Afterload is the pressure or resistance that the ven­tricles must overcome to eject blood through the semilunar valves and into the peripheral blood vessels. The amount of resistance is directly related to arterial blood pressure and the diameter of the blood vessels.

Impedance, the peripheral component of afterload, is the pressure that the heart must overcome to open the aortic valve. The amount of impedance depends on aortic compli­ance and total systemic vascular resistance, a combination of blood viscosity and arteriolar constriction. A decrease in stroke volume can result from an increase in afterload without the benefit of compensatory mechanisms.

CONTRACTILITY. Contractility also affects stroke volume and CO. Myocardial contractility is the force of cardiac con­traction independent of preload. Contractility is increased by factors such as sympathetic stimulation and calcium release. Factors such as hypoxia and acidemia decrease contractility.

 

The heart is the pump responsible for maintaining adequate circulation of oxygenated blood around the vascular network of the body. It is a four-chamber pump, with the right side receiving deoxygenated blood from the body at low presure and pumping it to the lungs (the pulmonary circulation) and the left side receiving oxygenated blood from the lungs and pumping it at high pressure around the body (the systemic circulation).

The myocardium (cardiac muscle) is a specialised form of muscle, consisting of individual cells joined by electrical connections. The contraction of each cell is produced by a rise in intracellular calcium concentration leading to spontaneous depolarisation, and as each cell is electrically connected to its neighbour, contraction of one cell leads to a wave of depolarisation and contraction across the myocardium.

This depolarisation and contraction of the heart is controlled by a specialised group of cells localised in the sino-atrial node in the right atrium- the pacemaker cells.

1.     These cells generate a rhythmical depolarisation, which then spreads out over the atria to the atrio-ventricular node. 2.     The atria then contract, pushing blood into the ventricles. 3.     The electrical conduction passes  via  the Atrio-ventricular node to the bundle of His, which divides into right and left branches and then spreads out from the base of the ventricles across the myocardium. 4.     This leads to a 'bottom-up' contraction of the ventricles, forcing blood up and out into the pulmonary artery (right) and aorta (left). 5.     The atria then re-fill as the myocardium relaxes.

The 'squeeze' is called systole and normally lasts for about 250ms. The relaxation period, when the atria and ventricles re-fill, is called diastole; the time given for diastole depends on the heart rate.

The ECG

The Electrocardiograph (ECG) is clinically very useful, as it shows the electrical activity within the heart, simply by placing electrodes at various points on the body surface. This enables clinicians to determine the state of the conducting system and of the myocardium itself, as damage to the myocardium alters the way the impulses travel through it.

When looking at an ECG, it is often helpful to remember that an upward deflection on the ECG represents depolarisation moving towards the viewing electrode, and a downward deflection represents depolarisation moving away from the viewing electrode. Below is a normal lead II ECG.

The P wave represents atrial depolarisation- there is little muscle in the atrium so the deflection is small. The Q wave represents depolarisation at the bundle of His; again, this is small as there is little muscle there. The R wave represents the main spread of depolarisation, from the inside out, through the base of the ventricles. This involves large ammounts of muscle so the deflection is large. The S wave shows the subsequent depolarisation of the rest of the ventricles upwards from the base of the ventricles. The T wave represents repolarisation of the myocardium after systole is complete. This is a relatively slow process- hence the smooth curved deflection.

The Coronary Circulation

The heart needs its own reliable blood supply in order to keep beating- the coronary circulation. There are two main coronary arteries, the left and right coronary arteries, and these branch further to form several major branches (see image). The coronary arteries lie in grooves (sulci) running over the surface of the myocardium, covered over by the epicardium, and have many branches which terminate in arterioles supplying the vast capillary network of the myocardium. Even though these vessels have multiple anastomoses, significant obstruction to one or other of the main branches will lead to ischaemia in the area supplied by that branch.

 

Functions of the Cardiovascular System

Knowing the functions of the cardiovascular system and the parts of the body that are part of it is critical in understanding the physiology of the human body. With its complex pathways of veins, arteries, and capillaries, the cardiovascular system keeps life pumping through you. The heart, blood vessels, and blood help to transport vital nutrients throughout the body as well as remove metabolic waste. They also help to protect the body and regulate body temperature.

 

The cardiovascular system consists of the heart, blood vessels, and blood. This system has three main functions:

·         Transport of nutrients, oxygen, and hormones to cells throughout the body and removal of metabolic wastes (carbon dioxide, nitrogenous wastes).

·         Protection of the body by white blood cells, antibodies, and complement proteins that circulate in the blood and defend the body against foreign microbes and toxins. Clotting mechanisms are also present that protect the body from blood loss after injuries.

·         Regulation of body temperature, fluid pH, and water content of cells.

 

Circulatory Pathways

Blood is confined to a closed system of blood vessels and to the four chambers of the heart (essentially dilated vessels). Blood travels away from the heart through arteries, which branch into smaller vessels, the arterioles. Arterioles branch further into the smallest vessels, the capillaries. Gas, nutrient, and waste exchange occurs across the capillary walls. The blood returns to the heart as capillaries merge to form venules, which further merge to form large veins, which connect to the heart. Blood circulates through the following two separate circuits:

 

·         In the pulmonary circulation, deoxygenated blood travels from the right side of the heart to each of the two lungs. Within the lungs, O ₂ enters and CO ₂ leaves the capillaries by diffusion. Oxygenated blood returns from the lungs to the left side of the heart.

·         In the systemic circulation, oxygenated blood travels from the left side of the heart to the various areas of the body. Gas, nutrient, and waste exchange occurs across the capillary walls into the interstitial fluids outside the capillaries and then into the surrounding cells. The deoxygenated blood returns to the right side of the heart.

Cardiac Conduction

Unlike skeletal muscle fibers (cells), which are independent of one another, cardiac muscle fibers (contractile muscle fibers) are linked by intercalated discs, areas where the plasma membranes intermesh. Within the intercalated discs, the adjacent cells are structurally connected by desmosomes, tight seals that weld the plasma membranes together, and electrically connected by gap junctions, ionic channels that allow the transmission of a depolarization event. As a result, the entire myocardium functions as a single unit with a single contraction of the atria followed by a single contraction of the ventricles.

 

Action potentials (electrical impulses) in the heart originate in specialized cardiac muscle cells, called autorhythmic cells. These cells are self‐excitable, able to generate an action potential without external stimulation by nerve cells. The autorhythmic cells serve as a pacemaker to initiate the cardiac cycle (pumping cycle of the heart) and provide a conduction system to coordinate the contraction of muscle cells throughout the heart. The autorhythmic cells are concentrated in the following areas:

·         The sinoatrial (SA) node, located in the upper wall of the right atrium, initiates the cardiac cycle by generating an action potential that spreads through both atria through the gap junctions of the cardiac muscle fibers.

·         The atrioventricular (AV) node, located near the lower region of the interatrial septum, receives the action potential generated by the SA node. A slight delay of the electrical transmission occurs here, allowing the atria to fully contract before the action potential is passed on to the ventricles.

·         The atrioventricular (AV) bundle (bundle of His) receives the action potential from the AV node and transmits the impulse to the ventricles by way of the right and left bundle branches. Except for the AV bundle, which provides the only electrical connection, the atria are electrically insulated from the ventricles.

·         The Purkinje fibers are large‐diameter fibers that conduct the action potential from the interventricular septum, down to the apex, and then upward through the ventricles.

Cardiac Muscle Contraction

The sarcolemma (plasma membrane) of an unstimulated muscle cell is polarized—that is, the inside of the sarcolemma is negatively charged with respect to the outside. The unstimulated state of the muscle cell, called the resting potential, is created by the presence of large, negatively charged proteins and nucleic acids inside the cell. A balance between K ⁺ inside the cell and Na ⁺ outside the cell contributes to the polarization. During an action potential, the balance of Na ⁺ and K ⁺ is upset so that the cell becomes depolarized. The series of events that occurs during and following an action potential in contractile muscle fibers of the heart is similar to that in skeletal muscle. Here is a description of these events:

 

1.     Rapid depolarization occurs when fast‐opening Na ⁺ channels in the sarcolemma open and allow an influx of Na ⁺ ions into the cardiac muscle cell. The Na ⁺ channels rapidly close.

2.     A plateau phase occurs during which Ca ²⁺ enters the cytosol of the muscle cell. Ca ²⁺ enters from the sarcoplasmic reticulum (endoplasmic reticulum) within the cell and also from outside the cell through slow‐opening Ca ²⁺channels in the sarcolemma. Within the cell, Ca ²⁺ binds to troponin, which in turn triggers the cross‐bridge binding that leads to the sliding of actin filaments past myosin filaments. The sliding of the filaments produces cell contraction. At the same time that the Ca ²⁺ channels open, K ⁺ channels, which normally leak small amounts of K ⁺ out of the cell, become more impermeable to K ⁺ leakage. The combined effects of the prolonged release of Ca ²⁺ and the restricted leakage of K ⁺ lead to an extended depolarization that appears as a plateau when membrane potential is plotted against time.

3.     Repolarization occurs as K ⁺ channels open and K ⁺ diffuses out of the cell. At the same time, Ca ²⁺ channels close. These events restore the membrane to its original polarization, except that the positions of K ⁺ and Na ⁺ on each side of the sarcolemma are reversed.

4.      A refractory period follows, during which concentration of K ⁺ and Na ⁺ are actively restored to their appropriate sides of the sarcolemma by Na ⁺/K ⁺ pumps. The muscle cell cannot contract again until Na ⁺ and K ⁺ are restored to their resting potential states. The refractory period of cardiac muscle is dramatically longer than that of skeletal muscle. This prevents tetanus from occurring and ensures that each contraction is followed by enough time to allow the heart chamber to refill with blood before the next contraction.

Cardiac Output

The following variables are measures of the capacity of the heart:

 

·         Stroke volume (SV) is the volume of blood ejected by each ventricle during a single contraction.

·         Heart rate (HR) is the number of heartbeats per minute.

·         Cardiac output (CO) is the volume of blood pumped out of the right or left ventricle per minute. CO = SV × HR.

Cardiac output varies widely with the health of the individual and the state of activity at the time of measurement. Cardiac output in exercising athletes may exceed their resting cardiac output seven times. The ratio between the maximum and resting cardiac output of an individual is the cardiac reserve. Note that cardiac output changes when either stroke volume or heart rate changes.

Stroke volume is regulated by the following three factors:

·         Preload is the degree to which cardiac muscle cells are stretched by the blood entering the heart chambers. According to the Frank‐Starling law of the heart, the more the chamber is stretched, the greater the force of its contraction. Because the end‐diastolic volume (EDV) is a measure of how much blood enters the ventricles, the EDV is an indicator of ventricle preload.

·         Contractility is the degree to which cardiac muscle cells contract as a result of extrinsic influences. Positive inotropic factors, such as certain hormones (epinephrine or thyroxin), drugs (digitalis), or elevated levels of Ca ²⁺, increase contractility, while negative inotropic factors, such as certain drugs (calcium channel blockers) or elevated levels of K ⁺, decrease contractility.

·         Afterload is a measure of the pressure that must be generated by the ventricles to force the semilunar valves open. The greater the afterload, the smaller the stroke volume. Arteriosclerosis (narrowing of the arteries) and high blood pressure increase afterload and reduce stroke volume.

Heart rate is regulated by the following three factors:

·         The autonomic nervous system may influence heart rate when the sympathetic nervous system stimulates cardiac muscle contractions or when the parasympathetic system inhibits cardiac muscle contractions.

·         Chemicals such as hormones and ions can influence heart rate. Epinephrine, secreted by the adrenal medulla, and thyroxin, secreted by the thyroid gland, increase heart rate. Abnormal blood concentrations of Na ⁺, K ⁺, and Ca ²⁺ interfere with muscle contraction.

·         Other factors such as age, gender, body temperature, and physical fitness may influence heart rate.

The Heart Sounds

The closure of the heart valves and the contraction of the heart muscle produce sounds that can be heard through the thoracic wall by the unaided ear, although they can be heard better when amplified by a stethoscope. The sounds of the heart may be represented as lubb-dubb-pause-lubb-dubb-pause. The lubb sound indicates the closing of the valves between the atria and ventricles and the contracting ventricles; the dubb sound indicates the closing of the semilunar valves. In addition, there may also be cardiac murmurs, especially when the valves are abnormal. Some heart murmurs, however, may also occur in healthy persons, mainly during rapid or pronounced cardiac action. The study of heart sounds and murmurs furnishes valuable information to physicians regarding the condition of the heart muscle and valves.

Coronary Circulation

The coronary arteries supply blood to the heart muscle. These vessels originate from the aorta immediately after the aortic valve and branch out through the heart muscle. The coronary veins transport the deoxygenated blood from the heart muscle to the right atrium. The heart's energy supply is almost completely dependent on these coronary vessels. When the coronary vessels become blocked, as inarteriosclerosis or hardening of the arteries, blood flow to the cardiac muscle is compromised. This is when the common "bypass surgery" is performed where the coronary arteries are "bypassed" by replacing them with, for example, a vein from the leg. A "double bypass" is when two coronary arteries are bypassed. A "triple bypass" is when three are bypassed, etc.

The Heartbeat

The heart muscle pumps the blood through the body by means of rhythmical contractions (systole) and relaxations or dilations (diastole). The heart's left and right halves work almost synchronously. When the ventricles contract (systole), the valves between the atria and the ventricles close as the result of increasing pressure, and the valves to the pulmonary artery and the aorta open. When the ventricles become flaccid during diastole, and the pressure decreases, the reverse process takes place.

The Pulmonary Circulation

From the right atrium the blood passes to the right ventricle through the tricuspid valve, which consists of three flaps (or cusps) of tissue. The tricuspid valve remains open during diastole, or ventricular filling. When the ventricle contracts, the valve closes, sealing the opening and preventing backflow into the right atrium. Five cords attached to small muscles, called papillary muscles, on the ventricles' inner surface prevent the valves' flaps from being forced backward.

From the right ventricle blood is pumped through the pulmonary or semilunar valve, which has three half-moon-shaped flaps, into the pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the pulmonary artery blood is pumped to the lungs where it releases carbon dioxide and picks up oxygen.

The Systemic Circulation

From the lungs, the blood is returned to the heart through pulmonary veins, two from each lung. From the pulmonary veins the blood enters the left atrium and then passes through the mitral valve to the left ventricle. As the ventricles contract, the mitral valve prevents backflow of blood into the left atrium, and blood is driven through the aortic valve into the aorta, the major artery that supplies blood to the entire body. The aortic valve, like the pulmonary valve, has a semilunar shape.

The aorta has many branches, which carry the blood to various parts of the body. Each of these branches in turn has branches, and these branches divide, and so on until there are literally millions of small blood vessels. The smallest of these on the arterial side of the circulation are called arterioles. They contain a great deal of smooth muscle, and because of their ability to constrict or dilate, they play a major role in regulating blood flow through the tissues.

Blood Vessels 
Blood vessels are the body’s highways that allow blood to flow quickly and efficiently from the heart to every region of the body and back again. The size of blood vessels corresponds with the amount of blood that passes through the vessel. All blood vessels contain a hollow area called the lumen through which blood is able to flow. Around the lumen is the wall of the vessel, which may be thin in the case of capillaries or very thick in the case of arteries.

All blood vessels are lined with a thin layer of simple squamous epithelium known as the endothelium that keeps blood cells inside of the blood vessels and prevents clots from forming. The endothelium lines the entire circulatory system, all the way to the interior of the heart, where it is called the endocardium.

There are three major types of blood vessels: arteries, capillaries and veins. Blood vessels are often named after either the region of the body through which they carry blood or for nearby structures. For example, the brachiocephalic artery carries blood into the brachial (arm) and cephalic (head) regions. One of its branches, the subclavian artery, runs under the clavicle; hence the name subclavian. The subclavian artery runs into the axillary region where it becomes known as the axillary artery.

1.     Arteries and Arterioles: Arteries are blood vessels that carry blood away from the heart. Blood carried by arteries is usually highly oxygenated, having just left the lungs on its way to the body’s tissues. The pulmonary trunk and arteries of the pulmonary circulation loop provide an exception to this rule – these arteries carry deoxygenated blood from the heart to the lungs to be oxygenated.

Arteries face high levels of blood pressure as they carry blood being pushed from the heart under great force. To withstand this pressure, the walls of the arteries are thicker, more elastic, and more muscular than those of other vessels. The largest arteries of the body contain a high percentage of elastic tissue that allows them to stretch and accommodate the pressure of the heart.

Smaller arteries are more muscular in the structure of their walls. The smooth muscles of the arterial walls of these smaller arteries contract or expand to regulate the flow of blood through their lumen. In this way, the body controls how much blood flows to different parts of the body under varying circumstances. The regulation of blood flow also affects blood pressure, as smaller arteries give blood less area to flow through and therefore increases the pressure of the blood on arterial walls.

Arterioles are narrower arteries that branch off from the ends of arteries and carry blood to capillaries. They face much lower blood pressures than arteries due to their greater number, decreased blood volume, and distance from the direct pressure of the heart. Thus arteriole walls are much thinner than those of arteries. Arterioles, like arteries, are able to use smooth muscle to control their aperture and regulate blood flow and blood pressure.

2.     Capillaries: Capillaries are the smallest and thinnest of the blood vessels in the body and also the most common. They can be found running throughout almost every tissue of the body and border the edges of the body’s avascular tissues. Capillaries connect to arterioles on one end and venules on the other.

Capillaries carry blood very close to the cells of the tissues of the body in order to exchange gases, nutrients, and waste products. The walls of capillaries consist of only a thin layer of endothelium so that there is the minimum amount of structure possible between the blood and the tissues. The endothelium acts as a filter to keep blood cells inside of the vessels while allowing liquids, dissolved gases, and other chemicals to diffuse along their concentration gradients into or out of tissues.

Precapillary sphincters are bands of smooth muscle found at the arteriole ends of capillaries. These sphincters regulate blood flow into the capillaries. Since there is a limited supply of blood, and not all tissues have the same energy and oxygen requirements, the precapillary sphincters reduce blood flow to inactive tissues and allow free flow into active tissues.

3.     Veins and Venules: Veins are the large return vessels of the body and act as the blood return counterparts of arteries. Because the arteries, arterioles, and capillaries absorb most of the force of the heart’s contractions, veins and venules are subjected to very low blood pressures. This lack of pressure allows the walls of veins to be much thinner, less elastic, and less muscular than the walls of arteries.

Veins rely on gravity, inertia, and the force of skeletal muscle contractions to help push blood back to the heart. To facilitate the movement of blood, some veins contain many one-way valves that prevent blood from flowing away from the heart. As skeletal muscles in the body contract, they squeeze nearby veins and push blood through valves closer to the heart.

When the muscle relaxes, the valve traps the blood until another contraction pushes the blood closer to the heart. Venules are similar to arterioles as they are small vessels that connect capillaries, but unlike arterioles, venules connect to veins instead of arteries. Venules pick up blood from many capillaries and deposit it into larger veins for transport back to the heart.

Coronary Circulation 
The heart has its own set of blood vessels that provide the myocardium with the oxygen and nutrients necessary to pump blood throughout the body. The left and right coronary arteries branch off from the aorta and provide blood to the left and right sides of the heart. The coronary sinus is a vein on the posterior side of the heart that returns deoxygenated blood from the myocardium to the vena cava.

Hepatic Portal Circulation
The veins of the stomach and intestines perform a unique function: instead of carrying blood directly back to the heart, they carry blood to the liver through the hepatic portal vein. Blood leaving the digestive organs is rich in nutrients and other chemicals absorbed from food. The liver removes toxins, stores sugars, and processes the products of digestion before they reach the other body tissues. Blood from the liver then returns to the heart through the inferior vena cava.

Blood
The average human body contains about 4 to 5 liters of blood. As a liquid connective tissue, it transports many substances through the body and helps to maintain homeostasis of nutrients, wastes, and gases. Blood is made up of red blood cells, white blood cells, platelets, and liquid plasma.

·         Red Blood Cells: Red blood cells, also known as erythrocytes, are by far the most common type of blood cell and make up about 45% of blood volume. Erythrocytes are produced inside of red bone marrow from stem cells at the astonishing rate of about 2 million cells every second. The shape of erythrocytes is biconcave—disks with a concave curve on both sides of the disk so that the center of an erythrocyte is its thinnest part. The unique shape of erythrocytes gives these cells a high surface area to volume ratio and allows them to fold to fit into thin capillaries. Immature erythrocytes have a nucleus that is ejected from the cell when it reaches maturity to provide it with its unique shape and flexibility. The lack of a nucleus means that red blood cells contain no DNA and are not able to repair themselves once damaged.

Erythrocytes transport oxygen in the blood through the red pigment hemoglobin. Hemoglobin contains iron and proteins joined to greatly increase the oxygen carrying capacity of erythrocytes. The high surface area to volume ratio of erythrocytes allows oxygen to be easily transferred into the cell in the lungs and out of the cell in the capillaries of the systemic tissues. 

·         White Blood Cells: White blood cells, also known as leukocytes, make up a very small percentage of the total number of cells in the bloodstream, but have important functions in the body’s immune system. There are two major classes of white blood cells: granular leukocytes and agranular leukocytes.

1.     Granular Leukocytes: The three types of granular leukocytes are neutrophils, eosinophils, and basophils. Each type of granular leukocyte is classified by the presence of chemical-filled vesicles in their cytoplasm that give them their function. Neutrophils contain digestive enzymes that neutralize bacteria that invade the body. Eosinophils contain digestive enzymes specialized for digesting viruses that have been bound to by antibodies in the blood. Basophils release histamine to intensify allergic reactions and help protect the body from parasites.

2.     Agranular Leukocytes: The two major classes of agranular leukocytes are lymphocytes and monocytes. Lymphocytes include T cells and natural killer cells that fight off viral infections and B cells that produce antibodies against infections by pathogens. Monocytes develop into cells called macrophages that engulf and ingest pathogens and the dead cells from wounds or infections. 

·         Platelets : Also known as thrombocytes, platelets are small cell fragments responsible for the clotting of blood and the formation of scabs. Platelets form in the red bone marrow from large megakaryocyte cells that periodically rupture and release thousands of pieces of membrane that become the platelets. Platelets do not contain a nucleus and only survive in the body for up to a week before macrophages capture and digest them. 

·         Plasma: Plasma is the non-cellular or liquid portion of the blood that makes up about 55% of the blood’s volume. Plasma is a mixture of water, proteins, and dissolved substances. Around 90% of plasma is made of water, although the exact percentage varies depending upon the hydration levels of the individual. Theproteins within plasma include antibodies and albumins. Antibodies are part of the immune system and bind to antigens on the surface of pathogens that infect the body. Albumins help maintain the body’s osmotic balance by providing an isotonic solution for the cells of the body. Many different substances can be found dissolved in the plasma, including glucose, oxygen, carbon dioxide, electrolytes, nutrients, and cellular waste products. The plasma functions as a transportation medium for these substances as they move throughout the body.

Cardiovascular System Physiology

Functions of the Cardiovascular System 
The cardiovascular system has three major functions: transportation of materials, protection from pathogens, and regulation of the body’s homeostasis.

·         Transportation: The cardiovascular system transports blood to almost all of the body’s tissues. The blood delivers essential nutrients and oxygen and removes wastes and carbon dioxide to be processed or removed from the body. Hormones are transported throughout the body via the blood’s liquid plasma.

·         Protection: The cardiovascular system protects the body through its white blood cells. White blood cells clean up cellular debris and fight pathogens that have entered the body. Platelets and red blood cells form scabs to seal wounds and prevent pathogens from entering the body and liquids from leaking out. Blood also carries antibodies that provide specific immunity to pathogens that the body has previously been exposed to or has been vaccinated against.

·         Regulation: The cardiovascular system is instrumental in the body’s ability to maintain homeostatic control of several internal conditions. Blood vessels help maintain a stable body temperature by controlling the blood flow to the surface of the skin. Blood vessels near the skin’s surface open during times of overheating to allow hot blood to dump its heat into the body’s surroundings. In the case of hypothermia, these blood vessels constrict to keep blood flowing only to vital organs in the body’s core. Blood also helps balance the body’s pH due to the presence of bicarbonate ions, which act as a buffer solution. Finally, the albumins in blood plasma help to balance the osmotic concentration of the body’s cells by maintaining an isotonic environment.

The Circulatory Pump 
The heart is a four-chambered “double pump,” where each side (left and right) operates as a separate pump. The left and right sides of the heart are separated by a muscular wall of tissue known as the septum of the heart. The right side of the heart receives deoxygenated blood from the systemic veins and pumps it to the lungs for oxygenation. The left side of the heart receives oxygenated blood from the lungs and pumps it through the systemic arteries to the tissues of the body. Each heartbeat results in the simultaneous pumping of both sides of the heart, making the heart a very efficient pump.

Regulation of Blood Pressure 
Several functions of the cardiovascular system can control blood pressure. Certain hormones along with autonomic nerve signals from the brain affect the rate and strength of heart contractions. Greater contractile force and heart rate lead to an increase in blood pressure. Blood vessels can also affect blood pressure. Vasoconstriction decreases the diameter of an artery by contracting the smooth muscle in the arterial wall. The sympathetic (fight or flight) division of the autonomic nervous system causes vasoconstriction, which leads to increases in blood pressure and decreases in blood flow in the constricted region. Vasodilation is the expansion of an artery as the smooth muscle in the arterial wall relaxes after the fight-or-flight response wears off or under the effect of certain hormones or chemicals in the blood. The volume of blood in the body also affects blood pressure. A higher volume of blood in the body raises blood pressure by increasing the amount of blood pumped by each heartbeat. Thicker, more viscous blood from clotting disorders can also raise blood pressure.

Hemostasis
Hemostasis, or the clotting of blood and formation of scabs, is managed by the platelets of the blood. Platelets normally remain inactive in the blood until they reach damaged tissue or leak out of the blood vessels through a wound. Once active, platelets change into a spiny ball shape and become very sticky in order to latch on to damaged tissues. Platelets next release chemical clotting factors and begin to produce the protein fibrin to act as structure for the blood clot. Platelets also begin sticking together to form a platelet plug. The platelet plug will serve as a temporary seal to keep blood in the vessel and foreign material out of the vessel until the cells of the blood vessel can repair the damage to the vessel wall.

 

The Path of Blood through the Human Body

When a heart contracts and forces blood into the blood vessels, there is a certain path that the blood follows through the body. The blood moves through pulmonary circulation and then continues on through systemic circulation. Pulmonary and systemic are the two circuits in the two-circuit system of higher animals with closed circulatory systems.

Humans and other mammals have two-circuit circulatory systems: one circuit is forpulmonary circulation (circulation to the lungs; pulmo = lungs), and the other circuit is forsystemic circulation (the rest of the body). As each atrium and ventricle contract, blood is pumped into certain major blood vessels, and from there, continues through the circulatory system.

The intertwined circulatory system pathways: Pulmonary circulation and systemic circulation work together.

Pulmonary circulation

Blood that is lacking oxygen is said to be deoxygenated. This blood has just exchanged oxygen for carbon dioxide across cell membranes, and now contains mostly carbon dioxide. Deoxygenated blood enters the right atrium through the superior vena cava and the inferior vena cava.

Superior means higher, and inferior means lower, so the superior vena cava is at the top of the right atrium, and the inferior vena cava enters the bottom of the right atrium.

From the right atrium, the deoxygenated blood drains into the right ventricle through the rightatrioventricular (AV) valve, which is so named because it is between the atrium and the ventricle. This valve is also referred to as the tricuspid valve because it has three flaps in its structure. When the ventricles contract, the AV valve closes off the opening between the ventricle and the atrium so that blood does not flow back up into the atrium.

As the right ventricle contracts, it forces the deoxygenated blood through the pulmonary semilunar valve and into the pulmonary artery. Semilunar means half-moon and refers to the shape of the valve. Note that this is the only artery in the body that contains deoxygenated blood; all other arteries contain oxygenated blood. The semilunar valve keeps blood from flowing back into the right ventricle once it is in the pulmonary artery.

The pulmonary artery carries the blood that is very low in oxygen to the lungs, where it becomes oxygenated.

Systemic circulation

Freshly oxygenated blood returns to the heart via the pulmonary veins. Note that these are the only veins in the body that contain oxygenated blood; all other veins contain deoxygenated blood.

The pulmonary veins enter the left atrium. When the left atrium relaxes, the oxygenated blood drains into the left ventricle through the left AV valve. This valve is also called the bicuspid valve because it has only two flaps in its structure.

Now the heart really squeezes. As the left ventricle contracts, the oxygenated blood is pumped into the main artery of the body — the aorta. To get to the aorta, blood passes through the aortic semilunar valve, which serves to keep blood flowing from the aorta back into the left ventricle.

The aorta branches into other arteries, which then branch into smaller arterioles. The arterioles meet up with capillaries, which are the blood vessels where oxygen is exchanged for carbon dioxide.

Capillary exchange

Capillaries bridge the smallest of the arteries and the smallest of the veins. Near the arterial end, the capillaries allow materials essential for maintaining the health of cells to diffuse out (water, glucose, oxygen, and amino acids).

To maintain the health of cells, it is also necessary for the capillaries to transport wastes and carbon dioxide to places in the body that can dispose of them. The waste products enter near the venous end of the capillary. Water diffuses in and out of capillaries to maintain blood volume, which adjusts to achieve homeostasis.

Capillaries are only as thick as one cell, so the contents within the cells of the capillaries can easily pass out of the capillary by diffusing through the capillary membrane. And, because the capillary membrane abuts the membrane of other cells all over the body, the capillary’s contents can easily continue through the abutting cell’s membrane and get inside the adjoining cell.

The process of capillary exchange is how oxygen leaves red blood cells in the bloodstream and gets into all the other cells of the body. Capillary exchange also allows nutrients to diffuse out of the bloodstream and into other cells. At the same time, the other cells expel waste products that then enter the capillaries, and carbon dioxide diffuses out of the body’s cells and into the capillaries.

How capillary exchange works.

After the capillaries “pick up” the garbage from other cells, the capillaries carry the wastes and carbon dioxide through the deoxygenated blood to the smallest of the veins, which are called venules. The venules branch into bigger vessels called veins. The veins then carry the deoxygenated blood toward the main vein, which is the vena cava. The two branches of the vena cava enter the right atrium, which is where pulmonary circulation begins.

Circulation and Blood Vessels

Your heart and blood vessels make up your overall blood circulatory system. Your blood circulatory system is made up of four subsystems.

Arterial Circulation

Arterial circulation is the part of your circulatory system that involves arteries, like the aorta and pulmonary arteries. Arteries are blood vessels that carry blood away from your heart. (The exception is the coronary arteries, which supply your heart muscle with oxygen-rich blood.)

Healthy arteries are strong and elastic (stretchy). They become narrow between heartbeats, and they help keep your blood pressure consistent. This helps blood move through your body.

Arteries branch into smaller blood vessels called arterioles (ar-TEER-e-ols). Arteries and arterioles have strong, flexible walls that allow them to adjust the amount and rate of blood flowing to parts of your body.

Venous Circulation

Venous circulation is the part of your circulatory system that involves veins, like the vena cavae and pulmonary veins. Veins are blood vessels that carry blood to your heart.

Veins have thinner walls than arteries. Veins can widen as the amount of blood passing through them increases.

Capillary Circulation

Capillary circulation is the part of your circulatory system where oxygen, nutrients, and waste pass between your blood and parts of your body.

Capillaries are very small blood vessels. They connect the arterial and venous circulatory subsystems.

The importance of capillaries lies in their very thin walls. Oxygen and nutrients in your blood can pass through the walls of the capillaries to the parts of your body that need them to work normally.

Capillaries' thin walls also allow waste products like carbon dioxide to pass from your body's organs and tissues into the blood, where it's taken away to your lungs.

Pulmonary Circulation

Pulmonary circulation is the movement of blood from the heart to the lungs and back to the heart again. Pulmonary circulation includes both arterial and venous circulation.

Oxygen-poor blood is pumped to the lungs from the heart (arterial circulation). Oxygen-rich blood moves from the lungs to the heart through the pulmonary veins (venous circulation).

Pulmonary circulation also includes capillary circulation. Oxygen you breathe in from the air passes through your lungs into your blood through the many capillaries in the lungs. Oxygen-rich blood moves through your pulmonary veins to the left side of your heart and out of the aorta to the rest of your body.

Capillaries in the lungs also remove carbon dioxide from your blood so that your lungs can breathe the carbon dioxide out into the air.

The primary function of the heart is to pump blood through blood vessels to the body's cells. Imagine a simple machine like a water pump working for perhaps 70 or more years without attention and without stopping. Impossible? Yet this is exactly what the heart can do in our bodies. The heart is really a muscular bag surrounding four hollow compartments, with a thin wall of muscle separating the left hand side from the right hand side. The muscles in the heart are very strong because they have to work harder than any of the other muscles in our body, pushing the blood to our head and feet continuously.

The blood flow around our body is called our circulation. The heart connects the two major portions of the circulation's continuous circuit, the systemic circulation and the pulmonary circulation. The blood vessels in the pulmonary circulation carry the blood through the lungs to pick up oxygen and get rid of carbon dioxide, while the blood vessels in the systemic circulation carry the blood throughout the rest of our body

The heart actually has two separate sides, one designed to pump deoxygenated blood into the pulmonary circulation where the blood becomes oxygenated, and one designed to pump the oxygenated blood into the systemic circulation where the blood flows throughout the body. Each side of the heart has two chambers or compartments. The top chamber on each side is called the atrium. The right atrium receives incoming deoxygenated blood from the body and the left atrium receives incoming oxygenated blood from the lungs. The thin-walled atrium on each side bulges as it fills with blood, and as the lower heart muscle relaxes, the atrium contracts and squeezes the blood into a second chamber, the thick muscular ventricle. The ventricle is the pumping chamber that, with each muscular contraction, pushes the blood forcefully out and into the lungs (right ventricle) and the rest of the body (left ventricle).

The atrium and ventricle on each side of the heart are separated by tissue flaps called valves. The structure of these valves prevents blood from flowing backward into the atrium as the ventricle squeezes blood out. The valve on the right side, between the atrium and the ventricle, is called the tricuspid valve. The valve on the left side, between the atrium and the ventricle, is called the bicuspid or mitral valve. There are two other important valves that help to keep the blood Rowing in the proper direction. These two valves are located at the two points where blood exits the heart. The pulmonary valve is located between the right ventricle and the pulmonary artery that carries the deoxygenated blood from the heart to the lungs, and the aortic valve is located between the left ventricle and the aorta, the major artery that carries the oxygenated blood from the heart to the rest of the body.

The arteries are the blood vessels that transport blood out of the heart under high pressure to the tissues. The arterioles are the last small branch of the arterial system through which blood is released into the capillaries. The capillaries are very small, thin-walled blood vessels where the exchange of gases, nutrients, and waste takes place between the cells and the blood. Blood flows with almost no resistance in the larger blood vessels, but in the arterioles and capillaries, considerable resistance to flow does occur because these vessels are so small in diameter that the blood must squeeze all its contents through them. The venules collect blood from the capillaries and gradually feed into progressively larger veins. The veins transport the blood from the tissues back to the heart. The walls of the veins are thin and very elastic and can fold or expand to act as a reservoir for extra blood, if required by the needs of the body.

Let us follow a single red blood cell (RBC) through one full cycle along the circulatory pathway. Remember that RBCs carry oxygen throughout the body. Since the blood travels endlessly, an arbitrary choice must be made of a starting point to describe the RBC's route. We will begin at the point where the RBC has delivered its oxygen to a cell in need and is on its return back to the heart.

1. Once the deoxygenated red blood cell (RBC) returns to the heart, it enters either through the superior vana cava or the inferior vena cava. The superior vena cava returns deoxygenated blood from the upper part of the body to the heart. The inferior vena cava returns deoxygenated blood from the lower part of the body to the heart. These large veins lead into the right atrium.

2. The RBC passes through the tricuspid valve into the right ventricle.

3. The RBC is then pumped through the pulmonary valve into the pulmonary artery and on to the lungs. There the RBC gives off carbon dioxide and picks up oxygen.

4. The RBC returns to the heart through a pulmonary vein, enters the left atrium, passes through the mitral valve, and flows into the left ventricle.

5. The left ventricle pumps the fully oxygenated RBC through the aortic valve, into the aorta, the body's main artery, and out to the body.

6. From the aorta, the RBC flows into one of the many arteries of the body, through the arterioles, and then to the capillaries, where the RBC will deliver oxygen and nutrients to the cells and remove wastes and carbon dioxide. Next it moves through the venules, veins, and on to the vena cava in a deoxygenated state, and returns to the heart, only to begin its repetitive journey once again. This whole process has taken approximately 20 seconds!

That single RBC will travel about 950 miles (more than 1500 kilometers) in its brief 4-month lifetime!