RN Expert Guides: Cardiovascular Care, 1st Edition (2008)

Chapter 1. Anatomy and Physiology

The cardiovascular system's job is to deliver oxygenated blood to tissues and remove waste products. The heart pumps blood to all organs and tissues of the body, and the autonomic nervous system (ANS) controls how the heart pumps.

CARDIOVASCULAR ANATOMY

Consisting of the arteries and veins, the vascular network:

·

carries blood throughout the body

·

keeps the heart filled with blood

·

maintains blood pressure.

Structures of the heart

The heart is a hollow, muscular organ about the size of a closed fist. It's about 5″ (12.5 cm) long, 3½″ (9 cm) in diameter at its widest point and weighs 1 to 1¼ 1b (453.5 to 567 g). The heart is located between the lungs in the mediastinum, behind and to the left of the sternum.

The heart spans the area from the second to the fifth intercostal space. The heart's right border lines up with the right border of the sternum. The heart's left border lines up with the left midclavicular line. The heart's base, its posterior surface, is the widest part of the heart. It's formed mainly by the left atrium, which is located at the top of the heart. The apex, formed mainly by the left ventricle, is located at the fifth left intercostal space at the bottom of the heart. The apex is the point of maximum impulse, where heart sounds are the loudest. (See Locating the heart, page 2

Leading into and out of the heart are the great vessels:

·

inferior vena cava

·

superior vena cava

 

·

aorta

·

pulmonary artery

·

four pulmonary veins.

LOCATING THE HEART

c1-tt1

HEART WALL

The heart wall consists of three layers:

·

endocardium, a thin layer of endothelial tissue

·

myocardium, composed of interlacing bundles of thick cardiac muscle fibers, which forms most of the heart wall

·

epicardium, which makes up the outside layer and is made of connective tissue covered by the epithelium.

The pericardium is a fibroserous sac that surrounds the heart and the roots of the great vessels. It consists of the serous pericardium and the fibrous pericardium. The serous pericardium contains two layers:

·

parietal layer, which lines the inside of the fibrous pericardium

·

visceral layer, which adheres to the surface of the heart.

The fibrous pericardium is a thicker layer that protects the heart. The space between the two layers, called the pericardial space, contains 10 to 30 ml of serous fluid, which prevents friction between the layers as the heart pumps. (See A look at the layers of the heart wall.)

A LOOK AT THE LAYERS OF THE HEART WALL

This cross section of the heart wall shows its various layers.

c1-tt2

HEART CHAMBERS

The heart contains four hollow chambers: two atria and two ventricles. The right atrium lies in front of and to the right of the smaller but thicker-walled left atrium. An interatrial septum separates the two chambers and helps them contract. The right and left atria serve as volume reservoirs for blood being sent into the ventricles. The right atrium receives deoxygenated blood returning from the body through the inferior and superior vena cavae and from the heart through the coronary sinus. The left atrium receives oxygenated blood from the lungs through the four pulmonary veins. Contraction of the atria forces blood into the ventricles. (See A look at the internal structures of the heart, page 4.)

The right and left ventricles make up the two lower chambers. The right ventricle:

·

lies behind the sternum

 

·

forms the largest part of the sternocostal surface and inferior border of the heart

·

receives deoxygenated blood from the right atrium

·

pumps the deoxygenated blood through the pulmonary arteries to the lungs where it's reoxygenated.

A LOOK AT THE INTERNAL STRUCTURES OF THE HEART

The heart's internal structure consists of the four chambers and four valve walls.

c1-tt3

The left ventricle:

·

forms the apex and most of the left border of the heart and its posterior and diaphragmatic surfaces

·

receives oxygenated blood from the left atrium

·

pumps oxygenated blood through the aorta into the systemic circulation.

The interventricular septum separates the ventricles and helps them pump.

The thickness of a chamber's walls is determined by the amount of pressure needed to eject its blood. Because the atria act as reservoirs for the ventricles and pump the blood a shorter distance, their walls are considerably thinner than the walls of the ventricles. Likewise, the left ventricle has a much thicker wall than the right ventricle because the left ventricle pumps blood against the higher pressures in the aorta.

The right ventricle pumps blood against the lower pressures in the pulmonary circulation.

BLOOD FLOW THROUGH THE HEART

Deoxygenated venous blood returns to the right atrium through three vessels:

·

superior vena cava, which carries blood from the upper body

·

inferior vena cava, which carries blood from the lower body

·

coronary sinus, which carries blood from the heart muscle.

The increasing volume of blood in the right atrium raises the pressure in that chamber above the pressure in the right ventricle. Then the tricuspid valve opens, allowing blood to flow into the right ventricle.

The right ventricle pumps blood through the pulmonic valve into the pulmonary arteries and lungs, where oxygen is picked up and excess carbon dioxide is released. From the lungs, the oxygenated blood flows through the pulmonary veins and into the left atrium. This completes a circuit called pulmonary circulation.

systemic circulation.

Coronary circulation

Like the brain and other organs, the heart needs an adequate supply of oxygenated blood to survive. The main coronary arteries lie on the surface of the heart, with smaller arterial branches penetrating the surface into the cardiac muscle mass. The heart receives its blood supply almost entirely through these arteries. Only a small percentage of the heart's endocardial surface can obtain sufficient amounts of nutrition directly from the blood in the cardiac chambers. (See A look at coronary circulation, page 6.)

Understanding coronary blood flow can help you provide better care to a patient with coronary artery disease because you'll be able to predict which areas of the heart will be affected by a narrowing or occlusion of a particular coronary artery.

The left main and right coronary arteries arise from the coronary ostia, small orifices located just above the aortic valve cusps. The right coronary artery fills the groove between the atria and ventricles, giving

rise to the acute marginal artery and ending as the posterior descending artery. The right coronary artery supplies blood to:

A LOOK AT CORONARY CIRCULATION

The coronary circulation involves the arterial system of blood vessels that supply oxygenated blood to the heart and the venous system that removes oxygen-depleted blood from it.

c1-tt4

·

right atrium

·

right ventricle

·

inferior wall of the left ventricle

·

sinoatrial (SA) node in about 50% of the population

·

atrioventricular (AV) node in about 90% of the population.

The posterior descending artery supplies the posterior wall of the left ventricle in most people.

interventricular) and the left circumflex arteries. The left anterior descending artery runs down the anterior surface of the heart toward the apex. This artery and its branches—the diagonal arteries and the septal perforators—supply blood to:

·

anterior wall of the left ventricle

·

anterior interventricular septum

·

bundle of His

·

right bundle branch

·

 

The circumflex artery circles the left ventricle, ending on its posterior surface. The obtuse marginal artery arises from the circumflex artery. The circumflex artery provides oxygenated blood to:

·

lateral wall of the left ventricle

·

left atrium

·

posterior wall of the left ventricle in 10% of people

·

posterior fasciculus of the left bundle branch

·

SA node in about 50% of people

·

AV node in about 10% of people.

right coronary dominance or a dominant right coronary artery. Likewise, when the left coronary artery supplies the posterior wall via the posterior descending artery, the terms left coronary dominancedominant left coronary artery are used.

When two or more arteries supply the same region, they usually connect through anastomoses, junctions that provide alternative routes of blood flow. This network of smaller arteries, called collateral circulation

In contrast to the other vascular beds in the body, the heart receives its blood supply primarily during ventricular relaxation or diastole, when the left ventricle is filling with blood. This is because the coronary ostia lie near the aortic valve and become partially occluded when the aortic valve opens during ventricular contraction or systole. However, when the aortic valve closes, the ostia are unobstructed, allowing blood to fill the coronary arteries. Because diastole is the time when the coronary arteries receive their blood supply, anything that shortens diastole, such as periods of increased heart rate or tachycardia, will also decrease coronary blood flow.

In addition, the left ventricle compresses intramuscular blood vessels during systole. During diastole, the cardiac muscle relaxes, and blood flow through the left ventricular capillaries is no longer obstructed.

Just like the other parts of the body, the heart has its own veins, which remove oxygen-depleted blood from the myocardium. About three quarters of the total coronary venous blood flow leaves the left ventricle by way of the coronary sinus, an enlarged vessel that returns blood to the right atrium. Most of the venous blood from the right ventricle flows directly into the right atrium through the small anterior cardiac veins, not by way of the coronary sinus. A small amount of coronary blood flows back into the heart through the thebesian veins, minute veins that empty directly into all chambers of the heart.

Heart valves

Valves in the heart keep blood flowing in only one direction through the heart, preventing blood from traveling the wrong way. Healthy valves open and close passively as a result of pressure changes within the four chambers.

The heart contains four valves: two AV valves (tricuspid and mitral) and two semilunar valves (aortic and pulmonic). Each valve consists of cusps, or leaflets, that open and close in response to pressure changes within the chambers they connect. The primary function of the valves is to keep blood flowing through the heart in a forward direction. When the valves close, they prevent backflow, or regurgitation, of blood from one chamber to another. Closure of the valves is associated with heart sounds.

bicuspid valve because of its two cusps, separates the left atrium from the left ventricle. Closure of the AV valves is associated with S1, or the first heart sound.

A look at the mitral valve.) Disruption of either of these structures may prevent complete valve closure, allowing blood to flow backward into the atria. This backward blood flow may cause a heart murmur.

These illustrations show the mitral valve and the attached papillary muscles and chordae tendineae. In the illustration on the left, the mitral valve is open, the papillary muscles are relaxed, and the chordae tendineae are slack. In the illustration on the right, the mitral valve is closed, the papillary muscles are contracted, and the chordae tendineae are tight to prevent the valve leaflets from entering the atria.

c1-tt5

The semilunar valves are so called because their three cusps resemble half moons. The pulmonic valve, located where the pulmonary artery meets the right ventricle, permits blood to flow from the right ventricle to the pulmonary artery and prevents backflow into the right ventricle. The aortic valve, located where the left ventricle meets the aorta, allows blood to flow from the left ventricle to the aorta and prevents blood backflow into the left ventricle.

Increased pressure within the ventricles during ventricular systole causes the pulmonic and aortic valves to open, allowing ejection of blood into the pulmonary and systemic circulation. Loss of pressure as the ventricular chambers empty causes the valves to close. Closure of the semilunar valves is associated with S2, or the second heart sound.

CARDIOVASCULAR PHYSIOLOGY

The ANS controls how the heart pumps by an electrical conduction system, which regulates myocardial contraction. This system includes the nerve fibers of the ANS and specialized nerves and fibers in the heart. The ANS involuntarily increases or decreases heart action to meet the individual's metabolic needs.

Both sympathetic and parasympathetic nerves participate in the control of cardiac function. With the body at rest, the parasympathetic nervous system controls the heart through branches of the vagus nerve (cranial nerve X). Heart rate and electrical impulse propagation slow down.

In times of activity or stress, the sympathetic nervous system takes control. It stimulates the heart's nerves and fibers to fire and conduct more rapidly and the ventricles to contract more forcefully.

Cardiac cycle

The cardiac cycle describes the period from the beginning of one heartbeat to the beginning of the next. During this cycle, electrical and mechanical events must occur in the proper order and to the proper degree to provide adequate blood flow to all body parts. Basically, the cardiac cycle has two phases: systole and diastole.

At the beginning of systole, the ventricles contract, increasing pressure and forcing the mitral and tricuspid valves to close. This valvular closing prevents blood backflow into the atria and coincides with the first heart sound (S1), also known as the “lub” of “lub-dub.” As the ventricles contract, ventricular pressure builds until it exceeds that in the pulmonary artery and the aorta. Then the aortic and pulmonary semilunar valves open, and the ventricles eject blood into the aorta and the pulmonary artery.

When the ventricles empty and relax, ventricular pressure falls below that in the pulmonary artery and the aorta. At the beginning of diastole, the semilunar valves close to prevent backflow into the ventricles. This coincides with the second heart sound, S2, also known as the “dub” of “lub-dub.”

As the ventricles relax, the mitral and tricuspid valves open and blood begins to flow into the ventricles from the atria. When the ventricles become full, near the end of diastole, the atria contract to send the remaining blood to the ventricles. A new cardiac cycle begins as the heart enters systole again. (See Understanding the phases of the cardiac cycle.)

Cardiac output and stroke volume

Cardiac output refers to the amount of blood the left ventricle pumps into the aorta in 1 minute. Cardiac output is measured by multiplying heart rate by stroke volume. Stroke volume refers to the amount of blood ejected with each ventricular contraction and is usually about 70 ml.

Normal cardiac output is 4 to 8 L per minute, depending on body size. The heart pumps only as much blood as the body requires, based upon metabolic requirements. During exercise, for example, the heart increases cardiac output accordingly.

UNDERSTANDING THE PHASES OF THE CARDIAC CYCLE

1. Isovolumetric ventricular contraction

In response to ventricular depolarization, tension in the ventricles increases. The rise in pressure within the ventricles leads to closure of the mitral and tricuspid valves. The pulmonic and aortic valves stay closed during the entire phase.

2. Ventricular ejection

When ventricular pressure exceeds aortic and pulmonary artery pressures, the aortic and pulmonic valves open and the ventricles eject blood.

3. Isovolumetric relaxation

When ventricular pressure falls below the pressures in the aorta and pulmonary artery, the aortic and pulmonic valves close. All valves are closed during this phase. Atrial diastole occurs as blood fills the atria.

4. Ventricular filling

Atrial pressure exceeds ventricular pressure, which causes the mitral and tricuspid valves to open. Blood then flows passively into the ventricles. About 70% of ventricular filling takes place during this phase.

5. Atrial systole

Known as the atrial kick, atrial systole (coinciding with late ventricular diastole) supplies the ventricles with the remaining 30% of the blood for each heartbeat.

c1-tt6

DEFINING PRELOAD AND AFTERLOAD

Preload refers to a passive stretching exerted by blood on the ventricular muscle fibers at the end of diastole. According to Starling's law, the more the cardiac muscles are stretched in diastole, the more forcefully they contract in systole.

Afterload refers to the pressure that the ventricles need to generate to overcome higher pressure in the aorta to eject blood into the systemic circulation. This systemic vascular resistance corresponds to the systemic systolic pressure.

c1-tt7

Three factors determine stroke volume:

·

preload

·

afterload

·

myocardial contractility. (See Defining preload and afterload.)

Preload is the degree of stretch or tension on the muscle fibers when they begin to contract. It's usually considered to be the end-diastolic pressure when the ventricle has become filled.

Afterload is the load or amount of pressure the left ventricle must work against to eject blood during systole and corresponds to the systolic pressure: the greater this resistance, the greater the heart's workload. Afterload is also called the systemic vascular resistance.

Myocardial contractility is the ventricle's ability to contract, which is determined by the degree of muscle fiber stretch at the end of diastole. The more the muscle fibers stretch during ventricular filling, up to an optimal length, the more forceful the contraction. (See Determining cardiac output.)

Nerves of the two branches of the ANS—the sympathetic (or adrenergic) and the parasympathetic (or cholinergic heart. Sympathetic fibers innervate all areas of the heart, whereas parasympathetic fibers primarily innervate the SA and AV nodes.

DETERMINING CARDIAC OUTPUT

This illustration shows how preload, afterload, and contractility work together to determine cardiac output.

c1-tt8

·

increases the heart rate by increasing SA node discharge

·

accelerates AV node conduction time

·

increases the force of myocardial contraction and cardiac output.

Parasympathetic (vagal) stimulation causes the release of acetylcholine, which produces the opposite effects:

·

decreases the rate of SA node discharge, thus slowing heart rate and conduction through the AV node

·

reduces cardiac output.

Transmission of electrical impulses

For the heart to contract and pump blood to the rest of the body, an electrical stimulus needs to occur first. Generation and transmission of electrical impulses depend on the four key characteristics of cardiac cells:

·

automaticity

·

excitability

·

 

·

contractility.

Automaticity refers to a cell's ability to spontaneously fire off an electrical impulse. Pacemaker cells usually possess this ability. Excitability results from ion shifts across the cell membrane and refers to the cell's ability to respond to an electrical stimulus. Conductivity is the ability of a cell to transmit an electrical impulse from one cell to another. Contractility refers to the cell's ability to contract after receiving a stimulus by shortening and lengthening its muscle fibers.

It's important to remember that the first three characteristics are electrical properties of the cells, while contractility represents a mechanical response to the electrical activity. Of the four characteristics, automaticity has the greatest effect on the start of cardiac rhythms.

Depolarization and repolarization

As impulses are transmitted, cardiac cells undergo cycles of depolarization and repolarization. (See Understanding the depolarization-repolarization cycle.) Cardiac cells at rest are considered polarized, meaning that no electrical activity takes place. Cell membranes separate different concentrations of ions, such as sodium and potassium, and create a more negative charge inside the cell. This is called the resting potential. After a stimulus occurs, ions cross the cell membrane and cause an action potential, or cell depolarization. When a cell is fully depolarized, it attempts to return to its resting state in a process called repolarization. Electrical charges in the cell reverse and return to normal.

A cycle of depolarization-repolarization consists of five phases— 0 through 4. The action potential is represented by a curve that shows voltage changes during the five phases. (See A look at action potential curves, page 16.)

During phase 0 (or rapid depolarization), the cell receives a stimulus, usually from a neighboring cell. The cell becomes more permeable to sodium, the inside of the cell becomes less negative, the cell is depolarized, and myocardial contraction occurs. In phase 1 (or early repolarization), sodium stops flowing into the cell, and the transmembrane potential falls slightly. Phase 2 (the plateau phase) is a prolonged period of slow repolarization, when little change occurs in the cell's transmembrane potential.

During phases 1 and 2 and at the beginning of phase 3, the cardiac cell is said to be in its absolute refractory period. During that period, no stimulus, no matter how strong, can excite the cell.

Phase 3 (or rapid repolarization) occurs as the cell returns to its original state. During the last half of this phase, when the cell is in its relative refractory period, a very strong stimulus can depolarize it.

UNDERSTANDING THE DEPOLARIZATION-REPOLARIZATION CYCLE

The depolarization-repolarization cycle consists of these phases.

c1-tt9

Phase 4 is the resting phase of the action potential. By the end of phase 4, the cell is ready for another stimulus.

The electrical activity of the heart is represented on an electrocardiogram (ECG). Keep in mind that the ECG represents electrical activity only, not the mechanical activity or actual pumping of the heart.

Electrical conduction system of the heart

After depolarization and repolarization occur, the resulting electrical impulse travels through the heart along a pathway called the conduction system. (See Illustrating the cardiac conduction system, page 17.)

Impulses travel out from the SA node and through the internodal tracts and Bachmann's bundle to the AV node. From there, they travel through the bundle of His, the bundle branches, and finally to the Purkinje fibers.

A LOOK AT ACTION POTENTIAL CURVES

An action potential curve shows the changes in a cell's electrical charge during the five phases of the depolarization-repolarization cycle. These graphs show electrical changes for nonpacemaker and pacemaker cells.

c1-tt10

As the graph below shows, the action potential curve for pacemaker cells, such as those in the sinoatrial node, differs from that of other myocardial cells. Pacemaker cells have a resting membrane potential of -60 mV (instead of -90 mV), and they begin to depolarize spontaneously. Called diastolic depolarization, this effect results mainly from calcium and sodium leaking into the cell.

c1-tt11

ILLUSTRATING THE CARDIAC CONDUCTION SYSTEM

Specialized fibers propagate electrical impulses throughout the heart's cells, causing the heart to contract. This illustration shows the elements and the order of the cardiac conduction system.

c1-tt12

The SA node, located in the right atrium where the superior vena cava joins the atrial tissue mass, is the heart's main pacemaker. Under resting conditions, the SA node generates impulses 60 to 100 beats/ minute. When initiated, the impulses follow a specific path through the heart. Electrical impulses usually don't travel in a backward or retrograde direction because the cells can't respond to a stimulus immediately after depolarization.

From the SA node, an impulse travels through the right and left atria. In the right atrium, the impulse is believed to be transmitted along three internodal tracts:

·

anterior

·

middle (or Wenckebach's)

·

posterior (or Thorel's).

The impulse travels through the left atrium via Bachmann's bundle, the interatrial tracts of tissue extending from the SA node to the left atrium. Impulse transmission through the right and left atria occurs so rapidly that the atria contract almost simultaneously.

The AV node is located in the inferior right atrium near the ostium of the coronary sinus. Although the AV node doesn't possess pacemaker cells, the tissue surrounding it, referred to as junctional tissue, contains pacemaker cells that can fire at a rate of 40 to 60 beats/minute. As the AV node conducts the atrial impulse to the ventricles, it causes a 0.04-second delay. This delay allows the ventricles to complete their filling phase as the atria contract. It also allows the cardiac muscle to stretch to its fullest for peak cardiac output.

Rapid conduction then resumes through the bundle of His, which divides into the right and left bundle branches and extends down either side of the interventricular septum. The right bundle branch extends down the right side of the interventricular septum and through the right ventricle. The left bundle branch extends down the left side of the interventricular septum and through the left ventricle. As a pacemaker site, the bundle of His has a firing rate between 40 and 60 beats/minute. The bundle of His usually fires when the SA node fails to generate an impulse at a normal rate or when the impulse fails to reach the AV junction.

The left bundle branch then splits into two branches, or fasciculations. The left anterior fasciculus extends through the anterior portion of the left ventricle. The left posterior fasciculus extends through the lateral and posterior portions of the left ventricle. Impulses travel much faster down the left bundle branch, which feeds the larger, thicker-walled left ventricle, than the right bundle branch, which feeds the smaller, thinner-walled right ventricle. The difference in the conduction speed allows both ventricles to contract simultaneously. The entire network of specialized nervous tissue that extends through the ventricles is known as the His-Purkinje system.

Purkinje fibers comprise a diffuse muscle fiber network beneath the endocardium that transmits impulses quicker than any other part of the conduction system. This pacemaker site usually doesn't fire unless the SA and AV nodes fail to generate an impulse or when the normal impulse is blocked in both bundle branches. The automatic firing rate of the Purkinje fibers ranges from 20 to 40 beats/minute.

ABNORMAL IMPULSE CONDUCTION

Causes of abnormal impulse conduction include:

·

altered automaticity

·

retrograde conduction of impulses

·

reentry abnormalities

·

ectopy.

Automaticity, a special characteristic of pacemaker cells, allows them to generate electrical impulses spontaneously. If a cell's automaticity is increased or decreased, an arrhythmia—or abnormality in the cardiac rhythm—can occur. Tachycardia and premature beats are commonly caused by an increase in the automaticity of pacemaker cells below the SA node. Likewise, a decrease in automaticity of cells in the SA node can cause the development of bradycardia or escape rhythms generated by lower pacemaker sites.

Impulses that begin below the AV node can be transmitted backward toward the atria. This backward, or retrograde, conduction usually takes longer than normal conduction and can cause the atria and ventricles to lose synchrony.

Reentry occurs when cardiac tissue is activated two or more times by the same impulse. This may happen when conduction speed is slowed or when the refractory periods for neighboring cells occur at different times. Impulses are delayed long enough that cells have time to repolarize. In those cases, the active impulse reenters the same area and produces another impulse.

Injured pacemaker (or nonpacemaker) cells may partially depolarize, rather than fully depolarizing. Partial depolarization can lead to spontaneous or secondary depolarization, repetitive ectopic firings called triggered activity.

The resultant depolarization is called afterdepolarization. Early afterdepolarization occurs before the cell is fully repolarized and can be caused by:

·

hypokalemia

·

slow pacing rates

·

drug toxicity.

If it occurs after the cell has been fully repolarized, it's called delayed afterdepolarization, and tachycardia may result. This can be caused by:

·

digoxin toxicity

·

hypercalcemia

·

increased catecholamine release.

Functions of the vascular system

About 60,000 miles of arteries, arterioles, capillaries, venules, and veins keep blood circulating to and from every functioning cell in the body. This network has two branches that include the pulmonary circulation and systemic circulation. (See A look at the major blood vessels, page 20, and A look at the pulmonary and systemic circulation, page 21.)

PULMONARY CIRCULATION

Pulmonary circulation refers to the movement of the blood as it travels back through the heart to the lungs to exchange carbon dioxide for oxygen. Following are the actions that occur during pulmonary circulation:

A LOOK AT THE MAJOR BLOOD VESSELS

This illustration shows the body's major arteries and veins.

c1-tt13

A LOOK AT THE PULMONARY AND SYSTEMIC CIRCULATION

This illustration shows the relationship between the pulmonary and systemic circulation. The blood begins in the right ventricle (1), travels through the lungs to the left atrium (4) and ventricle (5), and enters the systemic circulation before returning to the right atrium (9).

c1-tt14

·

Deoxygenated blood travels from the right ventricle through the pulmonic valve into the pulmonary arteries.

     

·

Blood passes through progressively smaller arteries and arterioles into the capillaries of the lungs.

·

Blood reaches the alveoli and exchanges carbon dioxide for oxygen.

·

The oxygenated blood then returns via venules and veins to the pulmonary veins, which carry it back to the left atrium of the heart.

SYSTEMIC CIRCULATION

Through the systemic circulation, blood carries oxygen and other nutrients to body cells and transports waste products for excretion. At specific sites, the pumping action of the heart that forces blood through the arteries becomes palpable. This regular expansion and contraction of the arteries is called the pulse.

The major artery—the aorta—branches into vessels that supply blood to specific organs and areas of the body. The left common carotid, left subclavian, and innominate arteries arise from the arch of the aorta and supply blood to the brain, arms, and upper chest. As the aorta descends through the thorax and abdomen, its branches supply blood to the GI and genitourinary organs, spinal column, and lower chest and abdominal muscles. Then the aorta divides into the iliac arteries, which further divide into femoral arteries.

As the arteries divide into smaller units, the number of vessels increases dramatically, thereby increasing the area of perfusion. Arteries are thick-walled because they transport blood under high pressure. Arterial walls contain a tough, elastic layer to help propel blood through the arterial system. At the end of the arterioles and the beginning of the capillaries, strong sphincters control blood flow into the tissues. These sphincters:

·

dilate to permit more flow when needed

·

close to shunt blood to other areas

·

constrict to increase blood pressure.

Although the capillary bed contains the smallest vessels, it supplies blood to the largest area. Capillary pressure is extremely low to allow for the exchange of nutrients, oxygen, and carbon dioxide with body cells. From the capillaries, blood flows into venules and, eventually, into veins. Only about 5% of the circulating blood volume at any given moment is contained within the capillary network.

Nearly all veins carry oxygen-depleted blood, the sole exception being the pulmonary vein, which carries oxygenated blood from the lungs to the left atrium. Veins serve as a large reservoir for circulating blood. Valves in the veins prevent blood backflow, and the pumping action of skeletal muscles assists venous return. The wall of a vein is thinner and more pliable than the wall of an artery. That pliability allows the vein to accommodate variations in blood volume. The veins merge until they form two main branches—the superior and inferior venae cavae—that return blood to the right atrium.

LOCATING PULSE SITES

This illustration shows the locations of the major arterial pulses and the apical pulse.

c1-tt15

PULSES

Arterial pulses are pressure waves of blood generated by the pumping action of the heart. All vessels in the arterial system have pulsations, but the pulsations can be felt only where an artery lies near the skin. You can palpate for these peripheral pulses: temporal, carotid, brachial, radial, ulnar, femoral, popliteal, posterior tibial, and dorsalis pedis. (See Locating pulse sites.)


Previous
Page
Next
Page