Mammal

The heart as a mechanical pump

The heart pumps 1.3 gallons of blood per minute, or 700,000 gallons per year, at rest and four times that amount at peak exercise. It weighs less than a pound and is the size of a closed fist.The heart is a powerful pump. It propels blood (a very viscous liquid) through 50,000 miles of vessels. It rests 0.25 second per beat, and it can increase its output by a factor of 5× to 8× under stress. The cardiac reserve is the ratio of maximum output to resting output.

Types of circulation in the anatomy

• In systemic circulation, arteries carry oxygenated blood from the heart to the rest of the body. Veins then return oxygen-depleted blood to the heart.
• In pulmonary circulation, pulmonary arteries carry oxygen-depleted blood to the lungs, where it is oxygenated and returned to the heart through pulmonary veins.
• Arteries have thick walls and carry blood away from the heart.
• Veins have thin walls and return blood to the heart

The thoracic cavity contains three separate enclosures, one for each lung and one for the heart. The mediastinum is the heart’s enclosure and is the space between the right and left lungs, between the sternum and the vertebral column, and between the clavicles and the diaphragm. The heart’s right-to-left axis is between the lungs. Its front-to-back axis is between the sternum and backbone. Its up-and down axis is between the diaphragm and neck. It sits on the diaphragm.

Location of the Heart

Figure 1. Hart is located in the center of the chest in the thoracic cavity, which is bounded by the ribs and the diaphragm. 

Contrary to common belief, the heart is not located on the left side but in the center of the chest in the thoracic cavity, which is bounded by the ribs and the diaphragm. CPR uses the flexible ribs to compress the heart between the sternum and backbone and pump blood in cases of cardiac arrest. Before the advent of CPR, open cardiac massage was used.

The thoracic cavity contains three separate enclosures, one for each lung and one for the heart. The mediastinum is the heart’s enclosure and is the space between the right and left lungs, between the sternum and the vertebral column, and between the clavicles and the diaphragm. The heart’s right-to left axis is between the lungs. Its front-to-back axis is between the sternum and backbone. Its up-and down axis is between the diaphragm and neck. It sits on the diaphragm.
The heart’s covering is the pericardium (“around the heart”). The pericardium is a saccular structure of tough  fibrous tissue that holds the heart in place and protects it from overexpansion. The parietal (“wall”) pericardium lines the walls of the heart’s enclosure. The visceral pericardium is a thin layer that covers and adheres to the heart. The space between the parietal and visceral pericardia is filled with lubricating fluid.

Clinical applications: Infections and inflammations of the pericardium (pericarditis) can cause friction between the heart and pericardial surfaces, causing severe pain. (See video on what is pericarditis?).

Trauma can cause blood to fill the pericardium, resulting in pericardial tamponade (blood accumulating between the visceral and parietal pericardium and pressing on the heart). Emergency surgery is necessary to relieve pressure on the heart.

Heart structure

Figure 2. Heart: Base and the Diaphragmatic Surface. Image credit: Netter's Atlas of Human Anatomy

The heart wall

The heart wall has three layers. The outer layer is the epicardium (epi means “outer”), which is actually the visceral pericardium. It is a shiny, transparent, lubricating layer that is an integral part of the heart wall. The thin inner layer is the endocardium (endo means “inner”). It covers the muscle and valves, and it continues out into the vessels, where it becomes the endothelium.

The middle layer, the myocardium (myo means “muscle”), is powerful, thick, and tireless. It is a specialized form of skeletal muscle that constantly pumps rhythmically without “instruction” from the nerves or blood. It consists of swirled layers of muscle that envelop the heart so that contraction squeezes the chambers to empty them. There are two sets of muscles: one for the ventricles and one for the atria. The two sets contract independently. The myocardium is an autonomic (involuntary) muscle.

The anatomy of the heart

The heart is a small, powerful, and untiring organ that pumps blood through more than 50,000 miles of vessels. The nomenclature of “right and left hearts” indicates an artificial separation of heart function. The right (venous) heart receives deoxygenated blood from the body and delivers it to the lungs. The left heart receives oxygenated blood from the lungs and delivers it to the body. In the embryo, the right and left hearts start out as two separate tubes that eventually fuse. The right and left hearts are anatomically different and operate at different pressures. The muscles of the left side are four times thicker and more powerful than those on the right side, which pumps blood through a much shorter distance and at lower pressures.

Schematic Diagram of Double Circulation

Figure 3. Double Circulation Diagram. Mammals have separate circulations for oxygenated and  de-oxygenated blood. Inferion vena cava collects oxygen-depleted blood from the lower parts of the body and limbs; superior vena cava collects oxygen depleted blood from the upper parts of the body and arms. Pulmonary veins take oxygen rich blood form the lungs to the heart. Pulmonary arteries crry oxygen-depleted blood from the heart to the lungs. Image credit: PPP Healthcare


Both sides of the heart have upper and lower chambers. The atria (upper chambers) receive blood from the veins and pump it into the ventricles (atrium = “waiting room”). The ventricles (lower chambers) receive blood from the atria and pump it to the body and lungs (ventricle = “little belly”). Because the two circulatory systems are connected, both sides of the heart must pump the same amount of blood. Even the smallest mismatch in circulation causes blood to accumulate in one side of the system, such as in heart failure.

Our four-chambered heart is at the top of the evolutionary line. The right atrium (plural is atria) receives deoxygenated blood returning from the body to the heart. Blood returns via the superior and inferior venae cavae. The right atrium pumps blood into the right ventricle. The right atrium is a thinwalled, low-pressure system.

The right ventricle receives deoxygenated blood from the right atrium. It pumps this deoxygenated blood to the lungs via the pulmonary artery. The right ventricle is a thin-walled, low-pressure system. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. It pumps blood into the left ventricle. It is a thin-walled, low-pressure system.

The left ventricle receives oxygenated blood from the left atrium. It pumps blood out to the whole body via the aorta. It is a thick-walled, highpressure system. The intra-atrial septum separates the atria. It contains a closed embryologic opening (the foramen ovale). The intraventricular septum separates the ventricles. It contains part of the conduction system (see below).
The valves of the heart prevent back flow. The mitral or bicuspid (“two leaves”) valve is located between the left atrium and left ventricle. It prevents back flow into the atrium during systole. The tricuspid valve is located between the right atrium and the right ventricle. It prevents back flow into the atrium during systole. The chordae tendineae and papillary muscles hold these valves tightly closed during back flow. The aortic (semilunar, i.e., shaped like a half-moon) valve is located between the left ventricle and the aorta. It prevents back flow into the ventricle during diastole. The pulmonic (semilunar) valve is located between the right ventricle and the pulmonary artery. It prevents back flow into the ventricle during diastole.

Clinical applications

Damage to the valves can cause stenosis (difficulty in opening), insufficiency (inadequate closure), or both. Mitral valve prolapse (a slight insufficiency) is common in young women. It is often asymptomatic. These conditions can be treated with valve replacements.

The coronary arteries supply blood to the heart itself. The left coronary artery comes off the left side of the aorta immediately after leaving the heart. It supplies blood to the muscles of both ventricles, the septum, and some of the left atrium. The right coronary artery comes off the aorta on the right side, level with the left coronary artery. It supplies blood to the right ventricle, left ventricle, right atrium, and septum. Both arteries supply both sides of the heart with plentiful anastomoses (places where blood vessels join), which provide redundant blood pathways.

Coronary arteries and Cardiac Veins

Figure 5. Rich blood supply to the heart is provided by numerous coronary arteries and cardiac veins. None Numerous anastomoses. Image Credit: Netter's Atlas of Human Anatomy

Cessation of coronary artery flow for more than a few minutes causes death of the heart muscle, called myocardial infarction (“heart attack” or “coronary”). However, more than 70% of the blood flow must be stopped before tissue damage becomes a problem or any symptoms appear. The coronary veins drain both sides of the myocardium. They empty progressively into the great cardiac vein, the middle cardiac vein, and from there into the coronary sinus in the right atrium.

Conduction system

Muscle is inherently conductive, but during embryologic development, about 1% of the heart muscle differentiates to become able to conduct electrical signals almost as well as nerves. these cells constitute a network nown as the conducting system of the heart and are in contact with the cardiac muscle via gap junctions. Chemical processes in the body produce electrical signals that cause the heart to contract. The conducting system initiates the heart beat and helps spread the impulse rapidly throughout the heart. The sinoatrial node is the heart’s primary pacemaker. It is located high in the right atrium.

In the old days, pacemakers had a fixed rate and patients had to come in for battery changes as they had a very short life. Today they’re highly sophisticated; the batteries almost never have to be changed.

Conducting System of the Heart

Figure 6. The atrioventricular node is located just above the atrioventricular septum in the base of the right atrium. The conduction system continues into the ventricles via the atrioventricular bundle (the bundle of His). The atrioventricular bundle then spreads throughout the ventricles and the septum via the conduction myofibers of Purkinje. This system of conduction is necessary because the myocardium has different inherent contractility in atria and ventricles, and uncoordinated contraction would occur without the conduction system.

Now we examine the physiology of the heart, starting with the functioning of the cardiac cycle, in which deoxygenated blood flows into the heart from the body, is pumped out to the lungs for oxygenation, and is then returned to the heart for distribution to the body We also examine the functioning of the heart’s conduction system, the functioning of the valves (which produce the heart’s distinctive “lub-dub” sound), and possible complications, notably atherosclerosis.

Systole and diastole

Systole is the active compression, or squeezing, of the ventricles that pushes blood outward to the lungs or the body. Diastole is the relaxation of the ventricles when they are filling with blood. The terms systole and diastole refer only to the ventricles. Atrial systole and diastole exist but are never referred to as simply systole or diastole.

Physiology of the cardiac cycle

Each side of the heart has two chambers. The atria are low-pressure systems, thin walled on both sides. They deliver blood to the ventricles. They contract during diastole to help fill the ventricles. Much of ventricular filling is passive; thus, atrial failure decreases cardiac output by only a small percentage (20–30%).

The ventricles

The left side of the ventricle is four times thicker and more powerful than the right side. The left (systemic) side is also a high-pressure system. It receives oxygenated blood from the lungs and pumps it out to the body. The right (pulmonary) side is a low-pressure system. It receives deoxygenated blood from the body and pumps it out to the lungs.

The conduction system

During embryologic development, 1% of the muscle mass of the heart is designated as autorhythmic (self-exciting). These muscles differentiate to form a conduction system. The conduction system establishes the fundamental rhythm. Hormones, chemicals, and nerve impulses can alter the heartbeat strength and heart rate.

The sinoatrial (SA) node initiates the rhythm. It has an inherent rhythm of 60–100 beats per minute. It is located high in the right atrial wall. Impulses spread from the SA node to both atria, causing atrial contraction.

The impulses then spread to the atrioventricular (AV) node, located at the medial base of the right atrium in the atrial septum above the ventricles. AV stimulation sends impulses to the AV bundle of His. The AV bundle of His provides the only electrical connection between the atria and the ventricles. A bundle of His impulses travel through the right and left bundle branches to the conduction myofibers of Purkinje, which conduct impulses to the right and left ventricular muscles, causing ventricular contraction.


The SA node is the normal pacemaker for the entire heart. Its depolarization normally initiates the cardiac excitation sequence, generating the action potential that leads to depolarization of all other cardiac muscle cells, and so its discharge rate determines the heart rate, the number of times the heart contracts per minute. The action potential initiated in the SA node spreads throughout the myocardium, passing from cell to cell by way of gap junctions. The spread throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system. The conduction through atrial muscle cells is rapid enough that the two atria are depolarized and contract at essentially the same time. However, the spread of the action potential to the ventricles is more complicated and involves the rest of the conducting system. The link between atrial depolarization and ventricular depolarization is a portion of the conducting system called the atrioventricular (AV) node, which is located at the base of the right atrium. The action potential spreading through the muscle cells of the right atrium causes depolarization of the AV node. This node has a particularly important characteristic: the propagation of action potentials through the AV node is relatively slow (requiring approximately 0.1 s). This results in a delay that allows atrial contraction to be completed before ventricular excitation occurs.

Figure 8 Sequence of cardiac excitation. The yellow color denotes areas that are depolarized. Impulses spread from right atrium to left atrium. Image credit: Science Natural phenomena

After leaving the AV node, the impulse enters the wall—the interventricular septum—between the two ventricles. This pathway has conducting-system fibers termed the bundle of His (or atrioventricular bundle) after its discoverer (pronounced Hiss). It should be emphasized that the AV node and the bundle of His constitute the only electrical link between the atria and the ventricles. Except for this pathway, the atria are completely separated from the ventricles by a layer of nonconducting connective tissue.

Within the interventricular septum the bundle of His divides into right and left bundle branches, which eventually leave the septum to enter the walls of both ventricles. These fibers in turn make contact with Purkinje fibers, large conducting cells that rapidly distribute the impulse throughout much of the ventricles. Finally, the Purkinje fibers make contact with ventricular myocardial cells, by which the impulse spreads through the rest of the ventricles. The rapid conduction along the Purkinje fibers and the diffuse distribution of these fibers cause depolarization of all right and left ventricular cells more or less simultaneously and ensure a single coordinated contraction. Actually, though, depolarization and contraction begin slightly earlier in the bottom (apex) of the ventricles and spread upward. The result is a more efficient contraction, like squeezing a tube of toothpaste from the bottom up.


Timing


The AV node fibers are small, and this delay allows atrial contraction to be completed before the next heartbeat. Conduction speeds up in the AV bundle. The AV node’s inherent rhythm is much slower (40–50 beats per minute). Disruption of the SA-AV node sequence will result in a slowed heart rate of less than 60 beats per minute.

Patients with SA-AV node disturbances (bundle branch blocks) can be fitted with pacemakers, which electrically stimulate the heart. Pacemakers can be programmed for the demands of variable pacing. Valve stenosis (a tight valve) may cause the valve to overexpand and weaken, increasing the conduction distance. A pacemaker can correct this delay.


The cardiac cycle

The first phase of the cardiac cycle consists of the relaxation period. Both the atria and ventricles are in relaxation (diastole). Blood fills the atria from the body and the lungs; when the pressure is high enough, the mitral and tricuspid valves (between the atria and ventricles) open, and blood begins to passively fill the ventricles. The atrial contraction assists in filling the ventricles but is not critical.
Starling’s Law (or Frank-Starling’s Law) relates muscular stretching to muscle power. Up to a certain
point, stretching a muscle makes it contract with greater power. Atrial contraction stretches the ventricles for greater systolic power.

The second phase consists of ventricular filling, which begins. after the valves open. When 30%
of the blood is left in the atria, atrial contraction (atrial systole) occurs, emptying the last of the atrial blood into the ventricles. If no atrial contraction occurs, 30% of ventricular filling is lost. An increase in the heart rate can compensate for this loss. The AV valves are still open, and the semilunar valves are still closed.

The third phase consists of ventricular systole (ejection). The ventricles contract, closing the A-V valves. All valves are closed for a fraction of a second. The aortic and pulmonary valves open. Blood is ejected out into the systemic and pulmonary circulations via the aorta and pulmonary artery. When all ventricular force is spent, the aortic and pulmonic valves close, preventing systemic and pulmonary pressure from causing back flow into the ventricles. Once the third phase is complete, the cycle begins again.

Clinical application

During ventricular systole, coronary flow is obstructed by squeezing of the myocardium. Therefore, coronary flow to the heart muscle occurs during ventricular diastole, not ventricular systole. During periods of increased heart rate, such as exercise, speed comes at the expense of diastole. In a heart with narrow blood vessels, coronary filling may be compromised to a sufficient extent to cause angina pectoris (“pain in the chest”). Patients with only slightly narrowed vessels may be asymptomatic until a significant exertion that sufficiently raises the heart rate. Treadmill, thallium, and echocardiogram tests can identify areas with absent or decreased blood flow by comparing a patient’s stressed and unstressed coronary blood flows.

Cardiac output Stroke volume is measured by the milliliters (ml) of blood pumped out of the left ventricle in one stroke (beat).

Calculating cardiac output Cardiac output (ml/minute) = stroke volume (ml/beat) × heart rate (beats/ minute) = ml/minute. At rest, cardiac output (ml/beat) = stroke volume (70 ml/beat) × heart rate (75 beats/min) = 5,250 ml/minute. With moderate exercise, cardiac output = stroke volume (140 ml/beat) × heart rate (150 beats/min) = 21,000 ml/min. Cardiac output can safely go much higher in extreme stress in a trained athlete. A young, healthy person will have a maximum safe heart rate of about 200 beats per minute. The safe maximum heart rate declines with age. A pathological state of arrhythmia can produce rates of up to 300 beats per minute.

The big picture

An electrocardiogram (EKG) provides an electrical picture of the cardiac cycle as seen from the surface of the chest. The P-wave is an electrical signature of atrial depolarization. Atrial contraction occurs 0.1 second after the P-wave. The QRS complex marks the beginning of ventricular systole. There is no sign of atrial recovery because it is buried in the electrical portrait of the large QRS wave (atrial recovery is synchronous with ventricular contraction). The T-wave indicates ventricular repolarization, recovery, and rest. The P-R interval is the length of time between the beginning of atrial contraction and the beginning of ventricular contraction. Heart block (A-V dissociation) produces a prolongation or interruption in the P-R interval. Irregularities in an EKG can diagnose many heart problems.

Symptoms

The most common symptom of a heart attack is chest pain. The second most common symptom is denial. Almost everybody denies they’re having a heart attack. “I’m having indigestion.” And many people end up dying with a pack of Rolaids in their hand because they thought they were having indigestion. About 75% narrowing will cause symptoms. And some of these people are correctable merely by diet, some with exercise, and some need more mechanical intervention, the kind of intervention that we’ve talked about.

Heart sounds—“lub-dub”

These sounds are made by blood turbulence as valves close. The sounds are not splashes, because there is no air in the heart, but rather turbulence associated with valve closure. The first sound (lub) comes with the closure of the mitral and tricuspid valves as ventricular systole begins in full. The
second sound (dub) comes with the closure of the aortic and pulmonary semilunar valves as systole ends and diastole begins. Mitral and aortic sounds dominate over tricuspid and pulmonic sounds because of the higher pressures in the arterial side (left heart). An important diagnostic tool is the changes of the rhythm and intensities of the sounds in disease. Damaged valves can produce “gallop rhythms,” or murmurs, which are the sounds of turbulence when valves are damaged. Heart sounds are not heard best directly over the valves but when projected back into the chambers. Stents, angioplasties, and chemicals can repair damaged valves. Bypass surgery reroutes blood around the damaged area.

Source: Understanding the Human Body: An Introduction to Anatomy and Physiology by Professor Anthony A. Goodman, Montana State University.