Functional anatomy of the heart

The heart is located in the center of the thoracic cavity. It sits directly above the muscles of the diaphragm, which separates the thorax from the abdomen, and lies beneath the sternum between the two lungs. The heart is enclosed and anchored in place by a double-walled fibrous sac referred to as the pericardium. The membranes of the pericardium produce a small amount of pericardial fluid that minimizes friction produced by the movement of the heart when it beats. To function mechanically as a pump, the heart must have:

• Receiving chambers

• Delivery chambers

The atria (sing. atrium) are chambers that receive blood returning to the heart through the veins. The blood then moves to the ventricles, or delivery chambers, of the heart. The powerful contractions of the ventricles generate a force sufficient to propel blood through the systemic or the pulmonary circulations. Valves ensure the one-way, or forward, flow of the blood.

The route of blood flow through the heart begins with the venae cavae, which return blood from the peripheral tissues to the right side of the heart (see Figure 13.1; Table 13.1). The superior vena cava returns blood from the head and arms to the heart and the inferior vena cava returns blood from the trunk of the body and the legs to the heart. As this blood has already passed through the tissues of the body, it is low in oxygen. Blood from the venae cavae first enters the right atrium and then the right ventricle. Contraction of the right ventricle propels this blood to the lungs through the pulmonary circulation by way of the pulmonary artery. As it flows through the lungs, blood becomes enriched with oxygen and eliminates carbon dioxide to the

Figure 13.1 Route of blood flow through the heart. Systemic blood returns to the heart by way of the superior (SVC) and inferior (IVC) venae cavae. This blood, which is low in oxygen and high in carbon dioxide, enters the right atrium (RA) and then the right ventricle (RV). The right ventricle pumps the blood through the pulmonary artery (PA) to the pulmonary circulation. It is within the lungs that gas exchange takes place. Next, this blood, which is high in oxygen and low in carbon dioxide, returns to the heart by way of the pulmonary veins (PV). It enters the left atrium (LA) and then the left ventricle (LV). The left ventricle pumps the blood through the aorta to the systemic circulation and the peripheral tissues.

Arterial Blood

Figure 13.1 Route of blood flow through the heart. Systemic blood returns to the heart by way of the superior (SVC) and inferior (IVC) venae cavae. This blood, which is low in oxygen and high in carbon dioxide, enters the right atrium (RA) and then the right ventricle (RV). The right ventricle pumps the blood through the pulmonary artery (PA) to the pulmonary circulation. It is within the lungs that gas exchange takes place. Next, this blood, which is high in oxygen and low in carbon dioxide, returns to the heart by way of the pulmonary veins (PV). It enters the left atrium (LA) and then the left ventricle (LV). The left ventricle pumps the blood through the aorta to the systemic circulation and the peripheral tissues.

Arterial Blood

Table 13.1 Route of Blood Flow through the Heart

Systemic Organs

Cavae

Aorta

Right ^k Right Atrium Ventricle

Pulmonary Artery

Pulmonary Veins

atmosphere. Blood then returns to the heart through the pulmonary veins. The blood first enters the left atrium and then the left ventricle. Contraction of the left ventricle propels the blood back to the peripheral tissues through systemic circulation by way of the aorta, the largest arterial vessel.

In summary, the heart is a single organ consisting of two pumps; the right heart delivers blood to the lungs and the left heart delivers blood to the rest of the body. Both pumps work simultaneously. The atria fill with blood and then contract at the same time and the ventricles fill with blood and then contract at the same time. Contraction of the atria occurs prior to contraction of the ventricles in order to ensure proper filling of the ventricles with blood.

Two sets of valves in the heart maintain the one-way flow of blood as it passes through the heart chambers:

• Atrioventricular (AV) valves

• Semilunar valves

Each of these valves consists of thin flaps of flexible but tough fibrous tissue whose movements are passive. The atrioventricular (AV) valves are found between the atria and the ventricles. The right AV valve is a tricuspid valve and has three cusps or leaflets. The left AV valve (also referred to as the mitral valve) is a bicuspid valve because it has two cusps. When the ventricles contract, the pressure within them increases substantially, creating a pressure gradient for blood flow from the ventricles back into the atria where the pressure is very low. Closure of the AV valves prevents this potential backward flow of blood. However, what prevents this increased ventricular pressure from causing eversion of the valves or opening of the valves in the opposite direction, which would also allow blood to flow backward into the atria? Strong fibrous ligaments, the chordae tendinae, are attached to the flaps of the valves. The chordae tendinae arise from cone-shaped papillary muscles that protrude into the ventricles. These muscles are continuous with the ventricular muscle so, when the ventricles are stimulated to contract, the papillary muscles also contract, pulling downward on the chordae tendinae. In this way, the flaps of the valves are not pushed open into the atria, but instead are held in place in the closed position. Blood is now forced to continue its forward progression and move from the ventricles into their respective arteries.

The semilunar valves separate the ventricles from their associated arteries. The pulmonary valve is found between the right ventricle and the pulmonary artery and the aortic valve is found between the left ventricle and the aorta. These valves prevent backward flow of blood from the pulmonary artery or the aorta into their preceding ventricles when the ventricles relax. The semilunar valves also have three cusps. There are no valves between the venae cavae or the pulmonary veins and the atria into which they deliver blood. The closure of the valves causes the "lub-dub" associated with the heart beat. The first heart sound, or the "lub," occurs when the ventricles contract and the AV valves close. The second heart sound, or the "dub," occurs when the ventricles relax and the semilunar valves close.

The wall of the heart has three layers:

• Epicardium

• Endocardium

• Myocardium

The outermost layer, the epicardium, is the thin membrane on the external surface of the heart. The innermost layer, the endocardium, consists of a thin delicate layer of cells lining the chambers of the heart and the valve leaflets. The endocardium is continuous with the endothelium, which lines the blood vessels.

The middle layer is the myocardium, which is the muscular layer of the heart. This is the thickest layer, although the thickness varies from one chamber to the next. Thickness of the myocardium is related to the amount of work that a given chamber must perform when pumping blood. The atria, which serve primarily as receiving chambers, perform little pumping action. Under normal resting conditions, most of the blood (75%) moves passively along a pressure gradient (higher pressure to lower pressure) from the veins, into the atria and ventricles where the pressure is close to zero. Therefore, it follows that the atria have relatively thin layers of myocardium because powerful contractions are not necessary. On the other hand, when the ventricles contract, they must develop enough pressure to force open the semilunar valves and propel the blood through the entire pulmonary or systemic circulations. Under normal resting conditions, between heart beats, the pressure in the pulmonary artery is approximately 8 mmHg and pressure in the aorta is approximately 80 mmHg. Therefore, in order to eject blood into the pulmonary artery, the right ventricle must generate a pressure greater than 8 mmHg and, in order to eject blood into the aorta, the left ventricle must generate a pressure greater than 80 mmHg. Because the left ventricle performs significantly more work, its wall is much thicker than that of the right ventricle.

Table 13.2 Distinguishing Features of Cardiac Muscle and Skeletal Muscle

Cardiac muscle

Organized into sarcomeres Sliding-filament mechanism of contraction Source of calcium:

Sarcoplasmic reticulum Tranverse tubules

Resting length of sarcomere less than optimal length Gap junctions provide electrical communication between cells, forming a functional syncytium Myogenic

Contraction modified by autonomic nervous system

Skeletal muscle

Organized into sarcomeres

Sliding-filament mechanism of contraction

Source of calcium:

Sarcoplasmic reticulum

Resting length of sarcomere equal to optimal length

No gap junctions

Neurogenic

Contraction elicited by somatic nervous system

Cardiac muscle has many structural and functional similarities with skeletal muscle (Chapter 11; also see Table 13.2). The contractile elements, composed of thin actin filaments and thick myosin filaments, are organized into sarcomeres. Therefore, as with skeletal muscle, tension development within the myocardium occurs by way of the sliding filament mechanism. As the action potential travels along the surface of the muscle cell membrane, the impulse also spreads into the interior of the cell along the transverse (T) tubules. This stimulates the release of calcium from the sarcoplasmic reticulum. Calcium promotes the interaction of actin and myosin resulting in cross-bridge cycling and muscle shortening. Unlike skeletal muscle whose only source of calcium is the sarcoplasmic reticulum, cardiac muscle also obtains calcium from the T tubules, which are filled with extracellular fluid. This added calcium results in a much stronger contraction.

The arrangement of the myofilaments into sarcomeres renders the cardiac muscle subject to the length-tension relationship. When the resting sar-comere length is altered, the amount of tension developed by the myocardium upon stimulation is altered as well. In the heart, the resting sarcomere length is determined by the volume of blood within the ventricle immediately prior to contraction. This length-tension relationship is described by the Frank-Starling mechanism and is discussed in more detail in the next chapter on cardiac output.

Skeletal and cardiac muscles also have important differences. Skeletal muscle cells are elongated and run the length of the entire muscle; furthermore, these cells have no electrical communication between them. Cardiac muscle cells, on the other hand, branch and interconnect with each other. Intercellular junctions found where adjoining cells meet end-to-end are referred to as intercalated discs. Two types of cell-to-cell junctions exist within these discs. Desmosomes hold the muscle cells together and provide the structural support needed when the heart beats and exerts a mechanical stress that would tend to pull the cells apart. Gap junctions are areas of very low electrical resistance (1/400 of the resistance of the outside membrane) that allow free diffusion of ions. It is through the gap junctions that the electrical impulse, or heart beat, spreads rapidly from one cell to another. As a result, the myocardium is a syncytium in which the initiation of a heart beat in one region of the heart results in stimulation and contraction of all cardiac muscle cells at essentially the same time. The heart is actually composed of two syncytiums: atrial and ventricular. In each case, but particularly in the ventricles, simultaneous stimulation of all the muscle cells results in a more powerful contraction, facilitating the pumping of blood.

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory; this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker" region is the sinoatrial node.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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