bioch22 c

To maintain homeostasis, blood must circulate continuously throughout the body. The nonstop pumping action of the heart is essential for maintaining blood circulation. If the heart fails to pump adequate volumes of blood, cells are deprived of needed oxygen and nutrients, waste products accumulate, and cell death may occur. In a healthy, 80-kilogram resting adult, the heart beats about 75 times per minute (about 4500 times per hour or 108,000 times per day). The amount of blood pumped from one ventricle per minute (about 5.25 liters [L] at rest) is called the cardiac output. When the body is more active, and the cells need oxygen and nutrients delivered at a faster pace, the heart can increase its output up to five- or six-fold.

As the center of the cardiovascular system, the heart connects to blood vessels that transport blood between the heart and all body tissues. The two basic types of blood vessels are arteries (ar′ter-ē), which transport blood away from the heart, and veins (vān), which transport blood back to the heart. The differences between these types of vessels are discussed in section 23.1. Most arteries carry blood high in oxygen (except for the pulmonary arteries, as explained in section 22.1a), whereas most veins carry blood low in oxygen (except for the pulmonary veins). The arteries and veins entering and leaving the heart are called the great vessels because of their relatively large diameter. The heart exhibits several related characteristics and functions: The heart's anatomy ensures the unidirectional flow of blood through it. Backflow of blood is prevented by valves within the heart. The heart acts like two side-by-side pumps that work at the same rate and pump the same volume of blood; one directs blood to the lungs for respiratory gas exchange, whereas the other directs blood to body tissues for nutrient and respiratory gas delivery. The heart develops blood pressure through alternate cycles of heart wall contraction and relaxation. Blood pressure is the force of the blood pushing against the inside walls of the vessels. A minimum blood pressure is essential for pushing blood through the blood vessels.

The right ventricle receives deoxygenated venous blood from the right atrium. An interventricular septum forms a thick wall between the right and left ventricles. The internal wall surface of each ventricle displays characteristic large, smooth, irregular muscular ridges, called the trabeculae carneae (tră-bek′yū-lē; trabs = beam; kar′nē-ē; carne = flesh) (see figure 22.6). The right ventricle typically has three cone-shaped, muscular projections called papillary (pap′i-lăr′ē; papilla = nipple) muscles, which anchor numerous thin strands of collagen fibers called chordae tendineae (kōr′dē ten′di-nē-ē). The chordae tendineae attach to the lower surface of cusps of the right AV valve and prevent the valve from everting and flipping into the atrium when the right ventricle is contracting. A muscle bundle called the septomarginal trabecula (see figure 22.6), or moderator band, connects the base of the anterior papillary muscle to the interventricular septum. At its superior end, the right ventricle narrows into a smooth-walled, conical region called the conus arteriosus. Beyond the conus arteriosus is the pulmonary semilunar valve, which separates the right ventricle and the pulmonary trunk. The pulmonary trunk divides shortly into right and left pulmonary arteries, which carry deoxygenated blood to the lungs. Semilunar valves are located within the walls of both ventricles immediately before the connections of the right and left ventricles to the pulmonary trunk and aorta, respectively (see figure 22.6). Each valve is composed of three thin, half-moon-shaped, pocketlike semilunar cusps. As blood is pumped into the arterial trunks, it pushes against the cusps, forcing the valves open. When ventricular contraction ceases, blood is prevented from flowing back into the ventricles from the arterial trunk by first entering the pockets of the semilunar valves between the cusps and the chamber wall. This causes the cusps to "fill and expand" and meet at the artery center, effectively blocking blood backflow.

Left Atrium Once gas exchange occurs in the lungs, the oxygenated blood travels through the pulmonary veins to the left atrium (see figure 22.6). The smooth posterior wall of the left atrium contains openings for typically four pulmonary veins. Sometimes two of these vessels fuse prior to reaching the left atrium, thus decreasing the number of openings through the atrial wall. Like the right atrium, the left atrium also has pectinate muscles along its anterior wall as well as an auricle. Separating the left atrium from the left ventricle is the left atrioventricular opening, covered by the left atrioventricular valve (also called the bicuspid valve, because it has two triangular cusps). This valve is also sometimes called the mitral (mī′trăl) valve, because the two triangular cusps resemble a miter (the headpiece worn by a bishop). Oxygenated blood flows from the left atrium, through the left atrioventricular opening when the valve is open, into the left ventricle. The left AV valve is forced closed when the left ventricle contracts, preventing blood backflow into the left atrium.

Left Ventricle The left ventricular wall is typically three times thicker than the right ventricular wall (figure 22.8). The left ventricle requires thicker walls to generate enough pressure to force the oxygenated blood that has returned to the heart from the lungs into the aorta and then through the entire systemic circulation. (The right ventricle, in contrast, merely has to pump blood to the nearby lungs.) The trabeculae carneae in the left ventricle are Page 660more prominent than in the right ventricle. Typically, two large papillary muscles project from the ventricle's inner wall and anchor the chordae tendineae that attach to the cusps of the left AV valve. At the superior end of the ventricular cavity, the aortic semilunar valve marks the end of the left ventricle and the entrance into the aorta

Left and right coronary arteries travel within the coronary sulcus of the heart to supply the heart wall (figure 22.9a). These arteries are the only branches of the ascending aorta. The openings for these arteries are located in the wall of the ascending aorta immediately superior to the aortic semilunar valve. The right coronary artery typically branches into the right marginal artery, which supplies the right border of the heart, and the posterior interventricular artery, which supplies the posterior surface of both the left and right ventricles. The left coronary artery typically branches into the anterior interventricular artery (also called the left anterior descending artery), which supplies the anterior surface of both ventricles and most of the interventricular septum, and the circumflex (ser′kŭm-fleks; around) artery, which supplies the left atrium and ventricle. This arterial pattern can vary greatly among individuals. For example, some people may have a posterior interventricular artery that is a branch of the left coronary artery. Knowledge of this variation is essential when treating individuals for coronary artery disease.

The pericardium is composed of two parts. The outer portion is a tough, dense connective tissue layer called the fibrous pericardium. This layer is attached inferiorly to the diaphragm and superiorly to the base of the great vessels. The inner portion is a thin, double-layered serous membrane called the serous pericardium (see figure 1.10b). The serous pericardium may be subdivided into (1) a parietal layer of serous pericardium that lines the inner surface of the fibrous pericardium, and (2) a visceral layer of serous pericardium (also called the epicardium) that covers the outside of the heart. The parietal and visceral layers reflect (fold back) along the great vessels, where these layers become continuous with one another. The pericardial cavity is a thin space between the parietal and visceral layers of the serous pericardium. Serous fluid is secreted into this space to lubricate the serous membranes and decrease friction when the heart beats. The pericardial cavity is a potential space with just a thin lining of serous fluid. However, it may become a real space

Pericarditis (per′i-kar-dī′tis; itis = inflammation) is an inflammation of the pericardium, typically caused by viruses, bacteria, or fungi. The inflammation is associated with an increase in permeability of the capillaries, which become more "leaky," resulting in fluid accumulation in the pericardial cavity. At this point, the potential space of the pericardial cavity becomes a real space as it fills with fluid. In severe cases, the excess fluid accumulation limits the heart's movement and keeps the heart chambers from filling with an adequate amount of blood. The heart is unable to pump blood, leading to a medical emergency called cardiac tamponade (tam′pŏ-nād′) and possibly resulting in heart failure and death. A helpful physical finding in diagnosing pericarditis is friction rub, a crackling or scraping sound heard with a stethoscope that is caused by the movement of the inflamed pericardial layers against each other.

depicts the internal anatomy and structural organization of the four heart chambers: the right atrium, right ventricle, left atrium, and left ventricle. Each of these chambers plays a role in the continuous process of blood circulation. Important to their function are valves, endothelium-lined dense connective tissue cusps that permit the passage of blood in one direction and prevent its backflow (table 22.1). Valve cusps are the tapering projection of a cardiac valve, also called flaps or leaflets. When the flaps of the valves are forced closed during the cardiac cycle, they produce the "lubb-dupp" heart sounds. The first sound heard with a stethoscope is the result of the atrioventricular (AV) valves closing; producing a "lubb" sound. The second sound is produced when the semilunar valves close; producing a "dupp" sound

Right atrioventricular valve Between right atrium and right ventricle Three triangular-shaped cusps of dense connective tissue covered by endothelium; chordae tendineae attached to free edges Prevents backflow of blood into right atrium when ventricles contract Pulmonary semilunar valve Between right ventricle and pulmonary trunk Three semilunar cusps of dense connective tissue covered by endothelium; no chordae tendineae Prevents backflow of blood into right ventricle when ventricles relax Left atrioventricular valve Between left atrium and left ventricle Two triangular-shaped cusps of dense connective tissue covered by endothelium; chordae tendineae attached to free edges Prevents backflow of blood into left atrium when ventricles contract Aortic semilunar valve Between left ventricle and ascending aorta Three semilunar cusps of dense connective tissue covered by endothelium; no chordae tendineae Prevents backflow of blood into left ventricle when ventricles relax

The heart is a relatively small, conical organ about the size of a person's clenched fist. In the average normal adult, it weighs about 250 to 350 grams, but certain diseases may cause heart size to increase dramatically. The heart wall consists of three distinctive layers: an external epicardium, a middle myocardium, and an internal endocardium Organization of the Heart Wall. The heart wall is composed of an outer epicardium (visceral layer of the serous pericardium), a middle myocardium (cardiac muscle), and an inner endocardium (composed of areolar connective tissue and an endothelium). The heart is composed of four hollow chambers: two smaller atria and two larger ventricles (figure 22.5). The left and right atria (ā′trē-ă; sing., atrium; entrance hall) are thin-walled chambers located superiorly. The anterior part of each atrium is a wrinkled, flaplike extension, called an auricle (aw′ri-kl; auris = ear) because it resembles an ear. The atria receive blood returning to the heart through both circulations: The right atrium receives blood from the systemic circulation, and the left atrium receives blood from the pulmonary circulation. Blood that enters an atrium is passed to the ventricle on the same side of the heart. The left and right ventricles (ven′tri-kĕl; venter = belly) are the inferior chambers. Two large arteries, the pulmonary trunk and the aorta (ā-ōr′tă), exit the heart at its superior border. The pulmonary trunk transports blood from the right ventricle into the pulmonary circulation, whereas the aorta conducts blood from the left ventricle into the systemic circulation. Both ventricles pump the same volume of blood per minute.

The epicardium (ep′i-kar′dē-ŭm; epi = upon, kardia = heart) is the outermost heart layer and is also known as the visceral layer of the serous pericardium. The epicardium is composed of a serous membrane and areolar connective tissue (see section 4.3). As we age, more fat is deposited in the epicardium, and so this layer becomes thicker and more fatty. The myocardium (mī′ō-kar′dē-ŭm; mys = muscle) is the middle layer of the heart wall and is composed of cardiac muscle tissue (see sections 4.4a and 10.9a). The myocardium is the thickest of the three heart wall layers. It lies deep to the epicardium and superficial to the endocardium. Contraction of cardiac muscle composing the myocardium generates the force necessary to pump blood. The ventricular myocardium may change in thickness as we age. For example, it hypertrophies in response to narrowing of systemic arteries because the heart must work harder to pump the blood. We consider the microscopic anatomy of cardiac muscle in section 22.4a. The internal surface of the heart chambers and the external surfaces of the heart valves are covered by endocardium (en′dō-kar′dē-ŭm; endon = within). The endocardium, like the epicardium, is composed of a simple squamous epithelium and an underlying layer of areolar connective tissue. The epithelial layer of the endocardium is continuous with the epithelial layer called the endothelium, which lines the blood vessels

There are two normal sounds associated with each heartbeat that collectively form the lubb-dupp sound. The "lubb" sound is also known as the S1 sound and represents the closing of the atrioventricular valves. The "dupp" sound is also known as the S2 sound and is the closing of the semilunar valves. These heart sounds provide clinically important information about heart activity and the action of heart valves. The place where sounds from each AV valve and each semilunar valve may best be heard does not correspond with the location of the valve, because some overlap of valve sounds occurs near their anatomic locations. The aortic semilunar valve is best heard in the second intercostal space to the right of the sternum. The pulmonary semilunar valve is best heard in the second intercostal space to the left of the sternum. The right AV valve is best heard at the inferior left sternal border in the fifth intercostal space. The left AV valve is best heard near the apex of the heart (at the level of the left fifth intercostal space, about 9 centimeters from the midline of the sternum). An abnormal heart sound, generally called a heart murmur, is the first indication of heart valve problems. A heart murmur is usually the result of turbulence of the blood as it passes through the heart, and may be caused by valvular leakage, decreased valve flexibility, or a misshapen valve. Sometimes heart murmurs are of little consequence, but all of them need to be evaluated to rule out a more serious heart problem. Two types of heart murmurs are valvular insufficiency and valvular stenosis. Valvular insufficiency, also termed valvular incompetence, occurs when one or more of the cardiac valves leaks because the valve cusps do not close tightly enough. Inflammation or disease may cause the free edges of the valve cusps to become scarred and constricted, allowing blood to regurgitate back through the valve and may cause heart enlargement. Valvular stenosis (ste-nō′sis; narrowing) is scarring of the valve cusps so that they become rigid or partially fused and cannot completely open. A stenotic valve is narrowed and presents resistance to the flow of blood, decreasing chamber output. Often the affected chamber undergoes hypertrophy and dilates—both conditions that may have dangerous consequences. A primary cause of valvular stenosis is rheumatic (rū-mat′ik) heart disease, which may follow a streptococcal infection of the throat.

The fibrous skeleton of the heart is formed from dense regular connective tissue and is located between the atria and the ventricles (figure 22.7). The fibrous skeleton performs the following functions: Provides structural support at the boundary between the atria and the ventricles Forms supportive fibrous rings to anchor the heart valves Provides a rigid framework for the attachment of cardiac muscle tissue Acts as an electrical insulator because it does not conduct action potentials and thus prevents the ventricles from contracting at the same time as the atria The right atrium receives venous blood from the systemic circulation and the heart muscle itself. Three major vessels empty into the right atrium: (1) The superior vena cava (vē′nă kā′vă; pl: vē′nē ca′vē) drains blood from the head, neck, upper limbs, and superior regions of the trunk; (2) the inferior vena cava drains blood from the lower limbs and trunk; and (3) the coronary sinus drains blood from the heart wall. The interatrial (in′ter-ā′trē-ăl) septum forms a thin wall between the right and left atria. The posterior atrial wall is smooth, but the auricle and anterior wall exhibit obvious muscular ridges, called pectinate (pek′ti-nāt; teeth of a comb) muscles. The structural differences in the anterior and posterior walls occur because the two walls formed from separate structures during embryonic development. Inspection of the interatrial septum reveals an oval depression called the fossa (fos′ă; trench) ovalis, also called the oval fossa. It occupies the former location of the fetal foramen ovale, which shunted blood from the right atrium to the left atrium during fetal life, as described later in section 22.8. Separating the right atrium from the right ventricle is the right atrioventricular opening. This opening is covered by a right atrioventricular valve (also called the tricuspid valve, because it has three triangular cusps). Deoxygenated venous blood flows from the right atrium, through the right atrioventricular opening when the valve is open, into the right ventricle. The right AV valve is forced closed when the right ventricle begins to contract, preventing blood from flowing back into the right atrium.

The heart is located left of the body midline posterior to the sternum in the mediastinum (figure 22.2). The heart is slightly rotated such that its right side or right border (primarily formed by the right atrium and ventricle) is located more anteriorly, whereas its left side or left border (primarily formed by the left atrium and ventricle) is located more posteriorly. The posterosuperior surface of the heart, formed primarily by the left atrium, is called the base (not shown in figure 22.2). The pulmonary veins that enter the left atrium border this base. The superior border is formed by the great arterial trunks (ascending aorta and pulmonary trunk) and the superior vena cava. The inferior, conical end is called the apex (ā′peks; tip). It projects slightly anteroinferiorly toward the left side of the body. The inferior border is formed by the right ventricle.

The heart is contained within the pericardium (per′i-kar′dē-ūm), a fibrous sac and serous lining (figure 22.3; see also figure 1.10). The pericardium restricts the heart's movements so that it doesn't bounce and move about in the thoracic cavity, and prevents the heart from overfilling with blood.

The cardiovascular system consists of two circulations: the pulmonary circulation and the systemic circulation (figure 22.1). The pulmonary (pul′mō-nār′ē; pulmo = lung) circulation conveys deoxygenated blood from the right side of the heart through blood vessels to the lungs for the pickup of oxygen and the release of carbon dioxide, and then back through blood vessels to the left side of the heart. The cardiovascular system is composed of the pulmonary circulation and the systemic circulation. The pulmonary circulation pumps blood from the right side of the heart through pulmonary vessels, to the lungs, and back to the left side of the heart. The systemic circulation pumps blood from the left side of the heart, through systemic vessels in peripheral tissues, and back to the right side of the heart.

The systemic (sis-tem′ik) circulation moves oxygenated blood from the left side of the heart through blood vessels to the systemic cells Page 652such as those of the liver, skin, muscle, and brain. Nutrients, respiratory gases, and wastes are exchanged with these systemic cells before blood is returned to the right side of the heart. Thus, the basic pattern of blood flow is the right side of the heart → lungs → the left side of the heart → systemic tissues of the body → back to the right side of the heart.