Cardiovascular System PDF

7645_Ch15_294-315 24/07/19 12:18 PM Page 294 Cardiovascular System CHAPTER OUTLINE Structures of the Heart Vessel Structure Arteries Veins Capillaries LEARNING OUTCOM...

7645_Ch15_294-315 24/07/19 12:18 PM Page 294 Cardiovascular System CHAPTER OUTLINE Structures of the Heart Vessel Structure Arteries Veins Capillaries LEARNING OUTCOMES 1. Describe the size and location of the heart. 2. Describe the layers of the pericardial sac surrounding the heart. 3. Identify the three layers of the heart wall and describe the characteristics of each. 4. Name the four chambers of the heart. 5. Describe the basic role and characteristics of each of the heart’s chambers. 6. Identify the name, location, and function of each of the heart’s four valves. 7. Identify the purpose of the skeleton of the heart. 8. Identify the two main coronary arteries and describe the parts of the myocardium nourished by each. 9. Describe the structure and function of cardiac muscle. 10. Discuss the heart’s conduction system. 11. Describe the structure of the walls of arteries and veins. 12. Describe the characteristics that make veins distinct from arteries. 13. Describe the structure of capillaries. 7645_Ch15_294-315 24/07/19 12:18 PM Page 295 Heart Heart composed of a type of muscle found nowhere else in the body, the heart works to pump blood throughout the body, delivering oxygen-rich blood to organs and tissues and returning oxygen-poor blood to the lungs. About the size of a fist, the heart lies in the thoracic cavity in the mediastinum, a space between the lungs and beneath the sternum. The heart tilts toward the left, so that two-thirds of it extends to the left of the body’s midline. The broadest part of the heart, called the base, is at the upper right, while the pointed end, called the apex, is at the lower left. Base: Where the great vessels enter and leave the heart Fifth intercostal space Apex: The point of maximum impulse, where the strongest beat can be felt or heard Right Midline Left midclavicular midclavicular line line 7645_Ch15_294-315 24/07/19 12:18 PM Page 296 Structures of the Heart Key structures of the heart include the pericardium, the heart wall, the chambers, and the valves. The Pericardium Surrounding the heart is a double-walled sac called the The fibrous pericardium—a loose- pericardium. Anchored by ligaments and tissue to fitting sac of strong connective surrounding structures, the pericardium has two layers: tissue—is the outermost layer. the fibrous pericardium and serous pericardium. The serous pericardium, which consists of two layers, covers the heart’s surface. At the heart’s base, the serous pericardium folds back on itself to form the: parietal layer, which lines the inside of the fibrous pericardium, and the visceral layer, which covers the heart’s surface. Between these two layers is the pericardial cavity. This cavity contains a small amount of serous fluid, which helps prevent Pericardium friction as the heart beats. The Heart Wall The heart wall consists of three layers: The endocardium lines the heart’s The myocardium, composed of cardiac The epicardium, which consists of a thin chambers, covers the valves, and muscle, forms the middle layer. It’s the layer of squamous epithelial cells, covers continues into the vessels. It consists of a thickest of the three layers and performs the heart’s surface. Also known as the thin layer of squamous epithelial cells. the work of the heart. visceral layer of the serous pericardium, the epicardium is closely integrated with the myocardium. 7645_Ch15_294-315 24/07/19 12:18 PM Page 297 The Heart Chambers and Great Vessels The heart contains four hollow chambers. The two upper chambers are called atria (singular: atrium); the two lower chambers are called ventricles. Attached to the heart are several large vessels that transport blood to and from the heart. Called great vessels, they include the superior and inferior vena, pulmonary artery (which branches into a right and left pulmonary artery), four pulmonary veins (two for each lung), and the aorta. Aorta Right pulmonary Left pulmonary artery artery (branches) Superior vena cava Pulmonary valve Left pulmonary veins Right pulmonary veins LEFT ATRIUM Interatrial septum Mitral valve RIGHT ATRIUM Aortic valve LEFT VENTRICLE Tricuspid valve Papillary muscle Chordae tendineae Interventricular septum RIGHT VENTRICLE Inferior vena cava Atria Ventricles The atria serve primarily as reservoirs, receiving blood from the The ventricles serve as pumps, receiving blood from the atria and body or lungs. The right and left atria are separated by a common then pumping it either to the lungs (right ventricle) or the body wall of myocardium called the interatrial septum. Because the (left ventricle). The right and left ventricles are separated by the atria move blood only a short distance—from the atria to the interventricular septum. Because the ventricles pump rather than ventricles—they don’t have to generate much force. receive blood, they must generate more force than the atria. Consequently, the walls of the atria are not very thick. Therefore, the walls of the ventricles are thicker than those of the atria. 7645_Ch15_294-315 24/07/19 12:18 PM Page 298 The Heart Valves To ensure that blood moves in a forward direction through the heart, the heart contains four valves: one between each atrium and its ventricle and another at the exit of each ventricle. Each valve is formed by two or three flaps of tissue called cusps or leaflets. The atrioventricular (AV) valves regulate flow The semilunar valves regulate flow between the between the atria and the ventricles. ventricles and the great arteries. There are two The right AV valve— also called the tricuspid semilunar valves: valve (because it has three leaflets)— prevents The pulmonary valve prevents backflow from backflow from the right ventricle to the right atrium. the pulmonary artery to the right ventricle. The left AV valve—a lso called the bicuspid valve The aortic valve prevents backflow from the (because it has two leaflets), or, more commonly, aorta to the left ventricle. the mitral valve— prevents backflow from the left ventricle to the left atrium. Pulmonary valve Aortic valve Skeleton of heart Tricuspid valve Mitral valve Ventricles relaxed Ventricles contracted The Heart Skeleton The Heart Skeleton skeleton of the heart encircles each valve. Besides A semi-rigid, fibrous, connective tissue called the skeleton of the heart encircles each valve. Besides insulating offering support for the heart, the skeleton keeps the valves from stretching; it also acts as an insulating barrier between the the ventricles atria and the ventricles, preventing electrical impulses from reaching the ventricles other than through a normal conduction pathway. Pulmonary valve Aortic valve Skeleton of heart, including fibrous rings around valves Tricuspid valve Mitral valve Posterior view 7645_Ch15_294-315 24/07/19 12:18 PM Page 302 Coronary Arteries Just like any other organ or tissue in the body, the heart muscle requires an abundant supply of oxygen and nutrients. Because of its high demands, the heart has its own vascular system, known as the coronary circulation, to keep it well supplied with oxygenated blood. Coronary arteries deliver oxygenated blood to the myocardium, while cardiac veins collect the deoxygenated blood. Two main coronary arteries— the right and the left— arise from the ascending aorta and serve as the principle routes for supplying blood to the myocardium. The right coronary artery supplies blood to the right atrium, The left coronary artery, which part of the left atrium, branches into the anterior most of the right ventricle, descending and circumflex and arteries, supplies blood to the inferior part of the left the left atrium, ventricle. most of the left ventricle, and most of the interventricular septum. Circumflex artery Left anterior descending artery Anterior 7645_Ch15_294-315 24/07/19 12:18 PM Page 303 Life lesson: Angina and myocardial infarction Coronary artery disease is the leading cause of death in America today, causing more than 600,000 deaths each year. The disease results when the coronary arteries become blocked or narrowed by a buildup of cholesterol and fatty deposits (atherosclerosis). Any interruption in blood supply to the myocardium deprives the heart tissue of oxygen (ischemia), causing pain. Within minutes, cell death (necrosis) occurs. Sometimes the interruption is temporary, such as in angina pectoris. What happens in this condition is that a partially blocked vessel spasms—or the heart demands more oxygen than the narrowed vessel can supply (such as during a period of exertion). When the demand for oxygen exceeds the supply, ischemia and chest pain result. With rest, the heart rate slows and adequate circulation resumes. Chest pain stops and permanent myocardial damage is avoided. A more serious condition is a myocardial infarction (MI). In this situation, blood flow is completely blocked by a blood clot or fatty deposit, resulting in the death of myocardial cells in the area fed by the artery. Once the cells die, they produce an area of necrosis. Symptoms of an MI, or “heart attack,” vary widely, particularly between women and men. Men commonly experience chest pain or pressure, discomfort in the upper body (including either arm, the back, neck, jaw, or stomach), shortness of breath, nausea, profuse sweating, or anxiety. Women are more likely to complain of sudden extreme fatigue (not explained by a lack of sleep), abdominal pain or “heartburn,” dizziness, or weakness. After flowing through the capillaries in the myocardium, the cardiac veins collect the now deoxygenated blood. Most cardiac veins empty into the coronary sinus, a large transverse vein on the heart’s posterior, which returns the blood to the right atrium. (The exception is the anterior cardiac veins, which empty directly into the right atrium.) Posterior 7645_Ch15_294-315 24/07/19 12:18 PM Page 304 Cardiac Muscle Cardiac muscle is striated and contracts Found only in the heart, cardiac cells are short, fat, and branched. Each cell usually contains only one nuclei. Because of their branched shape, each cell can connect with three or four other cardiac muscle cells, resulting in a giant network of cells. 7645_Ch15_294-315 24/07/19 12:18 PM Page 305 Cardiac Conduction Cardiac muscle is unique in that it doesn’t depend on stimulation by extrinsic nerves to contract. Rather, it contains specialized cells, called pacemaker cells, that generate action potentials to stimulate contraction—a trait called automaticity. Also, because the heart beats regularly, it is said to have rhythmicity. The electrical impulses generated by the heart follow a very specific route through the myocardium, as shown below. 1 Normal cardiac impulses arise in the sinoatrial (SA) node from its spot in the wall of the right atrium just below the 2 An interatrial bundle of conducting fibers rapidly conducts the impulses to the left atrium, and both atria begin to opening of the superior vena cava. contract. 3 The impulse travels along three internodal bundles to the atrioventricular (AV) node (located near the right AV valve at the lower end of the interatrial septum). There, the impulse slows considerably to allow the atria time to contract completely and the ventricles to fill with blood. The heart’s skeleton insulates the ventricles, ensuring that only impulses passing through the AV node can enter. 4 After passing through the AV node, the impulse picks up speed. It then travels down the bundle of His, also called the atrioventricular (AV) bundle. 5 The AV bundle soon branches into right and left bundle branches. 6 Purkinje fibers distribute the impulses to the muscle cells of both ventricles, causing them to contract AV node almost simultaneously. The Body AT WORK The SA node is the heart’s primary pacemaker. If the SA node fails to fire, pacemaker cells in the AV node or Purkinje fibers can initiate impulses, although at a slower rate. Pacemakers other than the SA node are called ectopic pacemakers. The heart’s pacemakers, and their firing rates when the heart is at rest, are as follows: SA node: Fires at 60 to 80 beats per minute Has a AV node: firing rate of 40 to 60 beats per minute Purkinje fibers: Have a firing rate of 20 to 40 beats per minute 7645_Ch16_316-339 25/07/19 4:29 PM Page 317 Vascular System Every organ, every tissue, and every cell in the body depends on a continual supply of blood to provide it with oxygen and nutrients and to remove waste products. The body has an elaborate system of vessels—the vascular system—to meet this need. The demands on this system are great: it must penetrate every square inch of the body, from the skin’s surface to the deepest recesses of the internal structures. It must adapt to changes in body position (from lying down to standing up…or even being upside down), changes in activity, and changes in fluid volume. It must also work with the heart to keep the blood constantly moving. The framework of this system consists of three types of blood vessels: Arteries carry blood away from the heart. 1 Artery Veins return blood to the heart. Vein 2 1 Left ventricle 2 Right ventricle Capillaries connect the smallest arteries 3 to the smallest veins. ! That Makes Sense To remember the difference between the various blood vessels, remember: Arteries Away: arteries carry blood away from the 3 heart. Capillaries Capillaries Connect: capillaries serve to connect arteries and veins. 7645_Ch16_316-339 25/07/19 4:29 PM Page 318 Vessel Structure The walls of both arteries and veins consist of three layers, called tunics. The basic layers include the tunica intima, tunica media, and tunica externa. Tunica intima, the innermost layer, is exposed to the blood. It consists of a simple squamous epithelium—called endothelium—that is continuous with the endothelium that lines the heart. Its smooth surface keeps blood flowing freely, without sticking to the vessel wall. This layer also produces chemicals that cause blood vessels to dilate or constrict. Tunica media, the middle layer, is the thickest layer. Composed of smooth muscle and elastic tissue, it allows the blood vessel to change diameter. The smooth muscle in this layer is innervated by the autonomic nervous system. Tunica externa, the outer layer, is made of strong, flexible, fibrous connective tissue. This layer supports and protects the blood vessel. In veins, this is the thickest of the three layers. In arteries, it’s usually a little thinner than the middle layer. 7645_Ch16_316-339 25/07/19 4:29 PM Page 319 Arteries Arteries carry blood away from the heart. Every time the heart contracts, it forcefully ejects blood into the arteries. Therefore, arteries must be strong as well as resilient to withstand these high pressures. As they travel farther away from the heart, the arteries branch and divide,becoming ever smaller. Finally, they become arterioles, which are the smallest arteries. Arteries can be divided into conducting arteries, distributing arteries, and arterioles. Tunica Tunica Tunica intima: externa media Basement membrane Endothelium Conducting Arteries The body’s largest arteries, these arteries expand as blood surges into them and recoil when the ventricles relax. Because of the large number of elastic fibers embedded in the tunica media, they are also called elastic arteries. Examples: Aorta, common carotid artery, subclavian artery Tunica Internal Tunica Tunica intima: externa elastic media Internal elastic lamina lamina Basement membrane Endothelium Distributing Arteries These arteries carry blood farther away from the heart to specific organs and areas of the body. Also called muscular arteries, these arteries are smaller in diameter than elastic arteries. Examples: Brachial, femoral, and renal arteries Tunica Tunica Tunica intima: externa media Basement membrane Endothelium Arterioles These are the smallest arteries. They’re also called resistance vessels because, through the contraction of smooth muscle in their walls, they can resist the flow of blood, thus helping regulate blood pressure as well as control how much blood enters an organ. They are too numerous to be named. Arterioles are connected to capillaries by short connecting vessels called metarterioles. 7645_Ch16_316-339 25/07/19 4:29 PM Page 320 Veins Veins, which carry blood to the heart, converge on their path to the heart to form progressively larger and fewer vessels,with the vessels closest to the heart being the largest. Veins are distinct from arteries in other ways: Because they aren’t subjected to the same high pressures as arteries, the walls of veins are thinner. Veins have a great ability to stretch, which allows them to carry varying amounts of blood with almost no change in pressure. Because of this, they’re sometimes called capacitance vessels. Veins can constrict extensively. Veins lie closer to the body’s surface. Tunica Tunica Tunica intima: externa media Basement membrane Endothelium Lumen Large Veins Formed as medium-sized veins converge, these veins have a thick tunica externa. Examples: Vena cavae, pulmonary veins, internal jugular veins Tunica Tunica Tunica intima: externa media Basement membrane Endothelium Valve Medium-Sized Veins Formed by the convergence of venules on their route toward the heart, medium- sized veins have thicker, more elastic walls. These veins contain one-way valves. Formed from the thin endothelium lining, valves keep blood moving toward the heart and prevent backflow. Veins in the legs, which must fight the forces of gravity as they transport blood to the heart, contain the most valves. Examples: Radial and ulnar veins of the forearm, saphenous veins in the legs Tunica Tunica Tunica intima: externa media Basement membrane Endothelium Venules These are the smallest veins and collect blood from capillaries. The endothelium consists of squamous epithelial cells and acts as a membrane; the tunica media is poorly developed, giving venules thinner walls. They are porous and can exchange fluid with surrounding tissues. 7645_Ch16_316-339 25/07/19 4:29 PM Page 321 Capillaries Capillaries are microscopic vessels that link arterioles to venules. More importantly, it’s within capillaries that nutrients, wastes, and hormones are transferred between blood and tissues. These are the exchange vessels of the circulatory system. Fibrous connective tissues, such as tendons, have lower metabolic rates and contain fewer capillaries. Still other tissues—such as the epidermis, cartilage,and the lens and cornea of the eye—don’t have any capillaries. Composed of only an endothelium and basement membrane, capillaries have extremely thin walls through which substances can filter. Capillaries have very small diameters, barely wide enough for blood cells to pass. Capillary Organization Capillaries are organized into networks called capillary beds. Connecting arterioles to venules, capillaries form what is called the microcirculation. Arteriole Precapillary sphincters (open) Arterial capillaries Precapillary sphincters (closed) Venous capillaries Venule At the beginning of each capillary bed is a precapillary sphincter that regulates the flow of blood into the network. During exercise, when skeletal muscles require more oxygen, the precapillary sphincters open, blood fills the capillary network, and the During a time of rest, the precapillary sphincters close. Blood exchange of oxygen, nutrients, and wastes occurs with the tissue bypasses the capillary bed and flows directly into a venule to begin fluid. its journey back to the heart and lungs. 7645_Ch16_316-339 25/07/19 4:29 PM Page 322 Sinusoid Some organs—such as the liver, bone marrow, and spleen—contain a unique capillary called a sinusoid. These irregular, blood-filled spaces are more permeable, allowing for the passage of large substances such as proteins and blood cells. This is how blood cells formed in bone marrow as well as clotting factors and other proteins synthesized in the liver enter the bloodstream. 7645_Ch16_316-339 25/07/19 4:29 PM Page 326 Principal Arteries As previously mentioned, all of the systemic arteries arise, either directly or indirectly, from the aorta. The thoracic aorta and its branches Branching off the aortic arch is the: supply the chest wall and the organs Subclavian artery, within the thoracic cavity. which supplies blood to the arm The abdominal aorta gives rise to the: Axillary artery, which Celiac trunk, which divides into the is the continuation of gastric artery (which supplies the the subclavian artery in stomach), the splenic artery (which the axillary region supplies the spleen), and the hepatic artery (which supplies the liver) Brachial artery, which is the continuation of the Renal arteries, which supply the axillary artery and the kidneys artery most often used Superior mesenteric artery, which for routine blood supplies most of the small intestine pressure measurement and part of the large intestine Inferior mesenteric artery, which supplies the other part of the large Radial artery, which intestine is often palpated to measure a pulse The distal end of the abdominal aorta splits into the right and left common iliac arteries, which supply the pelvic organs, thigh, and lower extremities. Major arteries branching off the iliac arteries include the: Internal iliac artery External iliac artery Femoral artery Popliteal artery Anterior tibial artery Posterior tibial artery Dorsalis pedis artery 7645_Ch16_316-339 25/07/19 4:29 PM Page 327 Arteries of the Head and Neck The brain requires a constant supply of blood. An interruption of blood flow for just a few seconds causes loss of consciousness. If the brain is deprived of oxygen for 4 or 5 minutes, irreversible brain damage occurs. Because of this critical need for oxygen, two arteries supply blood to the brain. Remember: Arterial blood flows from the heart. So, to trace the path of blood to the brain, begin at the bottom of the illustration and work upward. At about the level of the Adam’s apple, each common carotid branches into the external carotid artery (which supplies most of the external head structures) and the internal carotid artery (which enters the cranial cavity and supplies the orbits and 80% of the cerebrum). Internal carotid artery The right common carotid artery arises from the External carotid artery brachiocephalic artery. The left common carotid arises from the aortic arch. The vertebral arteries arise from the right and left subclavian arteries. Each extends up the neck, through the cervical vertebrae, and enters the cranium. Anterior into brain A single anterior communicating artery Anterior communicating Two anterior cerebral arteries Anterior cerebral Two posterior into brain into brain communicating arteries Posterior communicating Two posterior cerebral Posterior cerebral into ain bra Basilar artery arteries br in o Right internal carotid t in Left internal carotid Right common carotid Left common carotid Left vertebral Posterior Right vertebral Right subclavian Left subclavian Brachiocephalic Aortic arch 7645_Ch16_316-339 25/07/19 4:29 PM Page 328 Principal Veins Veins drain blood from the organs and other parts of the body and carry it to the vena cava, which,in turn, delivers it to the heart’s right atrium. The vena cava is the body’s main vein. It’s divided into two branches: Superior vena cava (SVC), which receives blood from the head, shoulders, and arms Inferior vena cava (IVC), which receives blood from the lower part of the body The internal jugular vein drains most of the blood from the brain. In right-sided heart failure, blood backs up from the heart and causes jugular Brachiocephalic vein vein distension. Subclavian vein External jugular vein Superior vena cava The cephalic vein, at its distal end, is a common site for the Axillary vein administration of intravenous fluids. Inferior vena cava Basilic vein The hepatic veins drain the liver. Because of its proximity to the heart, The median cubital right-sided heart failure can cause vein is the most congestion in the liver. common site for drawing blood. Common iliac vein Radial vein Internal iliac vein External iliac vein Femoral vein The great saphenous vein is the longest vein in the body; it’s often harvested for use as grafts in coronary artery bypass surgery. The popliteal vein runs behind the knee. Fibular (peroneal) vein Anterior tibial vein Posterior tibial vein 7645_Ch16_316-339 25/07/19 4:29 PM Page 329 Veins of the Head and Neck Most of the blood of the head and neck is drained by the internal jugular, external jugular, and vertebral veins. The internal jugular vein receives most of the blood from the brain as well as from the face. The internal jugular vein merges into the subclavian vein, which, in turn, becomes the brachiocephalic vein. The brachiocephalic vein drains into the superior vena cava. The external jugular vein—the more superficial of the jugular veins— drains blood from the scalp, facial muscles, and other superficial structures. It, too, drains into the subclavian vein. The vertebral vein drains the cervical vertebrae, spinal cord, and some of the muscles of the neck. Right subclavian vein Right brachiocephalic vein