Chapter 7: CIRCULATION—THE HEART AND BLOOD VESSELS

7 circulation—the heart and blood vessels Chapter Outline the Cardiovascular SYSTEM—Moving Blood Through the Body The heart and blood vessels make up the cardiovascular system The cardiovascular system is linked to the lymphatic system Blood circulation is essential for maintaining homeostasis the Heart: A muscular DOUBLE PUMP The heart has two halves and four chambers In a “heartbeat,” the heart’s chambers contract, then relax THE TWO CIRCUITS OF Blood FLOW In the pulmonary circuit, blood picks up oxygen in the lungs In the systemic circuit, blood travels to and from tissues Blood from the digestive tract is shunted through the liver for processing how CARDIAC muscle contractS Electrical signals from “pacemaker” cells drive the heart’s contractions The nervous system adjusts heart activity blood pressure Blood exerts pressure against the walls of blood vessels structure AND FUNCTIONS OF BLOOD VESSELS Arteries are large, strong blood pipelines Arterioles are control points for blood flow Capillaries are specialized for diffusion Venules and veins return blood to the heart Many vessels have roles in homeostatic mechanisms that help control blood. capillaries: WHERE substances move between blood and TISSUES A vast network of capillaries brings blood close to nearly all body cells Some substances pass through “pores” in capillary walls Blood in capillaries flows onward to venules Cardiovascular Diseases and disorders Arteries can clog or weaken Heart damage can lead to heart attack and heart failure Arrhythmias are abnormal heart rhythms A heart-healthy lifestyle may help prevent cardiovascular disease infections, cancer, and Heart defects Infections may seriously damage the heart Is there such a thing as heart cancer? Inborn heart defects are fairly common Connections: The cardiovascular system and blood in homeostasis SUMMARY Review questions self-quiz critical thinking explore on your own your future Objectives List the basic components of the human circulatory system. Correlate the structures of the human heart with their function in the cardiac cycle. Compare and contrast the structure and functions of arteries, arterioles, capillaries, venules, and veins. Trace the routes of blood flow in the human cardiovascular system. Explain the factors that cause blood to exist under different pressures. Describe the major cardiovascular disorders and their causes. Key Terms cardiovascular system heart arteries arterioles capillaries venules veins myocardium atrium ventricle atrioventricular valve (AV) pulmonary valve aortic valve coronary circulation aorta cardiac cycle systole diastole pulmonary circuit systemic circuit hepatic portal system cardiac conduction system sinoatrial (SA) node atrioventricular (AV) node cardiac pacemaker blood pressure hypertension pulse capillary beds vasodilation vasoconstriction carotid arteries baroreceptor reflex precapillary sphincter atherosclerosis plaque aneurysm heart attack heart failure (HF) arrhythmias ventricular fibrillation rheumatic fever endocarditis Lyme disease myocarditis Lecture Outline During sudden cardiac arrest the heart stops its regular beating. CPR (cardiopulmonary resuscitation) is an immediate life-saving technique. As soon as possible a defibrillator should be used to shock the heart back to its usual rhythm; AEDs (automated external defibrillators) are now available in many public places. The heart, and its associated blood vessels, is a complicated life-sustaining system. 7.1 The Cardiovascular System—Moving Blood Through the Body The heart and blood vessels make up the cardiovascular system. The cardiovascular system has two major elements: The heart is the muscular pump that generates the pressure required to move the blood through the body. Blood vessels are the distribution tubes of varying diameters. The route of circulation: heart >>> arteries >>> arterioles >>> capillaries >>> venules >>> veins, and finally back to the heart. The cardiovascular system is linked to the lymphatic system. Because of the pressure in the cardiovascular system, water and proteins leak out to become part of the interstitial fluid. The lymphatic system vessels pick up the fluid and return it to the general circulation. Blood circulation is essential to maintain homeostasis. The cardiovascular system is the body’s internal rapid-transport system for oxygen, nutrients, secretions, and wastes via the blood. Homeostasis depends on the reliable supply of blood to all of the body. The Heart: A Double Pump The heart is a durable pump made mostly of cardiac muscle (myocardium). The heart is surrounded by a tough, fibrous sac (pericardium). The inner lining of the heart is the endocardium; it is composed of connective tissue and epithelial cells (endothelium). The heart has two halves and four chambers. The septum divides the heart into two halves, right and left. Each half of the heart consists of an atrium (receiving chamber) and a ventricle (pumping chamber) separated by an atrioventricular valve (AV valve). The AV valve on the right is a tricuspid valve; the one on the left is the bicuspid, or mitral valve. Chordae tendineae (“heartstrings”) connect the AV valve flaps to the ventricle wall to prevent eversion. Blood exits each ventricle through a semilunar valve. Heart muscle cells are serviced by the coronary circulation; coronary arteries branch off the aorta, forming a capillary bed around the heart. In a “heartbeat,” the heart’s chambers contract, and then relax. The cardiac cycle is a sequence of contraction (systole) and relaxation (diastole). As the atria fill, the ventricles are relaxed and begin filling passively. Pressure of the blood in the atria forces the AV valves open; the ventricles fill completely as the atria contract. When the ventricles contract, the AV valves close, and blood flows out through the semilunar valves. The cardiac output is the amount of blood each ventricle can pump in a minute; on average the output from each ventricle is about 5 liters. The heart sound “lub” is made by the closing of the AV valves; the “dup” sound is the closure of the aortic and pulmonary valves. The Two Circuits of Blood Flow In the pulmonary circuit: blood picks up oxygen in the lungs. The pulmonary circuit receives blood from the tissues, taking it through the lungs for gas exchange. The path of blood flow: blood from tissues enters the right atrium >>> tricuspid valve >>> right ventricle >>>right semilunar valve >>> pulmonary arteries >>> lungs >>> pulmonary veins >>> left atrium. Blood returning from the body tissues is high in carbon dioxide and low in oxygen; these concentrations are reversed after passage through the lung capillaries. In the systemic circuit, blood travels to and from tissues. In the systemic circuit, oxygenated blood is pumped through the body. Blood moves from the left atrium >>> bicuspid valve >>> left ventricle >>>left semilunar valve >>> aorta >>> body tissues. Blood from the upper body returns to the heart through the superior vena cava; blood from the lower body returns to the heart through the inferior vena cava. Blood from the digestive tract is shunted through the liver for processing. After a meal, blood laden with nutrients is carried from the digestive tract in the hepatic portal vein to the liver capillaries. There it passes through the liver capillary beds before leaving via the hepatic vein to return to the general circulation; oxygenated blood reaches the liver through the hepatic artery. How Cardiac Muscle Contracts Electrical signals from “pacemaker” cells drive the heart’s contractions. Cardiac muscle cells are linked by intercalated discs, which rapidly pass signals to contract throughout the heart. The cardiac conduction system consists of noncontractile cells that are self-excitatory (pacemaker cells). Excitation for a heartbeat is initiated in the sinoatrial (SA) node; it then passes to the atrioventricular (AV) node and on to the Purkinje fibers, which make contact with the muscle cells that result in ventricular contraction. It is the action of the cardiac pacemaker (SA node) that produces our normal heartbeat. The nervous system adjusts heart activity. The nervous system can adjust the rate and strength of cardiac muscle contraction; stimulation by one set of nerves increases the rate and strength while stimulation by other nerves decreases heart rate. Centers for nervous control of the heart lie in the spinal cord and the brain. Blood Pressure Blood exerts pressure against the walls of blood vessels. The force of blood against the vessel walls can be measured as blood pressure. Normal systolic pressure (peak pressure in the aorta) is 120 mm of Hg; normal diastolic pressure (lowest pressure in the aorta) is 80 mm. Blood pressure values give important clues as to the condition of the vessels and the flow of blood through them. In hypertension, the blood pressure is chronically high, which can lead to stroke or heart attack. In hypotension, the blood pressure is too low; loss of water or blood volume can lead to circulatory shock. Structure and Functions of Blood Vessels Arteries are large, strong blood pipelines. Because of their elastic walls, arteries tend to “bulge” in response to the pressure changes associated with the discontinuous pumping cycle of the heart (felt as a pulse). Because of their large diameters, arteries present little resistance to flow; blood pressure does not decrease very much in them. Arterioles are control points for blood flow. Arteries branch into smaller arterioles, where the greatest pressure drop occurs. The wall of an arteriole has rings of smooth muscle over a single layer of elastic fibers. The smooth muscle can dilate or constrict the blood vessel. Arterioles serve as control points where adjustments can be made in blood volume distribution. Capillaries are specialized for diffusion. A capillary is the smallest and thinnest tube in the path of circulation and is specialized for exchange of substances with interstitial fluid. Venules and veins return blood to the heart. Capillaries merge into venules; venules merge into veins. Veins are blood volume reservoirs (50–60% of blood volume) because their walls can distend or contract. Skeletal muscles adjacent to veins squeeze the walls to move the blood along on its way back to the heart; valves prevent backflow. Varicose veins can form when the veins have become overstretched, and the valves weakened. Vessels help control blood pressure. The brain monitors signals from various arteries to determine the rate of heartbeat and any changes needed in vessel diameters. If the blood pressure increases, the arterioles are instructed to relax (vasodilation). If the pressure decreases, the diameter of the arterioles decreases (vasoconstriction). In the baroreceptor reflex, special receptors in the carotid arteries monitor changes in blood pressure and send the information to the brain for action. Capillaries: Where Substances Move Between Blood and Tissues A vast network of capillaries brings blood close to nearly all body cells. Capillaries comprise most of the length of the cardiovascular system. The velocity of blood flow slows as the diameter of the vessels decreases. It is slowest in the capillaries to provide for maximum exchange. Diffusion is a slow process and is not efficient over long distances. Billions of capillaries ensure that all cells are near enough to a capillary to receive nutrients and give up wastes; blood flow is slow enough here to allow diffusion. Some substances pass through “pores” in capillary walls. Water-filled, slit-like areas between the cells of capillary walls allow water-soluble substances to exit the blood due to pressure (bulk flow). This movement of fluids and solutes is important to homeostasis and maintaining blood pressure. Blood in capillary beds flows onward to venules. Capillaries are the “turnaround points” for the cardiovascular system. Precapillary sphincters regulate the flow of blood into capillary beds. Cardiovascular Disease Many factors may influence your chance of developing a cardiovascular disorder. Some risk factors include: family history, hypertension, obesity, smoking, lack of exercise, or simply age. Inflammation, which leads to the production of C-reactive protein by the liver, may also play a role in cardiovascular disease. Arteries can clog or weaken. Arteriosclerosis is a hardening of the arteries. When cholesterol and other lipids build up in these hardened arteries, atherosclerosis occurs. Atherosclerotic plaques can impede blood flow. Coronary arteries are narrow and vulnerable to clogging with these plaques; chest pain (angina pectoris) or heart attack may occur. High blood levels of cholesterol can lead to atherosclerosis. Low-density lipoproteins (LDL or “bad” cholesterol) carry cholesterol into the arterial walls; high-density lipoproteins (HDL or “good” cholesterol) remove it. A total of 200 mg of cholesterol per milliliter of blood or less is considered acceptable for most people. Surgery may be needed to clear blocked arteries. Coronary bypass involves using a large vessel from elsewhere in the body to bypass a completely blocked artery in the heart. Laser angioplasty uses a laser to vaporize plaques while balloon angioplasty uses small balloons to flatten the plaques to open room in the artery; a wire “stent” may be inserted to keep the ballooned area open. Statins are drugs designed to reduce the amount of cholesterol in the blood. Disease, injury, or defects can weaken artery walls so they bulge outward due to blood pressure, forming an aneurysm; aneurysms can be fatal if the artery wall bursts. Heart damage can lead to heart attack and heart failure. A heart attack is damage to or death of heart muscle. In heart failure (HF), the heart is weak and does not pump blood as efficiently. Arrhythmias are abnormal heart rhythms. Electrocardiograms (ECGs) are recordings of the electrical activity of the cardiac cycle and can be used to reveal irregular heart rhythms. Arrhythmias are irregular heart rhythms; bradycardia is a below-normal rhythm, while tachycardia is an above-normal rhythm. Ventricular fibrillation occurs when the ventricles contract haphazardly so that blood is not pumped correctly; this can lead to cardiac arrest. A heart-healthy lifestyle may help prevent cardiovascular disease. Lifestyle changes can greatly reduce the risk of cardiovascular disease. Diets low in fat and cholesterol, regular exercise, and not smoking are three key strategies. Infections, Cancer, and Heart Defects Infections may seriously damage the heart. Bacteria can directly attack the heart, as in endocarditis or myocarditis. The immune system may attack the heart following certain infections due to the creation of autoresponsive antibodies. Alcohol and drug use can cause inflammation of heart tissue. Is there such a thing as heart cancer? Cancer rarely starts in the heart, but may spread there from other tissues. Inborn heart defects are fairly common. These typically require surgery to restore sufficient functioning. Connections: The Cardiovascular System and Blood in Homeostasis The cardiovascular system brings blood to most major organ systems, and allow for nutrient delivery and waste removal. Suggestions for Presenting the Material Because of the complexity of the entire cardiovascular system, an introduction based on Figure 7.1 should be used to survey the pathways throughout the body. The concept of oxygen-rich and oxygen-poor blood can be emphasized here. This can be followed by the heart pathways and contraction sequence. Make sure to note to students that the colors of blood vessels represented in diagrams and animations have nothing to do with the color of the blood in those vessels. Stress that vessels may contain deoxygenated (designated by blue) OR oxygenated blood (designated by red) and that they should know the direction of blood flow through the major circuits. The heartbeat, pulse, and blood pressure are measurable quantities of interest to students; therefore, these deserve special, and practical, emphasis. Capillary exchange is a topic that allows you to review previous lectures on diffusion and active transport. Classroom and Laboratory Enrichment Ask a physician to demonstrate an artificial pacemaker; include comments on the limitations and usefulness of such an artificial device versus the natural SA node. Borrow a sphygmomanometer and ask a person skilled in its use to explain how blood pressure is determined, using members of the class as volunteers. Obtain a recording of heart sounds as they would be heard through a stethoscope. Relate sounds to events of the normal and abnormal cardiac cycle. It is also possible to find videos of various EKGs of heart rhythm to show students visually. Use models of the heart or preserved beef hearts to show the actual interior of the heart, demonstrating the flow of blood and the arrangement of the chambers and valves. Classroom Discussion Ideas Heart disease in some families has been shown to be traceable to genetics. Should more people be tested for possible “heart attack genes”? Should those who test positive for the bad gene then be barred from eating high-fat foods? The essay at the beginning of the chapter mentions “trained bystanders” when referring to the use of the automated external defibrillators in public places. How many of these persons do you think there are? Have you ever met one? Will such a person be available if you have a heart attack? Could an amateur use the device or would that be dangerous? The heart is really a “double pump.” It is also, of course, divided into four chambers. Does this mean that one pump consists of atria, the other of ventricles, or does it mean the left and right sides are pumping to separate circulations? Explain your reasoning. Statin drugs are becoming increasingly popular for the treatment of high cholesterol. Like other drugs, however, they are not risk free. Before you are prescribed a statin, do you think you should have to try changing your habits to reduce the cholesterol on your own? Heart attacks often occur without warning and can be fatal. In the absence of advance clues, how then do we make sure we don’t have one? Term Paper Topics, Library Activities, and Special Projects Evaluate the reported links between lipids and cardiovascular diseases. Survey the various corrective surgical procedures that are routinely performed on the heart and its vessels. Select one or two for an in-depth report. It is often said that “the heart never rests.” True, it does beat continuously from before birth until death, usually at old age. However, it does rest for about 0.3 seconds after each beat. Assuming a steady pulse of 70 beats per minute, calculate the total amount of time the heart has “rested” in a person 75 years old. Videos, Animations, and Websites VIDEOS Films for the Humanities and Sciences Reducing the risks of cardiovascular disease. http://ffh.films.com/id/1202/Reducing_the_Risks_of_Cardiovascular_Disease.htm EMedTV - Types of Blood Vessels An overview of the major types of blood vessels. http://www.youtube.com/watch?v=CjNKbL_-cwA Structure of Blood Vessels Overview of the structure of blood vessels. http://www.youtube.com/watch?v=3ffSxq9iyB8&feature=related The Children’s Hospital of Philidelphia How a normal heart pumps blood. http://www.youtube.com/watch?v=JA0Wb3gc4mE&feature=related Electrocardiogram What is an ECG? http://www.youtube.com/watch?v=SNrTbeL2h84 ANIMATIONS MedTropolis – The Virtual Body Interactive animations of the human heart. http://medtropolis.com/virtual-body/ PBS NOVA – The Human Body Interactive animation of the human heart. http://www.pbs.org/wgbh/nova/body/map-human-heart.html Texas Heart Institute Interactive animations of the heart http://www.texasheartinstitute.org/HIC/Anatomy/ WEBSITES The National Heart, Lung, and Blood Institute Provides research, training, and education program to promote the prevention and treatment of heart, lung, and blood diseases http://www.nhlbi.nih.gov/index.htm Possible Responses to Review Questions The cardiovascular system exists to pump blood throughout the body; this is necessary to ensure nutrient and waste exchange between cells of the body, regulate other aspects of homeostasis, ensure immune protection, and in general simply to ensure proper functioning of the body as a unit. A “heartbeat” is the contraction (systole) and relaxation (diastole) of the heart’s chambers during the cardiac cycle. The atria fill when the ventricles are relaxed. Pressure of the blood in the atria forces the AV valves open; the ventricles fill as the atria contract. When the ventricles contract, the AV valves close (the “lub” of the beat), and blood flows out through the semilunar valves. The semilunar valves close to make the “dub” of the beat. The systemic circuit takes oxygenated blood away from the heart to the rest of the body. The pulmonary circuit picks up the un-oxygenated blood from the rest of the body and passes it through the lungs to become re-oxygenated. The SA (sinoatrial) node generates waves of excitation to the atria to cause them to contract. The AV (atrioventricular) node receives the signals from the SA node, passes them through the Purkinje fibers, and ultimately causes contraction of the ventricles. The cardiac pacemaker, otherwise known as the SA node, determines the rate of the heartbeat. Blood capillaries function in transport of nutrients from blood to cells and tissues, and pick up wastes. The exchange is mediated by diffusion through the membranes of cells and through bulk flow of fluids out via splits between cells lining the capillaries. The main function of venules and veins is to return blood back to the heart. Valves and back pressure from the heart beat help to push blood towards the heart. Reference figure 7.4. Possible Responses to Critical Thinking Questions A person with hypertension would receive drugs to decrease the heart’s output in order to put less strain on vessels that are already fragile. Other drugs would dilate the arterioles to allow more blood to flow in a less constricted lumen. Increased urine production would have the overall effect of reducing (within specified limits) the amount of fluid in the body’s systems, thereby reducing pressure in the blood vessels. Nicotine acts on the smooth muscles in the walls of arterioles to constrict the diameter of the vessel and thus slow the blood flow and increase the pressure on the walls of the vessel. Rheumatic heart damage is the result of an autoimmune response to streptococcal infections. The most serious effects are to the heart valves, which may be thickened and scarred, leading to narrowing and/or leaking. Because these effects are permanent and progressive, heart valve surgery may be needed. Blood pressure changes with the diameter of the artery, arteriole, venule, or vein the blood is traveling in, with the lowest pressure being in the capillary beds that link the artery system to the venous system. Sitting physically still in cramped airline seats can result in physical compression of blood vessels, narrowing the space so that blood pools and cells within the blood clump under pressure at the compression point. If you sit too long, the body starts trying to repair the damage, which could lead to increased clotting and more problems. By getting up and moving around, you don’t allow the conditions in the compression zones to get to the point that an actual clot develops; getting up allows the diameter of the vessels to return to normal, allowing blood flow to return to normal. In addition, getting up and moving around allows the muscular pumps to help move the blood through the veins. Possible Responses to Explore on Your Own Questions With each change in activity (sitting after lying down, running after sitting) the pulse rate should increase from the baseline of lying down. Changes in body posture (sitting after lying) change the effect gravity has on the flow of blood in the body, and the short “burst” of activity places energy demands on the cells. The heart has to pump harder to move blood vertically and ensure blood supply to the body. Running places a higher oxygen demand on cells, requiring increased blood flow to sustain the activity. In both types of activity, sitting and running, the brain senses the changes in posture and demand, and responds accordingly. When you stop running, the stimulus goes away, and the heart will eventually return to its normal rate. 86 Chapter Seven Circulation—The Heart and Blood Vessels 85 86 Chapter Seven Circulation—The Heart and Blood Vessels 75 88 Chapter Seven Circulation—The Heart and Blood Vessels 75 88 Chapter Seven Circulation—The Heart and Blood Vessels 75 88 Chapter Seven Circulation—The Heart and Blood Vessels 87