Chapter 12

major arteries

After the aorta leaves the heart, it divides into the ascending aorta, the aortic arch, and the descending aorta (Fig. 12.16). The left and right coronary arteries, which supply blood to the heart, branch off the ascending aorta (Table. 12.1). Three major arteries branch off the aortic arch: the brachiocephalic artery (also called the brachiocephalic trunk), the left common carotid artery, and the left subclavian artery. The brachiocephalic artery is also correctly referred to as the brachiocephalic trunk because it divides into two branches: the right common carotid and the right subclavian arteries. These blood vessels serve the head (right and left common carotids) and the shoulders and upper limbs (right and left subclavians). The descending aorta is divided into the thoracic aorta, which branches off to the organs within the thoracic cavity, and the abdominal aorta, which branches off to the organs in the abdominal cavity. The descending aorta ends when it divides into the common iliac arteries that branch into the internal iliac arteries and the external iliac arteries. Each internal iliac artery serves the pelvic organs, and the external iliac artery serves the lower limbs. These and other arteries are shown in Figure 12.16.

art...

Arteries (Fig. 12.8) transport blood away from the heart. They have thick, strong walls composed of three layers: (1) The tunica intima (sometimes also referred to as tunica interna) is an endothelium layer with a basement membrane. (2) The tunica media is a thick middle layer of smooth muscle and elastic fibers. (3) The tunica externa is an outer connective tissue layer composed principally of elastic and collagen fibers. Arterial walls are sometimes so thick that they are supplied with their own blood vessels. The radius of an artery allows the blood to flow rapidly, and the elasticity of an artery allows it to expand when the heart contracts and recoil when the heart rests. This means that blood continues to flow in an artery even when the heart is in diastole. Arterioles are small arteries just visible to the naked eye. The middle layer of these vessels has some elastic tissue but is composed mostly of smooth muscle whose fibers encircle the arteriole. If the muscle fibers contract, the lumen (cavity) of the arteriole decreases; if the fibers relax, the lumen of the arteriole enlarges. Whether arterioles are constricted or dilated affects the distribution of blood flow and blood pressure throughout the body. When a muscle is actively contracting, for example, the arterioles in the vicinity dilate to allow additional blood flow to the muscle. In this way, the muscle's need for oxygen and glucose is met. Constriction and dilation of arterioles throughout the body is controlled by the autonomic nervous system and helps to determine blood pressure. The greater the number of arterioles that are constricted, the higher the resistance to blood flow. Higher resistance causes higher blood pressure. By contrast, the greater the number of dilated arterioles, the lower the resistance to blood flow. Blood pressure falls as a result.

capillaries

Arterioles branch into capillaries (Fig. 12.8), which are extremely narrow, microscopic blood vessels. Their walls are composed of only one layer of endothelial cells connected by tight junctions. These thin walls easily allow gases, nutrients, and wastes to diffuse between the capillary and the surrounding cells. Capillaries are extremely numerous: The body probably contains a billion, and their combined surface area is estimated at 6,300 square meters. Capillary beds (networks of many capillaries) are present in all regions of the body, and each supplies the needs of neighboring cells. Therefore, most of the body's cells are near a capillary, and the heart and other vessels of the cardiovascular system can be considered a means for conducting blood to and from the capillaries. Not all capillary beds are open or in use at the same time. For instance, after a meal, the capillary beds of the digestive tract are usually open, while during muscular exercise, the capillary beds of the skeletal muscles are open. Most capillary beds have a shunt that allows blood to move directly from an arteriole to a venule (a small vessel leading to a vein) when the capillary bed is closed. Sphincter muscles, called precapillary sphincters, encircle the entrance to each capillary (Fig. 12.8). When the precapillary sphincters are constricted, the capillary bed is closed, preventing blood from entering the capillaries. Conversely, when the precapillary sphincters are relaxed, the capillary bed is open. As would be expected, the larger the number of open capillary beds, the lower the blood pressure in the body.

diseases

Arteriosclerosis, Atherosclerosis, and Coronary Artery Disease The leading cause of heart attack and stroke, and the number-one killer in North America and Western Europe, is arteriosclerosis. Arteriosclerosis is the older, more generalized term for abnormal thickening and hardening of the arterial wall over time. (However, this term is now seldom used in clinical literature.) The most common form of arteriosclerosis is atherosclerosis. Scientists agree that atherosclerosis begins with injury to the arterial wall. Research has suggested several possible causes for injury: smoking, high blood pressure (called hypertension), low levels of HDL cholesterol (high-density lipoprotein, often referred to as "good cholesterol") and elevated levels of blood lipids, LDL cholesterol (low-density lipoprotein, or "bad cholesterol"), and homocysteine (a by-product of protein metabolism). Diabetics (especially those with type II or non-insulin-dependent diabetes mellitus) are at increased risk for atherosclerosis, probably because their disease causes high levels of blood lipids and LDL cholesterol. Research also indicates that low-level bacterial or viral infection that spreads to the blood may injure arterial walls and start the atherosclerosis process. This infection may originate with gum disease, or it can be caused by a bacterium called Helicobacter pylori (the microbe that also causes ulcers in the stomach). Antibodies specific to these microbes are found in people with atherosclerosis. In addition, a blood protein called C-reactive protein, or CRP, is an important piece of evidence suggesting an infection. For example, blood CRP levels rise if you suffer from a cold or are recovering from a wound. High blood levels of CRP in an otherwise healthy person imply that the arteries are damaged. Indeed, recent studies show that people with the highest blood levels of CRP have double the risk of heart attack. Further, new research has shown that excesses of additional blood components may also be linked to an increased risk of atherosclerosis. These components include fibrinogen (the inactive clotting protein you learned about in Chapter 11) and a specific lipid called lipoprotein (a). Once the arterial wall is injured, the body's defense mechanisms respond. White blood cells called macrophages invade the injured area and stick to the arterial wall. These macrophages ingest LDL and are then called foam cells (because mixing fat with the cell's watery cytoplasm creates a foamy appearance). A collection of foam cells creates a fatty streak. Sadly, post-mortem studies on the arteries of young people have shown that these fatty streaks begin to develop during the early teenage years. Over time, the artery's smooth muscle cells migrate to cover the fatty streak. Finally, fibroblasts and scar tissue will cover the smooth muscle cells. Calcium ions will invade the tissue, causing it to harden into an atherosclerotic plaque. Atherosclerotic plaques can grow so large that they completely block arterial blood flow, causing the tissue supplied by the artery to die. Because the plaque's surface is very rough compared to the smooth endothelium, the plaque may also trigger the clotting mechanism and cause a stationary blood clot, or thrombus, to form inside a blood vessel. Thrombi may also form if the surface of the plaque ulcerates (cracks open and bleeds). As mentioned in Chapter 11, thromboembolism occurs when a blood clot breaks away from its place of origin and is carried to a new location. Further, the plaque can prevent the arterial wall cells from receiving oxygen and nutrients. The cells die, causing the wall itself to weaken, which might result in an aneurysm. Aneurysms are weakened areas in the arterial wall, which balloon outward and may even rupture. Page 268Coronary artery disease is the term for atherosclerosis of the coronary arteries (Fig. 12A). If the coronary artery is partially occluded (blocked) by atherosclerosis, the individual may suffer from ischemic heart disease. Although enough oxygen normally reaches the resting heart, the person's heart is oxygen-deprived during periods of exercise or stress. Some patients experience silent ischemia during the earliest stage of the disease; that is, they don't detect any symptoms of a problem. However, most patients suffer from angina pectoris, chest pain that is often accompanied by a radiating pain in the left arm. Angina pain is a warning sign of reduced blood flow to the heart and must not be ignored. Should the coronary artery become completely blocked by atherosclerotic plaque or thrombus, a portion of the heart will die from lack of oxygen. Dead tissue is called an infarct and, therefore, a heart muscle attack is termed a myocardial infarction. Two surgical procedures can reopen occluded coronary arteries. In balloon angioplasty, a plastic tube is threaded into an artery of an arm or thigh, then guided through a major blood vessel toward the heart. Once the tube reaches a blockage, a balloon attached to the end of the tube can be inflated to break up a blood clot or open a vessel clogged with plaque (Fig. 12B). Often, a small metal-mesh cylinder called a vascular stent is inserted into a blood vessel during balloon angioplasty. The stent holds the vessels open and decreases the risk of future occlusion. Stent devices currently in use have built-in medication, which is slowly released in the artery. These medications prevent blood clots and additional scar tissue from forming, thus helping to keep the stent open and blood flowing. In a similar fashion, a stent can be placed into an artery weakened by an aneurysm (Fig. 12C). This type of stent supports the arterial wall and can be monitored externally, allowing a physician to ensure that it is working properly. Figure 12B Balloon angioplasty. A balloon inserted in an artery can be inflated to open up a clogged coronary blood vessel. Courtesy of nasaimages.org/NASA Figure 12C Abdominal aortic aneurysm. (a) Before and (b) after stent placement. In a coronary bypass operation, a portion of a blood vessel from another body part (usually one of the mammary arteries from the chest) is sutured from the aorta to the coronary artery, past the obstruction point. Blood can then flow normally again from the aorta to the heart muscle. Figure 12D shows a triple bypass in which three blood vessels connect the aorta to the coronary artery, restoring blood flow to the myocardium. Figure 12D Coronary bypass surgery. During this operation, the surgeon grafts segments of another vessel between the aorta and the coronary vessels, bypassing areas of blockage. Page 269

Blood pressures

Blood pressure is the force of blood against the walls of blood vessels. You would expect arterial blood pressure to be highest in the aorta. Why? Because the pumping action of the powerful, thick-muscled left ventricle forces blood into the aorta. Further, Figure 12.11 shows that systemic blood pressure decreases progressively with distance from the left ventricle. Blood pressure is lowest in the venae cavae because they are farthest from the left ventricle. Note also in Figure 12.11 that blood pressure fluctuates in the arterial system between systolic blood pressure and diastolic blood pressure. Certainly, we can correlate this with the action of the heart. During systole, the left ventricle is pumping blood out of the heart, and during diastole the left ventricle is resting. More important than the systolic and diastolic pressure is the mean arterial blood pressure (MABP), the pressure in the arterial system averaged over time. It is important to note that MABP is not determined by taking the average of systolic and diastolic pressures. Rather, MABP is the product of cardiac output (CO) times peripheral resistance. (Recall that cardiac output equals heart rate Page 275times stroke volume; see page 266). To put it as a simple math equation, MABP = CO × PR. According to this equation, increasing CO will also increase MABP. In other words, the greater the amount of blood leaving the left ventricle, the greater the pressure of blood against the wall of an artery. Another factor that determines blood pressure is peripheral resistance, which is the resistance to flow between blood and the walls of a blood vessel. All things being equal, the smaller the blood vessel diameter, the greater the resistance and the higher the blood pressure. As an analogy, imagine a skinny, 1-inch-diameter garden hose (high resistance) compared to a firefighter's 12-inch canvas hose (low resistance). Similarly, total blood vessel length increases blood pressure because a longer vessel offers greater resistance. For this reason, an obese person is apt to have high blood pressure because about 200 miles of additional blood vessels develop for each extra pound of adipose (fat) tissue. Blood Pressure and Cardiac Output Our previous discussion on pages 266 and 269 emphasized that the heart rate and the stroke volume determine cardiac output. We learned that the heart rate is intrinsic but is under extrinsic (nervous and endocrine) control. Therefore, heart rate can speed up. The faster the heart rate, the greater the cardiac output. As cardiac output increases, blood pressure increases as well (assuming constant peripheral resistance). Similarly, the larger the stroke volume, the greater the blood pressure. However, stroke volume and heart rate increase blood pressure only if the venous return is adequate. Venous Return Venous return depends on three factors: a blood pressure difference—blood pressure is higher in systemic veins than in the right atrium, which enables venous blood to empty into the heart the skeletal muscle pump and the respiratory pump, both of which are effective because of the presence of valves in veins; total blood volume in the cardiovascular system. Here's how the skeletal muscle pump works: When skeletal muscles contract, they compress the weak walls of the veins. This causes the blood to move past a valve (Fig. 12.12). Once past the valve, backward pressure of blood closes the valve and prevents its return. Blood in veins will always return to the heart. As you might suspect, gravity can assist the return of venous blood from the head to the heart but not the return of blood from the extremities and trunk to the heart. The importance of the skeletal muscle pump in maintaining CO and blood pressure can be demonstrated by forcing a person to stand rigidly still for a number of hours. Frequently, the person faints because blood collects in the limbs, limiting venous return to the heart. Cardiac output decreases and hypotension (low blood pressure) develops, robbing the brain of oxygen. In this case, fainting helps: the horizontal body position caused by the faint aids in getting blood to the brain. The respiratory pump works like this: When inhalation occurs, thoracic pressure falls and abdominal pressure rises as the chest expands. This aids in the flow of venous blood back to the heart because blood flows from areas of higher pressure (in the abdominal cavity) to areas of lower pressure (in the thoracic cavity). Page 276During exhalation, the pressure reverses, but the valves in the veins prevent backward flow. As stated, the amount of venous return also depends on the total blood volume in the cardiovascular system. As you know, this volume in the pulmonary circuit and the systemic circuit is 5 L. If this amount of blood decreases (for example, due to a hemorrhage), blood pressure falls. On the other hand, if blood volume increases (due to water retention, for example), blood pressure rises. Blood Pressure and Peripheral Resistance The nervous system and the endocrine system both affect peripheral resistance. Neural Regulation of Peripheral Resistance A vasomotor center in the medulla oblongata controls vasoconstriction. This center is under the control of the cardioregulatory center. As mentioned on pages 269-270, if blood pressure falls, baroreceptors in the blood vessels signal the cardioregulatory center. The cardioregulatory center will activate its vasomotor center. The vasomotor center then stimulates sympathetic nerve fibers, which cause the heart rate to increase and the arterioles to constrict. Increasing heart rate increases cardiac output; constricting the arterioles increases peripheral resistance. The result is a rise in blood pressure. What factors lead to a reduction in blood pressure? If blood pressure rises above normal, the baroreceptors signal the cardioregulatory center in the medulla oblongata. Subsequently, the heart rate decreases and the arterioles dilate. Nervous control of blood vessels also causes blood to be shunted from one area of the body to another. During exercise, arteries in the viscera and skin are more constricted than those in the exercising muscles. Therefore, blood flow to the muscles increases. Also, dilation of the precapillary sphincters in muscles means that blood will flow to the muscles and not to the viscera. Hormonal Regulation of Peripheral Resistance Certain hormones cause blood pressure to rise. Epinephrine and norepinephrine, the hormones from the adrenal medulla, increase the heart rate. As heart rate increases, so too does cardiac output and blood pressure. Epinephrine and norepinephrine also constrict arterioles in the capillary beds supplying the skin, abdominal viscera, and kidneys. Arteriolar vasoconstriction increases blood pressure by increasing peripheral resistance in these large vascular beds. When the blood volume and blood sodium level are low, the kidneys secrete the enzyme renin. Renin converts the plasma protein angiotensinogen to angiotensin I, which is changed to angiotensin II by a converting enzyme found in the lungs. Angiotensin II stimulates the adrenal cortex to release aldosterone. The effect of this system, called the renin-angiotensin-aldosterone system, is to raise the blood volume and pressure in two ways. First, angiotensin II constricts the arterioles directly. Second, aldosterone causes the kidneys to reabsorb sodium. When the blood sodium level rises, water is reabsorbed, and blood volume and pressure are maintained. Two other hormones play a role in the homeostatic maintenance of blood volume and blood pressure. As discussed in Chapter 10, antidiuretic hormone (ADH) helps increase blood volume by causing the kidneys to reabsorb water. ADH also causes vasoconstriction of smooth muscle in arteries and veins throughout the body. Thus, ADH boosts blood pressure by simultaneously increasing blood volume (which increases cardiac output) and by causing vasoconstriction (which increases peripheral resistance). The hormonal mechanism for decreasing blood pressure involves an endocrine hormone secreted by the heart. You'll remember (from Chapter 10) that when the atria of the heart are stretched due to increased blood volume, cardiac cells release a hormone called atrial natriuretic hormone (ANH, or atriopeptide), which inhibits renin secretion by the kidneys and aldosterone secretion by the adrenal cortex. The effect of ANH, therefore, is to cause sodium excretion—that is, natriuresis (a term that means "excretion of sodium in the urine"). When sodium is excreted, so is water, and therefore blood volume and blood pressure decrease (Fig. 12.13).

circ. routes

Blood vessels belong to either the pulmonary circuit or the systemic circuit. The path of blood through the pulmonary circuit Page 279can be traced as follows: Blood from all regions of the body first collects in the right atrium and then passes into the right ventricle, which pumps it into the pulmonary trunk. The pulmonary trunk divides into the pulmonary arteries, which in turn divide into the many arterioles of the lungs. These pulmonary arterioles then take blood to the pulmonary capillaries where carbon dioxide and oxygen are exchanged. The blood then enters the pulmonary venules and flows through the pulmonary veins back to the left atrium. Because the blood in the pulmonary arteries is O2-poor but the blood in the pulmonary veins is O2-rich, it is not correct to say that all arteries carry blood that is high in oxygen and that all veins carry blood that is low in oxygen. In fact, just the opposite is true in the pulmonary circuit. The systemic circuit includes all of the other arteries and veins of the body. The largest artery in the systemic circuit is the aorta, and the largest veins are the superior vena cava and inferior vena cava. The superior vena cava collects blood from the head, chest, shoulders, and upper limbs, and the inferior vena cava collects blood from the lower body regions. Both venae cavae enter the right atrium. The aorta and venae cavae are the major pathways for blood in the systemic system. The path of systemic blood to any organ in the body begins in the left atrium, which pumps blood into the left ventricle. In turn, the left ventricle pumps blood into the aorta. Branches from the aorta go to the major body regions and organs. Tracing the path of blood to any organ in the body requires mentioning only the aorta, the proper branch of the aorta, the organ, and the returning vein to the vena cava. In many instances, the artery and vein that serve the same organ have the same name. For example, the path of blood to and from the kidneys is: left ventricle; aorta; renal artery; arterioles, capillaries, venules; renal vein; inferior vena cava; right atrium. In the systemic circuit, unlike the pulmonary circuit, arteries contain O2-rich blood and appear bright red, while veins contain O2-poor blood and appear dark maroon.

Coronary Circulation

Cardiac muscle fibers and the other types of cells in the wall of the heart are not nourished by the blood in the chambers; diffusion of oxygen and nutrients from this blood to all the cells that make up the heart would be too slow. Instead, these cells receive nutrients and rid themselves of wastes at capillaries embedded in the heart wall. As mentioned previously, the right and left coronary arteries branch from the aorta just superior to the aortic semilunar valve (Fig. 12.4). Each of these arteries branches and then rebranches, until the heart is encircled by small arterial blood vessels. Some of these join so that there are several routes to reach any particular capillary bed in the heart. Alternate routes are helpful if an obstruction should occur along the path of blood reaching cardiac muscle cells. The Medical Focus on pp. 267-268 describes the cause, and some treatment options, for obstruction of the coronary arteries. After blood has passed through cardiac capillaries, it is taken up by vessels that join to form veins. The coronary veins are specifically called cardiac veins. The cardiac veins enter a coronary sinus, which is essentially a thin-walled vein. The coronary sinus enters the right atrium, where deoxygenated blood mixes with the blood from the superior vena cava and inferior vena cava. This blood will then be sent to the lungs for oxygenation.

cardiac output

Cardiac output (CO) is the volume of blood pumped out of a ventricle in one minute. (The same amount of blood is pumped out of each ventricle in one minute.) Cardiac output is dependent on two factors: heart rate (HR) = beats per minute stroke volume (SV) = amount of blood pumped by a ventricle each time it contracts Thus, cardiac output = HR × SV.

conduction system of heart

Conduction System of the Heart The conduction system of the heart is a route of specialized cardiac muscle fibers that initiate and stimulate contraction of the atria and ventricles. The conduction system is said to be intrinsic, meaning that the heart beats automatically without the need for external nervous stimulation. The conduction system coordinates the atria and ventricles so they work as a "team," that is, the atria contract simultaneously, and the ventricles then contract simultaneously. Thus, the heart is an effective pump. Without this conduction system, the atria and ventricles would contract at different rates. Nodal Tissue The heartbeat is controlled by nodal tissue, which has both muscular and nervous characteristics. This unique type of cardiac muscle is located in two regions of the heart: The SA (sinoatrial) node is located in the upper posterior wall of the right atrium. The AV (atrioventricular) node is located in the base of the right atrium very near the interatrial septum (Fig. 12.5). he SA node initiates the heartbeat and automatically sends out an excitation signal every 0.85 second. The SA node normally functions as the pacemaker for the entire heart because its intrinsic rate is the fastest in the system. From the SA node, signals spread out over the atria, causing them to contract. When the signals reach the AV node, there is a slight delay that allows the atria to finish their contraction before the ventricles begin their contraction. The signal for the ventricles to contract travels from the AV node through the two branches of the atrioventricular bundle (AV bundle) before reaching the numerous and smaller Purkinje fibers. The AV bundle, its branches, and the Purkinje fibers consist of specialized cardiac muscle fibers that Page 265efficiently spread an electrical signal throughout the ventricles. Recall from Chapter 4 (p. 73) that cardiac muscle cells are bound end-to-end at intercalated disks, and within the disks are adhesion junctions and gap junctions. Gap junctions are specialized intercellular connections that allow electrical current to flow directly from cell to cell. Once stimulated electrically, the ventricular muscle contracts purposefully to pump blood. The SA node pacemaker usually keeps the heartbeat regular. If the SA node fails to work properly, the ventricles still beat due to impulses generated by the AV node. But the beat is slower (40 to 60 beats per minute). This condition is referred to as a heart block. An area other than the SA node can become the pacemaker when it develops a rate of contraction that is faster than the SA node. This site, called an ectopic pacemaker, may cause an extra beat, if it operates only occasionally, or it can even pace the heart for a while. Caffeine and nicotine are two substances that can stimulate an ectopic pacemaker. Occasionally, the cardiac conduction system fails to operate properly. As a result, the heartbeat will be abnormal: failing to beat at regular intervals, or perhaps beating too slowly or too quickly. To correct this, an artificial electronic pacemaker can be surgically implanted to restore a normal heart rate.

circulation/pressure

Evaluating Circulation Taking a patient's pulse and arterial blood pressure are two ways to quickly acquire important information needed to assess the status of the cardiovascular system. Pulse The surge of blood entering the arteries causes their elastic walls to stretch, but then they almost immediately recoil. This alternating expansion and recoil of an arterial wall can be felt as a pulse in any artery that runs close to the body's surface. These Page 277superficial arteries are called pulse points (Fig. 12.14). It is customary to feel the pulse by placing several fingers on the radial artery, which lies near the outer border of the palm side of a wrist. The common carotid artery, on either side of the trachea in the neck, is another accessible location for feeling the pulse. Normally, the pulse rate indicates the rate of the heartbeat because the arterial walls pulse whenever the left ventricle contracts. The resting pulse is usually 70 times per minute, but can vary between 60 and 80 times per minute. In addition to the carotid and radial arteries, pulse points can also be found at the superficial temporal, facial, brachial, femoral, popliteal, posterior tibial and dorsalis pedis artery. The dorsalis pedis artery is on the dorsum, or top of the foot. Figure 12.14 Pulse points. Pulse points are the locations where the pulse can be taken. Each pulse point is named after the appropriate artery. Blood Pressure Blood pressure is usually measured in the brachial artery with a sphygmomanometer, an instrument that records changes in terms of millimeters (mm) of mercury (Fig. 12.15). A blood pressure cuff connected to the sphygmomanometer is wrapped around the patient's arm, and a stethoscope is placed over the brachial artery. The blood pressure cuff is inflated until no blood flows through it; therefore, no sounds can be heard through the stethoscope. The cuff pressure is then gradually lowered. As soon as the cuff pressure declines below systolic pressure, blood flows through the brachial artery each time the left ventricle contracts. The blood flow is turbulent below the cuff. This turbulence produces vibrations in the blood and surrounding tissues that can be heard through the stethoscope. These sounds are called Korotkoff sounds, and the cuff pressure at which the Korotkoff sounds are heard the first time is the systolic pressure. As the pressure in the cuff is lowered still more, the Korotkoff sounds change tone and loudness. When the cuff pressure no longer constricts the brachial artery, no sound is heard. The cuff pressure at which the Korotkoff sounds disappear is the diastolic pressure. Figure 12.15 Use of a sphygmomanometer. The technician inflates the cuff with air, gradually reduces the pressure, and listens with a stethoscope for the sounds that indicate blood is moving past the cuff in an artery. This is systolic blood pressure. The pressure in the cuff is further reduced until no sound is heard, indicating that blood is flowing freely through the artery. This is diastolic pressure. © Ariel Skelley/Blend Images/Getty Images RF Normal resting blood pressure for a young adult is 120/80. The higher number is the systolic pressure, the pressure recorded in an artery when the left ventricle contracts. The lower number is the Page 278diastolic pressure, the pressure recorded in an artery when the left ventricle relaxes.

major veins

Figure 12.17 shows the major veins of the body. The external and internal jugular veins drain blood from the brain, head, and neck. Page 280Each external jugular vein enters a subclavian vein. In turn, the subclavian veins join with the internal jugular veins to form the brachiocephalic veins. Right and left brachiocephalic veins merge, giving rise to the superior vena cava. Figure 12.17 Major veins (v.) of the body. Module 9: Animation Pulmonary and systemic circulation In the abdominal cavity, paired veins return blood from bilateral structures such as the kidneys and gonads. In addition, as discussed in more detail later, the hepatic portal vein receives blood from the stomach, spleen, intestines, and other abdominal organs, and then enters the liver. Emerging from the liver, the hepatic veins enter the inferior vena cava. In the pelvic region, veins from the various organs enter the internal iliac veins, while the veins from the lower limbs enter the external iliac veins. The internal and external iliac veins become the common iliac veins that merge, forming the inferior vena cava. Table 12.2 lists the principal veins that enter the venae cavae.

strokes

Imagine you are a physician, nurse, or other health-care provider in a busy family practice center, when a 68-year-old patient comes into your office complaining that the right side of his face is numb and his right arm is tingling. His wife reports that he has had trouble answering her questions all morning. On physical examination, you can see that his right-side facial muscles are functioning weakly, and his face is drooping. When asked to smile, he can only raise the corner of his mouth on the left side, and when asked to stick his tongue straight out, you can see that it bends to the left. His previous history includes all of the risk factors for atherosclerosis: he's an overweight, sedentary type II diabetic, with a 20-year history of cigarette smoking and poorly controlled hypertension. During his most recent physical exam several months ago, you lectured him about his elevated blood LDL, triglyceride, homocysteine, and CRP levels. You explained that his lifestyle might lead to hardening and narrowing of his inflamed arteries, and warned him that atherosclerosis causes heart attacks and strokes. You know that he is currently experiencing a transient ischemic attack (TIA), and his symptoms are a warning that he is at tremendous risk for a cerebrovascular accident (CVA), or stroke. Without hesitating, you send him to the nearby hospital known for its excellent stroke care. Acting fast in this scenario is critical for the patient's survival and continued quality of life. Just as a myocardial infarction causes death to heart muscle, a cerebrovascular accident results in the death of nerve cells. Scientists estimate that for each minute that delicate brain tissue is deprived of oxygenated blood, up to 1.9 million neurons are lost. Stroke is a leading cause of death and disability in the United States, affecting more than 800,000 people every year. A small percentage of strokes occur when an intracranial blood vessel bursts, causing a hemorrhagic stroke. However, the vast majority of strokes are caused by clots that form in cerebral arteries. As you know from the Medical Focus on pages 267-268, atherosclerotic plaques often trigger the clotting cascade. Thrombotic stroke results from a stationary clot in a cerebral artery. If the clot forms elsewhere in the body, then breaks off and blocks a cerebral vessel, it causes embolic stroke. Patients with smaller clots can often be successfully treated with tPA, a drug that dissolves clots. However, because large clots often don't respond to tPA treatment, the stroke victim may die or be seriously impaired. A new catheter stent device shows promise for removing these large clots (Fig. 12F). The catheter is inserted into the femoral artery and threaded up to the blocked cerebral artery until it reaches the clot. Next, a tiny metal cage at the end of the stent is opened and pushed into the clot. Once the clot has been captured, the catheter is withdrawn, pulling the clot along with it. In a recent clinical study of stroke patients, 20% of the patients who received tPA alone recovered enough to return to independent living. However, of those patients treated with both tPA and the clot-removing stent, fully 33% were able to resume their normal daily activities. Furthermore, subsequent CT scans showed less brain damage in patients who received the stent treatment. Neurologists and neurosurgeons are optimistic that the device will greatly improve available care for those patients at greatest risk of brain damage. The stent is a tiny coiled wire cylinder, just large enough to be introduced into an artery. Figure 12F Clot-Removing Stent It's important to remember that stroke and heart attack risk can be lessened by minimizing or avoiding their known risk factors. See the Medical Focus on pages 285-286 for a complete discussion on preventing cardiovascular disease.

capilary exchange

In the tissues of the body, metabolically active cells require oxygen and nutrients and give off wastes, including carbon dioxide. During capillary exchange between tissue capillaries and body cells, oxygen and nutrients leave a capillary. Cellular wastes, including carbon dioxide, enter a capillary. For this reason, systemic arterial blood contains more oxygen and nutrients than does systemic venous blood, and venous blood contains more wastes than does arterial blood. In pulmonary capillaries—the capillaries supplying the lung—the exchange is reversed: Oxygen enters the blood and carbon dioxide leaves. The internal environment of the body consists of blood and tissue fluid. Tissue fluid is simply the fluid that surrounds the cells of the body. In other words, substances that leave a capillary pass through tissue fluid before entering the body's cells, and substances that leave the body's cells pass through tissue fluid before entering a capillary. Water and other small molecules can cross through the cells of a capillary wall or through tiny clefts that occur Page 272between the cells. Large molecules in plasma, such as the plasma proteins, are too large to pass through capillary walls. Yet, the composition of tissue fluid stays relatively constant because of capillary exchange. Tissue fluid is a water-based solution that contains sodium chloride, other electrolytes, and scant protein. Any excess tissue fluid is collected by lymphatic capillaries, which are always found near blood capillaries. Three processes influence capillary exchange—blood pressure, diffusion, and osmotic pressure: Blood pressure, which is created by the pumping of the heart, pushes the blood through the capillary. Blood pressure also pushes blood against a vessel's (e.g., capillary) walls. Diffusion, as you know, is simply the movement of substances from the area of higher concentration to the area of lower concentration. Osmotic pressure is a force caused by a difference in solute concentration on either side of a membrane. To understand osmotic pressure, consider that water will diffuse across a membrane toward the side that has the greater concentration of solutes, and the accumulation of this water results in a pressure. The presence of the plasma proteins, and also salts to some degree, means that blood has a greater osmotic pressure than does tissue fluid. Therefore, the osmotic pressure of blood pulls water into and retains water inside a capillary. Notice in Figure 12.9 that a capillary has an arterial end (contains arterial blood) and a venous end (contains venous blood). In between, a capillary has a midsection. We will now consider the exchange of molecules across capillary walls at each of these locations. Figure 12.9 Capillary exchange. The capillary shows the exchanges that take place and the forces that aid the process. At the arterial end of a capillary, the blood pressure is higher than the osmotic pressure. Tissue fluid tends to leave the bloodstream. In the midsection, solutes, including oxygen (O2) and carbon dioxide (CO2), diffuse from high to low concentration. Carbon dioxide and wastes diffuse into the capillary while nutrients and oxygen enter the tissues. At the venous end of a capillary, the osmotic pressure is higher than the blood pressure. Tissue fluid tends to re-enter the bloodstream. The red blood cells and the plasma proteins are too large to exit a capillary. Module 9: Animation Capillary exchange Arterial End of Capillary When arterial blood enters tissue capillaries, it is bright red because the hemoglobin in red blood cells is carrying oxygen. Blood is also rich in nutrients, which are dissolved in plasma. At the arterial end of a capillary, blood pressure, an outward force, is higher than osmotic pressure, an inward force. Pressure is measured in terms of mm Hg (mercury). In this case, blood pressure is 30 mm Hg, and osmotic pressure is 21 mm Hg. Because blood pressure is higher than osmotic pressure at the arterial end of a capillary, water and other small molecules (e.g., glucose and amino acids) filter out of a capillary at its arterial end. Red blood cells and a large proportion of the plasma proteins generally remain in a capillary because they are too large to pass through its wall. The exit of water and other small molecules from a capillary creates tissue fluid. Therefore, tissue fluid consists of all the components of plasma, except that it contains fewer plasma proteins. Page 273 Midsection of Capillary Diffusion takes place along the length of the capillary, as small molecules follow their concentration gradient by moving from the area of higher to the area of lower concentration. In the tissues, the area of higher concentration of oxygen and nutrients is always blood, because after these molecules have passed into tissue fluid, they are constantly being taken up and metabolized by cells. The cells use oxygen and glucose in the process of cellular respiration, and they use amino acids for protein synthesis. As a result of metabolism, tissue cells constantly give off carbon dioxide and other wastes. Because tissue fluid is always the area of greater concentration for waste materials, they diffuse from tissue fluid into a capillary. Venous End of Capillary At the venous end of the capillary, blood pressure is much reduced to only about 15 mm Hg, as shown in Figure 12.9. Blood pressure is reduced at the venous end because capillaries have a greater cross-sectional area at their venous end than their arterial end. However, there is no reduction in osmotic pressure, which remains at 21 mm Hg and is now higher than blood pressure. Therefore, water tends to diffuse into a capillary at the venous end. As water enters a capillary, it brings with it additional waste molecules. Blood that leaves the capillaries to drain into veins is deep maroon in color because red blood cells now contain reduced hemoglobin—hemoglobin that has given up its oxygen and taken on hydrogen ions. In the end, about 85% of the water that left a capillary at the arterial end returns to it at the venous end. Therefore, retrieving fluid by means of osmotic pressure is not completely effective. The body has an auxiliary means of collecting tissue fluid; any excess usually enters lymphatic capillaries.

Operation of the Heart Valves and Heart Sounds

Let's take a look at how the valves of the heart operate to create a one-way flow of blood from the atria to the ventricles to the arteries. The AV valves (tricuspid and mitral valve) are open when the ventricles are filling with blood. When a ventricle contracts, however, the increasing pressure of the blood inside the ventricle forces the cusps of the AV valve to slam shut. The force of the blood can be compared to a strong wind that can blow a door (the valve cusps) shut. However, when the ventricle contracts, the papillary muscles also contract, causing the chordae tendineae to tighten and pull on the valve. Thus, AV valves in the normal heart are prevented from inverting back up into the atrium. The semilunar valves (pulmonary and aortic) are normally closed while the ventricles are filling with blood. However, the contraction of the ventricles pushes blood at high pressure against the valve cusps, forcing the valves open. Then, when the ventricle relaxes, the blood in the artery falls backward toward the valve, closing the valve once again. A heartbeat produces the familiar "LUB-DUP" sounds as the chambers contract and the valves close. The first heart sound, "lub," is heard when the ventricles begin to contract and the atrioventricular (tricuspid and mitral) valves close. This sound lasts the longest and has a lower pitch. The second heart sound, "dup," is heard when the relaxation of the ventricles allows the semilunar (pulmonary and aortic) valves to close. Like mechanical valves, the heart valves sometimes leak—in the language of medicine, the valves are incompetent. When valves don't close properly there is a backflow of blood. Blood leaks from the ventricle to the atrium (if an atrioventricular valve is incompetent) or from the pulmonary artery or aorta back into the ventricle (if the semilunar valve fails). Most often, the valves of the systemic circulation—bicuspid (or mitral) valves and the aortic semilunar valve—are affected. The backflow causes a heart murmur, which is typically a clicking or swishing sound heard after the first heart sound ("lub"). A trained physician or health professional can diagnose heart murmurs from their sound and timing. A person can be born with a valve deformity (called a congenital valve defect), or the valve can be damaged by bacterial infection. For example, rheumatic fever begins in the throat, and then spreads throughout the body. The bacteria attack connective tissue in the heart valves as well as other organs. Further, antibodies produced by the infected individual can also damage the valves. Fortunately, defective heart valves can be repaired surgically, or replaced with a synthetic valve or one taken from a pig's heart.

homeostatis

Maintaining Blood Composition, pH, and Temperature As you study Human Systems Work Together, it's important to understand that the composition of the blood is maintained by the other systems of the body. The red bone marrow of the skeletal system contributes red blood cells for O2 and CO2 transport. The respiratory Page 289system is responsible for gas exchange. Both the respiratory and urinary systems cooperate to maintain proper pH. The digestive system supplies nutrients, and the liver detoxifies waste. Skeletal muscle contraction supplies heat to keep us warm, and the integumentary system allows us to eliminate any excess heat. The cardiovascular system is the body's transportation mechanism: O2 and nutrients to body tissues, CO2 and wastes to lungs and kidneys, cooler blood to the skeletal muscles to be warmed, warmed blood to the skin to be cooled, and so on. Thus, the body's O2-CO2 level, nutrient distribution, pH, and temperature remain relatively constant. Page 288 Human Systems Work Together CARDIOVASCULAR SYSTEM Maintaining Blood Pressure The pumping of the heart is critical to creating the blood pressure that moves blood throughout the body. The importance of the heart to survival can be seen in the speed with which it develops during prenatal life. Long before other major organs, the heart and its vessels have taken shape and are ready to function. The body has multiple ways to maintain blood pressure. Blood vessel sensory receptors signal the brain when blood pressure falls. The brain's regulatory center subsequently increases heartbeat and constricts blood vessels, and blood pressure is restored. The lymphatic system collects excess tissue fluid, returning it to the cardiovascular system. In this way, the lymphatic system makes an important contribution to regulating blood volume and pressure. The endocrine system assists the nervous system in regulating blood pressure. Epinephrine and norepinephrine act on blood vessels in variable ways, depending on the region of the body in which the blood vessels are found. Norepinephrine causes vasoconstriction, while epinephrine can cause either vasoconstriction or vasodilation. Aldosterone, ADH, and ANH (atriopeptide) regulate urine excretion. Retention of water helps to raise blood pressure, while water excretion helps to lower blood pressure. Venous return from the capillaries to the heart is assisted by the muscular and respiratory systems. Skeletal muscle contraction pushes blood past the vein valves, and breathing movements encourage the flow of blood toward the heart. Smooth muscle in the walls of arterioles constricts and helps raise blood pressure. Platelets are necessary to blood clotting, preventing excessive bleeding and loss of pressure. An individual who loses more than 10% of his or her blood will suffer a sudden drop in blood pressure and go into shock. The decreased pressure triggers the body's last defense: A powerful wave of sympathetic impulses constricts the veins and arterioles throughout the body to slow the drop in blood pressure. Heart rate soars as high as 200 beats a minute to maintain blood flow, especially to the brain and the heart itself. Because of this reflex, you might lose as much as 40% of your total blood volume and still live.

cardiovas. system

Page 259As you'll recall from Chapter 5, tattoos have been part of human cultural practice for centuries. However, it may surprise you to hear that tattoos once had an interesting medical application as well: ensuring the health of the original Mercury 7 astronauts before their space voyages in the early 1960s. John Glenn, pictured here, was one of the original pioneer astronauts, and the first man to orbit the Earth. He and his fellow astronauts underwent hundreds of electrocardiograms throughout the years of the program. An electrocardiogram is a record of the heart's electrical activity, and is an important indicator of heart health. Cardiologists (physicians who specialize in heart care) tattooed Glenn's chest so that when each repeated electrocardiogram was taken, the recording pads (called electrodes) would always be placed in the same position. By creating these permanent markings, the doctors hoped to ensure the accuracy of the recordings. Glenn was a pioneer once again, as the oldest space traveler—at age 77, he flew on the space shuttle Discovery, launched on October 29, 1998. This time, however, improved ECG technology eliminated the need for his tattoos. Instead, computers provided constant, real-time information about every aspect of his heart's performance: electrocardiogram, heart rate, strength of contraction, blood pressure, etc. You can find out more about how electrocardiograms are recorded and used in the Medical Focus on page 269. Chapter 11 described the components of blood and detailed the role that blood has in transport, defense, and regulation. In this chapter, we'll study the body's transportation system: the cardiovascular system. We'll begin with the anatomy and physiology of the heart and of the blood vessels. Then, we'll take a look at various branches of the circulatory system. A crucial function of circulation is to connect the body's trillions of cells so that each organ system can carry out its homeostatic function. In the lungs, oxygen enters and carbon dioxide exits the blood, and this exchange helps to maintain acid-base balance in a normal range. (You'll read more about acid-base balance in Chapters 14 and 16.). The small intestine absorbs nutrient molecules into the blood. The kidneys allow metabolic wastes to exit the blood, while reabsorbing critical ions, molecules, and nutrients and helping to maintain a normal pH. If the body becomes too cool, blood can be directed away from the surface to be warmed by the muscles. Conversely, blood that is too warm can be diverted to the skin so that heat can be lost to the surroundings. None of these homeostatic functions—maintaining O2/CO2, nutrient, pH, and temperature balance—would be possible without transportation from the circulatory system. The cardiovascular system consists of three components: (1) the heart, which pumps blood so that it flows to tissue capillaries and lung capillaries, (2) the blood vessels through which the blood flows, and (3) the blood itself, which as you learned in the previous chapter, is a tissue. As you can see in Figure 12.1, the cardiovascular system is divided into two functional systems. The right side of the heart and its blood vessels form the pulmonary circuit, which pumps blood to the lungs. There, oxygen diffuses into the bloodstream, and CO2 diffuses into the alveoli (the lung's air sacs) to be exhaled. The left side of the heart and its vessels form the systemic circuit, which supplies blood containing oxygen and nutrients to the entire body.

phy. of heart

Page 264The physiology of the heart pertains to its pumping action—that is, the heartbeat. It is estimated that the heart beats almost three billion times in a lifetime, continuously recycling some 5 to 6 liters (L) of blood to keep us alive. In this section, we will consider what causes the heartbeat, what it consists of, and its consequences.

preventing disease

Preventing Cardiovascular Disease All of us can take steps to prevent cardiovascular disease, the most frequent cause of death in the United States. Genetic factors that predispose an individual to cardiovascular disease include family history of heart attack under age 55, male gender, and ethnicity (African Americans are at greater risk). However, people with one or more of these risk factors don't have to give up. It only means that they need to pay particular attention to the following guidelines for a heart-healthy lifestyle. The Don'ts Smoking Hypertension is recognized as a major contributor to cardiovascular disease. When a person smokes, the drug nicotine, present in cigarette smoke, enters the bloodstream. Nicotine causes the arterioles to constrict and the blood pressure to rise. Restricted blood flow and cold hands are associated with smoking in most people. Cigarette smoke also contains carbon monoxide, and hemoglobin combines preferentially and nonreversibly with carbon monoxide. Therefore, the presence of carbon monoxide lowers the oxygen-carrying capacity of the blood, and the heart must pump harder to propel the blood through the lungs. Smoking also damages the arterial wall and accelerates the formation of atherosclerosis and plaque. Drug Abuse Stimulants, such as cocaine and amphetamines, can cause an irregular heartbeat and lead to heart attacks even in people who are using drugs for the first time. Intravenous drug use may also result in a cerebral blood clot and stroke. Too much alcohol can destroy just about every organ in the body, the heart included. But investigators have discovered that people who take an occasional drink have a 20% lower risk of heart disease than do those who completely abstain from alcohol. Two to four drinks a week is the recommended limit for men; one to three drinks is the recommendation for women. Research has shown that wines (and especially red wine) contain antioxidants which can further help to reduce the risk of cardiovascular damage. Weight Gain Hypertension also occurs more often in persons who are more than 20% above the recommended weight for their height. Because more tissue requires servicing, the heart must send extra blood out under greater pressure in those who are overweight. It may be very difficult to lose weight once it is gained, and therefore weight control should be a lifelong endeavor. Even a slight decrease in weight can bring a reduction in hypertension. A 4.5-kilogram weight loss doubles the chance that blood pressure can be normalized without drugs. The Do's Healthy Diet Of all the possible dietary changes one could make to prevent cardiovascular disease, the most important will be to switch to a diet low in saturated fats, trans fats, and cholesterol. These three are found most often as the "solid" forms of fat: butter, margarine, shortening, lard, and the marbling found on fatty meat. Trans fats can be found in baked goods as well. Instead, replace these fats with monounsaturated fats (found in nuts, olives, and olive oil) polyunsaturated fats (found in vegetable oils) and omega-3 fatty acids (found in fish, especially fatty fish such as salmon, mackerel, and trout). Healthy fats like these can help to lower total cholesterol and low-density lipoprotein (LDL), while boosting high-density lipoprotein (HDL). Those changes are very important: In our bodies, cholesterol is ferried in the blood by LDL and HDL. LDL (often referred to as "bad cholesterol" by physicians) takes cholesterol from the liver to the tissues. Recall (from the Medical Focus on pages 267-268) that in blood vessels, LDL is oxidized by macrophages to form foam cells, and later, a fatty streak that begins an atherosclerotic plaque. HDL ("good" cholesterol) transports cholesterol out of the tissues to the liver, where cholesterol is metabolized. When the LDL level in the blood is abnormally high or the HDL level is abnormally low, cholesterol accumulates in artery walls. Substituting heart-healthy fats in the diet has been shown to decrease the risk of cardiovascular disease—but it's important to recognize that all fats are very calorie-dense, and must be used in moderation to avoid weight gain. Limiting sodium intake is another important step to take in modifying diet. In 2009, data from the Centers for Disease Control showed that approximately 70% of Americans are salt-sensitive, and their blood pressure tends to rise after consumption of excess sodium. In response, the American Heart Association lowered its recommended daily sodium intake to less than 1500 milligrams (mg). Almost all processed food contains sodium, so watch those nutrition labels! Here's another recommendation from the American Heart Association, and it may surprise you how few Americans actually do it. Only about 27% of us eat those five servings of fruits and vegetables daily. Evidence is mounting to suggest a role for antioxidant vitamins (A, E, and C) in the prevention of cardiovascular disease, and the best sources are in plant-based food. Antioxidants protect the body from free radicals that may damage HDL cholesterol through oxidation or damage the lining of an artery, leading to a blood clot that can block the vessel. It's not all about vitamins either: By creating a feeling of fullness, all that fiber found in fruits and vegetables can help with weight loss. Regular Health Screenings Page 286It is recommended that everyone know his or her blood cholesterol level, as well as levels of HDL, LDL, and triglyceride. Individuals with a high total blood cholesterol level (240 mg/100 ml) should be further tested to determine their LDL cholesterol level. Blood tests can also determine high levels of homocysteine and C-reactive protein, two important indicators that atherosclerosis is occurring. Levels of these two markers, along with LDL cholesterol level, must be considered together with other cardiovascular risk factors such as age, family history, general health, and whether the patient smokes. Further, for those with a strong family history of cardiovascular disease, testing may be recommended for elevated levels of fibrinogen and lipoprotein (a), two blood components that may indicate atherosclerosis. A person with a moderate risk of cardiovascular disease may have success with dietary therapy to lower LDL. Cholesterol-lowering drugs are reserved for high-risk patients. Proper Dental Care Periodontal disease, the inflammation of the gums caused by poor dental hygiene, has been suggested as a cause for atherosclerosis. Scientists suspect that people with tooth decay and gum disease may have a low-level bacterial infection in the blood—not severe enough to cause illness, but enough to injure the endothelial lining and start the formation of atherosclerotic plaques. Proper care of the teeth and gums, along with regular visits to the dentist, just might prevent cardiovascular disease. Exercise People who exercise are less apt to have cardiovascular disease. Exercise helps keep weight under control, may help minimize stress, and reduces hypertension. The heart beats faster when exercising, but exercise slowly increases the heart's capacity. This means that the heart can beat more slowly when we're at rest and still do the same amount of work. The American Heart Association recommends at least 150 minutes per week of moderate exercise: 30 minutes a day, for 5 days per week. Those 30 minutes can even be broken into three 10-minute sessions, making regular daily exercise an attainable goal for practically everyone. In addition, practice meditation and yoga-like stretching and breathing exercises to reduce stress. For more information about cardiovascular disease prevention, visit the American Heart Association website: http://www.heart.org

special circulation

Special Systemic Circulations Hepatic Portal System The hepatic portal system (Fig. 12.18) carries blood from the stomach, intestines, spleen, and other organs to the liver. The term portal system is used to describe the following unique pattern of circulation: capillaries → vein → capillaries → vein Figure 12.18 Hepatic portal system. This system provides venous drainage of the digestive organs and takes venous blood to the liver. Module 9: Animation Hepatic portal system Capillaries of the digestive tract drain into the superior mesenteric and inferior mesenteric veins and the splenic vein, which join to form the hepatic portal vein. The gastric veins empty directly into the hepatic portal vein. The hepatic portal vein carries blood to capillaries in the liver. The hepatic capillaries allow nutrients and wastes to diffuse into liver cells for further processing. In addition, colonies of white blood cells destroy any pathogens that may be present in the blood. Then, hepatic capillaries join to form venules that enter a hepatic vein. The hepatic veins enter the inferior vena cava. In addition to receiving venous blood from the intestine, the liver also receives arterial blood via the hepatic artery. The hepatic artery is not a part of the hepatic portal system. Hypothalamus-Pituitary Portal System The hypothalamus-pituitary portal system (illustrated in the Visual Focus, Fig. 10.4) is an important endocrine portal system. This portal venous arrangement links the hypothalamus to the anterior pituitary. Through it, the hypothalamus sends releasing hormones to the anterior pituitary. Blood Supply to the Brain The brain is supplied with O2-rich blood by the anterior and posterior cerebral arteries and the carotid arteries. These arteries give off Page 283branches that join to form the cerebral arterial circle (circle of Willis), a vascular route in the region of the pituitary gland (Fig. 12.19). Because the blood vessels form a circle, alternate routes are available for bringing arterial blood to the brain and thus supplying the brain with oxygen. The presence of the cerebral arterial circle also equalizes blood pressure in the brain's blood supply. Figure 12.19 Cerebral arterial circle. The arteries that supply blood to the brain form the cerebral arterial circle (circle of Willis). Module 9: Animation Blood flow through the brain Fetal Circulation As Figure 12.20 shows, the fetus has four circulatory features that are not present in adult circulation: The foramen ovale, or oval window, is an opening between the two atria. This window is covered by a flap of tissue that acts as a valve. The ductus arteriosus, or arterial duct, is a connection between the pulmonary artery and the aorta. It is found on the superior pulmonary trunk near the origin of the left pulmonary artery. The umbilical arteries and vein are vessels that travel to and from the placenta, leaving waste and receiving nutrients. The ductus venosus, or venous duct, is a connection between the umbilical vein and the inferior vena cava. Figure 12.20 Fetal circulation. Arrows indicate the direction of blood flow. The lungs are not functional in the fetus, but they are developing. The blood passes directly from the right atrium to the left atrium via the foramen ovale or from the right ventricle to the aorta via the pulmonary trunk and ductus arteriosus. The umbilical arteries take fetal blood to the placenta where exchange of molecules between fetal and maternal blood takes place. Oxygen and nutrient molecules diffuse into the fetal blood, and carbon dioxide and urea diffuse from the fetal blood. The umbilical vein returns blood from the placenta to the fetus. All of these features can be related to the fact that the fetus does not use its lungs for gas exchange—it receives oxygen and nutrients from the mother's blood at the placenta. During development, the lungs receive only enough blood to supply their developmental need for oxygen and nutrients. The path of blood in the fetus can be traced, beginning from the right atrium (Fig. 12.20). Most of the blood that enters the right atrium passes directly into the left atrium by way of the foramen ovale because the blood pressure in the right atrium is somewhat greater than that in the left atrium. The rest of the fetal blood entering the right atrium passes into the right ventricle and out through the pulmonary trunk. However, because of the ductus arteriosus, most pulmonary trunk blood passes directly into the aortic arch. Notice that whatever route blood takes, most of it reaches the aortic arch instead of the pulmonary circuit vessels. Blood within the aorta travels to the various branches, including the iliac arteries, which connect to the umbilical arteries leading to the placenta. Diffusion of oxygen, nutrients, carbon dioxide, and metabolic wastes between maternal and fetal blood takes place at the placenta. There, oxygen and nutrients diffuse from mother to fetus, while carbon dioxide and waste simultaneously travel from fetus to mother. It's important to note that maternal and fetal blood do not mix at the placenta during a normal pregnancy. Oxygenated blood from the placenta then returns to the fetus's body through the umbilical vein. Thus, blood in the umbilical arteries is O2-poor, but blood in the umbilical vein, which travels from the placenta, is O2-rich. The umbilical vein enters the ductus venosus, which passes directly through the liver. The ductus venosus then joins with the inferior vena cava, a vessel that contains O2-poor blood. The vena cava returns this mixture to the right atrium. Page 284 Changes at Birth Sectioning and tying the umbilical cord permanently separates the newborn from the placenta. The newborn's first breath inflates the lungs, and oxygen enters the blood at the lungs instead of the placenta. O2-rich blood returning from the lungs to the left side of the heart usually causes a flap on the left side of the interatrial septum to close the foramen ovale. What remains is a depression called the fossa ovalis. Incomplete closure occurs in nearly one out of four individuals, but even so, blood rarely passes from the right atrium to the left atrium because either the opening is small or it closes when the atria contract. In a small number of cases, the passage of O2-poor blood from the right side to the left side of the heart is sufficient to cause cyanosis, a bluish cast to the skin. This condition can now be corrected by open-heart surgery. The fetal blood vessels and shunts constrict and become fibrous connective tissue called ligamentums in all cases except the distal portions of the umbilical arteries, which become the medial umbilical ligaments. Regardless, these structures run between internal organs. For example, the ligamentum teres (which is the remnant of the umbilical vein) attaches the umbilicus to the liver.

stroke volume

Stroke volume, which is the amount of blood that leaves a ventricle, depends on the strength of heart contraction. As with heart rate, the autonomic nervous system helps to determine contraction force and stroke volume. Sympathetic activity strengthens each contraction and increases stroke volume. By contrast, parasympathetic activity decreases both heart rate and contractile force, which in turn decreases stroke volume. Further, the degree of contraction also depends on the correct blood electrolyte concentration. Recall from Chapter 8 that the proper concentration of ions, or electrolytes, is essential to create cardiac muscle action potentials. Without these electrolytes, cardiac conduction and contraction are impaired and stroke volume decreases. Two additional factors, venous return and difference in blood pressure, also influence the strength of contraction. Venous Return Venous return is the amount of blood entering the heart by way of the venae cavae (right side of heart) or pulmonary veins (left side of heart). The heart adjusts the strength of its own contraction beat by beat, based upon venous return. This principle is called the Frank-Starling law. The more blood returned to the heart before a given beat, the more the ventricles stretch. As the ventricles are stretched, they contract more and more forcefully. Thus, any event that increases the volume of blood entering the heart will increase the stroke volume leaving the heart. For example, exercise increases the strength of cardiac contraction because skeletal muscle contraction squeezes the veins within muscles and increases venous return. The opposite is also true: If venous return decreases, stroke volume decreases for the next beat. A low venous return, as might happen if there is blood loss, decreases the strength of cardiac contraction. Difference in Blood Pressure The strength of ventricular contraction has to be strong enough to overcome the blood pressure within the attached arteries. If a person has hypertension or atherosclerosis, the opposing arterial pressure may reduce the effectiveness of contraction and the stroke volume.

heart rate

The CO of an average human is 5,250 ml (or 5.25 L) per ventricle, per minute, which equates to about the total volume of blood in the human body. Each minute, the right ventricle pumps about 5.25 L through the pulmonary circuit, while the left ventricle pumps about 5.25 L through the systemic circuit. And this is only the resting cardiac output! During exercise, cardiac output can increase tremendously to meet the body's need for more oxygen. Cardiac output can vary because stroke volume and heart rate can vary, as discussed next. In this way, the heart regulates the blood supply, dependent on the body's needs. For example, increases in heart rate and stroke volume during exercise can increase cardiac output as much as seven to eight times the normal resting amount. Heart Rate A cardioregulatory center in the medulla oblongata of the brain can alter the heart rate by way of the autonomic nervous system (Fig. 12.7). Parasympathetic motor signals conducted by the vagus nerve cause the heart rate to slow, and sympathetic motor signals conducted by sympathetic motor fibers cause the heart rate to increase. Figure 12.7 Control of heart activity. The cardioregulatory center regulates the heart rate, and the vasomotor center regulates constriction of blood vessels, according to input received from baroreceptors in the carotid artery and aortic arch. Module 9: Animation Baroreceptor reflex The cardioregulatory center receives sensory input from receptors within the cardiovascular system. For example, stretch Page 270receptors called baroreceptors are present in the aorta just after it leaves the heart, and also in the carotid arteries, which take blood from the aorta to the brain. If blood pressure falls, as it sometimes does when we stand up quickly, the baroreceptors signal the cardioregulatory center. In response, sympathetic motor signals to the heart cause the heart rate to increase. Once blood pressure begins to rise above normal, nerve signals from the cardioregulatory center cause the heart rate to decrease. Such reflexes help control cardiac output and, therefore, blood pressure, as discussed in section 12.4. The cardioregulatory center is under the influence of the cerebrum and the hypothalamus. Therefore, when we feel anxious, the sympathetic motor nerves are activated. In addition, the adrenal medulla releases the hormones norepinephrine and epinephrine. The result is an increase in heart rate. On the other hand, activities such as yoga and meditation lead to activation of the vagus nerve, which slows the heart rate. Other factors affect the heart rate as well. For example, a low body temperature slows the rate. Also, the proper electrolyte concentrations are needed to keep the heart rate regular.

electro..

The Electrocardiogram A graph that records the electrical activity of the myocardium during a cardiac cycle is called an electrocardiogram, or ECG.* An ECG is obtained by placing several electrodes on the patient's skin, then wiring the electrodes to a voltmeter (an instrument for measuring voltage). As the heart's chambers contract and then relax, the change in polarity is measured in millivolts. An ECG consists of a set of waves: the P wave, a QRS complex, and a T wave (Fig. 12E). The P wave represents depolarization of the atria as a signal started by the SA node travels throughout the atria. The P wave signals that the atria are going to be in systole and that the atrial myocardium is about to contract. The QRS complex represents depolarization of the ventricles following excitation of the Purkinje fibers. It signals that the ventricles are going to be in systole and that the ventricular myocardium is about to contract. The QRS complex shows greater voltage changes than the P wave because the ventricles have more muscle mass than the atria. The T wave represents repolarization of the ventricles. It signals that the ventricles are going to be in diastole and that the ventricular myocardium is about to relax. Atrial diastole (repolarization) does not show up on an ECG as an independent event because the voltage changes are masked by the QRS complex. A P wave is a small upward curve on the graph that resembles an inverted lower-case letter U. The QRS complex is a narrow spike on the graph. It begins with a short downward deflection on the graph, continues with a tall upward deflection, and then finishes with another downward deflection. The T wave is an upward curve on the graph that is slightly longer in duration than the P wave. Figure 12E Electrocardiogram. (a) A portion of an electrocardiogram. (b) An enlarged normal cycle. An ECG records the duration of electrical activity and therefore can be used to detect arrhythmia, an irregular or abnormal heartbeat. A rate of fewer than 60 heartbeats per minute is called bradycardia, and more than 100 heartbeats per minute is called tachycardia. Another type of arrhythmia is fibrillation, in which the heart beats rapidly but the contractions are uncoordinated. Fibrillation can be very dangerous and potentially deadly, because if the heart muscle is not contracting properly, the heart will not pump blood. Cells and tissues will subsequently die of oxygen starvation. The heart can sometimes be defibrillated by briefly applying a strong electrical current to the chest. It is important to understand that an ECG only supplies information about the heart's electrical activity. To be used in diagnosis, an ECG must be coupled with other information, including X rays, studies of blood flow, and a detailed history from the patient.

effects of aging

The heart generally grows larger with age, primarily because of fat deposition in the epicardium and myocardium. In many middle-aged people, the heart is covered by a layer of fat, and the number of collagenous fibers in the endocardium increases. With age, the valves, particularly the aortic semilunar valve, become thicker and more rigid. As a person ages, the myocardium loses some of its contractile power and some of its ability to relax. The resting heart rate decreases throughout life, and the maximum possible rate during exercise also decreases. With age, the contractions become less forceful; the heart loses about 1% of its reserve pumping capacity each year after age 30. In the elderly, arterial walls tend to thicken with plaque and become inelastic, signaling that atherosclerosis and arteriosclerosis are present. Increased blood pressure was once believed to be inevitable with age, but now hypertension is known to result from other conditions, such as kidney disease and atherosclerosis. The Medical Focus on pages 285-286 describes how diet and exercise in particular can help prevent atherosclerosis. Myocardial infarction (described in the Medical Focus box on pages 267-268) and other diseases related to atherosclerosis increase in frequency as a person ages. Congestive heart failure can result from myocardial infarction. In congestive heart failure, a damaged left side of the heart fails to pump adequate blood, and blood backs up in the pulmonary veins. Therefore, pulmonary blood vessels have become congested. The congested vessels leak fluid into tissue spaces, causing pulmonary edema. The result is shortness of breath, fatigue, and a constant cough with pink, frothy sputum. Treatment consists of the three Ds: diuretics (to increase urinary output), digoxin (to increase the heart's contractile force), and dilators (to relax the blood vessels). If necessary, a surgically implanted mechanical pump called a left ventricular assist device (LVAD) can help to maintain the pumping ability of the damaged left ventricle until it can recover. In some cases, heart transplant is also an option. The occurrence of varicose veins increases with age, particularly in people who are required to stand for long periods. Thromboembolism as a result of varicose veins can lead to death if a blood clot settles in a major branch of a pulmonary artery. (This disorder is called pulmonary embolism.)

chambers of heart

The heart has four hollow chambers: two superior atria (sing., atrium) and two inferior ventricles (Fig. 12.3). Each atrium has an anterior pocket-like flap called an auricle. The auricles expand fully when the atrium fills with blood. Auricles also contain cells that produce atrial natriuretic hormone (see p. 228), as well as cardiac stem cells. Internally, the atria are separated by the interatrial septum, and the ventricles are separated by the interventricular septum (plural, septa). Therefore, the heart's pulmonary circuit (its right side) is completely separated from its systemic circuit (the left side) by the septa. However, it's important to note that though they are physically separated, the pulmonary and systemic circuits perform their work together. Thus, the two atria contract simultaneously, and then the two ventricles contract simultaneously. The thickness of each chamber's myocardium is suited to its function. The atria have thin walls, and each pumps blood into the Page 262ventricle below. The ventricles are thicker, and they pump blood into blood vessels that travel to other parts of the body. The thinner myocardium of the right ventricle is suited for pumping blood to the lungs, which are nearby in the thoracic cavity. The left ventricle has a thicker wall than the right ventricle. Thicker myocardium enables the left ventricle to pump its blood to all other parts of the body. Right Atrium At its posterior wall, the right atrium receives O2-poor blood from three veins: the superior vena cava, the coronary sinus, and the inferior vena cava. Venous blood passes from the right atrium into the right ventricle through an atrioventricular (AV) valve. This valve, like the other heart valves, directs the flow of blood and prevents any backflow. The AV valve on the right side of the heart is specifically called the tricuspid valve because it has three cusps, or flaps. Right Ventricle In the right ventricle, the cusps of the tricuspid valve are connected to fibrous cords, called the chordae tendineae (meaning "heart strings"). The chordae tendineae in turn are connected to the papillary muscles, which are conical extensions of the myocardium. Blood from the right ventricle passes through a semilunar valve into the pulmonary trunk. Semilunar valves are so called because their cusps are thought to resemble half-moons. This particular semilunar valve, called the pulmonary semilunar valve, prevents blood from flowing back into the right ventricle. In the Lungs Within the right and left lungs, the pulmonary arteries divide to form smaller and smaller arterioles. The smallest arterioles supply pulmonary capillaries: tiny blood vessels which cover the alveoli, or air sacs of the lungs. As you know from Chapter 4, both capillaries and alveoli are composed of simple squamous epithelium, an exceedingly thin tissue. The respiratory gases oxygen and carbon dioxide freely diffuse between the alveoli and pulmonary capillaries. The capillaries then empty into pulmonary venules, which join to form larger veins. The largest of these veins are the four pulmonary veins. Page 263 Left Atrium At its posterior wall, the left atrium receives O2-rich blood from the pulmonary veins. Two veins come from each lung. Blood passes from the left atrium into the left ventricle through an AV valve. The AV valve on the left side is specifically called the bicuspid valve because it has two cusps. (In the United States, the bicuspid valve is more commonly referred to as the mitral valve, so called because the valve is similar in shape to a bishop's hat, or mitre.) Left Ventricle The left ventricle forms the apex of the heart. The papillary muscles in the left ventricle are quite large, and the chordae tendineae attached to the AV valve are thicker and stronger than those in the right ventricle. Blood passes from the left ventricle through a semilunar valve into the aorta. This semilunar valve is appropriately called the aortic semilunar valve. The semilunar cusps of this valve are larger and thicker than those of the pulmonary semilunar valve. Just beyond the aortic semilunar valve lie the first branches from the aorta. These are the coronary arteries—blood vessels that lie on and nourish the heart itself. The rest of the blood stays in the aorta, which continues as the arch of the aorta and then the descending aorta.

wall/coverings of heart

The heart is composed of three layers, as shown in Figure 12.2. The innermost layer, the endocardium, is a single layer of simple squamous epithelium, called endothelium. Endothelium not only lines the heart but it also continues into and lines the blood vessels. Its smooth nature helps prevent blood from clotting unnecessarily. The central myocardium is the thickest part of the heart wall and is made up of cardiac muscle (see Fig. 4.15). When cardiac muscle fibers contract, the heart beats. The outermost layer is the epicardium, which is also called the visceral serous pericardium (the term visceral means organ, and refers to the fact that this layer covers the heart). After covering the heart, the visceral pericardium folds back over the heart, creating the parietal serous pericardium. The two serous membranes (epicardium and parietal pericardium) secrete pericardial fluid (a fluid similar to plasma). The pericardial fluid reduces friction as the heart beats. The parietal pericardium is fused to the outermost fibrous pericardium. The fibrous pericardium is a thick layer of fibrous connective tissue that adheres to the great blood vessels at the heart's base and anchors the heart to the diaphragm and the mediastinal wall. The coverings of the heart protect the heart, confine it to its location, and prevent it from overfilling, while still allowing the heart to contract and carry out its function of pumping the blood.

anatomy of heart

The heart is located in the thoracic cavity within the mediastinum, a serous membrane sac between the lungs. It is a hollow, cone-shaped, muscular organ. To approximate the size of your heart, make a fist and then clasp the fist with your opposite hand. Figure 12.1 shows that the base (the widest part) of the heart is superior to its apex (the pointed tip), which rests on the diaphragm. Also, the heart lies on a slant; the base is directed toward the right shoulder, and the apex points to the left hip. The base is deep to the second rib, and the apex is at the level of the fifth intercostal space (though these positions can vary depending on a person's size).

blood vessel anatomy

There are three types of blood vessels: arteries, capillaries, and veins (Fig. 12.8). These vessels function to: transport blood and its contents (see page 260); carry out exchange of gases in the pulmonary capillaries and exchange of gases plus nutrients for waste at the systemic capillaries; regulate blood pressure; and direct blood flow to those systemic tissues that most require it at the moment.

veins

Veins and Venules Veins and smaller vessels called venules return blood from the capillary beds to the heart. The venules first drain the blood from the capillaries and then join together to form a vein. The wall of a vein is much thinner than that of an artery because the middle layer of muscle and elastic fibers is thinner (see Fig. 12.8). Within some veins, especially the major veins of the arms and legs, valves allow blood to flow only toward the heart when they are open and prevent the backward flow of blood when they are closed. At any given time, more than half of the total blood volume is found in the veins and venules. If blood is lost due to, for example, hemorrhaging, sympathetic nervous stimulation causes the veins to constrict, providing more blood to the rest of the body. In this way, the veins act as a blood reservoir. Varicose Veins and Phlebitis Varicose veins are abnormal and irregular dilations in superficial (near the surface) veins, particularly those in the lower legs. Varicose veins in the rectum, however, are commonly called piles, or more properly, hemorrhoids. Varicose veins develop when the valves of the veins become weak and ineffective due to backward pressure of the blood. Page 274Phlebitis, or inflammation of a vein, is a more serious condition because thromboembolism can occur. In this instance, the embolus may eventually come to rest in a pulmonary arteriole, blocking circulation through the lungs. This condition, termed pulmonary embolism, can be fatal.

BLOOD FLOW

Velocity of Blood Flow The velocity of blood flow is slowest in the capillaries. What might account for this? Consider that the aorta branches into the other arteries, and these in turn branch into the arterioles, and so forth until blood finally flows into the capillaries. As you know, the diameter of these vessels decreases, getting smaller and smaller with each branching. Capillary diameter is so small that blood cells must travel through in single file. In addition, each time an artery branches, the total cross-sectional area of the blood vessels increases, reaching the maximum cross-sectional area in the capillaries (Fig. 12.10). The slow rate of blood flow in the capillaries is beneficial because it allows time for the exchange of gases in pulmonary capillaries and for the exchange of gases and nutrients for wastes in systemic capillaries (see Fig. 12.9). Once blood has left the capillaries, blood velocity increases as venules combine to form larger and larger veins. Thus, velocity in the venous system is greatest in the venae cavae, which are the largest veins. However, the velocity of venous blood flow returning to the heart is always lower than that of arterial blood leaving the heart. Contractions of the powerful left ventricle generate a greater velocity for arterial blood. The velocity of the arterial and venous systems working together is very high—in a resting individual, it takes only about a minute for a drop of blood to go from the heart to the foot and back again to the heart!

ectrocardiogram/cardiac cycle

With the contraction of any muscle, including the myocardium, electrolyte changes occur that can be detected by electrical recording devices. These changes occur as a muscle action potential sweeps over the cardiac muscle fibers. The resulting record, called an electrocardiogram, helps a physician detect and possibly diagnose the cause of an irregular heartbeat. There are many types of irregular heartbeats, called arrhythmias. The Medical Focus on page 269 discusses the electrocardiogram and some types of arrhythmias. Cardiac Cycle A cardiac cycle includes all the events that occur during one heartbeat. On average, the heart beats about 70 times a minute, although a normal adult heart rate can vary from 60 to 100 beats per minute. After tracing the path of blood through the heart, it might seem that the right and left sides of the heart beat independently of one another, but as you know, they actually contract together. First the two atria contract simultaneously; then the two ventricles contract together. The term systole refers to contraction of heart muscle, and the term diastole refers to relaxation of heart muscle. During the cardiac cycle, atrial systole is followed by ventricular systole. As shown in Figure 12.6, the three phases of the cardiac cycle are: Phase 1: Atrial Systole. Time = 0.15 sec. During this phase, both atria are in systole (contracted), while the ventricles are in diastole (relaxed). Rising blood pressure in the atria forces the blood to enter the two ventricles through the AV valves. At this time, both atrioventricular valves are open, and the semilunar valves are closed. Atrial systole ends when the atrioventricular valves (tricuspid and bicuspid/mitral) slam shut. Closure of the AV valves is caused by the rising pressure of blood filling the ventricle. Remember that closure of the AV valves causes the first heart sound, "lub" (page 263). Page 266Phase 2: Ventricular Systole. Time = 0.30 sec. During this phase, both ventricles are in systole (contracted), while the atria are in diastole (relaxed). Rising blood pressure in the ventricles forces the semilunar valves (aortic and pulmonary) to open. Blood in the right ventricle exits through the pulmonary artery trunk to the right and left pulmonary arteries. Simultaneously, blood in the left ventricle exits into the aorta. During ventricular systole, both semilunar valves are open, and the atrioventricular valves are closed. Ventricular systole ends as the ventricles complete their pumping job; recall that backflow of blood in the pulmonary artery and aorta forces the semilunar valves to slam shut once more (page 263). Closure of the semilunar valves causes the second heart sound "dup." Phase 3: Atrial and Ventricular Diastole. Time = 0.40 sec. During this period, both atria and both ventricles are in diastole (relaxed). At this point, pressure in all the heart chambers is low. Blood returning to the heart from the superior and inferior venae cavae, the coronary sinus, and the pulmonary veins fills the right and left atria and flows passively into the ventricles. At this time, both atrioventricular valves are open and the semilunar valves are closed.

pink box

ardiopulmonary Resuscitation and Automated External Defibrillation How many people's lives have been saved during cardiac arrest by cardiopulmonary resuscitation (CPR)? The exact number would probably be impossible to track, according to the American Heart Association (AHA). Regrettably, it's easier to track the tragedies that occur without it: Fewer than 30% of people who suffer cardiac arrest ever receive bystander help. Onlookers simply don't know what to do, or perhaps are afraid that they might do something wrong. For this reason, the AHA has published the Chain of Survival, a new set of four guidelines that make it easier to help a person in cardiac arrest: Recognize that you're in an emergency, and immediately call for help. Call 9-1-1, or use the EMS service in your area. Start CPR. The traditional method alternates chest compressions with mouth-to-mouth breathing, also known as rescue breathing. If you're not sure how to do rescue breathing, simply pump fast (100 or more times a minute) and hard (at a depth of least 2 inches) on the body of the sternum. Let the patient's chest rise in between each pump, but don't stop—keep any interruptions to 10 seconds or fewer. Recent research from the AHA has shown that simple manual chest compression can be as effective as traditional CPR (chest compression accompanied by rescue breathing). If you know how to do rescue breathing, use 2 quick breaths for every 30 chest compressions. Continue CPR until you're relieved by emergency responders. If you have access to an automated external defibrillator (AED), use it. This computerized device is available in many public places, such as airports and shopping malls, and is prominently labeled. An AED will explain, step by step, how to check for the person's breathing and a pulse first. Next, it will describe how to apply pads to the victim's chest so that the computer can analyze the heart activity. If a shock is needed to restart the victim's heart, the AED delivers a burst of intense electrical current to the chest. The rescuer simply moves back and pushes a button when prompted. Voice instructions will also detail how to do CPR until paramedics arrive. Transfer to advanced care. Clinicians refer to the first hour after admission to an emergency room as the "golden hour." It's not surprising that research has shown that patients who receive the best care during this critical time are the most likely to survive. The AHA recommends that everyone learn both CPR and how to use an AED. The Red Cross and many hospitals regularly offer introductory and refresher classes in traditional classrooms and online. With training, you just might be able to save a person's life! Hypotension, or low blood pressure, may be caused by a number of factors. Simply standing up very quickly causes orthostatic hypotension, and a person may temporarily feel light-headed. Normally, the brain's blood pressure control mechanisms rapidly compensate. However, factors such as hemorrhage or excessive fluid loss (for example, following a burn, or as a consequence of vomiting and/or diarrhea) can cause severe hypotension and shock (see page 252). If uncontrolled, shock is fatal. Currently, an estimated 33% of all Americans suffer from hypertension, or high blood pressure. Most have essential hypertension, for which the cause is not precisely known. Hypertension is present when systolic blood pressure is 140 or greater, or diastolic blood pressure is 90 or greater. Hypertension is sometimes a silent killer because it may not be detected until a stroke or heart attack occurs. Among other factors, one's genetic makeup affects the development of hypertension. Two genes are involved: the first codes for the plasma protein angiotensinogen (see page 276), while the second gene's product helps activate angiotensinogen into a powerful vasoconstrictor. Gene therapy might one day cure individuals with this gene combination. Meanwhile, regular blood pressure checks and a lifestyle that lowers hypertension risk are our best safeguards against hypertension (see the Medical Focus on pages 285-286). Medication might also be necessary—often for an entire lifetime.