Overview of the Cardiovascular System
The cardiovascular system consists of
1) the heart, which pumps blood, and
(2) the blood vessels, through which the blood flows.
The beating of the heart sends blood into the blood vessels. In humans, blood is always contained within blood vessels.
Circulation Performs Exchanges
Even though circulation of blood depends on the beating of the heart, the purpose of circulation is to serve cells. Remember that cells are surrounded by tissue fluid that is used to exchange substances between the blood and cells. Blood removes waste products from tissue fluid. Blood also brings tissue fluid the oxygen and nutrients cells require to continue their existence. Blood would not be able to continue to perform this function if it did not become oxygenated in the lungs (exchanging carbon dioxide waste for needed oxygen) and cleansed by the kidneys . At the lungs, blood drops off carbon dioxide and picks up oxygen, as indicated by the two arrows in and out of the lungs in below figure. Gas exchange is not the only function of blood. Nutrients enter the bloodstream at the intestines and transport much needed substances to the body’s cells. Blood is purified of its wastes at the kidneys, and water and salts are retained as needed. The liver is important, because it takes up amino acids from the blood and returns needed proteins. Liver proteins transport substances such as fats in the blood. The liver also removes toxins and chemicals that may have entered the blood at the intestines, and its colonies of white blood cells destroy bacteria and other pathogens. Thousands of miles of blood vessels, which form an intricate circuit reaching almost every cell of the body, move the blood and its contents through the body to and from all the body’s organs.
Functions of the Cardiovascular System
1. contractions of the heart generate blood pressure, which moves blood through blood vessels
2. blood vessels transport the blood from the heart into arteries, capillaries, and veins; and blood then returns to the heart so the circuit can be completed
3. gas exchange (pickup of carbon dioxide waste and drop-off of oxygen for the cells) occurs at the smallest diameter vessels, the capillaries
4. the heart and blood vessels regulate blood flow, according to the needs of the body
The Arteries: From the Heart
The arterial wall has three layers. The innermost layer is a thin layer of cells called endothelium. Endothelium is surrounded by a relatively thick middle layer of smooth muscle and elastic tissue. The artery’s outer layer is connective tissue. The strong walls of an artery give it support when blood enters under pressure; the elastic tissue allows an artery to expand to absorb the pressure. Arterioles are small arteries barely visible to the naked eye. Whereas the middle layer of arterioles has some elastic tissue, it is composed mostly of smooth muscle. These muscle fibers encircle the arteriole. When the fibers contract, the vessel constricts; when these muscle fibers relax, the vessel dilates. The constriction or dilation of arterioles controls blood pressure. When arterioles constrict, blood pressure rises. Dilation of arterioles causes blood pressure to fall.
The Capillaries: Exchange
Arterioles branch into capillaries. Each capillary is an extremely narrow, microscopic tube with a wall composed only of endothelium. Capillary endothelium is formed by a single layer of epithelial cells with a basement membrane. Although capillaries are small, their total surface area in humans is about 6,300 square meters. Capillary beds (networks of many capillaries) are present in all regions of the body, so no cell is far from a capillary and thus not far from gas exchange with blood. In the tissues, only certain capillaries are open at any given time. For example, after eating, the capillaries supplying the digestive system are open, whereas most serving the muscles are closed. Rings of muscle called precapillary sphincters control the blood fl ow through a capillary bed. Constriction of the sphincters closes the capillary bed. When a capillary bed is closed, the blood moves to an area where gas exchange is needed, going directly from arteriole to venule through a pathway called an arteriovenous shunt.
The Veins: To the Heart
Venules are small veins that drain blood from the capillaries and then join to form a vein. The walls of venules (and veins) have the same three layers as arteries. However, there is less smooth muscle in the middle layer of a vein and less connective tissue in the outer layer. Therefore, the wall of a vein is thinner than that of an artery. Veins often have valves, which allow blood to fl ow only toward the heart when open and prevent backward fl ow of blood when closed. Valves are extensions of the inner-wall layer and are found in the veins that carry blood against the force of gravity, especially the veins of the lower extremities. The walls of veins are thinner, so they can expand to a greater extent. At any one time, about 70% of the blood is in the veins. In this way, the veins act as a blood reservoir. If blood is lost due to hemorrhaging, nervous stimulation causes the veins to constrict, providing more blood to the rest of the body.
The Heart Is a Double Pump
The heart is a cone-shaped, muscular organ located between the lungs, directly behind the sternum (breastbone). The heart is tilted so that the apex (the pointed end) is oriented to the left. To approximate the size of your heart, make a fist, then clasp the fist with your opposite hand. The major portion of the heart is the interior wall of tissue called the myocardium, consisting largely of cardiac muscle tissue. The muscle fibers of myocardium are branched. Each fiber is tightly joined to neighboring fibers by structures called intercalated disks. The intercalated disks also include cell junctions like gap junctions and desmosomes. Gap junctions are used to aid in simultaneous contractions of the cardiac fibers. Desmosomes include arrangements of protein fibers that tightly hold the membranes of adjacent cells together and prevent overstretching. The heart is surrounded by the pericardium, a thick, membranous sac that supports and protects the heart. The inside of the pericardium secretes pericardial fluid (a lubrication fluid), and the pericardium slides smoothly over the heart’s surface as it pumps the blood. Internally, a wall called the septum separates the heart into a right side and a left side. The heart has four chambers. The two upper, thin-walled atria (sing., atrium) are called the right atrium and the left atrium. Each atrium has a wrinkled, ear like flap on the outer surface called an auricle. The two lower chambers are the thick-walled ventricles, called the right ventricle and the left ventricle. Heart valves keep blood fl owing in the right direction and prevent its backward movement. The valves that lie between the atria and the ventricles are called the atrioventricular (AV) valves. These valves are supported by strong fibrous strings called chordae tendineae. The chordae tendineae are attached to papillary muscles that project from the ventricular walls. Chordae anchor the valves, preventing them from inverting when the heart contracts.
The AV valve on the right side is called the tricuspid valve because it has three flaps, or cusps. The AV valve on the left side is called the bicuspid valve because it has two flaps. The bicuspid valve is commonly referred to as the mitral valve, because it has a shape like a bishop’s hat, or miter. The remaining two valves are the semilunar valves, with flaps shaped like half-moons. These valves lie between the ventricles and their attached vessels. The semilunar valves are named for their attached vessels: The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk. The aortic semilunar valve lies between the left ventricle and the aorta.
Coronary Circulation: The Heart’s Blood Supply
The myocardium—the middle, muscular layer of the three layers of the walls of the heart— receives oxygen and nutrients from the coronary arteries. Likewise, wastes are removed by the cardiac veins. The blood that flows through the heart contributes little to either nutrient supply or waste removal. The coronary arteries serve the heart muscle itself. These arteries are the first branches off the aorta. They originate just above the aortic semilunar valve. They lie on the exterior surface of the heart, where they divide into diverse arterioles. The coronary capillary beds join to form venules which converge to form the cardiac veins, which empty into the right atrium. Because they have a very small diameter, they can become easily clogged leading to coronary artery disease . If these vessels become completely clogged, oxygen and nutrients such as glucose, will not reach the muscles of the heart. This may result in a myocardial infarction, or heart attack. Coronary artery disease may be treated with medication, coronary bypass surgery or angioplasty.
Passage of Blood Through the Heart
Recall that intercalated disks join fibers of cardiac muscle cells, allowing them to communicate with each other. By sending electrical signals between cells, both atria and then both ventricles contract simultaneously. We can trace the path of blood through the heart and body in the following manner.
• The superior vena cava and the inferior vena cava (see above figure) carry oxygen-poor blood from body veins to the right atrium.
• The right atrium contracts, sending blood through an atrioventricular valve (the tricuspid valve) to the right ventricle. (Remember, the left atrium contracts at the same time.)
• The right ventricle contracts, pumping blood through the pulmonary semilunar valve into the pulmonary trunk. The pulmonary trunk, which carries oxygen poor blood, divides into two pulmonary arteries, which go to the lungs.
• Pulmonary capillaries within the lungs allow gas exchange. Oxygen enters the blood; carbon dioxide waste is excreted from the blood.
• Four pulmonary veins, which carry oxygen-rich blood, enter the left atrium.
• The left atrium pumps blood through an atrioventricular valve (the bicuspid [mitral] valve) to the left ventricle.
• The left ventricle contracts, sending blood through the aortic semilunar valve into the aorta. (The right ventricle is contracting at the same time.)
• Large arteries, smaller arteries, and arterioles supply tissue capillaries. Tissue capillaries drain into increasingly larger veins. Veins drain into the superior and inferior vena cava, and the cycle starts again.
From this description, you can see that oxygen-poor blood never mixes with oxygen-rich blood and that blood must go through the lungs to pass from the right side to the left side of the heart. The heart is a double pump because the right ventricle of the heart sends blood through the lungs, and the left ventricle sends blood throughout the body. Thus, the left ventricle has the harder job of pumping blood. The atria have thin walls, and each pumps blood into the ventricle right below it. The ventricles are thicker, and they pump blood into arteries (pulmonary artery and aorta) that travel to other parts of the body. The thinner myocardium of the right ventricle pumps blood to the lungs nearby in the thoracic cavity. The left ventricle has a thicker wall with more cardiac muscle cells than the right ventricle, and this enables it to pump blood out of the heart with enough force to send it through the body. The pumping of the heart sends blood under pressure out into the arteries. The left side of the heart pumps with greater force, so blood pressure is greatest in the aorta. Blood pressure then decreases as the total cross-sectional area of arteries and then arterioles increases . A different mechanism, aside from blood pressure, is used to move blood in the veins
The Heartbeat Is Controlled
Each heartbeat is called a cardiac cycle (see below figure). Recall that when the heart beats, first the two atria contract at the same time. Next, the two ventricles contract at the same time. Then, all chambers relax. Systole, the working phase, refers to contraction of the chambers, and diastole, the resting phase, refers to relaxation of the chambers . The heart contracts, or beats, about 70 times a minute on average in a healthy adult, with each heartbeat lasting about 0.85 second with a normal resting rate varying from 60 to 80 beats per minute. There are two audible heartbeat sounds referred to as “lub-dup.” The first sound, “lub,” occurs when increasing pressure of blood inside a ventricle forces the cusps of the AV valves to slam shut . In contrast, the pressure of blood inside a ventricle causes the semilunar valves (pulmonary and aortic) to open. The “dup” occurs when the ventricles relax, and blood in the arteries flows backward momentarily, causing the semilunar valves to close . A heart murmur, or a slight swishing sound after the “lub,” is often due to leaky valves, which allow blood to pass back into the atria after the AV valves have closed. Faulty valves can be surgically corrected.
Internal Control of Heartbeat
The rhythmic contraction of the atria and ventricles is due to the internal (intrinsic) conduction system of the heart. Nodal tissue is a unique type of cardiac muscle located in two regions of the heart. Nodal tissue has both muscular and nervous characteristics. The SA (sinoatrial) node is located in the upper dorsal wall of the right atrium. The AV (atrioventricular) node is located in the base of the right atrium very near the septum . The SA node initiates the heartbeat and automatically sends out an excitation signal every 0.85 second. This causes the atria to contract. When signal impulses 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 (AV) bundle before reaching the numerous and smaller Purkinje fibers. The AV bundle, its branches, and the Purkinje fibers work efficiently because gap junctions (tiny channels built into intercalated disks) allow electrical current to fl ow from cell to cell .The SA node is called the pacemaker because it regulates heartbeat. If the SA node fails to work properly, the heart still beats due to signals generated by the AV node. But the beat is slower (40 to 60 beats per minute). To correct this condition, it is possible to implant an artificial pacemaker, which automatically gives an electrical stimulus to the heart every 0.85 second.
Features of the Cardiovascular System
When the left ventricle contracts, blood is sent out into the aorta under pressure. A progressive decrease in pressure occurs as blood moves through the arteries, arterioles, capillaries, venules, and finally the veins. Blood pressure is highest in the aorta. By contrast, pressure is lowest in the superior and inferior venae cavae, which enter the right atrium
1. Pulse Rate Equals Heart Rate
The surge of blood entering the arteries causes their elastic walls to stretch, but then they almost immediately recoil. This rhythmic expansion and recoil of an arterial wall can be felt as a pulse in any artery that runs close to the body’s surface. It is customary to feel the pulse by placing several fingers on either the radial artery (near the outer border of the palm side of the wrist) or the carotid artery (located on either side of the trachea in the neck).
Normally, the pulse rate indicates the heart rate because the arterial walls pulse whenever the left ventricle contracts. The pulse rate is usually 70 beats per minute in a healthy adult but can vary between 60 and 80 beats per minute.
2. Blood Flow Is Regulated
The beating of the heart is necessary to homeostasis because it creates the pressure that propels blood in the arteries and the arterioles. Arterioles lead to the capillaries where exchange with tissue fluid takes place.
3. Blood Pressure Moves Blood in Arteries
Blood pressure is the pressure of blood against the wall of a blood vessel. A sphygmomanometer (blood pressure instrument) can be used to measure blood pressure, usually in the brachial artery of the arm. The highest arterial pressure, called the systolic pressure, is reached during ejection of blood from the heart. The lowest arterial pressure, called the diastolic pressure, occurs while the heart ventricles are relaxing. Blood pressure is measured in millimeters mercury (mm Hg). Normal resting blood pressure for a young adult should be slightly lower than 120 mm Hg over 80 mm Hg, or 120/80, but these values can vary somewhat and still be within the range of normal blood pressure . The number 120 represents the systolic pressure, and 80 represents the diastolic pressure. High blood pressure is called hypertension, and low blood pressure is called hypotension. Both systolic and diastolic blood pressure decrease with distance from the left ventricle because the total cross-sectional area of the blood vessels increases—there are more arterioles than arteries. The decrease in blood pressure causes the blood velocity to gradually decrease as it flows toward the capillaries.
4. Blood Flow Is Slow in the Capillaries
There are many more capillaries than arterioles, and blood moves slowly through the capillaries . This is important because the slow progress allows time for the exchange of substances between the blood in the capillaries and the surrounding tissues. Any needed changes in fl ow rate are adjusted by the opening and closing of the pre-capillary sphincters.
5. Blood Flow in Veins Returns Blood to Heart
The velocity of blood fl ow increases from capillaries to veins. As an analogy, imagine a single narrow street emptying its cars into a fast multilane highway. Likewise, as capillaries empty into veins, blood can travel faster. However, it’s also known that blood pressure is minimal in venules and veins. Blood pressure thus plays only a small role in returning venous blood to the heart. Venous return is dependent upon three additional factors:
1. the skeletal muscle pump, dependent on skeletal muscle contraction;
2. the respiratory pump, dependent on breathing; and
3. valves in veins.
The skeletal muscle pump functions every time a muscle contracts. When skeletal muscles contract, they compress the weak walls of the veins. This causes the blood to move past a valve . Once past the valve, blood cannot fl ow backward. The importance of the skeletal muscle pump in moving blood in the veins can be demonstrated by forcing a person to stand rigidly still for an hour or so. Fainting may occur because lack of muscle contraction causes blood to collect in the limbs. Poor venous return deprives the brain of needed oxygen. In this case, fainting is beneficial because the resulting horizontal position aids in getting blood to the head.
Two Cardiovascular Pathways
The blood flows in two circuits: the pulmonary circuit, which circulates blood through the lungs, and the systemic circuit, which serves the needs of body tissues (see below figure). As we shall see, both circuits are necessary to homeostasis.
The Pulmonary Circuit: Exchange of Gases
The path of blood through the lungs can 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 right and left pulmonary arteries, which branch as they approach the lungs. The arterioles take blood to the pulmonary capillaries, where carbon dioxide is given off and oxygen is picked up. Blood then passes through the pulmonary venules, which lead to the four pulmonary veins that enter the left atrium. Blood in the pulmonary arteries is oxygen-poor but blood in the pulmonary veins is oxygen-rich, so it is not correct to say that all arteries carry blood high in oxygen and all veins carry blood low in oxygen (as people tend to believe). Just the reverse is true in the pulmonary circuit.
The Systemic Circuit: Exchanges with Tissue Fluid
The systemic circuit includes all of the arteries and veins shown in above Figure.(For simplicity, some blood vessels are not shown.) The heart pumps blood through 60,000 miles of blood vessels to deliver nutrients and oxygen and remove wastes from all body cells. The largest artery in the systemic circuit, the aorta, receives blood from the heart; the largest veins, the superior and inferior venae cavae, return blood to the heart. The superior vena cava collects blood from the head, the chest, and the arms, and the inferior vena cava collects blood from the lower body regions. Both enter the right atrium.
Tracing the Path of Blood
It’s easy to trace the path of blood in the systemic circuit by beginning with the left ventricle, which pumps blood into the aorta. Branches from the aorta go to the organs and major body regions. For example, this is the path of blood to and from the lower legs:
left ventricle—aorta—common iliac artery— femoral artery—lower leg capillaries— lower leg veins--femoral vein—common iliac vein— inferior vena cava—right atrium
When tracing blood, mention the aorta, the proper branch of the aorta, the region, and the vein returning blood to the vena cava. In many instances, the artery and the vein that serve the same region are given the same name (see fig below). What happens in between the artery and the vein? Arterioles from the artery branch into capillaries, where exchange takes place, and then venules join into the vein that enters a vena cava.
Exchange at the Capillaries
Two forces control movement of fluid through the capillary wall: blood pressure, which tends to cause fluids in the blood to move from capillary to tissue spaces, and osmotic pressure, which tends to cause water to move in the opposite direction. At the arterial end of a capillary, blood pressure (30 mm Hg) is higher than the osmotic pressure of blood (21 mm Hg) (see Fig. Below). Osmotic pressure is created by the presence of solutes dissolved in plasma, the liquid fraction of the blood. Dissolved plasma proteins are of particular importance in maintaining the osmotic pressure. Most plasma proteins are manufactured by the liver. Blood pressure is higher than osmotic pressure at the arterial end of a capillary, so water exits a capillary at the arterial end. Midway along the capillary, where blood pressure is lower, the two forces essentially cancel each other, and there is no net movement of fluid. Solutes now diffuse according to their concentration gradient: Oxygen and nutrients (glucose and amino acids) diffuse out of the capillary; carbon dioxide and wastes diffuse into the capillary. Red blood cells and almost all plasma proteins remain in the capillaries. The substances that leave a capillary contribute to tissue fluid, the fluid between the body’s cells. Plasma proteins are too large to readily pass out of the capillary. Thus, tissue fluid tends to contain all components of plasma, except much lower amounts of protein. At the venule end of a capillary, blood pressure has fallen even more. Osmotic pressure is greater than blood pressure, and fluid tends to move back. Almost the same amount of fluid that left the capillary returns to it, although some excess tissue fluid is always collected by the lymphatic capillaries. Tissue fluid contained within lymphatic vessels is called lymph. Lymph is returned to the systemic venous blood when the major lymphatic vessels enter the subclavian veins in the shoulder region.
Disorders of the Blood Vessels
Hypertension and atherosclerosis often lead to stroke or heart attack, due to an artery blocked by a blood clot or clogged by plaque. Treatment involves removing the blood clot or prying open the affected artery. Another possible outcome is an aneurysm, a burst blood vessel. An aneurysm can be prevented by replacing a blood vessel that is about to rupture with an artificial one.
High Blood Pressure
Hypertension occurs when blood moves through the arteries at a higher pressure than normal. Also known as high blood pressure, hypertension is sometimes called a silent killer. It may not be detected until it has caused a heart attack, stroke, or even kidney failure. Hypertension is present when the systolic blood pressure is 140 or greater or the diastolic blood pressure is 90 or greater. Though systolic and diastolic pressures are both important, diastolic pressure is emphasized when medical treatment is being considered. The best safeguard against developing hypertension is regular blood pressure checks and a lifestyle that lowers the risk of CVD. If hypertension is present, prescription drugs can help lower blood pressure. Diuretics cause the kidneys to excrete more urine, ridding the body of excess fluid. In addition, hormones (the body’s chemical messengers) that raise blood pressure can be inactivated. Drugs called betablockers and ACE (angiotensin-converting enzyme) inhibitors help to control hypertension caused by hormones. Hypertension is often seen in individuals who have atherosclerosis. Atherosclerosis is caused by formation of lesions, or atherosclerotic plaques, on the inside of blood vessels. The plaques narrow blood vessel diameter, choking off blood and oxygen supply to the tissues (see below Fig.). In most instances, atherosclerosis begins in early adulthood and develops progressively through middle age, but symptoms may not appear until an individual is 50 or older. To prevent the onset and development of atherosclerosis, the American Heart Association and other organizations recommend a diet low in saturated fat and cholesterol but rich in omega-3 polyunsaturated fatty acids. Atherosclerotic plaques can cause a clot to form on the irregular, roughened arterial wall. As long as the clot remains stationary, it is called a thrombus. If the thrombus dislodges and moves along with the blood, it is called an embolus. A thromboembolism consists of a clot first carried in the bloodstream that then becomes completely stationary when it lodges in a small blood vessel. If a thromboembolism is not treated, the life-threatening complications described in the next section can result. Research has suggested several possible causes for atherosclerosis aside from hypertension. Chief among these, as discussed in the aforementioned Bioethical Focus, Cardiovascular Disease Prevention, are smoking and a diet rich in lipids and cholesterol. Research also indicates that a low-level bacterial or viral infection that spreads to the blood may cause an injury that starts the process of atherosclerosis. Surprisingly, such an infection may originate with gum diseases or be due to Helicobacter pylori (the bacterium that causes ulcers). People who have high levels of C-reactive protein, which occur in the blood following a cold or injury, are more likely to have a heart attack.
Stroke, Heart Attack, and Aneurysm
Stroke, heart attack, and aneurysm are associated with hypertension and atherosclerosis. A cerebrovascular accident (CVA), also called a stroke, often results when a small cranial arteriole bursts or is blocked by an embolus. Lack of oxygen causes a portion of the brain to die, and paralysis or death can result. A person is sometimes forewarned of a stroke by a feeling of numbness in the hands or the face, difficulty in speaking, or temporary blindness in one eye. A myocardial infarction (MI), also called a heart attack, occurs when a portion of the heart muscle dies due to lack of oxygen. If a coronary artery becomes partially blocked, the individual may then suffer from angina pectoris. Characteristic symptoms of angina pectoris include a feeling of pressure, squeezing, or pain in the chest. Pressure and pain can extend to the left arm, neck, jaw, shoulder, or back. Nausea and vomiting, anxiety, dizziness, and shortness of breath may accompany the chest discomfort. Nitroglycerin or related drugs dilate blood vessels and help relieve the pain. When a coronary artery is completely blocked, perhaps because of a thromboembolism, a heart attack occurs. An aneurysm is a ballooning of a blood vessel, most often the abdominal artery or the arteries leading to the brain. Atherosclerosis and hypertension can weaken the wall of an artery to the point that an aneurysm develops. If a major vessel such as the aorta bursts, death is likely. It is possible to replace a damaged or diseased portion of a vessel, such as an artery, with a plastic tube. Cardiovascular function is preserved because exchange with tissue cells can still take place at the capillaries. In the future, it may be possible to use vessels made in the laboratory by injecting a patient’s cells inside an inert mold.
When a person has heart failure, the heart no longer pumps as it should. Heart failure is a growing problem because people who used to die from heart attacks now survive but are left with damaged hearts. Often the heart is oversized, not because the cardiac wall is stronger but because it is sagging and swollen. One idea is to wrap the heart in a fabric sheath to prevent it from getting too big. This might allow better pumping, similar to the way a weight lifter’s belt restricts and reinforces stomach muscles. But a failing heart can have other problems, such as an abnormal heart rhythm.
To counter that condition, it’s possible to place an implantable cardioverter-defi brillator (ICD) just beneath the skin of the chest. This device can sense both an abnormally slow and an abnormally fast heartbeat. If the former, the ICD generates the missing beat like a pacemaker does. If the latter, it sends the heart a sharp jolt of electricity to slow it down. If the heart rhythm becomes erratic, the ICD sends an even stronger shock—like a defibrillator does.