The
Transport
Systems
Chapter
23
Outline
23.1 Open and Closed Circulatory Systems
• An open circulatory system can be contrasted with a closed circulatory system.396–97
• Fishes have a single circulatory loop, whereas the other vertebrates have a double circulatory loop—to and from the lungs and also to and from the tissues.398
23.2 Transport in Humans
• In humans, the right side of the heart pumps blood to the lungs, and the left side pumps blood to the tissues.399
• The heartbeat results from the contraction of the heart’s chambers. The pulse can be used to measure the heart rate.400
• Arteries take blood away from the heart to the capillaries where exchange occurs, and veins take blood to the heart.401
• The pulmonary circuit moves blood from the heart to the lungs and back, while the systemic circuit moves blood from the heart to all regions of the body and returns it to the heart.402
• The lymphatic vessels form a one-way system that transports lymph from the tissues and fat from the digestive tract to certain veins.403
• Although the human cardiovascular system is very efficient, it is still subject to degenerative disorders.403–4
23.3 Blood: A Transport Medium
• In humans, blood is composed of cells and a fluid called plasma that contains proteins and various other molecules and ions.405
• Blood clotting is a series of reactions that produces a clot consisting of fibrin threads in which red blood cells are trapped.406–7
• Exchange of substances between blood and tissue fluid across capillary walls supplies cells with nutrients and removes wastes.408
The beat of the heart is under involuntary control, so we rarely consider the complex series of events that occurs when the heart moves blood to the lungs and the body tissues. Ordinarily, contraction of the heart is controlled by its natural pacemaker (referred to as the SA node), which automatically triggers the heart’s rhythmic beating. At rest, the heart contracts about 70 times per minute, and each contraction moves the entire blood volume of approximately 4–6 liters through the system once every minute. The nervous system gets involved when the body requires an increased amount of oxygen, as when you exercise vigorously. A particular nerve releases an adrenaline-like chemical in order to increase the activity of the SA node and therefore the heart rate. This action of the nervous system is mimicked whenever adrenaline is injected directly into the heart muscle, as is sometimes shown in the movies.
When damage occurs to the SA node, physicians often implant an artificial pacemaker. The heart responds to electrical signals generated by the SA node, and the artificial pacemaker emits electrical signals at the proper rate in order to duplicate the activity of a normal SA node. It is amazing to think that a simple electrical device can keep the entire cardiovascular system running.
The cardiovascular system is one of the body’s transport systems. It must move blood to the tissues at a rate appropriate to meet the oxygen and nutrient demands of the cells. In this chapter, you will learn how the cardiovascular system works and how its functioning can be adjusted to meet the body’s needs. Some of the consequences of a malfunctioning system are also discussed.
23.1 Open and Closed Circulatory Systems
In some animals, the body plan makes a circulatory system unnecessary (Fig. 23.1). In a hydra, cells are either part of an external layer, or they line the gastrovascular cavity. In either case, each cell is exposed to water and can independently exchange gases and rid itself of wastes. The cells that line the gastrovascular cavity are specialized to carry out digestion. They pass nutrient molecules to other cells by diffusion. In a planarian, a trilobed gastrovascular cavity branches throughout the small, flattened body. No cell is very far from one of the digestive branches, so nutrient molecules can diffuse from cell to cell. Similarly, diffusion meets the respiratory and excretory needs of the cells.
Some other animals, such as nematodes and echinoderms, rely on movement of fluid within a coelom to circulate substances.
Open Circulatory Systems
More complex animals have a circulatory system in which a pumping heart moves a fluid into blood vessels. The grasshopper is an example of an invertebrate animal that has an open -circulatory system (Fig. 23.2a). A tubular heart pumps a fluid called hemolymph through a network of channels and cavities in the body. Eventually, hemolymph (a combination of blood and tissue fluid) drains back to the heart. When the heart contracts, openings called ostia (sing., ostium) are closed; when the heart relaxes, the hemolymph is sucked back into the heart by way of the ostia. The -hemolymph of a grasshopper is colorless because it does not contain hemoglobin or any other respiratory pigment that combines with and carries oxygen. Oxygen is taken to cells, and carbon dioxide is removed from them by way of air tubes, called tracheae, which are found throughout the body. The tracheae provide efficient transport and delivery of respiratory gases, while at the same time restricting water loss.
Closed Circulatory Systems
All vertebrates and some invertebrates have a closed circulatory system (Fig. 23.2b), which is more properly called a cardiovascular system because it consists of a strong muscular heart and blood vessels. In humans, the heart has two receiving chambers, called atria (sing., atrium), and two pumping chambers, called ventricles. There are three kinds of vessels: arteries, which carry blood away from the heart; capillaries, which exchange materials with tissue fluid; and veins, which return blood to the heart. Blood is always contained within these vessels and never runs freely into the body unless an injury occurs.
As blood passes through capillaries, the pressure of blood forces some water out of the blood and into the tissue fluid. Some of this fluid returns directly to a capillary, and some is picked up by lymphatic capillaries in the vicinity. The fluid, now called lymph, is returned to the cardiovascular system by lymphatic vessels. The function of the lymphatic system is discussed in Section 23.2.
Comparison of Circulatory Pathways
Two different types of circulatory pathways are seen among vertebrate animals. In fishes, blood follows a one-circuit (single-loop) pathway through the body. The heart has a single atrium and a single ventricle (Fig. 23.3a). The pumping action of the ventricle sends blood under pressure to the gills, where gas exchange occurs. After passing through the gills, blood is under reduced pressure and flow. However, this single circulatory loop has advantages in that the gill capillaries receive O2-poor blood and the systemic capillaries receive O2-rich blood.
As a result of evolutionary changes, the other vertebrates have a two-circuit (double-loop) circulatory pathway. The heart pumps blood to the tissues, called a systemic circuit, and also pumps blood to the lungs, called a pulmonary circuit. This double pumping action is seen in terrestrial animals that utilize lungs to breathe air.
In amphibians, the heart has two atria, but only a single ventricle (Fig. 23.3b), and some mixing of O2-rich and O2-poor blood does occur. The same holds true for most reptiles, except that the ventricle has a partial septum. The hearts of crocodiles, which are reptiles, and those of all birds and mammals are divided into right and left halves (Fig. 23.3c). The right ventricle pumps blood to the lungs, and the left ventricle, which is larger than the right ventricle, pumps blood to the rest of the body. This arrangement provides -adequate blood pressure for both the pulmonary and systemic circuits.
23.2 Transport in Humans
In the human cardiovascular system, like that of other vertebrates, the heart pumps blood into blood vessels that take it to capillaries where exchanges take place. In the lungs, carbon dioxide is exchanged for oxygen, and in the tissues, nutrients and oxygen are exchanged for carbon dioxide and other wastes. These exchanges in the lungs and tissues are so important that if the heart stops pumping, death results.
The Human Heart
In humans, the heart is a double pump: The right side of the heart pumps O2-poor blood to the lungs, and the left side of the heart pumps O2-rich blood to the tissues (Fig. 23.4). (The “right side of the heart” refers to how the heart is positioned in your body and not to the right side of the diagram.) The heart can be a double pump because a septum separates the right from the left side. Further, the septum is complete and prevents O2-poor blood from mixing with O2-rich blood.
Each side of the heart has two chambers. The upper, thin-walled chambers are called atria (sing., atrium), and they receive blood. The lower chambers are the thick-walled ventricles, which pump the blood away from the heart.
Valves occur between the atria and the ventricles, and between the ventricles and attached vessels. Because these valves close after the blood moves through, they keep the blood moving in the correct direction. The valves between the atria and ventricles are called the atrioventricular valves, and the valves between the ventricles and their attached vessels are called semilunar valves because their cusps look like half moons.
The right atrium receives blood from attached veins (the venae cavae) that are returning O2-poor blood to the heart from the tissues. After the blood passes through an atrioventricular valve (also called the tricuspid valve), the right ventricle pumps it through the pulmonary semilunar valve into the pulmonary trunk and arteries that take it to the lungs. The pulmonary veins bring O2-rich blood to the left atrium. After this blood passes through an atrioventricular valve (also called the bicuspid valve), the left ventricle pumps it through the aortic semilunar valve into the aorta, which takes it to the tissues.
Like mechanical valves, the heart valves are sometimes leaky; they may not close properly, and there is a backflow of blood. A heart murmur is often due to leaky atrioventricular valves, which allow blood to pass back into the atria after they have closed. Rheumatic fever is a bacterial infection that begins in the throat and spreads throughout the body. The bacteria attack various organs, including the heart valves. When damage is severe, the valve can be replaced with a synthetic valve or one taken from a pig’s heart.
Another observation is in order. Some people associate O2-poor blood with all veins and O2-rich blood with all arteries, but this idea is incorrect: Pulmonary arteries and pulmonary veins are just the reverse. That’s why the pulmonary arteries are colored blue and the pulmonary veins are colored red in Figure 23.4. The correct definitions are that an artery is a vessel that takes blood away from the heart, and a vein is a vessel that takes blood to the heart.
The Heartbeat
The heart’s pumping action, known as the heartbeat or cardiac cycle, consists of a series of events: First the atria contract, then the ventricles contract, and then they both rest. Figure 23.5 lists and depicts the events of a heartbeat, using the term systole to mean contraction and diastole to mean relaxation. When the heart beats, the familiar lub-dub sound is caused by the closing of the heart valves. The longer and lower-pitched lub occurs when the atrioventricular valves close, and the shorter and sharper dub is heard when the semilunar valves close. The pulse is a wave effect that passes down the walls of the arterial blood vessels when the aorta expands and then recoils following the ventricular systole. Because there is one pulse per ventricular systole, the pulse rate can be used to determine the heart rate. The heart beats about 70 times per minute, although a normal adult heart rate can vary from 60 to 100 beats per minute.
The beat of the heart is regular because it has an intrinsic pacemaker, called the SA (sinoatrial) node. The nodal tissue of the heart, located in two regions of the atrial wall, is a unique type of cardiac muscle tissue. Every 0.85 seconds, the SA node automatically sends out an excitation impulse that causes the atria to contract (Fig. 23.6a). When this impulse is picked up by the AV (atrioventricular) node, it passes to fibers that cause the ventricles to contract. 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 (40–60 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 seconds.
Although the beat of the heart is intrinsic, it is regulated by the nervous system and various hormones. Activities such as yoga and meditation lead to activation of the vagus nerve, which slows the heart rate. Exercise or anxiety leads to release of the hormones norepinephrine and epinephrine, which cause it to speed up.
An electrocardiogram (ECG) is a recording of the electrical changes that occur in the wall of the heart during a cardiac -cycle. Body fluids contain ions that conduct electrical currents, and therefore the electrical changes in heart muscle can be detected on the skin’s surface. When an electrocardiogram is being taken, electrodes placed on the skin are connected by wires to an instrument that detects these electrical changes (Fig. 23.6b). Various types of abnormalities can be detected by an electrocardiogram. One of these, called ventricular fibrillation, is due to uncoordinated contraction of the ventricles (Fig. 23.6c). Ventricular fibrillation is of special interest because it can be caused by an injury or a drug overdose. It is the most common cause of sudden cardiac death in a seemingly healthy person. Once the ventricles are fibrillating, they must be defibrillated by applying a strong electrical current for a short period of time. Then the SA node may be able to reestablish a coordinated beat.
Blood Vessels
Arteries transport blood away from the heart. When the heart contracts, blood is sent under pressure into the arteries; thus, blood pressure accounts for the flow of blood in the arteries. Arteries have a much thicker wall than veins because of a well-developed middle layer composed of smooth muscle and elastic fibers. The elastic fibers allow arteries to expand and accommodate the sudden increase in blood volume that results after each heartbeat. The smooth muscle strengthens the wall and prevents overexpansion (Fig. 23.7a).
Arteries branch into arterioles. Arterioles are small arteries just visible to the naked eye, and their diameter can be regulated by the nervous system, depending on the needs of the body. When arterioles are dilated, more blood flows through them, and when they are constricted, less blood flows. The constriction of arterioles can also raise blood pressure.
Arterioles branch into capillaries, which are extremely narrow, microscopic tubes with a wall composed of only epithelium, often called endothelium because it’s inside the other layers (Fig. 23.7b). Capillaries, which are usually so narrow that red blood cells pass through in single file, allow exchange of nutrient and waste molecules across their thin walls. Capillary beds (many capillaries interconnected) are so prevalent that in humans, all cells are within 60–80 micrometers (m) of a capillary. The capillaries are so extensive that blood pressure drops and blood flows only slowly along. The entrance to a capillary bed is controlled by bands of muscle called precapillary sphincters. During muscular exercise, the sphincters relax, and the capillary beds of the muscles are open. Also, after an animal has eaten, the capillary beds in the digestive tract are open.
Venules and veins collect blood from the capillary beds and return it to the heart. First the venules drain the blood from the capillaries, and then they join to form a vein. Blood pressure is much reduced by the time blood reaches the veins. The walls of veins are much thinner and their diameter wider than those of arteries (Fig. 23.7c). This allows skeletal muscle contraction to push on the veins, forcing the blood past a valve (Fig. 23.8). Valves within the veins point, or open, toward the heart, preventing a backflow of blood when they close. When inhalation occurs, the thoracic pressure falls and abdominal pressure rises as the chest expands. This action also aids the flow of venous blood back to the heart because blood flows in the direction of reduced pressure.
The Pulmonary and Systemic Circuits
The human cardiovascular system includes two major circular pathways, the pulmonary circuit and the systemic circuit. The pulmonary circuit moves blood to and from the lungs, where O2-poor becomes O2-rich blood. The systemic circuit moves blood to and from the body proper. The function of the systemic circuit is to serve the needs of the body’s cells. Figure 23.9 traces the path of blood in both circuits.
Pulmonary CircuitO2-poor blood from all regions of the body collects in the right atrium and then passes into the right ventricle (RV). The pulmonary circuit begins when the RV pumps blood to the lungs via the pulmonary trunk and the pulmonary arteries. As blood passes through pulmonary capillaries, carbon dioxide is given off and oxygen is picked up. O2-rich blood returns to the heart via the pulmonary veins. Pulmonary veins enter the left atrium.
Systemic CircuitO2-rich blood enters the left atrium from the lungs and passes into the left ventricle (LV). The systemic circuit begins when the LV pumps the blood into the aorta. Arteries branching from the aorta carry blood to all areas and organs of the body, where it passes through capillaries and collects in veins. Veins converge on the venae cavae, which return the O2-poor blood to the right atrium. In the systemic circuit, -arteries contain O2-rich blood and have a bright red color, and veins contain O2-poor blood and appear dull red or, when viewed through the skin, blue.
A portal system begins and ends in capillaries. For example, the hepatic portal takes blood from the intestines to the liver. The liver, an organ of homeostasis, modifies substances absorbed by the intestines and monitors the normal composition of the blood. The hepatic vein leaves the liver and enters the inferior vena cava.
Lymphatic System
The lymphatic system consists of lymphatic vessels and various lymphatic organs. Just now we are interested in the lymphatic vessels that take up excess tissue fluid and return it to cardiovascular veins in the shoulders, namely the subclavian veins (Fig. 23.10). The lymphatic vessels also take up fat in the form of lipoproteins at the digestive tract and transport it to these veins. As you will see in Chapter 26, the lymphatic system works with the immune system to help defend the body against disease.
Lymphatic vessels are quite extensive; most regions of the body are richly supplied with lymphatic capillaries. The construction of the larger lymphatic vessels is similar to that of cardiovascular veins, including the presence of valves. Also, the movement of lymph within these vessels is dependent upon skeletal muscle contraction. When the muscles contract, the lymph is squeezed past a valve that closes, preventing the lymph from flowing backward.
The lymphatic system is a one-way system that -begins at the lymphatic capillaries. These capillaries take up fluid that has diffused from and not been reabsorbed by the blood capillaries. Once tissue fluid enters the lymphatic vessels, it is called lymph. The lymphatic capillaries join to form larger lymphatic vessels that merge before entering one of two ducts: the thoracic duct or the right lymphatic duct. The thoracic duct is much larger than the right lymphatic duct. It serves the lower limbs, the abdomen, the left arm, and the left side of both the head and the neck. The right lymphatic duct serves the right arm, the right side of both the head and the neck, and the right thoracic area. The lymphatic ducts enter the subclavian veins.
Cardiovascular Disorders
Cardiovascular disease is the leading cause of -untimely death in Western countries. In the United States, it is estimated that about 20% of the population suffers from hypertension, which is high blood pressure. Normal blood pressure is 120/80 mm Hg. The top number is called systolic blood pressure because it is due to the contraction of the ventricles, and the bottom number is called diastolic blood pressure because it is due to the resting of the ventricles. Hypertension occurs when blood pressure readings are higher than these numbers—say, 160/100. Hypertension is sometimes called a silent killer -because it may not be detected until a stroke or heart attack occurs.
Inheritance and lifestyle contribute to hypertension. For example, hypertension is often seen in individuals who have atherosclerosis (formerly called arteriosclerosis), an accumulation of soft masses of fatty materials, particularly cholesterol, -beneath the inner linings of arteries (Fig. 23.11). Such -deposits, called plaque, tend to protrude into the lumen of the vessel, interfering with the flow of blood. 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 its onset and development, the American Heart Association and other organizations recommend a diet low in saturated fat and cholesterol and rich in fruits and vegetables. Smoking, drug or alcohol abuse, obesity, and lack of exercise contribute to the risk of atherosclerosis.
Plaque can cause a clot to form on the irregular arterial wall. As long as the clot remains stationary, it is called a thrombus, but when and if it dislodges and moves along with the blood, it is called an embolus. If thromboembolism is not treated, serious health problems can result. A cardiovascular accident, also called a stroke, often occurs 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. If a coronary artery becomes completely blocked due to thromboembolism, a heart attack can result.
The coronary arteries bring O2-rich blood from the aorta to capillaries in the wall of the heart, and the cardiac veins return O2-poor blood from the capillaries to the right ventricle. If the coronary arteries narrow due to cardiovascular disease, the individual may first suffer from angina pectoris, chest pain that is often accompanied by a radiating pain in the left arm. When a coronary artery is completely blocked, a portion of the heart muscle dies due to a lack of oxygen. This is known as a heart attack. Two surgical procedures are frequently performed to correct a blockage or facilitate blood flow. In a coronary bypass operation, a portion of a blood vessel from another part of the body is sutured from the aorta to the coronary artery, past the point of obstruction (Fig. 23.12a). Now blood flows normally again from the aorta to the wall of the heart. In balloon angioplasty, a plastic tube is threaded through an artery to the blockage, and a balloon attached to the end of the tube is inflated to break through the blockage. A stent is often used to keep the vessel open (Fig. 23.12b).
23.3 Blood: A Transport Medium
The blood of mammals helps maintain homeostasis. Blood’s numerous functions include the following:
1. Transports substances to and from the capillaries, where exchanges with tissue fluid take place.
2. Helps defend the body against invasion by pathogens (e.g., disease-causing viruses and bacteria).
3. Helps regulate body temperature.
4. Forms clots, preventing a potentially life-threatening loss of blood.
In humans, blood has two main portions: the liquid portion, called plasma, and the formed elements, consisting of various cells and platelets (Fig. 23.13).
Plasma
Plasma is composed mostly of water (90–92%) and proteins (7–8%), but it also contains smaller quantities of many types of molecules, including nutrients, wastes, and salts. The salts and proteins are involved in buffering the blood, effectively keeping the pH near 7.4. They also maintain the blood’s -osmotic pressure so that water has an automatic tendency to enter blood capillaries. Several plasma proteins are involved in blood clotting, and others transport large organic molecules in the blood. Albumin, the most plentiful of the plasma proteins, transports bilirubin, a breakdown product of -hemoglobin. Lipoproteins transport cholesterol.
Formed Elements
The formed elements are red blood cells, white blood cells, and platelets. Among the formed elements, red blood cells, also called erythrocytes, transport oxygen. Red blood cells are small, biconcave disks that at maturity lack a nucleus and contain the respiratory pigment hemoglobin (Fig. 23.14). There are 6 million red blood cells per mm3 of whole blood, and each one of these cells contains about 250 million hemoglobin molecules. Hemoglobin contains iron, which combines loosely with oxygen; in this way, red blood cells transport oxygen. If the number of red blood cells is insufficient, or if the cells do not have enough hemoglobin, the individual suffers from anemia and has a tired, run-down feeling.
Red blood cells are manufactured continuously within certain bones, namely the skull, the ribs, the vertebrae, and the ends of the long bones. The hormone erythropoietin stimulates the production of red blood cells. The kidneys produce erythropoietin when they act on a precursor made by the liver. Now available as a drug, erythropoietin is helpful to persons with anemia and has also been abused by athletes to enhance their performance.
Before they are released from the bone marrow into the blood, red blood cells lose their nuclei and begin to synthesize -hemoglobin. After living about 120 days, they are destroyed chiefly in the liver and the spleen, where they are engulfed by large phagocytic cells. When red blood cells are -destroyed, hemoglobin is released. The iron is recovered and returned to the red bone marrow for reuse. Other portions of the molecules (i.e., heme) undergo chemical degradation and are excreted by the liver as bile pigments in the bile. The bile pigments are primarily responsible for the color of feces.
White blood cells, also called leukocytes, help fight infections. White blood cells differ from red blood cells in that they are usually larger and have a nucleus, they lack hemoglobin, and without staining, they appear translucent. Figure 23.15 shows the appearance of the various types of white blood cells. With staining, white blood cells appear light blue unless they have granules that bind with certain stains. The white blood cells that have granules also have a lobed nucleus. The agranular leukocytes have no granules and a spherical or indented nucleus. There are approximately 5,000–11,000 white blood cells per mm3 of blood. Growth factors are available to increase the production of all white blood cells, and these are helpful to people with low immunity, such as AIDS patients.
Red blood cells are confined to the blood, but white blood cells are able to squeeze between the cells of a capillary wall. Therefore, they are found in tissue fluid and lymph and in lymphatic organs. When an infection is present, white blood cells greatly increase in number. Many white blood cells live only a few days—they probably die while engaging pathogens. Others live months or even years.
When microorganisms enter the body due to an injury, the response is called an inflammatory response because swelling, reddening, heat, and pain occur at the injured site. Damaged tissue releases kinins, which dilate capillaries, and histamines, which increase capillary permeability. White blood cells called neutrophils, which are amoeboid, squeeze through the capillary wall and enter the tissue fluid, where they phagocytize foreign material. White blood cells called monocytes appear and are transformed into macrophages, large phagocytizing cells that release white blood cell growth -factors. Soon the number of white blood cells increases explosively. A thick, yellowish fluid called pus contains a large proportion of dead white blood cells that have fought the infection.
Lymphocytes, another type of white blood cell, also play an important role in fighting infection. Certain lymphocytes called T cells attack infected cells that contain viruses. Other lymphocytes, called B cells, produce antibodies. Each B cell produces just one type of antibody, which is specific for one type of antigen. An antigen, which is most often a protein but sometimes a polysaccharide, causes the body to produce an antibody because the antigen doesn’t belong to the body. Antigens are present in the outer covering of parasites or in their toxins. When antibodies combine with antigens, the complex is often phagocytized by a macrophage. An individual is actively immune when a large number of B cells are all producing the antibody needed for a particular infection.
Platelets and Blood Clotting
Platelets (also called thrombocytes) result from fragmentation of certain large cells, called megakaryocytes, in the red bone marrow. Platelets are produced at a rate of 200 -billion a day, and the blood contains 150,000–300,000 per mm3. These formed -elements are involved in blood clotting, or coagulation.
Blood contains at least 12 clotting factors that participate in the formation of a blood clot. Hemophilia is an inherited clotting disorder in which the liver is unable to produce one of the clotting factors. The slightest bump can cause the affected person to bleed into the joints, and this leads to degeneration of the joints. Bleeding into muscles can lead to nerve damage and muscular atrophy. The most frequent cause of death due to hemophilia is bleeding into the brain.
Prothrombin and fibrinogen, two proteins involved in blood clotting, are manufactured and deposited in blood by the liver. Vitamin K, found in green vegetables and also formed by intestinal bacteria, is necessary for the production of prothrombin, and if by chance this vitamin is missing from the diet, hemorrhagic disorders can develop.
A series of reactions leads to the formation of a blood clot (Fig. 23.16a–c). When a blood vessel in the body is damaged, platelets clump at the site of the puncture and form a plug that temporarily seals the leak. Platelets and the injured tissues release a clotting factor called prothrombin activator that converts prothrombin to thrombin. This reaction requires calcium ions (Ca21). Thrombin, in turn, acts as an enzyme that severs two short amino acid chains from each fibrinogen molecule. These activated fragments then join end to end, forming long threads of fibrin. Fibrin threads wind around the platelet plug in the damaged area of the blood vessel and provide the framework for the clot. Red blood cells also are trapped within the fibrin threads; these cells make a clot appear red (Fig. 23.16d). Clot retraction follows, during which the clot gets smaller as platelets contract. A fluid called serum is squeezed from the clot. A fibrin clot is present only temporarily. As soon as blood vessel repair is initiated, an enzyme called plasmin destroys the fibrin network and restores the fluidity of plasma.
Capillary Exchange in the Tissues
In Figure 23.1, we noted that in some animals, exchanges are carried out by each cell individually because there is no cardiovascular system. When an animal does have a cardiovascular system, tissue fluid (the fluid between the cells) makes exchanges with blood within a capillary. Notice in Figure 23.17 that amino acids, oxygen, and glucose exit a capillary and enter tissue fluid to be used by cells. On the other hand, carbon dioxide and wastes exit tissue fluid and enter a capillary to be taken away and excreted from the body. A chief purpose of the cardiovascular system is to take blood to the capillaries where exchange occurs. Without this exchange, homeostasis is not maintained, and the cells of the body perish.
Figure 23.17 illustrates certain mechanics of capillary exchange. Blood pressure and osmotic pressure are two opposing forces at work along the length of a capillary. Blood pressure is caused by the beating of the heart, while osmotic pressure is due to the salt and protein content of the blood. Blood pressure holds sway at the arterial end of a capillary and water exits. Blood pressure is reduced by the time blood reaches the venous end of a capillary, and osmotic pressure now causes water to enter. Midway between the arterial and venous ends of a capillary, blood pressure pretty much equals osmotic pressure, and passive diffusion alone causes nutrients to exit and wastes to enter. This works because tissue fluid always has the lesser amount of nutrients and the greater amount of wastes. After all, cells use nutrients and thereby create wastes.
The exchange of water at a capillary is not exact, and the result is always excess tissue fluid. Excess tissue fluid is collected by lymphatic capillaries, and in this way it becomes lymph (Fig. 23.18). Lymph contains all the components of plasma except much lesser amounts of protein. Lymph is returned to the cardiovascular system when the major lymphatic vessels enter the subclavian veins in the shoulder region.
In addition to nutrients and wastes, blood distributes heat to body parts. When you are warm, many capillaries that serve the skin are open, and your face is flushed. This helps rid the body of excess heat. When you are cold, skin capillaries close, conserving heat.
The THE Chapter in Review
Summary
23.1 Open and Closed Circulatory Systems
• Some invertebrates do not have a transport system because their body plan allows each cell to exchange molecules with the external environment.
• Other invertebrates do have a transport system. Some have an open circulatory system, and some have a closed circulatory system.
• All vertebrates have a closed circulatory system in which arteries carry blood away from the heart to capillaries, where exchange takes place, and veins carry blood to the heart.
Comparison of Vertebrate Circulatory Pathways
• Fishes have a one-circuit pathway because the heart, with a single atrium and ventricle, pumps blood only to the gills.
• Other vertebrates have both pulmonary and systemic circuits. Amphibians have two atria but a single ventricle. Birds and mammals, including humans, have a heart with two atria and two ventricles, in which O2-rich blood is always separate from O2-poor blood.
23.2 Transport in Humans
The cardiovascular system consists of the heart and the blood vessels.
The Human Heart
The heart has a right and a left side. Each side has an atrium and a ventricle. Valves keep the blood moving in the correct direction.
Right Side of the HeartThe atrium receives O2-poor blood from the tissues, and the ventricle pumps it to the lungs.
Left Side of the HeartThe atrium receives O2-rich blood from the lungs, and the ventricle pumps it to the tissues.
HeartbeatDuring a heartbeat, first the atria contract and then the ventricles contract. The heart sounds, lub-dub, are caused by the closing of valves.
SA NodeThe SA node (pacemaker) causes the two atria to contract. The SA node also stimulates the AV node.
AV NodeThe AV node causes the two ventricles to contract.
Transport Through Blood VesselsArteries, with thick walls, take blood away from the heart to arterioles, which take blood to capillaries that have walls composed only of epithelial cells. Venules take blood from capillaries and merge to form veins, which have thinner walls than arteries have.
• Blood pressure, created by the beat of the heart, accounts for the flow of blood in the arteries.
• Skeletal muscle contraction is largely responsible for the flow of blood in the veins, which have valves preventing backward flow.
Blood Has Two Circuits
• In the pulmonary circuit, blood travels to and from the lungs.
• In the systemic circuit, the aorta divides into blood vessels that serve the body’s cells. Venae cavae return O2-poor blood to the heart.
Cardiovascular DisordersHypertension and atherosclerosis are two conditions that lead to heart attack and stroke. Following a heart-healthy diet, getting regular exercise, maintaining a proper weight, and not smoking cigarettes are protective against these conditions.
The Lymphatic System
The lymphatic system is a one-way system that consists of lymphatic vessels and lymphatic organs. The lymphatic vessels receive fat at the digestive tract and excess tissue fluid at blood capillaries, and carry these to the subclavian veins (cardiovascular veins in the shoulders).
23.3 Blood: A Transport Medium
Blood has two main parts: plasma and formed elements.
Plasma
Plasma contains mostly water (90–92%) and proteins (7–8%), but also nutrients, wastes, and salts.
• The proteins and salts buffer the blood and maintain its osmotic pressure.
• The proteins also have specific functions, such as participating in blood clotting and transporting molecules.
Formed Elements
The red blood cells contain hemoglobin and function in oxygen transport. Defense against disease depends on the various types of white blood cells:
• Neutrophils and monocytes are phagocytic and are especially responsible for the inflammatory response.
• Lymphocytes
are involved in the development of specific immunity
to
disease.
The
platelets and two plasma proteins, prothrombin and fibrinogen,
function in
blood
clotting, an enzymatic process that results in
fibrin
threads.
Capillary Exchange in the Tissues
Capillary exchange in the tissues helps keep the internal environment constant. When blood reaches a capillary,
• Water moves out at the arterial end due to blood pressure.
• Water moves in at the venous end due to osmotic pressure.
• Nutrients diffuse out of and wastes diffuse into the capillary between the arterial end and the venous end.
• Lymphatic capillaries in the vicinity pick up excess tissue fluid and return it to cardiovascular veins.
Thinking Scientifically
1. Explain why the evolution of the four-chambered heart was critical for the development of an endothermic lifestyle—the generation of internal heat—in birds and mammals.
2. Provide a physiological explanation for the benefit gained by athletes who train at high altitudes.
Testing Yourself
Choose the best answer for each question.
1. In insects with an open circulatory system, oxygen is taken to cells by
a. blood.
b. hemolymph.
c. tracheae.
d. capillaries.
2. Label the components of the cardiovascular system in the following diagram.
3. Which of the following statements is true?
a. Arteries carry blood away from the heart, and veins carry blood to the heart.
b. Arteries carry blood to the heart, and veins carry blood away from the heart.
c. Arteries carry O2-rich blood, and veins carry O2-poor blood.
d. Arteries carry O2-poor blood, and veins carry O2-rich blood.
4. Label the following diagram of the heart.
5. In humans, blood returning to the heart from the lungs returns to
a. the right ventricle.
b. the right atrium.
c. the left ventricle.
d. the left atrium
e. both the right and left sides of the heart.
6. An artificial pacemaker replaces the effect of the
a. vagus nerve.
b. SA (sinoatrial) node.
c. AV (atrioventricular) node.
d. ventricular systole.
7. Which of the following lists the events of the cardiac cycle in the correct order?
a. contraction of atria, rest, contraction of ventricles
b. contraction of ventricles, rest, contraction of atria
c. contraction of atria, contraction of ventricles, rest
d. contraction of ventricles, contraction of atria, rest
8. The “lub,” the first heart sound, is produced by the closing of
a. the aortic semilunar valve.
b. the pulmonary semilunar valve.
c. the atrioventricular (tricuspid) valve.
d. the atrioventricular (bicuspid) valve.
e. both atrioventricular valves.
9. An electrocardiogram measures
a. chemical signals in the brain and heart.
b. electrical activity in the brain and heart.
c. chemical signals in the heart.
d. electrical changes in the wall of the heart.
10. Place the following blood vessels in order, from largest to smallest in diameter.
a. arterioles, capillaries, arteries
b. arteries, arterioles, capillaries
c. capillaries, arteries, arterioles
d. arterioles, arteries, capillaries
e. arteries, capillaries, arterioles
11. The lymphatic system
a. returns excess tissue fluid to cardiovascular veins.
b. is found in limited regions of the body.
c. relies most extensively on diffusion to move lymph.
d. is a two-way system.
e. More than one of these are correct.
For questions 12–15, identify the cardiovascular disorder in the key that matches the description. Each answer may be used more than once. Each question may have more than one answer.
Key:
a. hypertension
b. atherosclerosis
c. stroke
d. heart attack
12. May not be detected until after a stroke or heart attack.
13. Sometimes called a silent killer.
14. Results from the accumulation of plaque.
15. May be caused by an embolism.
16. Exchange of the gases oxygen and carbon dioxide occurs across the _______________ of the _______________.
a. veins, lungs
b. capillaries, tissues
c. arteries, tissues
d. All of these are correct.
17. The average heart rate is about _______________ beats per minute.
a. 100
b. 45
c. 60
d. 70
18. If the SA node fails and the AV node takes over, the result will be a heart rate that is
a. slower.
b. faster.
c. the same.
d. None of these are correct.
19. Which association is incorrect?
a. white blood cells—infection fighting
b. red blood cells—blood clotting
c. plasma—water, nutrients, and wastes
d. red blood cells—hemoglobin
e. platelets—blood clotting
20. In the tissues, nutrients and _______________ are exchanged for _______________ and other wastes.
a. blood, oxygen
b. oxygen, carbon dioxide
c. hemoglobin, tissue fluid
d. None of these are correct.
21. Lymph is formed from
a. urine.
b. fats.
c. excess tissue fluid.
d. All of these are correct.
22. A decrease in lymphocytes would result in problems associated with
a. clotting.
b. immunity.
c. oxygen transportation.
d. All of these are correct.
23. The best explanation for the slow movement of blood in capillaries is
a. skeletal muscles press on veins, not capillaries.
b. capillaries have much thinner walls than arteries.
c. there are many more capillaries than arterioles.
d. venules are not prepared to receive so much blood from the capillaries.
e. All of these are correct.
24. Which of the following assist in the return of venous blood to the heart?
a. valves
b. skeletal muscle contractions
c. respiratory movements
d. blood flow in the direction of reduced pressure
e. All of these are correct.
25. Water enters the venous end of capillaries because
a. osmotic pressure is higher than blood pressure.
b. of an osmotic pressure gradient.
c. of higher blood pressure on the venous side.
d. of higher blood pressure on the arterial side.
e. of higher red blood cell concentration on the venous side.
26. One-way conduction in lymphatic vessels is aided by
a. valves.
b. skeletal muscle contractions.
c. Both a and b are correct.
d. None of these are correct.
27. Which of the following is not a function of the lymphatic system?
a. produces blood cells
b. returns excess tissue fluid to the blood
c. transports lipids absorbed from the digestive system
d. defends the body against disease
28. Red blood cells
a. reproduce themselves by mitosis.
b. live for several years.
c. continually synthesize hemoglobin.
d. are destroyed in the liver and spleen.
e. More than one of these are correct.
29. Which of the following is not a formed element of blood?
a. white blood cells
b. red blood cells
c. fibrinogen
d. platelets
30. The last step in blood clotting
a. is the only step that requires calcium ions.
b. occurs outside the bloodstream.
c. is the same as the first step.
d. converts prothrombin to thrombin.
e. converts fibrinogen to fibrin.
31. Hemorrhagic disorders can result when which vitamin is missing from the diet?
a. A d. D
b. C e. thiamine
c. K
32. Water
a. exits the bloodstream at the arterial and venous ends of a capillary.
b. enters the bloodstream at the arterial and venous ends of a capillary.
c. enters the bloodstream at the arterial end and exits cells at the venous end of a capillary.
d. exits the bloodstream at the arterial end and enters cells at the venous end of a capillary.
33. In the following diagram, label arrows a.–d. as either blood pressure or osmotic pressure.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
In the highly competitive world of professional athletics, individuals are constantly striving to gain an edge over the competition. With six-figure salaries, international recognition, and multimillion-dollar endorsements as their incentives, some athletes resort to life-threatening measures to enhance their performance.
In recent years, various means of increasing the red blood cell count in athletes have been employed. By increasing the number of red blood cells in the body, more oxygen can be delivered to the muscles. This enhances athletic ability, especially in events requiring a high level of endurance.
In the past, blood doping was commonly used. Red blood cells (RBCs) were removed from the athlete’s body and stored for several weeks. During this time, the body would release increased levels of the hormone erythropoietin to stimulate RBC production. When the RBC count returned to normal, the blood cells previously removed were injected back into the body. This was done a few days before the competition to get maximum effect. In more recent times, injections of artificially produced erythropoietin are being used to stimulate RBC production far above normal levels.
The artificial form of erythropoietin is known as epoetin alfa and is structurally identical to the naturally occurring form. Therefore, it is very difficult to determine when it is being used.
However, many athletes have paid a high price for the competitive advantage offered by epoetin alfa. The use of this hormone causes the blood to become thicker than normal. Hypertension and an increased risk of heart attack and stroke result. In recent decades, the suspected use of epoetin alfa has been linked to the deaths of numerous competitive cyclists.
Understanding the Terms
antibody406
antigen406
aorta399, 402
arteriole401
artery399, 401
atrioventricular valve399
atrium (pl., atria)399
AV (atrioventricular) node400
blood405
blood pressure401
capillary401
cardiac cycle400
cardiovascular system397
closed circulatory system397
diastole400
electrocardiogram (ECG)400
formed elements405
heart396
heart attack404
heart murmur399
hemoglobin405
hypertension403
lymph403
lymphatic system403
lymphatic vessel403
lymphocyte406
macrophage406
monocyte406
neutrophil406
open circulatory system396
plaque404
plasma405
platelet406
portal system403
pulmonary artery399
pulmonary circuit398, 402
pulmonary trunk399
pulmonary vein399
pulse400
red blood cell405
SA (sinoatrial) node400
semilunar valve399
septum399
stroke404
systemic circuit398, 402
systole400
thrombin407
vein399, 401
vena
cava (pl., venae
cavae)399,
402
ventricle399
venule401
white blood cell406
Match the terms to these definitions:
a. _______________ Circuit that takes blood to and from the tissues.
b. _______________ Upper chambers of the heart.
c. _______________ Condition generally caused by leaky heart valves.
d. _______________ Heart’s intrinsic natural pacemaker.
e. _______________ Smallest of the blood vessels.
f. _______________ Major vessels in the systemic circuit.
g. _______________ Tissue fluid that has entered lymphatic vessels.
h. _______________ Liquid portion of blood.
i. _______________ Phagocytic white blood cells derived from monocytes.
j. _______________ Formed elements involved in blood clotting.
An artificial pacemaker emits electrical signals that keep the heart contracting.
When we exercise, a red blood cell can move through the body in 20 seconds.
Giving blood, which carries oxygen, can save a life.
Figure 23.2Open versus closed circulatory systems.
a. The grasshopper has an open circulatory system. Hemolymph freely bathes the internal organs. The heart, a pump, keeps the hemolymph moving, but an open system probably could not rapidly supply oxygen to wing muscles. These muscles receive oxygen directly from tracheae (air tubes). b. Vertebrates and some invertebrates have a closed circulatory system. The heart pumps blood into the arteries, which take blood away from the heart to the capillaries where exchange takes place. Veins then return blood to the heart.
Figure 23.1No circulatory systems.
In hydras and planarians, each cell makes exchanges directly with the fluid in the gastrovascular cavity or the external environment. Therefore, there is no need for a circulatory system.
Figure 23.4Heart anatomy and path of blood through the heart.
The right side of the heart receives and pumps O2-poor blood to the lungs, and therefore is colored blue. The left side of the heart receives and pumps O2-rich blood to tissues.
Check Your Progress
1. Compare and contrast an open circulatory system with a closed circulatory system.
2. List and describe the functions of the three types of vessels in a cardiovascular system.
3. Contrast a one-circuit circulatory pathway with a two-circuit pathway.
Answers:1. Both use a heart to pump fluid. An open system pumps hemolymph through channels and cavities. The hemolymph eventually drains back to the heart. A closed system pumps blood through vessels that carry blood both away from and back to the heart.2. Arteries carry blood away from the heart, capillaries exchange their contents with tissue fluid, and veins return blood back to the heart.3. The one-circuit pathway utilizes a heart with one atrium and one ventricle to send blood to the gill capillaries and then to the systemic capillaries in a single loop. The two-circuit pathway pumps blood to both the pulmonary and systemic capillaries simultaneously.
Check Your Progress
1. Contrast atria with ventricles.
2.
Trace
the path of blood through the heart from the venae cavae to the
aorta.
3. What is the purpose of the heart valves?
Answers:1. Atria are located in the upper part of the heart, are thin-walled, and receive blood. Ventricles, located in the lower part of the heart, are thick-walled, and pump blood.2. From the body: venae cavae, right atrium, tricuspid valve, right ventricle, pulmonary semilunar valve, pulmonary trunk and arteries. From the lungs: pulmonary veins, left atrium, bicuspid valve, left ventricle, aortic semilunar valve, aorta. 3. Heart valves prevent backward flow, and therefore keep the blood moving in the right direction.
Figure 23.7Blood vessels.
a.
Arteries have well-developed walls with a thick middle layer of
elastic fibers and smooth muscle.
b.
Capillary
walls are composed of an epithelium only one cell thick. c.
Veins have flabby walls, particularly because the middle layer is not
as thick as in arteries. Veins have valves, which point toward the
heart.
Figure 23.8Movement of blood in a vein.
Pressure on the walls of a vein, exerted by skeletal muscles, increases blood pressure within the vein and forces a valve open. Closure of the valves prevents the blood from flowing in the opposite direction.
Figure 23.5The heartbeat.
A heartbeat is a cycle of events. Phase 1: The atria contract and pass blood to the ventricles. Phase 2: The ventricles contract and blood moves into the attached arteries. Phase 3: Both the atria and ventricles relax while the heart fills with blood.
Figure 23.6Control of the heartbeat.
a.
The beat of the heart occurs regularly because the SA node (called
the pacemaker) automatically sends out an impulse that causes the
atria (RA, LA) to contract and is picked up by the AV node.
Thereafter, the ventricles (RV, LV) contract. b.
An electrocardiogram records the electrical changes that occur as the
heart beats. The large spike is associated with ventricular
activation.
c.
Ventricular fibrillation produces an irregular ECG.
Check Your Progress
1.
Explain
what happens during a heartbeat and what makes the familiar
lub-dub
sounds.
2. Outline the flow of blood through blood vessels.
Answers:1. First the atria contract, then the ventricles contract, and then they both rest. The lub sound occurs when the atrioventricular valves close, and the dub sound occurs when the semilunar valves close.2. Arteries carry blood away from the heart to small arteries called arterioles. The arterioles branch to form small capillaries. Venules collect blood from the capillaries and deliver it to veins, which transport it back to the heart.
Figure 23.10Lymphatic vessels.
Lymphatic vessels drain excess fluid from the tissues and return it to the cardiovascular system. The enlargement shows that lymphatic vessels, like cardiovascular veins, have valves to prevent backward flow. Lymph nodes filter lymph and remove impurities.
Figure 23.9Path of blood.
a. Overview of cardiovascular system. When tracing blood from the right to the left side of the heart in the pulmonary circuit, you must include the pulmonary vessels. When tracing blood from the digestive tract to the right atrium in the systemic circuit, you must include the hepatic portal vein, the hepatic vein, and the inferior vena cava. b. To move from an artery to a vein, blood must move through a capillary bed where exchanges occur between blood and tissue fluid. When precapillary sphincters shut down a capillary bed, blood moves through a thoroughfare vessel (called an arteriovenous shunt) from arteriole to venule.
Check Your Progress
1. The blockage of which vessels in the systemic circuit lead to a heart attack? Describe the two surgical procedures used to correct blockage of these arteries.
2. Describe the function of the lymphatic system.
3. What conditions might occur as a result of hypertension and plaque?
Answers:1.
The coronary arteries that serve the heart. In balloon angioplasty, a
balloon is inserted into the artery and inflated to break the
blockage. In a coronary bypass operation, a piece of a blood vessel
from another part of the body is attached to the artery so that blood
flows through the new section rather than through the blocked
area.2. The lymphatic system takes up excess tissue fluid and
returns it to cardiovascular veins.
3.
Thromboembolism, stroke, heart attack.
Figure 23.13Components of blood.
When blood settles in a test tube without clotting, it is apparent that it is composed of plasma and formed elements.
Figure 23.14Micrograph of red blood cells.
Red blood cells contain hemoglobin, a red pigment that accounts for the red color of blood.
Figure 23.11Plaque.
Plaque is an irregular accumulation of cholesterol and fat that closes off blood vessels. Of special concern is the closure of coronary arteries.
Figure 23.16Blood clotting.
a. When a capillary is injured, blood begins leaking out. b. Platelets congregate to form a platelet plug, and this temporarily seals the leak. c. Platelets and damaged tissue cells release an activator that sets in motion a series of reactions that end with (d) a blood clot.
Figure 23.15Formed elements.
White blood cells are quite varied and have different functions that are all associated with defense of the body against infections.
Figure 23.17Capillary exchange.
A capillary, illustrating the exchanges that take place and the forces that aid the process. At the arterial end of a capillary (top), the blood pressure is higher than the osmotic pressure; therefore, water tends to leave the bloodstream. In the midsection, molecules, including oxygen and carbon dioxide, follow their concentration gradients. At the venous end of a capillary (bottom), the osmotic pressure is higher than the blood pressure; therefore, water tends to enter the bloodstream. Notice that the red blood cells and the plasma proteins are too large to exit a capillary.
Figure 23.18Lymphatic capillary bed.
A lymphatic capillary bed lies near a blood capillary bed. The heavy black arrows show the flow of blood. The yellow arrows show that lymph is formed when lymphatic capillaries take up excess tissue fluid.
Check Your Progress
1. List the functions of blood.
2. Contrast red blood cells with white blood cells.
3. Describe how a blood clot forms.
Answers:1.
Blood transports substances to and from the capillaries, defends
against pathogen invasion, helps regulate body temperature, and forms
clots to prevent excessive blood loss.2. Red blood cells are
smaller, lack a nucleus, contain hemoglobin, and are red in color.
White blood cells are larger, have a nucleus, do not contain
hemoglobin, and are translucent in appearance.
3.
Platelets accumulate at the site of injury and release a clotting
factor that results in the synthesis of thrombin. Thrombin
synthesizes fibrin threads that provide a framework for the clot.
Figure 23.3Comparison of circulatory circuits in vertebrates.
a. In a fish, the blood moves in a single circuit. The heart has a single atrium and ventricle, and it pumps the blood into the gill region, where gas exchange takes place. Blood pressure created by the pumping of the heart is dissipated after the blood passes through the gill capillaries. b. Amphibians and most reptiles have a two-circuit system in which the heart pumps blood to both the lungs and the body itself. There is a single ventricle, and some mixing of O2-rich and O2-poor blood takes place. c. The pulmonary and systemic circuits are completely separate in crocodiles (a reptile) and in birds and mammals. The right side pumps blood to the lungs, and the left side pumps blood to the body proper.
Figure 23.12Treatment for clogged coronary arteries.
(left) Many heart procedures, such as coronary bypass and stent insertion, can be performed using robotic surgery techniques. a. During a coronary bypass operation, blood vessels (usually veins from the leg) are stitched to the heart, taking blood past the region of obstruction. b. During stenting, a cylinder of expandable metal mesh is positioned inside the coronary artery by using a catheter. Then, a balloon is inflated so that the stent expands and opens the artery.
grafted vessels
carry arterial
blood