The cardiovascular system includes the blood, heart, and blood vessels. The heart and blood vessels serve to transport the blood throughout the body. The blood, in turn, has the following functions: (1) transport of gases, nutrients, hormones, and wastes; (2) regulation of pH and electrolyte concentration of intercellular fluids; (3) control of heat transport;
(4) removal of pathogens and cell debris; and (5) restriction of fluid loss from injury, by clotting. The blood is the mechanism by which systemic toxins are carried around the body to the tissues they injure. Each of its parts may also be attacked by various toxins.
Blood makes up about 7% of human body weight. In females this averages about 4.5 L of blood; males about 5.5 L. When a tube of blood is centrifuged, the upper 55% of the volume will be a pale yellow liquid called plasma. Plasma is 90% water, with 10% solutes including proteins and electrolytes; nutrients such as glucose, amino acids, and lipids; and waste products such as urea and bilirubin, a waste product that gives plasma, as well as urine and feces, their color. About 96% of the plasma proteins consist of albumins and globulins. These have overlapping functions, including control of osmolarity and the transport of insoluble lipids, vitamins, metals, hormones, and so on. Globulins are an important component of the immune system, discussed in Section 9.7. The rest of the protein is fibrinogen, involved in blood clotting. The dominant electrolytes are sodium and chloride. Intercellular fluid is similar in composition to plasma.
The other 45% of blood volume consists of the blood cells and platelets (Figure 9.7). The great majority of these, about 5,000,000 per cubic millimeter, are the red blood cells, or erythrocytes. These are produced in the bone marrow under stimulation by erythro-poetin from the kidney. When mature, they lose their ribosomes, mitochondria, nucleus, and so on, giving them the appearance of a small (about 7 mm) disk indented in the center. Without their organelles, erythrocytes cannot long maintain their structure. They are replaced continuously, at the rate of a little less than 1% per day. Old red blood cells are destroyed and recycled by the spleen and liver.
The function of erythrocytes is to carry oxygen from the lungs to the other tissues and to carry carbon dioxide on the reverse path. These compounds are carried by the red pigment hemoglobin (Figure 9.8), which constitutes one-third of the mass of the erythrocyte. Hemoglobin is composed of four polypeptide chains complexed with four heme groups, each containing a ferrous iron ion at its center. Note the similarity between
Erythrocytes (red blood cells)
Leukocytes (white blood cells)
Figure 9.7 The formed elements of the blood. (From Van De Graaff and Rhees, 1997. © The McGraw-Hill Companies, Inc. Used with permission.)
the heme molecule and chlorophyll shown in Figure 5.10. Each of the four ferrous ions can complex with one molecule of O2. This gives blood an oxygen-carrying capacity of about 74 mg/L, about 10 times the solubility of oxygen in water at physiological temperature.
Just 0.1% of the blood volume is composed of white blood cells, or leukocytes, and platelets. All are descendants of the same cells in the bone marrow that produce erythro-cytes, although some leukocytes go through additional processing in the thymus and organs of the lymphatic system. There are four types of leukocytes, of which 60 to 70% are neutrophils and 20 to 25% are lymphocytes. Functionally, leukocytes are part of the immune system and are discussed in Section 9.7. Although transported by blood, they usually leave the blood vessels for sites of infection or damage to do their work.
Blood platelets are cytoplasmic cell fragments that are much smaller than erythro-cytes. They are involved in the formation of blood clots to prevent blood loss and promote tissue healing after a physical injury. Such injury disrupts the endothelial lining of the blood vessels, exposing underlying connective tissue. Platelets adhere to collagen, and a plug of platelets is formed rapidly. Simultaneously, a complex series of reactions, some of which involve platelets, results in the conversion of fibrinogen into a form that aggregates to form a mesh of interlocking strands called fibrin that forms the clot. Several factors limit clotting to the area of the wound, including prostaglandins produced by normal tissue. Hemophilia is a gender-linked genetic disease in which the victim cannot produce one of the factors involved in fibrin formation. It has been mentioned that aspirin inhibits prostaglandins. However, at low doses it inhibits thromboxane more. Thus, it has been found that men with no previous history of heart disease who take one aspirin every other day reduce their chance of heart attack (caused by clotting of the coronary arteries) by 50%.
The blood vessels are the tubes that conduct blood around the body. They are lined with a type of epithelial cells, which in this case are called endothelium. Arteries carry blood away from the heart toward the tissues. They contain muscles that help maintain blood pressure. Arteries branch successively into arterioles, which by their contraction and dilation control blood flow to various tissues. These divide further until they form a weblike network of tiny thin-walled vessels called capillaries. These are about 5 to 8 mm in diameter, just wide enough for erythrocytes to pass in single file. Capillaries permeate almost all tissues; few cells are more than 1 mm from a capillary. The capillaries are responsible for the transfer of heat and substances to and from tissues. Veins carry blood back to the heart. (Note that the definition does not depend on whether or not the blood is oxygenated. Specifically, the pulmonary arteries are deoxygenated but carry blood from the heart to the lungs, and the pulmonary veins are oxygenated but carry blood back to the heart.) Veins are less muscular and under much lower pressure than arteries. Thus, movement is aided by contraction of skeletal muscles surrounding them and by the presence of one-way check valves to prevent flow reversal.
The heart is actually two connected pumps. The right side pumps deoxygenated blood through the lungs, the left distributes it through the body. The heart of a resting adult pumps about 5.0 L/min in 72 beats/min, or about 70 mL/beat. When exercising, a non-athlete may pump 20 L/min at 192 beats/min. A well-trained athlete may pump up to 35 L/min. Blood is delivered to the right atrium of the heart by the superior and inferior vena cava. This collects a volume of blood to fill the more muscular right ventricle, which then provides the pressure to force the blood out through the pulmonary arteries to the lungs. After CO2 is exchanged for O2, the blood flows back into the left atrium. The left atrium fills the left ventricle, the most muscular part of the heart, which must develop enough pressure to circulate the blood throughout the body. The left ventricle discharges through the main artery, called the aorta. A pair of coronary arteries branch out from the aorta to provide blood for the heart's own use.
Although innervated by the autonomic nervous system, the heart can beat independently. Nervous stimulation can serve to increase or decrease heart rate as needed. The contraction phase of the cardiac cycle is called systole; the relaxation phase is called diastole. Pressure peaks at a normal 120 mmHg (over 2 m of hydraulic head) during systole. During diastole it drops to about 80 mmHg. These measurements are usually expressed in the form 120/80. Blood pressure above 140/90 is abnormally high. Surprisingly, the heart also produces a hormone to regulate blood volume in concert with the kidneys.
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