Blood is an opaque, red liquid consisting of several types of cells suspended in a complex, amber fluid known as plasma. When blood is allowed to clot or coagulate, the suspending medium is referred to as serum.
Blood is normally confined to the circulation, including the heart and the pulmonary and systemic blood vessels. Blood accounts for 6 to 8% of the body weight of a healthy adult. The blood volume is normally 5.0 to 6.0 L in men and 4.5 to 5.5 L in women.
The density (or specific gravity) of blood is approximately 1.050 g/mL. Density depends on the number of blood cells present and the composition of the plasma. The density of individual blood cells varies according to cell type and ranges from 1.115 g/mL for erythrocytes to 1.070 g/mL for certain leukocytes.
While blood is only slightly heavier than water, it is certainly much thicker. The viscosity of blood, a measure of resistance to flow, is 3.5 to 5.5 times that of water. Blood's viscosity increases as the total number of cells present increases and when the concentration of large molecules (macromolecules) in plasma increases. At pathologically high viscosity, blood flows poorly to the extremities and internal organs.
The Erythrocyte Sedimentation Rate and Hematocrit Are Important Diagnostic Measurements
Erythrocytes are the red cells of blood. Since erythrocytes have only a slightly higher density than the suspending plasma, they normally settle out of whole blood very slowly. To determine the erythrocyte sedimentation rate (ESR), anticoagulated blood is placed in a long, thin, graduated cylinder (Fig. 11.1). As the red cells sink, they leave behind the less dense leukocytes and platelets in the suspending plasma. Erythrocytes in the blood of healthy men sediment at a rate of 2 to 8 mm/hr,- those in the blood of healthy women sediment slightly faster (2 to 10 mm/hr).
The ESR can be an important diagnostic index, as values are often significantly elevated during infection, in patients with arthritis, and in patients with inflammatory diseases. In some diseases, such as sickle-cell anemia, polycythemia (abnormal increase in red cell numbers), and hyperglycemia (elevated blood sugar levels), the ESR is slower than normal. The reasons for alterations in the ESR in disease states are not always clear, but the cells tend to sediment faster when the concentration of plasma proteins increases.
Blood cells can be quickly separated from the suspending fluid by simple centrifugation. When anticoagulated blood is placed in a tube that is rotated about a central point, centrifugal forces pull the blood cells from the suspending plasma. The hematocrit is the portion of the total blood volume that is made up of red cells. This value is determined by the centrifugation of small capillary tubes of anticoagulated blood to pack the cells.
Determination of the erythrocyte sedimentation rate (ESR). Fresh, anticoagulated blood is allowed to settle at room temperature in a graduated cylinder. After a fixed time interval (1 hour), the distance (in millimeters) that the erythrocytes sediment is measured.
Determination of hematocrit values is a simple and important screening diagnostic procedure in the evaluation of hematological disease. Hematocrit values of the blood of healthy adults are 47 ± 5% for men and 42 ± 5% for women. Decreased hematocrit values often reflect blood loss as a result of bleeding or deficiencies in blood cell production. Low hematocrit values indicate the presence of anemia, a reduction in the number of circulating erythro-cytes. Increased hematocrit levels may likewise indicate a serious imbalance in the production and destruction of red cells. Increased production (or decreased rate of destruction) of erythrocytes results in polycythemia, as reflected by increased hematocrit values. Dehydration, which decreases the water content and, thus, the volume of plasma, also results in an increase in hematocrit.
While the cellular and plasma components of blood may act alone, they often work in concert to perform their functions. Working together, blood cells and plasma proteins play several important roles, including
• Transport of substances from one area of the body to another
• Immunity, the body's defense against disease
• Hemostasis, the arrest of bleeding
• Homeostasis, the maintenance of a stable internal environment
Transport. Blood carries several important substances from one area of the body to another, including oxygen, carbon dioxide, antibodies, acids and bases, ions, vitamins, cofactors, hormones, nutrients, lipids, gases, pigments, minerals, and water. Transport is one of the primary and most important functions of blood, and blood is the primary means of long-distance transport in the body. Substances can be transported free in plasma, bound to plasma proteins, or within blood cells.
Oxygen and carbon dioxide are two of the more important molecules transported by blood. Oxygen is taken up by the red cells as they pass through capillaries in the lung. In tissue capillaries, red cells release oxygen, which is then used by respiring tissue cells. These cells produce carbon dioxide and other wastes.
The blood also transports heat. By doing so, it maintains the proper temperature in different organs and tissues, and in the body as a whole.
Immunity. Blood leukocytes are involved in the body's battle against infection by microorganisms. While the skin and mucous membranes physically restrict the entry of infectious agents, microbes constantly penetrate these barriers and continuously threaten internal infection. Blood leukocytes, working in conjunction with plasma proteins, continuously patrol for microbial pathogens in the tissues and in the blood. In most cases, penetrating microbes are efficiently eliminated by the sophisticated and elaborate antimicrobial systems of the blood.
Hemostasis. Bleeding is controlled by the process of he-mostasis. Complex and efficient hemostatic mechanisms have evolved to stop hemorrhage after injury, and their failure can quickly lead to fatal blood loss (exsanguination). Both physical and cellular mechanisms participate in he-mostasis. These mechanisms, like those of the immune system, are complex, interrelated, and essential for survival.
Homeostasis. Homeostasis is a steady state that provides an optimal internal environment for cell function (see Chapter 1). By maintaining pH, ion concentrations, osmo-lality, temperature, nutrient supply, and vascular integrity, the blood system plays a crucial role in preserving home-ostasis. Homeostasis is the result of normal functioning of the blood's transport, immune, and hemostatic systems.
Plasma is composed mostly of water (93%) with various dissolved solutes, including proteins, lipids (fats), carbohydrates, amino acids, vitamins, minerals, hormones, wastes, cofactors, gases, and electrolytes (Table 11.1). The solutes in plasma play crucial roles in homeostasis, such as maintaining normal plasma pH and osmolality.
Blood cells include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes). Each microliter (a millionth of a liter) of blood contains 4 to 6 million erythrocytes, 4,500 to 10,000 leukocytes, and 150,000 to 400,000 platelets. There are several subtypes of leukocytes, defined by morphological differences (Fig. 11.2), each with vastly different functional characteristics and capabilities. Table 11.2 lists the normal circulating levels of different blood cell types.
Of the total leukocytes, 40 to 75% are neutrophilic, polymorphonuclear (multinucleated) cells, otherwise known as neutrophils. These phagocytic cells actively in-
^ABLE 11.1^ Some Components of Plasma
Normal Concentration Range
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.