Composition of Blood
Composition
of Blood
Among invertebrates that lack a circulatory system (such as flatworms and cnidarians) it is not possible to distinguish a true “blood.” These forms possess a clear, watery tissue fluid containing some phagocytic cells, a little protein, and a mixture of salts similar to seawater. The “blood” of invertebrates with open circulatory systems is more complex and is often called hemolymph (Gr. haimo, blood, + L. lympha, water). Invertebrates with closed circulatory systems, on the other hand, maintain a clear separation between blood contained within blood vessels and tissue (interstitial) fluid surrounding blood vessels.
In vertebrates, blood is a complex liquid tissue composed of plasma and formed elements, mostly red cells (also called corpuscles), suspended in plasma. If we separate red blood corpuscles and other formed elements from the fluid components by centrifugation, we find that blood is approximately 55% plasma and 45% formed elements.
The composition mammalian blood is as follows:
Plasma
Plasma proteins are a diverse group of large and small proteins that perform numerous functions. The major protein groups are (1) albumins, the most abundant group, constituting 60% of the total, which help to keep plasma in osmotic equilibrium with the cells of the body; (2) globulins, a diverse group of high-molecular weight proteins (35% of total) that includes immunoglobulins and various metal-binding proteins; and (3) fibrinogen, a very large protein that functions in blood coagulation. Blood serum is plasma minus the proteins involved in clot formation (see the following).
Red blood cells, or erythrocytes, are present in enormous numbers in blood, approximately 5.4 billion per milliliter of blood in adult men and 4.8 billion in adult women. In mammals and birds, red cells form continuously from large nucleated erythroblastsin red bone marrow (in other vertebrates kidneys and spleen are the principal sites of red blood cell production). During erythrocyte formation hemoglobin is synthesized and the precursor cells divide several times. In mammals the nucleus shrinks during development to a small remnant and eventually disappears altogether. Many other characteristics of a typical cell also are lost: ribosomes, mitochondria, and most enzyme systems. What is left is a biconcave disc consisting of a baglike membrane packed with about 280 million molecules of the bloodtransporting pigment hemoglobin. Approximately 33% of an erythrocyte by weight is hemoglobin. The biconcave shape (Figure 33-3) is a mammalian innovation that provides a larger surface for gas diffusion than would a flat or spherical shape. All other vertebrates have nucleated erythrocytes that are usually ellipsoidal in shape (Figure 33-4).
An erythrocyte enters the circulation for an average life span of approximately 4 months. During this time it may journey 11,000 km, squeezing repeatedly through capillaries, which are sometimes so narrow that the erythrocyte must bend to pass through. At last it fragments and is quickly engulfed by large scavenger cells called macrophages located in the liver, bone marrow, and spleen. Iron from hemoglobin is salvaged to be used again; the rest of the heme is converted to bilirubin, a bile pigment. It is estimated that a human body produces 10 million erythrocytes and destroys another 10 million every second.
White blood cells, or leukocytes, form a wandering system of protection for the body. In adults they number only approximately 7.5 million per milliliter of blood, a ratio of 1 white cell to 700 red cells. There are several kinds of white blood cells: granulocytes (subdivided into neutrophils, basophils, and eosinophils), and agranulocytes, the lymphocytes and monocytes (Figure 33-3). We discuss the role of leukocytes in the body’s defense mechanisms.
Among invertebrates that lack a circulatory system (such as flatworms and cnidarians) it is not possible to distinguish a true “blood.” These forms possess a clear, watery tissue fluid containing some phagocytic cells, a little protein, and a mixture of salts similar to seawater. The “blood” of invertebrates with open circulatory systems is more complex and is often called hemolymph (Gr. haimo, blood, + L. lympha, water). Invertebrates with closed circulatory systems, on the other hand, maintain a clear separation between blood contained within blood vessels and tissue (interstitial) fluid surrounding blood vessels.
In vertebrates, blood is a complex liquid tissue composed of plasma and formed elements, mostly red cells (also called corpuscles), suspended in plasma. If we separate red blood corpuscles and other formed elements from the fluid components by centrifugation, we find that blood is approximately 55% plasma and 45% formed elements.
The composition mammalian blood is as follows:
Plasma
- Water 90%
- Dissolved solids, consisting of plasma proteins (albumin, globulins, fibrinogen), glucose, amino acids, electrolytes, various enzymes, antibodies, hormones, metabolic wastes, and traces of many other organic and inorganic materials
- Dissolved gases, especially oxygen, carbon dioxide, and nitrogen
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| Figure 33-3 Formed elements of human blood. Hemoglobin-containing red blood cells of humans and other mammals lack nuclei, but those of all other vertebrates have nuclei. Various leukocytes provide a wandering system of protection for the body. Platelets participate in the blood’s clotting mechanism. |
- Red blood cells (erythrocytes), containing hemoglobin for transport of oxygen and carbon dioxide
- White blood cells (leukocytes), serving as scavengers and as defensive cells
- Cell fragments (platelets in mammals) or cells (thrombocytes in other vertebrates) that function in blood coagulation
Plasma proteins are a diverse group of large and small proteins that perform numerous functions. The major protein groups are (1) albumins, the most abundant group, constituting 60% of the total, which help to keep plasma in osmotic equilibrium with the cells of the body; (2) globulins, a diverse group of high-molecular weight proteins (35% of total) that includes immunoglobulins and various metal-binding proteins; and (3) fibrinogen, a very large protein that functions in blood coagulation. Blood serum is plasma minus the proteins involved in clot formation (see the following).
Red blood cells, or erythrocytes, are present in enormous numbers in blood, approximately 5.4 billion per milliliter of blood in adult men and 4.8 billion in adult women. In mammals and birds, red cells form continuously from large nucleated erythroblastsin red bone marrow (in other vertebrates kidneys and spleen are the principal sites of red blood cell production). During erythrocyte formation hemoglobin is synthesized and the precursor cells divide several times. In mammals the nucleus shrinks during development to a small remnant and eventually disappears altogether. Many other characteristics of a typical cell also are lost: ribosomes, mitochondria, and most enzyme systems. What is left is a biconcave disc consisting of a baglike membrane packed with about 280 million molecules of the bloodtransporting pigment hemoglobin. Approximately 33% of an erythrocyte by weight is hemoglobin. The biconcave shape (Figure 33-3) is a mammalian innovation that provides a larger surface for gas diffusion than would a flat or spherical shape. All other vertebrates have nucleated erythrocytes that are usually ellipsoidal in shape (Figure 33-4).
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| Figure 33-4 Mammalian and amphibian red blood cells. A, Erythrocytes of a gerbil are biconcave discs containing hemoglobin and surrounded by a tough stroma. B, Frog erythrocytes are convex discs, each containing a nucleus, which is plainly visible in the scanning electron micrograph as a bulge in the center of each cell. (Magnifications: mammalian erythrocytes, ×6300; frog erythrocytes, ×2400.) |
An erythrocyte enters the circulation for an average life span of approximately 4 months. During this time it may journey 11,000 km, squeezing repeatedly through capillaries, which are sometimes so narrow that the erythrocyte must bend to pass through. At last it fragments and is quickly engulfed by large scavenger cells called macrophages located in the liver, bone marrow, and spleen. Iron from hemoglobin is salvaged to be used again; the rest of the heme is converted to bilirubin, a bile pigment. It is estimated that a human body produces 10 million erythrocytes and destroys another 10 million every second.
White blood cells, or leukocytes, form a wandering system of protection for the body. In adults they number only approximately 7.5 million per milliliter of blood, a ratio of 1 white cell to 700 red cells. There are several kinds of white blood cells: granulocytes (subdivided into neutrophils, basophils, and eosinophils), and agranulocytes, the lymphocytes and monocytes (Figure 33-3). We discuss the role of leukocytes in the body’s defense mechanisms.
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