![]() In the narrowest vessels (the capillaries), erythrocytes are often distorted. When circulating through blood vessels, red blood cells are not evenly dispersed. Hemoglobin is involved in the transport of oxygen and carbon dioxide and plays a role in maintaining a constant pH in the blood. They contain hemoglobin, which imparts to them their color, and possess an envelope, which appears highly structured when viewed with an electron microscope. Human erythrocytes measure about 8 μm in diameter and about 2 μm at their thickest, and they have a biconcave shape. In normal adult men and women, the blood contains approximately 5,000,000 and 4,500,000 erythrocytes per cubic mm, respectively. There also are notable differences in the internal structure of the leukocytes, especially with regard to size, shape, and staining properties of granular leukocytes. ![]() ![]() They are also large in frogs, where they are somewhat less oval than in birds. For example, the red blood cells of fish are nucleated, ovoid or spindle-shaped, and may be larger than in any other vertebrate. See also: HematopoiesisĪlthough blood cells occur in all vertebrates, they differ in density, relative proportions, morphology (to some extent), and their sites of origin. All blood cells are formed in the hematopoietic organs (chiefly lymph nodes and spleen for agranular leukocytes, and bone marrow for erythrocytes, platelets, and granular leukocytes). The agranular leukocytes (monocytes and lymphocytes) may develop extravascularly into other cell types (macrophages, fibroblasts, or plasma cells). These include erythrocytes and granulocytes. Some of the blood cells are terminal cells, in the sense that they are fully differentiated and have little or no capacity for division. Hematopoiesis is the process by which the cellular elements of the blood are formed. See also: Blood vessel Circulation Connective tissue Homeostasis Immunology Because of its basic composition (cells surrounded by a matrix), development, and ability to modify into other forms of connective tissues, blood can be regarded as a special form of connective tissue. Finally, blood transports waste products to excretory organs for elimination from the body. Blood plasma also circulates cells of the immune system and contains several of the components essential for the formation of blood clots. In addition, blood carries hormones from their sites of origin throughout the body, and is thus the transmitter of the chemical integrators of the body. The circulating blood carries nutrients and oxygen to the body cells therefore, it is an important means of maintaining the homeostasis of the body. The fluid part of the blood is in equilibrium with the tissue fluids of the body. They frequently leave the blood vessels to enter the extravascular spaces, where some of them may be transformed into connective tissue cells. The cells are derived from extravascular sites and then enter the circulatory system. Beyond water, which constitutes about 90% of plasma, the chief components of plasma are proteins (albumin and globulins), anions (mainly chloride and bicarbonate), and cations (mainly sodium, with smaller concentrations of potassium, calcium, and magnesium). Blood consists of plasma (the fluid component) and cells floating (suspended) within it ( Fig. Blood circulates through the body via vascular blood vessels (arteries, veins, and capillaries). Hereditary factors and vitamin K deficiency can impair coagulation or other functions of the blood.Ī fluid connective tissue that circulates in the vascular system (blood vessels) of the body. Platelets enable coagulation, a process that involves thrombin enzyme, fibrinogen, and clotting factors. Leukocytes (white blood cells), which play critical roles in the immune response and in the repair of connective tissue, include granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes (T cells and B cells). Plasma is made up of water, proteins such as albumin and globulins, anions such as chloride and bicarbonate, and cations such as sodium.Įrythrocytes (red blood cells) contain hemoglobin, which allows blood to transport oxygen. Blood is a fluid tissue that circulates through the vascular system of living animals.
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