Blood is composed of both fluid and cellular constituents that can be separated by centrifugation. The fluid portion of blood is termed plasma. Plasma is an aqueous solution that contains proteins, small nutrients, metabolic waste products, gases, and electrolytes. With respect to the small solutes (other than proteins), plasma is similar to interstitial fluid in its composition; it is the amount of protein that separates plasma from interstitial fluid. Those plasma proteins can be organized into three groups:
albumin, globulins and fibrinogen. Albumin is synthesized by the liver and constitutes 60% of the total protein content of the blood. Globulins are a class of protein that comprise 36% of plasma proteins (they function in lipid transport, as steroid hormones, and in blood clotting). Fibrinogen is another protein synthesized in the liver, and is essential to formation of blood clots.
With respect to its cellular constituents, the proportion of blood volume that is composed of erythrocytes is termed the
hematocrit. The cellular constituents of the blood are produced within bone marrow. The most abundant cell type are the erythrocytes (45% of blood volume). These red blood cells contain large quantities of hemoglobin. Hemoglobin is essential for transport of oxygen and carbon dioxide in the blood. These cells no longer function like other cells since they lack nuclei, mitochondria, and ribosomes. They have a relatively short lifespan (120 days) and are stimulated to proliferate by erythropoietin. The remainder of the cellular components of blood include leukocytes and platelets.
Leukocytes function to defend the body against microorganisms and other foreign materials. Unlike red blood cells, these cells are not restricted to the circulation and can be found in most tissues. Platelets are important in blood clots. They are derived as cellular fragments from
megakaryocytes.
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