Plasma and Serum

When a sample of blood is treated with an anticoagulant to prevent clotting and permit­ted to stand in a tube undisturbed, the cells gradually settle to the bottom, leaving a straw­colored fluid above.

Plasma is about 92% water and 8% other substances. The kidneys are responsible for maintaining constant proportions of water and other constituents of the plasma by the selective filtration and reabsorption of water and other substances from the blood plasma. The total osmolality of plasma at normal body tempera­ture is about 290 mosm/kg. osmolality is a measure of the number of osmotically active particles (not the mass of the particles) per unit of solute. The two predominant particles in plasma are sodium and chloride ions (Table 15-1), and these contribute the most to the total osmolality of plasma or serum.

The plasma proteins consist of two major types: albumin and globulins. Albumin is the most prevalent plasma protein and is the pre­dominant protein synthesized by the liver. Many small compounds and electrolytes (e.g., calcium ions) bind to albumin and circulate in plasma in this bound form. This prevents their rapid loss in the urine. Because albumin and other large proteins do not readily pass through capillary walls, they also provide an effective osmotic force to prevent excessive fluid loss from capillaries into the interstitium.

The globulins in serum or plasma may be classified according to their migration (separa­tion) by electrophoresis. α-Globulins and β- globulins are classes that are synthesized in the liver. Members of these classes have a variety of functions, including transport in a manner similar to albumin, body defense (see Chapter 16), and blood clotting. Many of the globulin proteins are inactive precursors of enzymes or substrates for enzymes involved in blood clot­ting (discussed later in this chapter).

The γ-globulins are synthesized by cells of the immune system. Most of the known circulating antibodies are included in the γ-globulin frac­tion. The γ-globulin content of the blood there­fore increases following vaccination and during recovery from disease. Immune serum or hyper­immune serum can be produced by repeatedly inoculating an animal with a specific antigen. serum from that animal can then be injected into an animal susceptible to the same disease to provide passive protection for as long as the antibodies remain in the susceptible animal. This provides merely temporary immunity.