The major types of protein present in the plasma are albumin, globulin, and fibrinogen.
A major function of albumin is to provide colloid osmotic pressure in the plasma, which prevents plasma loss from the capillaries, as discussed in Chapter 16.
The globulins perform a number of enzymatic functions in the plasma, but equally important, they are principally responsible for the body's both natural and acquired immunity against invading organisms, discussed in Chapter 34.
Fibrinogen polymerizes into long fibrin threads during blood coagulation, thereby forming blood clots that help repair leaks in the circulatory system, discussed in Chapter 36.
Formation of the Plasma Proteins. Essentially all the albumin and fibrinogen of the plasma proteins, as well as 50 to 80 per cent of the globulins, are formed in the liver. The remainder of the globulins are formed almost entirely in the lymphoid tissues. They are mainly the gamma globulins that constitute the antibodies used in the immune system.
The rate of plasma protein formation by the liver can be extremely high, as much as 30 g/day. Certain disease conditions cause rapid loss of plasma proteins; severe burns that denude large surface areas of the skin can cause the loss of several liters of plasma through the denuded areas each day. The rapid production of plasma proteins by the liver is valuable in preventing death in such states. Occasionally, a person with severe renal disease loses as much as 20 grams of plasma protein in the urine each day for months, and it is continually replaced mainly by liver production of the required proteins.
In cirrhosis of the liver, large amounts of fibrous tissue develop among the liver parenchymal cells, causing a reduction in their ability to synthesize plasma proteins. As discussed in Chapter 25, this leads to decreased plasma colloid osmotic pressure, which causes generalized edema.
Plasma Proteins as a Source of Amino Acids for the Tissues.
When the tissues become depleted of proteins, the plasma proteins can act as a source of rapid replacement. Indeed, whole plasma proteins can be imbibed in toto by tissue macrophages through the process of pinocytosis; once in these cells, they are split into amino acids that are transported back into the blood and used throughout the body to build cellular proteins wherever needed. In this way, the plasma proteins function as a labile protein storage medium and represent a readily available source of amino acids whenever a particular tissue requires them.
Reversible Equilibrium Between the Plasma Proteins and the Tissue Proteins. There is a constant state of equilibrium, as shown in Figure 69-2, among the plasma proteins, the amino acids of the plasma, and the tissue proteins. It has been estimated from radioactive tracer studies that normally about 400 grams of body protein are synthesized and degraded each day as part of the continual state of flux of amino acids. This demonstrates the general principle of reversible exchange of amino acids among the different proteins of the body. Even during starvation or severe debilitating diseases, the ratio of total tissue proteins to total plasma proteins in the body remains relatively constant at about 33:1.
Because of this reversible equilibrium between plasma proteins and the other proteins of the body, one of the most effective therapies for severe, acute whole-body protein deficiency is intravenous transfusion of plasma protein. Within a few days, or sometimes within hours, the amino acids of the administered protein are distributed throughout the cells of the body to form new proteins where they are needed.
Ten of the amino acids normally present in animal proteins can be synthesized in the cells, whereas the other 10 either cannot be synthesized or are synthesized in quantities too small to supply the body's needs. This second group of amino acids that cannot be synthesized is called the essential amino acids. Use of the word "essential" does not mean that the other 10 "nonessential" amino acids are not required for the formation of proteins, but only that the others are not essential in the diet because they can be synthesized in the body.
Synthesis of the nonessential amino acids depends mainly on the formation of appropriate a-keto acids,
Tissue cells Liver cells
Tissue cells Liver cells
Reversible equilibrium among the tissue proteins, plasma proteins, and plasma amino acids.
II I . II Transaminase
(Glutamine) (Pyruvic acid)
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