Plasma Proteins

By weight, plasma proteins are the most abundant dissolved substances (solutes) in plasma. These proteins remain in the blood and interstitial fluids and ordinarily are not used as energy sources. The three main plasma protein groups are albumins, globulins, and fibrinogen. The groups differ in chemical composition and physiological function.

Albumins (al-bu'minz) are the smallest of the plasma proteins, yet account for 60% of these proteins by weight. They are synthesized in the liver, and because they are so plentiful, albumins are an important determinant of the osmotic pressure of the plasma.

Recall from chapter 3 (p. 87) that the presence of an impermeant solute on one side of a selectively permeable membrane creates an osmotic pressure and that water always diffuses toward a greater osmotic pressure. Because plasma proteins are too large to pass through the capillary walls, they are impermeant, and they create an osmotic pressure that tends to hold water in the capillaries despite the fact that blood pressure tends to force water out of capillaries by filtration (see chapter 3, p. 87). The term colloid osmotic pressure is often used to describe this osmotic effect due to the plasma proteins.

By maintaining the colloid osmotic pressure of plasma, albumins and other plasma proteins help regulate water movement between the blood and the tissues. In doing so, they help control blood volume, which in turn directly affects blood pressure. For this reason it is important that the concentration of plasma proteins remains relatively stable. Albumins also bind and transport certain molecules, such as bilirubin, free fatty acids, many hormones, and certain drugs.

If the concentration of plasma proteins falls, tissues swell, a condition called edema. This may result from starvation or a protein-deficient diet, either of which requires the body to use protein for energy, or from an impaired liver that cannot synthesize plasma proteins. As concentration of plasma proteins drops, so does the plasma osmotic pressure, sending fluids into the intercellular spaces.

Globulins (glob'u-linz), which make up about 36% of the plasma proteins, can be further subdivided into alpha, beta, and gamma globulins. The liver synthesizes alpha and beta globulins. They have a variety of functions, including transport of lipids and fat-soluble vitamins. Lymphatic tissues produce the gamma globulins, which are a type of antibody (see chapter 16, p. 666).

Fibrinogen (fi-brin'o-jen), which constitutes about 4% of the plasma protein, plays a primary role in blood coagulation. Synthesized in the liver, it is the largest of the plasma proteins. The function of fibrinogen is

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Essentials of Human Physiology

Essentials of Human Physiology

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