Plasma contains mostly water, which accounts for 91.5% of the plasma content. The water acts as a solvent for carrying other substances.
Proteins account for 7% of plasma. The most prevalent of these proteins in plasma is albumin, a protein also found in egg white. Albumin concentration is four times higher in the blood than in the interstitial fluid (the watery fluid that bathes tissues, but is located outside and between cells). This high concentration of albumin in plasma serves an important osmotic function. The higher concentration of protein in blood prevents water from moving from the blood into the interstitial fluid. Without this osmotic protection, water would move from the interstitial fluid into the blood, diluting the plasma and swelling the blood volume. A high blood volume could have disastrous consequences, because the circulatory system can only pump so much blood before it becomes overloaded.
Other proteins that are present in plasma are immunoglobins and fibrinogen. Immunoglobins, also called antibodies, are proteins that function in the immune response. Antibodies attach to invading bacteria and other microorganisms, marking them for destruction by other immune cells. Fibrinogen is a protein that functions in a complex series of reactions that leads to the formation of blood clots.
The other components of plasma are salts, nutrients, enzymes, hormones, and nitrogenous waste products. Together, these substances account for 1.5% of plasma. The salts present in plasma include sodium, potassium, calcium, magnesium, chloride, and bicarbonate. These salts function in many important body processes. For instance, calcium functions in muscle contraction; sodium, chloride, and potassium function in nerve impulse transmission in nerve cells; and bicarbonate regulates pH. These salts are also called electrolytes. An imbalance of electrolytes, which can be caused by dehydration, can be a serious medical condition. Many gastrointestinal illnesses, such as cholera, cause a loss of electrolytes through severe diarrhea. When electrolytes are lost, they must be replaced with intravenous solutions of water and salts or by having the patient drink solutions of salts and water.
The remaining substances present in plasma are elements that the plasma is transporting from one place to another. For instance, plasma contains nutrients that nourish tissues. The nutrients found in plasma include amino acids, the building blocks of proteins; glucose, or sugars; and fatty acids and glycerol, the components of lipids (fats). In addition to nutrients, plasma also contains enzymes, or small proteins that function in chemical reactions, and hormones, which are transported from glands to body tissues. Waste products from the breakdown of proteins are also found in plasma. These waste products include creatinine, uric acid, and ammonium salts. Blood transports these waste products from the body tissues to the kidneys, where they are filtered from the blood and excreted in the urine.
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