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Plasma is the liquid portion of blood which is about 90% water and transports nutrients, wastes, antibodies, ions, hormones, and other molecules throughout the body. Humans typically have about 1.3-1.5 gal (5-6 l) of blood, which is about 55% plasma and 45% cells-red blood cells, white blood cells, and platelets. The plasma of humans and other vertebrates is nearly colorless, since the red color of hemoglobin is sequestered inside red blood cells. In contrast, many invertebrates have hemoglobin or hemocyanin carried directly in their plasma, so that their plasma is red, green, or blue.

Proteins make up about 8% by weight of human plasma. Humans have over 60 different proteins in their plasma, but the major ones are albumins, globulins, and fibrinogen. Albumins constitute about half (by weight) of all plasma protein and are important as carriers of ions, fatty acids, and other organic molecules. The most important class of globulins is the immunoglobulins, which are the antibodies that defend the body against attack by foreign organisms. Fibrinogen is a plasma protein important in the formation of blood clots following damage to a blood vessel. In clotting, fibrinogen is converted into fibrin and the fibrin molecules form an insoluble polymer, a blood clot. Additional plasma proteins serve as carriers for lipids, hormones, vitamins and other molecules.

Ions make up only about 1% by weight of human plasma. However, they are the major contributors to plasma molarity, since their molecular weights are much less than those of proteins. Thus, ions are important in preventing blood cells from bursting by taking up excess water in osmosis. Sodium chloride (NaCl) constitutes more than 65% of the plasma ions. Bicarbonate, potassium, calcium, phosphate, sulfate, and magnesium are other plasma ions. The kidneys regulate the levels of plasma ion concentrations.

Plasma is also a transport medium for nutrients and wastes. The nutrients include amino acids (used to synthesize proteins), glucose (an energy source), and fatty acids (an energy source). The plasma transorts waste products such as urea and uricacid to the kidneys, where they are excreted.

Cholesterol and cholesterol esters are also present in plasma. Cholesterol is used as an energy source, as a metabolic precursor for the synthesis of steroid hormones, and is incorporated in cell membranes. Excess cholesterol and saturated fatty acids in the plasma can be deposited in arteries and can lead to arteriosclerosis (hardening of the arteries) and to heart disease.

The plasma of vertebrates also contains dissolved gases. Most of the oxygen in blood is bound to hemoglobin inside the red blood cells but some oxygen is dissolved directly in the plasma. Additional plasma gases include carbon dioxide (which forms bicarbonate ions) and nitrogen (which is inert).

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