Plasma is a clear, yellowish fluid (the color of straw). Plasma can sometimes appear milky after a very fatty meal or when people have a high level of lipids in their blood. Plasma is 90-percent water. The other 10 percent dissolved in plasma is essential for life. These dissolved substances are circulated throughout the body and diffuse into tissues and cells where they are needed. They diffuse from areas of high concentration to areas of lower concentration. The greater the difference in concentration, the greater the amount of material that diffuses. Waste materials flow in the opposite direction, from where they are created in the cells into the bloodstream, where they are removed either in the kidneys or lungs.
Hydrostatic pressure (blood pressure) pushes fluid out of blood vessels. Balancing this is something called oncotic pressure (caused by proteins dissolved in blood), which tends to keep fluid inside the blood vessels.
Proteins make up a large part of the 10 percent of material dissolved in plasma and are responsible for oncotic pressure. Protein molecules are much larger than water molecules and tend to stay in blood vessels. They have more difficulty fitting through the pores in capillaries, and therefore have a higher concentration in blood vessels. Proteins tend to attract water to keep their relative concentration in blood vessels more in line with fluid outside the blood vessels. This is one of the ways the body maintains a constant volume of blood.
Plasma contains 6.5 to 8.0 grams of protein per deciliter of blood. The main proteins in plasma are albumin (60 percent), globulins (alpha-1, alpha-2, beta, and gamma globulins (immunoglobulins)), and clotting proteins (especially fibrinogen). These proteins function to maintain oncotic pressure (especially albumin) and transport substances such as lipids, hormones, medications, vitamins, and other nutrients. These proteins are also part of the immune system (immunoglobulins), help blood to clot (clotting factors), maintain pH balance, and are enzymes involved in chemical reactions throughout the body.
Electrolytes are another large category of substances dissolved in plasma. They include:
- Sodium (Na+)
- Potassium (K+)
- Chloride (Cl-)
- Bicarbonate (HCO3-)
- Calcium (Ca+2)
- Magnesium (Mg+2)
Other materials dissolved in plasma are carbohydrates (glucose), cholesterol, hormones and vitamins. Cholesterol is normally transported attached to lipoproteins such as low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs). For more information on cholesterol, read How Cholesterol Works.
When plasma is allowed to clot, the fluid left behind is called serum. When blood is collected from a patient it is allowed to clot in a test tube, where the cells and clotting factors fall to the bottom and the serum is left on top. Serum is tested for all the numerous items discussed above to determine if any abnormalities exist.