Plasma (biology)

Plasma (biology), clear, yellowish liquid that forms the fluid portion of blood and lymph. Plasma transports red and white blood cells and platelets throughout the body. It also delivers nutrients to the body’s millions of cells and picks up cell waste products.

Plasma is composed chiefly of water. This watery solution contains small amounts of minerals, salts, sugar (glucose), fats, amino acids, hormones, enzymes, dissolved gases, wastes like urea, and several proteins. One such protein is fibrinogen, the substance chiefly responsible for blood clotting. Other important proteins transport vital chemical elements such as iron and copper through the bloodstream, which help maintain the fluid balance of the body. Plasma proteins known as gamma globulins are antibodies that help protect the body against invasion by bacteria or viruses.

When blood flows past individual cells at the level of the smallest of blood vessels, called capillaries, plasma passes through the capillary walls. It then becomes known as lymph (fluid that bathes tissues), which is rich in white blood cells. Lymph is much like plasma, except that it contains no proteins, which are too large to pass through the capillary walls. As it bathes the body’s cells, lymph delivers the nutrients transported in plasma and collects wastes from the cells. Lymph then rejoins the plasma in the circulatory system so that the wastes can be carried to the organs that excrete them from the body.

Plasma can be separated from blood and used in the treatment of many conditions. It has a longer shelf life than whole blood, and if frozen or dehydrated, can be stored for years. In an emergency, plasma may be more useful than blood because it can be given to anyone regardless of a person’s blood type. Transfusions of plasma may be administered to treat shock, in which vital organs are deprived of oxygen-carrying blood, or to restore blood pressure after loss of bodily fluids. Gamma globulin may be extracted from plasma and given to people who have been exposed to such diseases as measles, hepatitis, or mumps. The resistance to disease conferred by gamma globulin is available almost at once, but its effects are not long-lasting.