The liquid portion of blood---the part that is left after red cells, white cells, and platelets are removed---is called plasma. Although this transparent, pale-coloured liquid looks like water and is, in fact, 90 per cent water, the other 10 per cent plays a crucial part in maintaining the body's immune function, oxygen content and other functions. Plasma proteins make up about 6 per cent of plasma.
About 50 to 70 per cent of the protein content of blood plasma is made up of human serum albumin (HSA). HSA is a workhorse protein, serving a variety of purposes. It is a crucial transport protein, due to its ability to bind both mineral ions, such as calcium and zinc, and larger moieties, such as medications and hormones. Because of its abundance, HSA also plays a key role in maintaining the osmotic balance of the blood, which keeps the water content of blood from leaking out of vessels and into nearby tissue. HSA is a medium-sized protein of about 65 kilodaltons (kDa) with numerous crosslinks.
Immunoglobulins are more commonly known as antibodies. These proteins are abundant in blood plasma, making up 20 per cent of the total serum protein load. Immunoglobulins have a central role in immune defence, binding to foreign bodies in the blood (for instance, bacteria) and signalling cells in the bloodstream to eliminate them. Immunoglobulin units have a characteristic "Y" shape and consist of four protein chains, with "sticky ends" that allow them to bind to a specific type of foreign body. Other proteins in the plasma, called complement proteins, assist the immunoglobulins in their immune function.
Blood Clotting Proteins
Fibrinogen and prothrombin are two of several plasma proteins crucial for proper blood coagulation, or clotting. These proteins are inactive until they come in contact with an injury, at which point they link together into a three-dimensional mesh that results in a blood clot. Fibrinogen is a large protein (over 300 kDa) that consists of three separate protein chains crosslinked together. Prothrombin is about 70 kDa, and includes three loops called Kringle domains.
Hormones and Growth Factors
Because the circulatory system reaches into all parts of the body, it is ideally suited for transporting signalling proteins manufactured in the liver, glands and brain to peripheral parts of the body. Some of these are transported by blood cells, but others are transported through the plasma. Although these proteins make up only a small percentage of total plasma protein, they can have strong, systemic effects on the body. Sex hormones, such as follitropin, and growth hormones, such as insulin-like growth factor 1 (IGF-1), are examples of such proteins.
Many of the components that circulate in blood can participate in more than one biological reaction and can have powerful effects. Others are insoluble in water and unable to circulate on their own. For these and other reasons, many blood plasma molecules are chaperoned by transport proteins that regulate their interactions with cells. As described previously, albumin is a major transport protein in plasma. A closely related protein regulates vitamin D transport and metabolism. Retinol-binding protein is a plasma protein that chaperones another fat-soluble vitamin, vitamin A. The transferrin family of plasma proteins is involved in regulation of iron, which is essential for the proper oxygenation of blood. Other transport proteins are present transiently, depending on physiological conditions.
Although albumin, immunoglobulins and clotting factors are always present in healthy blood plasma, and make up the bulk of plasma protein content, the presence of other proteins in plasma can be important indicators of illness. For instance, high plasma levels of myoglobin indicate damage to the red blood cells and potential cardiovascular disease, while elevated levels of vascular endothelial growth factor (VEGF) and IGF-1 have been associated with cancer.