Plasma proteins are complex molecules dissolved within plasma, the pale-yellow, liquid component of blood that constitutes over half of its total volume. The total concentration of these proteins in a healthy adult typically falls between 6 and 8 grams per 100 milliliters of plasma. Their collective presence is essential for regulating fluid balance, transporting substances throughout the body, and defending against infection.
The Primary Classes
Plasma proteins are categorized into three major classes, each with distinct structures and roles, with the vast majority being synthesized in the liver. The most abundant of these is Albumin, which accounts for approximately 55 to 60 percent of the total protein content. Albumin is a relatively small, single-chain protein that the liver produces continuously.
The second major group is the Globulins, making up about 35 to 38 percent of the plasma proteins. Globulins are diverse and are subdivided into alpha, beta, and gamma categories based on their size and electrical charge. Alpha and beta globulins are primarily synthesized by the liver and function as transporters for various substances.
The third class, Fibrinogen, is a larger protein constituting about 7 percent of the total plasma protein content. Fibrinogen is also synthesized by the liver and is a precursor molecule that remains inactive until a blood vessel is damaged. The gamma globulins, a subdivision of the globulin class, are unique because they are not produced by the liver but by specialized immune cells called B lymphocytes in lymphoid tissues. These gamma globulins are specifically defined as immunoglobulins, which are the body’s primary antibodies.
Core Functions in Health
The functions of plasma proteins are diverse, but they center on maintaining the physical and chemical stability of the blood and body. One primary role is the regulation of osmotic pressure, which is maintained primarily by Albumin. This protein creates an oncotic pressure of about 25 millimeters of mercury, which pulls water from the surrounding tissues back into the capillaries. Without this pressure, fluid would leak excessively from the blood vessels into the tissues, leading to swelling known as edema.
Plasma proteins also function as transport vehicles for numerous substances that are not water-soluble. For instance, Albumin carries fatty acids, thyroid hormones, and certain drugs through the bloodstream. Other globulins, such as transferrin, are dedicated to binding and transporting metal ions, like iron, while lipoproteins carry lipids, including cholesterol and triglycerides.
The third function is the involvement of Fibrinogen in hemostasis, the process of stopping blood loss. When a blood vessel wall is injured, Fibrinogen is converted into Fibrin, a sticky, thread-like protein. These Fibrin strands form a meshwork that traps red blood cells and platelets, creating a stable clot to seal the damaged vessel.
Measuring Plasma Protein Levels
Clinicians frequently measure plasma protein levels to gain insight into a patient’s health status. A common diagnostic tool is the Total Protein Test, which quantifies the combined amount of Albumin and Globulins in the blood. Additionally, the ratio of Albumin to Globulin (A/G ratio) provides more specific information about the balance between these two major classes.
Abnormal levels of plasma proteins can signal various underlying health conditions. For example, low levels of Albumin, a condition called hypoalbuminemia, may indicate impaired protein synthesis due to liver disease or excessive protein loss from the body, often seen in kidney disease. Since Albumin is a smaller molecule, it is one of the first proteins to leak out when the filters in the kidneys are damaged.
Conversely, elevated levels of Globulins are often a sign of chronic inflammation, autoimmune disorders, or certain blood cancers, as the immune system produces a high volume of antibodies. Monitoring these protein concentrations helps physicians track disease progression, assess the effectiveness of treatment, and identify organs that may not be functioning correctly.