The liquid component of blood, known as plasma, is a complex fluid that carries essential substances throughout the body. Plasma proteins are fundamental biological molecules dissolved within this fluid, making up a significant portion of its composition. These proteins perform a variety of tasks necessary for maintaining health, including balancing fluid within blood vessels and supporting the body’s defense mechanisms.
Composition and Origin of Plasma Proteins
Plasma proteins represent a substantial concentration in the blood, typically accounting for about seven percent of the total plasma volume. The liver serves as the primary factory for the majority of these proteins, synthesizing and releasing them directly into the bloodstream in large quantities. This central role means that the liver is constantly monitoring and adjusting the production rate of these molecules based on the body’s needs.
Specific types of plasma proteins, however, originate from the immune system rather than the liver. Gamma globulins, also known as immunoglobulins or antibodies, are produced by specialized immune cells called plasma cells. These cells, derived from B-lymphocytes, create the proteins in response to foreign invaders like bacteria and viruses.
The Three Main Categories
Plasma proteins are broadly grouped into three major categories based on their structure and behavior in laboratory analysis. The most abundant of these is albumin, which constitutes approximately 60% of the total protein mass in the plasma. Albumin is a relatively small, single-chain protein that plays a disproportionately large role in plasma function.
The next major group is the globulins, which are a more structurally diverse set of proteins separated into alpha, beta, and gamma subtypes. Alpha and beta globulins are mostly produced by the liver and often function as carrier proteins for various substances. They are generally larger and more complex than albumin.
The last major category is fibrinogen, which is a large, filamentous glycoprotein that makes up about seven percent of the total plasma protein. Fibrinogen is notable because it is a precursor molecule that can be converted into a different form when needed.
Functions in Circulation
One of the most important roles of plasma proteins is maintaining the proper fluid balance between the blood vessels and the surrounding tissues. Albumin, due to its high concentration and molecular characteristics, generates a specific pressure known as colloidal osmotic pressure, or oncotic pressure. This pressure acts like a retaining force, helping to pull water back into the capillaries and preventing excessive fluid from leaking out into the body’s tissues, which would cause swelling.
These proteins also act as a crucial transport system for substances that cannot easily dissolve in the watery plasma on their own. Albumin can bind to and carry water-insoluble molecules, including fatty acids, steroid hormones, and certain therapeutic drugs, ensuring their distribution to various organs. Alpha and beta globulins also function as specialized carriers, such as transferrin, which is dedicated to transporting iron throughout the body.
The third major function involves the body’s systems for defense and repair. Fibrinogen is directly involved in hemostasis, the process of stopping bleeding after an injury. Upon activation, fibrinogen is converted into fibrin, a fibrous mesh that forms the structural foundation of a stable blood clot.
Gamma globulins, the antibodies, are the core components of the humoral immune response. They circulate throughout the blood, recognizing and neutralizing specific foreign pathogens like viruses and bacteria. By binding to these invaders, antibodies either directly disable them or mark them for destruction by other immune cells.
When Protein Levels Are Abnormal
Measuring the total level of plasma proteins can provide important clues about a person’s overall health status. When protein concentrations are abnormally high, a condition called hyperproteinemia, it is often a sign of underlying issues. A common non-disease-related cause is simple dehydration, which concentrates the proteins in a reduced volume of plasma.
In other cases, high levels may result from an overproduction of immunoglobulins due to chronic inflammation, infection, or a disorder of the plasma cells, such as multiple myeloma. Conversely, abnormally low levels, or hypoproteinemia, can also point to serious health problems. Low protein often results from the liver’s inability to synthesize sufficient amounts due to liver disease or cirrhosis.
The kidneys can also be responsible for protein loss if their filtering units become damaged, a condition known as nephrotic syndrome, which allows albumin to leak into the urine. A shift in the concentration of plasma proteins is a signal that one of the body’s major organs or systems is not functioning correctly.