Serum albumin

Serum albumin, or simply albumin, is a plasma protein made by the liver. Albumin's purpose in the human body is twofold. On one hand, it works to maintain proper fluid balance in the blood. On the other, it serves to transport fatty acids through plasma, which is the colorless fluid component of blood, and interstitial fluid, which is the solution that surrounds and bathes tissue cells. Albumin is the most abundant protein in plasma and accounts for approximately two-thirds of protein content in the average healthy adult. More broadly, albumin is important for tissue growth, healing, and maintaining adequate blood pressure. Albumin also aids in the transport of certain medications. Unusually low albumin levels are typically associated with problems such as liver disease, diabetes, and cardiovascular disease and can result in circulatory issues. When necessary, albumin can be administered via infusion for the treatment of hypoalbuminemia (abnormally low albumin levels), malnutrition, and hypotension.

Brief History

Historically, the term albumin arose from the more common albumen, which first appeared as a German word for protein. Both terms are rooted in the Latin term albus, which means white. Eventually, albumen became associated with the portion of an egg that turns white when cooked. Albumin, meanwhile, is used exclusively in association with the plasma protein described here.

It is widely believed that albumin, along with hemoglobin and fibrin, was the first bodily protein that scientists ever studied. The earliest recorded reference to albumin was made by the ancient Greek physician Hippocrates. In the Aphorisms, Hippocrates identified urine made foamy by the probable presence of albumin as a sign of chronic kidney disease. It was not until around the seventeenth or eighteenth century, however, that scientists fully understood that blood serum contained proteins they first referred to as albumen. By the mid-nineteenth century, scientists understood that albumen was also found in lymph and other body fluids. At the time, however, scientists made no distinction between the specific protein now known as albumin and the broader array of proteins they called albumen. Such a distinction was not made until the French physiologist C. Denis conducted a test in 1840 that demonstrated the difference between albumin and another key serum protein called globulin. The next major breakthrough came when A. Gürber first successfully crystalized albumin in 1894.

In the early twentieth century, researchers sought to develop a way to create preparations of pure albumin for use in transfusions. These efforts initially culminated in the 1940s as military forces faced a growing need for a stable substitute for blood plasma in World War II field hospitals. To meet that need, scientists worked to create purified albumin from a large supply of bovine albumin to which they had access at the time. Although they succeeded in this endeavor, many of the recipients of this bovine-derived albumin ultimately experienced serum sickness and a few even died. Regardless, this failure yielded an important discovery: the severe reaction many soldiers had to the bovine albumin was due to differences between the species, not the remaining impurities in the albumin itself. This led to the production of purified human albumin, which is still in use in the twenty-first century.

Overview

Synthesized in the liver, albumin is the most abundant of the various proteins found in blood plasma. In function, albumin serves to both maintain proper fluid balance in the blood and transport fatty acids and drugs such as penicillin in the plasma and interstitial fluids. Albumin is also tied directly to oncotic pressure, which is the pressure exerted by blood plasma proteins that pull fluids into the circulatory system through the capillaries. As a result, a decrease in production or an increase in losses of albumin leads to a disruption in intravascular oncotic pressure and the onset of edema, or swelling, due to the leakage of fluids from the blood vessels. In the 1990s, special x-ray studies allowed scientists to get their first look at the structure of albumin proteins and revealed that such proteins are heart-shaped.

People who have liver disease or certain other medical conditions may be likely to have an abnormally low level of albumin in their blood plasma. A person's albumin level can be determined through a total serum protein test. This test can be used to determine the health of a person's liver and kidneys, whether a person's diet includes an adequate amount of protein, or to diagnose the cause of swelling in the ankles or abdomen or fluid collection in the lungs. When a total serum protein test is performed, a blood sample is taken and analyzed in a laboratory. In healthy adults, the normal albumin level is approximately 35–50 grams per liter (g/L). High albumin levels are most often caused by severe dehydration. Low albumin levels can be caused by malnutrition, kidney disease, liver disease, autoimmune disease, gastrointestinal malabsorption syndromes like Crohn's disease, Hodgkin's lymphoma, severe diabetes, hyperthyroidism, or heart failure. Because low albumin levels are most commonly associated with liver problems, total serum protein tests are often included among the panel of other tests used to help diagnose liver conditions.

Patients with low albumin levels may need to undergo treatment to correct this problem. In such instances, patients may choose to undergo therapeutic plasma exchange (TPE), which is a transfusion of plasma materials. Albumin can be administered either on its own or as part of a full plasma transfusion. Administering albumin alone or with a replacement fluid like saline may help to reduce the likelihood of adverse reactions. Although such events are rare, they can be dangerous. Some adverse effects that may occur in response to the administration of albumin include changes in heart rate, blood pressure, or respiration; nausea; fever; chills; or allergic reactions. Because albumin can sometimes contain trace amounts of aluminum, transfusions can also lead to toxicity in infants or patients who have chronic kidney failure. It should also be noted that patients who object to receiving blood transfusions on religious grounds may also object to TPE for the same reasons. Some, however, do not view the administration of blood fractions as being the same as a full blood transfusion and may accept the transfusion of albumin as needed.

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