Albumin is the most abundant plasma protein (~40 g/L) in the normal adult. It accounts for 60% of plasma protein. It is synthesized in the liver as preproalbumin and secreted as albumin. It has a half-life in plasma of 20 days. It decreases rapidly under conditions of poor nutrition, in injury, infection, and surgery. Its main function is to maintain oncotic pressure in the blood. Oncotic pressure is the osmotic pressure exerted by plasma proteins that draws water into the circulatory system. It maintains the distribution of fluids in and out of cells and plasma volume. Eighty percent of plasma oncotic pressure is maintained by albumin. If we measure plasma proteins in a subject with lower extremity edema, we will find that they are more or less constant but the amount of albumin is definitely lower. A 10% decrease (from 4g to 3.7g) is enough to give rise to edematous manifestations. Albumin is also a non-specific transporter of hormones, calcium, free fatty acids, drugs. It also transports bilirubin, a derivative product of hemoglobin, synthesized in the spleen, marrow, and liver. Bilirubin is absolutely insoluble and has the tendency to cross membranes, and if this happens at the encephalic level it can cause serious damage. Tissue cells can take up albumin by pinocytosis, where it is hydrolyzed into amino acids. It is useful in the treatment of liver disease, hemorrhage, shock, and burns.
Hypoalbuminemia:
Decreased albumin synthesis (liver cirrhosis, malnutrition)
Increased albumin loss: may be caused by increased catabolism in infections, or excessive excretion by the kidneys (nephrotic syndrome) or excessive loss in the intestines or severe burns (plasma loss in the absence of a skin barrier).
The effects of hypoalbuminenia are:
- edema due to low oncotic pressure where albumin level decreases in liver disease causing low oncotic pressure; fluid moves into interstitial spaces causing edema;
- reduced transport of drugs and other substances in plasma; reduced protein-bound calcium;
- decrease in total plasma calcium level; ionized calcium level may remain normal.
Hyperalbuminemia
There are no known clinical conditions that cause the liver to produce large amounts of albumin. The only cause of hyperalbuminemia is dehydration.
