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Human serum albumin

Human serum albumin is the most abundant protein in human blood plasma. It is produced in the liver. Albumin comprises about half of the blood serum protein. It is soluble and monomeric.

Functions of albumin

  • Maintains oncotic pressure
  • Transports thyroid hormones
  • Transports other hormones, particularly ones that are fat soluble
  • Transports fatty acids ("free" fatty acids) to the liver
  • Transports unconjugated bilirubin
  • Transports many drugs; serum albumin levels can affect the half-life of drugs
  • Competitively binds calcium ions (Ca2+)
  • Buffers pH
  • Serum albumin, as a negative acute-phase protein, is down-regulated in inflammatory states. As such, it is not a valid marker of nutritional status; rather, it is a marker in inflammatory states

Hypoalbuminemia

Low blood albumin levels (hypoalbuminemia) can be caused by:

  • Liver disease; cirrhosis of the liver is most common
  • Excess excretion by the kidneys (as in nephrotic syndrome)
  • Excess loss in bowel (protein losing enteropathy e.g. Menetrier's)
  • Burns (plasma loss in the absence of skin barrier)
  • Redistribution (hemodilution [as in pregnancy], increased vascular permeability or decreased lymphatic clearance)
  • Acute disease states (referred to as a negative acute phase protein)
  • Mutation causing analbuminemia (very rare)

Hyperalbuminemia

Typically this condition is a sign of severe or chronic dehydration. Chronic dehydration needs to be treated with zinc as well as with water. Zinc reduces cell swelling caused by increased intake of water (hypotonicity) and also increases retention of salt. In the dehydrated state the body has too high of an osmolarity and apparently discards zinc to prevent this. Zinc also regulates transport of the cellular osmolyte taurine and albumin is known to increase cellular taurine absorption. Zinc has been shown to increase retinol (vitamin A) production from beta-carotene, and in lab experiments retinol reduced human albumin production. It is possible that a retinol (vitamin A) deficiency alone could cause albumin levels to become raised. Patients recovering from chronic dehydration may develop dry eyes as the body uses up its vitamin A store. Interestingly, retinol causes cells to swell with water (this is likely one reason that too much vitamin A is toxic).


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