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What is serum albumin?

Serum albumin is the most abundant circulating plasma protein. It constitutes about half of the total protein content (3.4 to 5.4 g/dL) of the plasma (i.e., component of blood without blood cells). Albumin is a protein synthesized by hepatocytes in the liver and excreted in very high quantities in the blood. Minimal amounts of albumin are also stored in the liver. 

Once albumin enters the circulation, about 30% to 40% remains in the bloodstream and the rest enters the interstitial space. The serum albumin levels are a very useful laboratory value and are easily measured with a serum albumin test, which is a common blood test.

What is the purpose of albumin in the body?

Serum albumin has multiple homeostatic functions in the body. Most importantly, it maintains the intravascular oncotic pressure, preventing fluid leakage into the extravascular space. In addition, albumin functions as a carrier of several different endogenous (e.g., long chain fatty-acids, steroids) and exogenous (e.g., various medications) compounds in the blood. The binding of those substances to albumin can reduce their toxicity as seen in the cases of unconjugated bilirubin and medications (e.g., methadone, propranolol, furosemide, warfarin, and methotrexate). 

Albumin binds at least 40% of the circulating calcium and is a transporter of hormones, such as thyroxine, cortisol, and testosterone. It is also the main carrier of fatty acids, has significant antioxidant properties, and is involved in maintaining acid-base balance, acting as a blood plasma buffer. Serum albumin can also be used as a highly sensitive marker for an individual’s nutritional status and severity of illness, particularly in chronically and critically ill individuals.