Human serum albumin (HSA) is the most abundant protein in human blood plasma, constituting about 60% of the total protein content. This protein is fundamental to maintaining the health of the circulatory system and performing numerous tasks necessary for survival. HSA is present in high concentrations, typically ranging from 3.5 to 5.0 grams per deciliter of serum in healthy adults.
Molecular Structure and Synthesis
The production of human serum albumin occurs exclusively within the liver. It is synthesized as a precursor molecule, processed through the endoplasmic reticulum and Golgi apparatus, and then secreted into the bloodstream. A healthy liver produces approximately 10 to 15 grams of albumin every day.
HSA is a single polypeptide chain composed of 585 amino acids, resulting in a molecular weight of about 66.5 kilodaltons (kDa). The molecule is water-soluble and globular, featuring a heart-shaped structure with three distinct domains. Its structure includes 17 disulfide bridges that provide stability and a single free thiol group that contributes to its antioxidant properties. Albumin has a long half-life in circulation, averaging about 21 days, due to its interaction with the neonatal Fc receptor (FcRn), which protects it from rapid degradation and allows for recycling.
Primary Physiological Functions
The primary function of HSA is regulating colloid osmotic pressure, also known as oncotic pressure, in the blood. Albumin accounts for 75-80% of this pressure, which is the force exerted by large proteins that prevents fluid from easily crossing capillary walls. This pressure pulls water back into the blood vessels from surrounding tissues, preventing the excessive accumulation of fluid, a condition known as edema.
HSA also functions as a non-specific transport molecule. Albumin has several binding pockets that allow it to carry both endogenous and exogenous compounds. The protein transports essential substances that are not water-soluble on their own, such as fatty acids, bilirubin (a waste product from red blood cell breakdown), and various hormones, including thyroid hormones and cortisol.
The transport capability extends to numerous pharmaceutical drugs, such as warfarin and ibuprofen. By binding to these drugs, albumin helps maintain their concentration in the bloodstream and prolongs their half-life. This binding mechanism also serves as a detoxification pathway, helping carry toxins and heavy metal ions away from sensitive organs.
Serum Albumin in Disease Diagnosis
Measuring the concentration of serum albumin provides a diagnostic marker for several underlying health conditions. Low albumin levels, referred to as hypoalbuminemia, are a common finding in hospitalized patients and are often associated with poor outcomes.
A decrease in albumin can point to issues with the protein’s production, loss, or distribution. Severe liver disease, such as cirrhosis, impairs the liver’s ability to synthesize albumin, leading to lower levels. Kidney diseases like nephrotic syndrome can cause the loss of albumin into the urine, as damaged filtering units fail to retain the protein. Hypoalbuminemia is also a feature of acute and chronic inflammation, where inflammatory mediators increase blood vessel permeability, causing albumin to leak into the extravascular space.
Elevated albumin levels, or hyperalbuminemia, do not typically result from overproduction. The most frequent cause of hyperalbuminemia is severe dehydration, which concentrates the blood components, leading to an artificially high reading. Identifying the cause of an abnormal albumin level is often an initial step in diagnosing and managing complex medical conditions.
Therapeutic Uses of Albumin
Purified human serum albumin is administered intravenously to treat specific clinical conditions. Administration aims to restore fluid balance and increase blood volume, a process known as plasma expansion. It is available in different concentrations, typically 5% (iso-oncotic) or 20-25% (hyper-oncotic), depending on the treatment goal.
Albumin infusions are a common treatment for hypovolemic shock resulting from severe blood loss or trauma. It is also used for patients with severe burns, where significant fluid loss occurs due to damaged capillaries. In patients with advanced liver disease and complications like ascites (fluid accumulation in the abdomen), albumin may be administered following large-volume fluid removal procedures to prevent drops in blood pressure and maintain adequate circulation.