In human blood, albumin is the plasma protein chiefly responsible for maintaining osmotic pressure.
Human serum albumin is made by the liver and is the most abundant protein in blood plasma. Its concentration and many charged sites allow it to attract and retain water within the bloodstream. This colloid osmotic, or oncotic, pressure helps balance the movement of fluid between blood vessels and surrounding tissues.
Albumin also acts as a transport protein. It binds and carries substances such as fatty acids, bilirubin, calcium and many medicines. That transport role is separate from its contribution to fluid balance, though both functions depend on the protein’s structure and abundance.
Low albumin levels, called hypoalbuminemia, may occur with liver disease, kidney protein loss, malnutrition or inflammation. Reduced oncotic pressure can contribute to oedema, but swelling has many possible causes. Albumin should not be confused with haemoglobin, which carries oxygen inside red blood cells, or fibrinogen, which helps blood clot.