Albumins are blood proteins that belong to the globular protein group. Their most important function in the human body is to maintain colloid osmotic pressure.
What is albumin?
Albumins are proteins that belong to the plasma protein group. Human albumins are also known as human albumin. The blood proteins have a molecular mass of approximately 66000 atomic mass units (Da). Each albumin is composed of nearly 600 amino acids. The amino acid cysteine is particularly abundant, so that albumins have a fairly high sulfur content. The blood proteins are water-soluble. They have a relatively high binding capacity for water. This is 18 milliliters per gram. Because of their water-binding properties, blood proteins play an important role in maintaining colloid osmotic pressure.
Function, effects, and roles
Albumin is the protein with the highest concentration in the blood. Thus, it provides the colloidosmotic pressure of the blood plasma. Colloidosmotic pressure is the pressure exerted by macromolecules in a solution. The amount of pressure is determined by the number of dissolved particles, in this case by the number of proteins. The colloid osmotic pressure keeps the fluid in the blood vessels. When the pressure in the blood drops, fluid enters the interstitium, causing edema. However, albumins also function as transport proteins. They bind various small-molecule and water-insoluble compounds and transport them via the bloodstream to their sites of action. Small-molecule compounds transported by albumins include calcium, the hormone progesterone, free fatty acids, the bile pigment bilirubin, magnesium, and drugs. Albumins exhibit ampholytic properties. They can absorb hydrogen ions and thus stabilize the pH of the blood. However, in contrast to the buffering capacities of hydrogen carbonate and hemoglobin, the buffering function of albumins plays a rather minor role.
Formation, occurrence, properties, and optimal values
Albumins are formed in the liver. Per day, the largest digestive gland of the body produces about twelve grams of albumins. A healthy person weighing 70 kilograms has an average of 250 to 300 grams of albumins. More than 50 percent of albumins are located in the tissues and thus outside the blood vessels. Only 40 percent circulate within the blood vessels in dissolved form in the blood plasma. In addition to albumin, other proteins are found in the blood. These plasma proteins are also known as globulins. However, they are outnumbered in terms of quantity. 60 percent of all blood proteins are albumins. This corresponds to an amount of 3.5 to 4.5 grams per deciliter. A healthy person should therefore have 35 to 62 grams of albumins per liter of blood. However, the reference values and the values determined can differ greatly from laboratory to laboratory. Individual laboratory values are also rarely meaningful, so the albumin value should always be considered by a physician in conjunction with other blood values. The albumin value is usually measured in the blood. Only a small amount of protein may be found in the urine. The maximum value is 30 milligrams within 24 hours. An elevated albumin concentration in the urine may indicate kidney damage.
Diseases and disorders
The renal corpuscles have what is called a fenestrated membrane. Small molecules, such as minerals, ions, or urinary substances, fit through the small gaps left by the cell wall of the renal corpuscles. The windows are too small for proteins and also for red blood cells. They therefore normally remain in the blood and enter the urine only in rare cases and in small concentrations. An increased albumin concentration in the urine is an indication of damage to the kidneys. The walls of the renal corpuscles are then so damaged that even larger molecules find their way into the urine. Albuminuria, i.e. the occurrence of albumins in the blood, is found in diabetic nephropathy, for example. Diabetic nephropathy is a kidney disease that occurs as a complication of diabetes mellitus. Due to the increased protein excretion in the urine, there is also a decrease in blood proteins. As a result, the colloid osmotic pressure in the blood vessels can no longer be maintained. Osmolarity in the vascular bed decreases and fluid from the blood vessels is shifted into the intercellular spaces.This results in water retention in the tissues (edema) and a reduced circulating blood volume. The edema is most apparent in the legs and eyelids. The combination of increased protein in the urine, decreased protein in the blood, increased blood lipids and edema is also known as nephrotic syndrome. Nephrotic syndrome occurs not only in diabetic nephropathy, but also in glomerulonephritis, sarcoidosis, and acute interstitial nephritis. A deficiency of albumins in the blood serum is called hypoalbuminemia. This can be caused by proteinuria as just described. However, the deficiency can also be caused by lack of production. The most common cause is liver disease such as cirrhosis or hepatitis. A deficiency of albumin in the blood therefore also serves as a marker for a synthesis defect of the liver. Albumin deficiency is also involved in the development of abdominal dropsy (ascites). Here, free fluid accumulates in the abdominal cavity. Abdominal dropsy is a typical symptom of advanced liver cirrhosis. Hyperalbuminemia, i.e., an increase in albumin levels in the blood serum, has little diagnostic relevance. Elevated albumin levels are really only found in cases of severe dehydration due to insufficient drinking or marked fluid losses.
