What is albumin?
Albumins are proteins that are counted among the group of plasma proteins. Human albumins are also referred to as human albumins. The blood proteins have a molecular mass of about 66, 000 atomic mass units (Da). Each albumin consists of almost 600 amino acids.
The amino acid cysteine is particularly frequently represented, 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. Due to their water-binding properties, the blood proteins play an important role in maintaining the colloid osmotic pressure.
Function, effect & tasks
Albumin is the protein with the highest concentration in the blood. Thus, it provides for the colloid osmotic pressure of the blood plasma. Colloid osmotic 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. But albumins also act as transport proteins. They bind various small-molecule and water-insoluble compounds and transport them via the bloodstream to their sites of action. The small molecule compounds that are transported with albumins include calcium, the hormone progesterone, free fatty acids, the biliary pigment bilirubin, magnesium, and medications.
Albumins have ampholytic properties. They can absorb hydrogen ions and stabilize the pH of the blood. In contrast to the buffering capacities of bicarbonate and hemoglobin, the buffering function of the albumins rather plays a minor role.
Education, occurrence, properties & optimal values
Albumins are made in the liver. The body's largest digestive gland produces around twelve grams of albumins per day. A healthy person weighing 70 kilograms has an average of 250 to 300 grams of albumins. More than 50 percent of the albumins are located in the tissue 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, there are other proteins in the blood. These plasma proteins are also referred to as globulins. However, they are in numbers outnumbered. 60 percent of all blood proteins are albumins. This corresponds to a mangle of 3.5 to 4.5 grams per deciliter. A healthy person should thus have 35 to 62 grams of albumins per liter of blood. However, the reference values and the determined values can vary greatly from laboratory to laboratory.
Individual laboratory values are also rarely meaningful, so albumin levels should always be considered by a physician in relation to other blood levels. The albumin level is usually measured in the blood. Only a few proteins can be found in the urine. The maximum value is 30 milligrams within 24 hours. An increased urinary albumin concentration may indicate renal damage.
Diseases & Disorders
The kidney bodies have a so-called windowed membrane. The small gaps in the cell wall of the kidney corpuscles make small molecules such as minerals, ions or urinary substances fit. For proteins and also for red blood cells, the windows are too small. They thus normally remain in the blood and enter the urine only in rare cases and in small concentrations.
An increased urinary albumin concentration is an indication of kidney damage. The walls of the kidney cells are then so damaged that even larger molecules find their way into the urine. Albuminuria, ie the appearance of albumins in the blood, is found, for example, in diabetic nephropathy. Diabetic nephropathy is a kidney disease that is a complication of diabetes mellitus. Increased protein excretion in the urine also leads to a reduction in blood proteins. As a result, the colloid osmotic pressure in the blood vessels can no longer be maintained. The osmolarity in the vascular bed drops and the fluid from the blood vessels is displaced into the cell spaces. This leads to accumulation of fluid in the tissue (edema) and to a reduced circulating blood volume.
The edema is especially evident on the legs and eyelids. The combination of increased protein in the urine, decreased protein in the blood, elevated blood lipid levels and edema is also referred to as nephrotic syndrome. The nephrotic syndrome occurs not only in diabetic nephropathy but also in glomerulonephritis, sarcoidosis, and acute interstitial nephritis.
A lack of albumin in blood serum is called hypoalbuminemia. This can be due to a proteinuria as just described. The lack can also be caused by a lack of production. The most common causes of this are liver diseases such as liver cirrhosis or hepatitis. A lack of albumins 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 ascites. Here free fluid accumulates in the abdominal cavity. The ascites is a typical symptom of advanced cirrhosis.
Hyperalbuminemia, ie an increase in serum albumin levels, has little diagnostic relevance. Increased albumin levels are actually only found in cases of severe dehydration due to low drinking volume or pronounced fluid loss.