ALBUMIN: FUNCTIONS, SYNTHESIS, DEFICIENCY, TYPES - BIOLOGY - 2025

The albumin is a protein synthesized by the liver which is found in the bloodstream, so is classified as a plasma protein. It is the main protein of its kind in humans, accounting for more than half of circulating proteins.

Unlike other proteins such as actin and myosin, which are part of solid tissues, plasma proteins (albumin and globulins) are suspended in plasma, where they perform various functions.

Albumin molecule

Features

Regulation of plasma oncotic pressure

One of the most important functions of albumin is to regulate the oncotic pressure of the plasma; that is, the pressure that draws water into the blood vessels (by osmotic effect) in order to counteract the capillary arterial pressure that forces the water outward.

The balance between capillary blood pressure (which pushes fluids out) and oncotic pressure generated by albumin (retaining water inside blood vessels) is what allows the circulating volume of plasma to remain stable and the extravascular space does not receive more fluids than it needs.

Maintenance of blood pH

In addition to its function as a regulator of oncotic pressure, albumin also acts as a buffer, helping to maintain the pH of the blood within a physiological range (7.35 to 7.45).

Main means of transportation

Finally, this protein with a molecular weight of 67,000 daltons is the main means of transport that plasma has to mobilize substances insoluble in water (main component of plasma).

For this, albumin has different binding sites where various substances can be temporarily "attached" to be transported in the bloodstream without having to dissolve in its aqueous phase.

Main substances transported by plasma

- Thyroid hormones.

- A wide range of drugs.

- Unconjugated bilirubin (indirect).

- Lipophilic compounds not soluble in water, such as certain fatty acids, vitamins and hormones.

Given its importance, albumin has different means of regulation in order to keep its plasma levels stable.

Albumin synthesis

Albumin is synthesized in the liver from amino acids obtained from dietary proteins. Its production occurs in the endoplasmic reticulum of hepatocytes (liver cells), from where it is released into the bloodstream where it will remain circulating for approximately 21 days.

For the synthesis of albumin to be efficient, two fundamental conditions are required: adequate supply of amino acids and healthy hepatocytes capable of converting such amino acids into albumin.

Although some proteins similar to albumin can be found in the diet - such as lactalbumin (milk) or ovalbumin (eggs) - these are not used directly by the body; in fact, they cannot be absorbed in their original form due to their large size.

In order to be used by the body, proteins such as lactalbumin and ovalbumin are digested in the digestive tract and reduced to their smallest components: amino acids. These amino acids will then be transported to the liver to manufacture albumin that will perform physiological functions.

Causes of albumin deficiency

As with almost any compound in the body, there are two main causes of albumin deficiency: insufficient synthesis and increased losses.

Insufficient synthesis

As already mentioned, in order for albumin to be synthesized in sufficient quantities and at a constant rate, it is necessary to have "raw material" (amino acids) and a "working factory" (hepatocytes). When one of these parts fails, albumin production goes into decline and its levels begin to decline.

Malnutrition is one of the main causes of hypoalbuminemia (as low levels of albumin in the blood are known). If the body does not have a sufficient supply of amino acids for a long time, it will not be able to maintain the synthesis of albumin. For this reason, this protein is considered a biochemical marker of nutritional status.

Compensation mechanisms

Even when the supply of amino acids in the diet is insufficient, there are compensation mechanisms, such as the use of amino acids obtained from the lysis of other available proteins.

However, these amino acids have their own limitations, so if the supply is kept restricted for a long period of time, albumin synthesis inexorably declines.

Importance of hepatocytes

The hepatocytes must be healthy and able to synthesize albumin; otherwise, the levels will drop because this protein cannot be synthesized in another cell.

Then, patients who suffer from liver diseases - such as liver cirrhosis, in which the dying hepatocytes are replaced by fibrous and non-functional tissue - begin to present a progressive decrease in the synthesis of albumin, whose levels decrease steadily and sustained.

Increased losses

As already mentioned, albumin has an average life of 21 days at the end, of which it breaks down into its basic components (amino acids) and waste products.

In general, the half-life of albumin remains unchanged, so an increase in losses would not be expected if it were not for the fact that there are points where it could escape from the body: the renal glomeruli.

Filtration through the glomeruli

The glomerulus is the structure of the kidney where the filtration of impurities from the blood occurs. Because of the blood pressure, the waste products are forced there through small openings that allow harmful elements to exit the bloodstream and keep proteins and blood cells inside.

One of the main reasons why albumin does not "escape" under normal conditions through the glomerulus is its large size, which makes it difficult for it to pass through the small "pores" where filtration takes place.

Action of the negative charge of albumin

The other mechanism that "protects" the body against the loss of albumin at the kidney level is its negative charge, which is equal to that of the basement membrane of the glomerulus.

Since they have the same electrical charge, the basement membrane of the glomerulus repels albumin, keeping it away from the filtration area and within the vascular space.

When this does not happen (as in cases of nephrotic syndrome or diabetic nephropathy), albumin begins to pass through the pores and escapes with the urine; first in small amounts, and then in greater amounts as the disease progresses.

At first, synthesis can make up for the losses, but as they increase, synthesis no longer manages to replace the lost proteins and albumin levels begin to decrease, so unless the cause of the losses is corrected, the amount of circulating albumin it will continue to go down irretrievably.

Consequences of low albumin

Decreased oncotic pressure

The main consequence of hypoalbuminemia is a decrease in oncotic pressure. This makes it easier for fluids to flow out of the intravascular space into the interstitial space (microscopic space that separates one cell from another), accumulating there and generating edema.

Depending on the area where the fluid accumulates, the patient will begin to present lower limb edema (swollen feet) and pulmonary edema (fluid within the pulmonary alveoli) with consequent respiratory distress.

You could also develop pericardial effusion (fluid in the sac that surrounds the heart), which can lead to heart failure and eventually death.

Decline in the function of some hormones

Furthermore, the functions of hormones and other substances that depend on albumin for transport decline when there is not enough protein to transport all the hormones from the site of synthesis to the area where they must act.

Decreased effect of drugs

The same occurs with medications and drugs, which are impaired by the inability to be transported in the blood by albumin.

To alleviate this situation, exogenous albumin can be administered intravenously, although the effect of this measure is usually transitory and limited.

The ideal, whenever possible, is to reverse the cause of hypoalbuminemia in order to avoid deleterious consequences for the patient.

Types of albumin

- Seroalbumin: important protein in human plasma.

- Ovalbumin: from the serpin protein superfamily, it is one of the proteins in egg white.

- Lactalbumin: protein found in whey. Its purpose is to synthesize or produce lactose.

- Conalbumin or ovotransferrin: with great affinity for iron, it is part of 13% of the egg white.

References

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