Albumin is the most abundant protein in human blood plasma, making up approximately half of the total protein content. It is exclusively synthesized by the liver and released into the bloodstream, typically ranging from 3.5 to 5 grams per deciliter in a healthy adult. Supplemental albumin administered to patients is derived from donated human plasma and processed into a therapeutic solution for intravenous use. The primary biological functions of this molecule are maintaining fluid balance within the circulation and serving as a transport vehicle for numerous substances.
Albumin’s Essential Role in the Body
The primary physiological function of albumin is regulating plasma oncotic pressure, also known as colloid osmotic pressure. Because the albumin molecule is large and cannot easily pass through healthy blood vessel walls, its presence creates a pressure gradient. This gradient draws water molecules from surrounding tissues back into the bloodstream. This mechanism is responsible for about 70 to 80% of the total oncotic pressure exerted by blood plasma.
By continually pulling fluid back into the vessels, albumin helps maintain the volume of circulating blood, which is necessary for stable blood pressure and effective circulation. A drop in albumin levels, called hypoalbuminemia, disrupts this balance, allowing fluid to leak out of the bloodstream and accumulate in the interstitial spaces. This fluid accumulation manifests as edema, or swelling, in the tissues.
Beyond fluid dynamics, albumin operates as a versatile transport molecule, binding to and carrying a wide array of ligands through the circulation. This function is necessary for distributing hormones, such as thyroid hormones, and various ions, including calcium and magnesium. It also carries fatty acids and bilirubin, a waste product, to the liver for processing.
Albumin’s ability to bind to substances extends to many pharmaceutical drugs, influencing their distribution and duration of action. The molecule also exhibits antioxidant properties, helping to scavenge harmful free radicals in the plasma. Low albumin levels reduce this carrier capacity, potentially leading to altered drug effects and a loss of antioxidant protection.
Use in Acute Volume Loss and Shock
In acute, life-threatening scenarios involving massive fluid loss, albumin is administered to rapidly expand circulating blood volume and treat shock. This hypovolemic shock can result from massive hemorrhage due to trauma or extensive plasma loss associated with severe burns. The goal of using albumin in these settings is rapid resuscitation, restoring perfusion to vital organs.
In trauma and hemorrhagic shock, the immediate need is to replace lost volume quickly to prevent circulatory collapse. Albumin is classified as a colloid solution, containing large protein molecules that remain largely confined to the intravascular space and draw fluid back into the vessels. This mechanism allows a smaller volume of albumin solution to achieve the same volume expansion as a larger volume of crystalloid solutions.
For patients with major burns, the injury causes widespread damage to the capillary walls, resulting in a massive leak of fluid and proteins into the burned tissues. This protein loss leads to severe hypoalbuminemia and hypovolemic shock. Administering exogenous albumin helps replace the lost protein and fluid, which reduces the overall volume of fluid needed for resuscitation.
Albumin solutions are available in 5% and 25% concentrations, chosen based on clinical need. The 5% solution is iso-oncotic, meaning its oncotic pressure is similar to normal plasma, and is often used for general volume replacement. The 25% solution is hyper-oncotic, exerting a powerful osmotic pull that draws fluid from the interstitial space into the circulation. This makes the 25% concentration useful in cases of severe edema or when rapid plasma volume expansion is necessary.
Managing Fluid Imbalances in Chronic Illness
Albumin infusion is used in chronic conditions where the body’s ability to produce or retain the protein is compromised, leading to persistent fluid imbalances. A primary use is in patients with advanced liver disease, such as cirrhosis. The damaged liver fails to synthesize sufficient albumin, and the resulting low plasma concentration contributes to the development of ascites, which is fluid accumulation in the abdominal cavity.
In cirrhosis, albumin is often administered after a large-volume paracentesis, the procedure used to remove excess fluid from the abdomen. This intervention prevents paracentesis-induced circulatory dysfunction (PICD), a complication where sudden fluid removal leads to severe circulatory instability. The recommended dose is typically 6 to 8 grams of albumin for every liter of ascitic fluid removed beyond five liters.
Albumin is also a component of treatment for complications like spontaneous bacterial peritonitis (SBP) and hepatorenal syndrome (HRS) in cirrhotic patients. In these situations, the protein acts not only as a volume expander but also provides non-oncotic benefits, such as reducing systemic inflammation and stabilizing the endothelium. This dual action helps preserve kidney function and improve overall outcomes in these severe infections and forms of kidney failure.
Another chronic condition requiring albumin is nephrotic syndrome, a kidney disorder where damage to the filtering units causes large amounts of protein to leak into the urine. This constant loss leads to severe hypoalbuminemia and widespread edema. While primary treatment targets the underlying kidney disease, albumin can be given temporarily to restore oncotic pressure. This helps mobilize excess fluid from the tissues back into the circulation, managing the severe swelling.