Albumin Physiology

      
       

 

          
       

Albumin is the most abundant extracellular protein, its distribution is primarily intravascular.

Albumin is the most abundant extracellular protein. It is a single polypeptide with 585 amino acids and a molecular weight of  66,200D. It is thus a medium sized compound (IgG is 150,000) which, in addition to being highly soluble, is small enough to pass through fenestrated endothelium, such as in the nephron. That proteinuria does not occur in normal people is consequent of a strong negative charge (–17), which rebuts the protein in the glomerulus.

Albumin is manufactured in the liver at a rate 9-12g/day. The normal serum albumin is 30 to 40 grams per litre. There is no storage, no reserve. Being the major source of oncotic pressure in health, it stands to reason that the rate of production of albumin is controlled by changes in colloid osmotic pressure and osmolality of extravascular liver space. The capacity for increased production is fairly low (can only increase by a factor of 2 or 3). Increased synthesis is increased by the neuroendocrine system, chiefly by insulin, thyroid hormones and cortisol.

Albumin is catabolized  at a rate of 9 - 12 g/day (the same rate as it was produced) by pinoctosis in cells adjacent to the vascular endothelium. Albumin is not catabolized in starvation: under these circumstances, protein is derived from muscle, following exhaustion of fat stores.

Although albumin is perceived as intravascular protein, the total extravascular albumin actually exceeds the total intravascular amount by 30%. The ratio of albumin to water is, however higher in the intravascular space (the extracellular fluid is 2/3 interstitial and 1/3 intravascular), hence the colloidal effect. Albumin cyclically leaves the circulation, through the endothelial barrier at the level of the capillaries, passes into the interstitium and returns to the bloodstream through the  lymph system  via thoracic duct.  The circulation half time for this process is 16 -18 hours. 4 - 5% of total intravascular albumin extravascates in this way per hour: this rate of movement is known as the Transcapillary Escape Rate (TER), and this is determined by:                        

  • Capillary and interstitial free albumin concentration.

  • Capillary permeability to albumin.

  • Movements of solvent / solute.

  • Electrical charges across the capillary wall.

The concentration of albumin in lymph protein content is approximately 80% that of plasma.

         
                   
       

         
     

       
       

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