Proximal tubule, albumin reabsorption

Hypoalbuminemia is a consequence of increased glomerular filtration of albumin. Loss of albumin is caused both by the loss of albumin into the urine (albuminuria) and by the reabsorption of albumin in the proximal tubule, which is mostly accompanied by its degradation. Increased tubular degradation of albumin may explain severe hypoalbuminemia in patients with only relatively moderate proteinuria, ca 3.5-4 g/day. Synthesis of albumin in the liver of nephrotic patients... 

In addition, the proportions of individual proteins excreted in the urine depend on the extent of their reabsorption by the proximal renal tubules, which is also inversely proportional to molecular size. The reabsorption of low molecular weight proteins involves binding of the protein to specific receptors on the tubule epithelial cells, uptake into the cells by pinocytosis, fusion of uptake vacuoles with lysosomes, and hydrolysis of the protein by lysosomal enzymes, followed by return of the constituent amino acids to the body pool. Very little of the total excreted urinary protein normally consists of small proteins. Only a small amount of protein is excreted normally (20 to 150mg/dL), and most of it is albumin because of its concentration—it is not completely removed from the filtrate by the tubular cells. The remainder is almost entirely the Tamm-Horsfall protein uromucoid, a constituent of urinary casts, probably secreted by the distal tubules. 

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