Urinary secretion is a mechanical process during which water and solutes are filtered from the blood through the glomerular membranes, while the plasma proteins are retained. Proteins in the urine in abnormal amounts are plasma proteins which have passed through permeable glomeruli; only insignificant amounts originate in the tubules. The mechanical process of glomerular filtration is further evidenced by the fact that protein molecules below a molecular weight of approximately 70,000 (that of hemoglobin) readily escape through the glomeruli, whereas larger protein molecules are usually retained; in other words, the larger the molecular size of the urinary protein, the greater is the glomerular lesion. The albumins, being the smallest of plasma proteins, should appear earliest and in greatest amounts in the urine under abnormal renal conditions. The globulins, which are much larger, should appear only when more severe lesions are present and in lesser amounts than the albumins. Fibrinogen, the
BERG S. URINARY PROTEIN PARTITIONS IN AMYLOID NEPHROSIS. Arch Intern Med (Chic). 1941;67(5):1050–1060. doi:10.1001/archinte.1941.00200050158011
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