Substances from the blood cross 3 barriers :
1 . Glomerular Endothelial Cells : very leaky because they have Fenestrations (pores). These cells permit all solutes in blood plasma to exit glomerular capillaries but prevents filtration of blood cells and platelets. 2. Basal Lamina: a layer of a cellular material between endothelium and the podocytes consisting minute fibres of collagen and proteoglycans in a glycoprotein matrix. This layer prevents filtration of larger plasma proteins. 3. Filtration Slits : extending from each podocyte are 1000s of foot like processes called Pedicels that wrap around glomerular capillaries. Filtration slits are the spaces between them. Across each slit is a thin membrane called the Slit Membrane and it permits the passage of molecules having a very small diameter, including water, glucose, vitamins, amino acids, very small plasma proteins, ammonia, urea, and ions.
Net Filtration Pressure
Glomerular filtration depends on 3 main pressures :
1. Glomerular Blood Hydrostatic Pressure (GBHP): is the blood pressure in the glomerular capillaries (generally about 55 mmHg ). It promotes FILTRATION by forcing water and solutes in blood plasma through the filtration membrane. 2. Capsular Hydrostatic Pressure (CHP): is the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule. CHP opposes filtration and represents “back pressure” of about 15 mmHg . 3. Blood Colloid Osmotic Pressure (BCOP): is due to the presence of proteins such as albumin, globulins, and fibrinogen in blood plasma; also opposes filtration. The average BCOP in glomerular capillaries is 30 mmHg .
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