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Just a filter keeps grounds out of your coffee, the glomerular filtration membrane keeps blood cells and proteins out of the urine passage ways. This filtration membrane is composed of three layers: Fenestrated glomerular endothelium, basement membrane and the many filtration slits formed by pedicels of the podocytes. Passage through the filtration membrane is limited not only on the basis of size, but also by charge. Glomerular filtration is a process of bulk flow driven by the hydrostatic pressure of the blood. As you can see, small molecules pass rapidly through the filtration membrane, while large proteins and blood cells are kept out of the capsular space. You can probably guess the severe effect on blood in urine composition if the filtration membrane is damaged or destroyed. The fluid and solutes collecting in the capsular space is called glomerular filtrate. Now we’ll take a sample of the glomerular filtrate and see what it contains. … and analyze the filtrate determine the contents of this sample. Our sample from a healthy person shows the common components of filtrate to be: water; ions such as sodium, potassium, and chloride; nitrogenous waste such as urea, uric acid, and creatinine; and useful organic molecules including glucose and amino acids. The concentration of each of these substances in the glomerular filtrate is similar to its concentration in plasma. How might the contents of the filtrate be altered if the filtration membrane is damaged or destroyed? Protein is increased, which produces a condition called proteinuria. The presence of blood cells can occur depending on the amount of damage. This condition is called hematuria. Just as gravity draws water through a coffee filter, so glomerular hydrostatic pressure promotes filtering of the blood. The blood pressure in the glomerulus averages 60 mm of mercury. This unusually high capillary pressure is the result of the short, large diameter afferent arterioles conveying blood at high arterial pressure directly to the glomerular capillaries. The smaller diameter of the efferent arterioles leaving the glomerulus also helps maintain the pressure by restricting the outflow of blood. However, the glomerular hydrostatic pressure is opposed by two forces that reduce the flow of fluid into the capsular space. Fluid already in the capsular space creates a back pressure that resist the incoming fluid. This capsular hydrostatic pressure averages 15 mm of mercury. The second opposing force is the osmotic pressure of the blood in the glomerular capillaries. Remember that the capillaries retain proteins, which become more concentrated in the blood as the filtrate flows out. Because of these proteins, the osmolarity of the blood is higher than the osmolarity of the filtrate. The osmotic pressure of the blood, or its tendency to draw the fluid back in, averages 28 mm of mercury. The algebraic sum of these three forces produces the net filtration pressure of 17 mm of mercury.