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# Glomerular Filtration Urine is the ultimate kidney product and based upon waste removed from the blood with the addition of some other fluids and ions. This process is initiated in the glomerulus when blood is filtered out under hydrostatic pressure leaving the small molecules of waste and other...

# Glomerular Filtration Urine is the ultimate kidney product and based upon waste removed from the blood with the addition of some other fluids and ions. This process is initiated in the glomerulus when blood is filtered out under hydrostatic pressure leaving the small molecules of waste and other compounds as glomerular filtrate, this process is called **ultra-filtration**. Filtrate primarily includes water, electrolytes, some amino acids, bicarbonate, and nitrogenous wastes, like urea, uric acid, and creatinine. Glomerular filtration rate (GFR) is directly proportional to the hydrostatic pressure exerted in its wall. This pressure is increased due to the difference in diameter of both afferent and efferent arterioles, as mentioned earlier. The kidney receives **180 liters of blood** by circulation in **24 hours** and after filtration it produces **2.5 liters of urine** in normal climatic conditions. GFR usually remains constant by autoregulation however it may change depending on the fluid intake or it's variable amount in the body. # Selective Reabsorption The composition of glomerular filtrate and the urine is different, it means that the fluid contents become change while passing through the renal tubules, including the PCT, loop of Henle and DCT in the nephron. For example, glucose if present in the filtrate but absent in the urine of a healthy person. The amount of urea and uric acid present more in urine than the filtrate. These changes are the outcome of selective reabsorption and the tubular secretion. **Selective reabsorption** is the process whereby certain molecules after being filtered out of the capillaries along with nitrogenous waste products (i.e. urea) and water in the glomerulus, are reabsorbed from the filtrate as they pass through the nephron and return back to the blood circulation. Most of the selective reabsorption of molecules takes place in the **proximal convoluted tubule (PCT)**. **Water (about 67%)**, Na and K variable quantities of Cl- (about 50%), Ca, Mg2+ and HPO42- ions, important nutrients like glucose (100%), amino acids, vitamins, and other organic substances are reabsorbed in the PCT, and given back to the blood circulation. Water is absorbed passively while glucose and sodium are absorbed actively. Na drags the other negatively charged ions due to opposite charge interaction. **Hormone aldosterone facilitates the sodium and ADH facilitates the water reabsorption**.