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# Chapter 15 - Homeostasis ## Glomerular Filtration - Urine is the ultimate kidney product and is based upon waste removed from the blood with the addition of some other fluids and ions. - This process is initiated in the glomerulus, where blood is filtered out under hydrostatic pressure, leaving sm...

# Chapter 15 - Homeostasis ## Glomerular Filtration - Urine is the ultimate kidney product and is based upon waste removed from the blood with the addition of some other fluids and ions. - This process is initiated in the glomerulus, where blood is filtered out under hydrostatic pressure, leaving small molecules of waste and other compounds as **glomerular filtrate**. - This process is called **ultrafiltration**. - Filtrate primarily includes water, electrolytes, some amino acids, bicarbonates and nitrogenous wastes like urea, uric acid, and creatinine. - Glomerular filtration rate (GFR) is directly proportional to the hydrostatic pressure exerted in the wall. - This pressure is increased due to the difference in diameter of both afferent and efferent arterioles. - Kidneys receive 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 upon the fluid intake or its variable amount in the body. ## Selective Reabsorption - The composition of glomerular filtrate and the urine is different since it means that the fluid contents become changed while passing through the renal tubules including PCT, loop of Henle, and DCT in the nephron. - For example, glucose is 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 tubular secretion. - 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%) - Ca2+ - Mg2+ - HPO42- ions - Important nutrients like glucose (100%) - Amino acids - Vitamins - Other organic substances - These are reabsorbed in 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 interactions. - Hormone aldosterone facilitates the sodium and ADH facilitates the water reabsorption.