Formation in Human · Part 4

To counteract this dilution the region of the ascending limb actively pumps solutes from the lumen into the interstitial fluid and the osmolarity increases to about 1200mOsm in medulla. This mismatch between water reabsorption from this point is dependent on the body’s need and is regulated by hormones. Reabsorption of bicarbonate (HCO ) takes place to regulate the blood pH. Homeostasis of K and Na in the blood is also regulated in this region. Aquaporins are water–permeable channels (membrane transport proteins) that allow water to move across the epithelial cells in relation to the osmotic difference from the lumen to the interstitial fluid. Collecting duct is permeable to water, secretes K (potassium ions are actively transported into the tubule) and reabsorbs Na to produce concentrated urine. The change in permeability to water is due to the presence of number of water- permeable channels called aquaporins . Tubular secretion- Substances such as H , K , NH , creatinine and organic acids move into the filtrate from the peritubular capillaries into the tubular fluid. Most of the water is absorbed in the proximal convoluted tubule and Na is exchanged for water in the loop of Henle. Hypotonic fluid enters the distal convoluted tubule and substances such as urea and salts pass from peritubular blood into the cells of DCT. The urine excreted contains both filtered and secreted substances. Once it enters the collecting duct, water is absorbed and concentrated hypertonic urine is formed. For every H secreted into the tubular filtrate, a Na is absorbed by the tubular cell. The H secreted combines with HCO , HPO and NH and gets fixed as H CO , H PO and NH respectively. Since H gets fixed in the fluid, reabsorption of H is prevented. XI Std Biology-Zoology Chapter- results in passive uptake of solutes and loss of water in descending vasa recta. As the blood enters the