Urinary 3

• The loop of Henle participates in a counter current mechanism where parallel passages in which the contents are in flow in opposite directions. This is important since it keeps the solute concentration of the medulla high
• The wall of the descending limb is thinner than the wall of the ascending limb. The thin wall allows water to diffuse out of the tubule and urea to diffuse into the tubule. The thick wall allows active transport of molecules (Na, Cl) out of the tubule and into the interstitial fluid
• Because of the exchange between the tubule and IF, the tubule fluid is hypotonic to the IF. This is called the countercurrent multiplier mechanism
• When filtrate comes from the PCT to the descending limb, it is high in water but low in sodium, chloride, glucose and urea. It is then hypertonic at the bottom of the loop because it loses water and gains salt. It is then hypotonic in the ascending limb because it loses salts but not water
• DCT reabsorbs sodium by active transport
• Cells that form the walls of DCT are impermeable to water. If the sodium can leave the DCT but water cannot, the solute concentration will continue to decrease and the kidney will produce a hypotonic urine
• ADH is a peptide hormone released by the posterior pituitary gland. It targets cells in the DCT and collecting ducts and makes them more permeable to water by adding more aquaporins. This results in some reabsorption of water in the DCT
• Secretion is the third step in urine formation. It consists of the movement of substances out of the blood and into the tubular fluid.
• The DCT and collecting ducts secrete potassium, hydrogen ions and drugs such as penicillin
• Potassium secretion increases with aldosterone, which is produced by the adrenal glands. Aldosterone production results in the activity of the sodium potassium pump, moving sodium out of the tubule and potassium inside
• Aquaporins (water channels) are absent in the DCT unless ADH is present. The sodium potassium pump is inactive unless aldosterone is present. This transport requires a pump because sodium goes from low to high
• Alcohol and caffeine are diuretics since they block aquaporins. The water then stays in the DCT and is evacuated so it makes you pee more
• Both aldosterone and ADH target the DCT and collecting ducts. ADH causes water reabsorption and alodsterone causes an increase in the activity of the sodium potassium pump