Structure of Kidney

• 1. Ultrafiltration: blood from renal artery enters the glomerulus. Water, urea, salts and glucose are forced into the Bowman’s capsule. Blood cells and large proteins cannot pass through.

  
• 2. Selective reabsorption: in the proximal tubule two thirds of the salt and water and all the glucose moves out of the nephron, by active transport. These substances are reabsorbed back into the blood capillary.

  
• 3. Loop of Henlé: this part of the loop of Henlé is permeable to water but not salt. Water is drawn out of the filtrate in the nephron by osmosis because of the low water potential of the medulla tissue fluid.

  
• 4. Loop of Henlé: this part is permeable to salt but not water. The loss of water from the filtrate in the previous stage increases the salt concentration. Some salt passively diffuses out of the loop here.

  
• 5. Collecting duct: the remaining substances move through the second coiled tubule (distal tubule), into the collecting duct. The permeability of this part of the nephron to water is controlled