Describe and explain the function of the loop of henle and suggest how its structure may differ in organisms adapted to a desiccated environment.

The loop of henle is a region of the kidney's nephrons which through active and passive transport of Na+ and Cl- ions lowers the water potential of the kidney's medulla, allowing greater reabsorbtion of water into the blood at both the loop of henle and collecting duct. This is achieved through differing permeability of the structure's descending and ascending limbs. The endothelium of the descending limb is permeable to water molecules, but impermeable to Na+ or Cl- ions. As the descending limb moves deeper into the kidney's medulla, the water potential of the medulla's interstitial fluid decreases. This allows water to move into the interstitial fluid passively by osmosis, before being reabsorbed into the blood. As water leaves the nephron filtrate and it becomes more concentrated, the water potential within the filtrate may begin to approach that of the interstitial fluid, reducing the rate at which water can leave and creating potential for filtrate ions to move into the medulla. As the descending limb transitions into the ascending limb the endothelium changes to become impermeable to water, but permeable to Na+ and Cl- ions. These ions move into the medulla by passive and (higher in the limb) active transport facilitated by ion transport proteins, where they lower water potential of the interstitial fluid, maintaining the conditions necessary for osmosis of water out of the descending limb. This feedback is known as the 'countercurrent multiplier mechanism'. With ions actively transported against their concentration gradient at the ascending limb the concentration of the filtrate is reduced, heightening the water potential gradient between the filtrate and medulla once again to allow further reabsorbtion of water at the collecting duct.Tetrapod organisms adapted for desiccated environments may have kidney nephrons with longer loops of henle moving deeper into the medulla, or more ion transport proteins. This would allow more ions to be pumped out of the nephron, creating an even greater water potential gradient and facilitating the reabsorbtion of more water into the blood. All this would prevent dehydration and improve the organism's water use efficiency.