Explain how water is re-absorbed in the kidney

It would be helpful for you to have a diagram of a kidney nephron in front of you as you read this explanation. The body has a clever mechanism to conserve water levels, it creates a strong salt concentration in the medulla of the kidney via the Loop of Henle. This means that water can be drawn out of the tube later on by osmosis and taken away by the blood. It's easier to understand if we start with the second part of the Loop of Henle which is called the ascending limb as it is moving back upwards, out of the medulla. In this part of the tube, sodium and chloride ions are actively pumped out of the filtrate into the tissue fluid around it. However no water follows as this part of the loop is impermeable to water. The consequence of this means that water flows out via osmosis of the descending limb which runs alongside the ascending limb in the opposite direction (the first part of the Loop of Henle) into the tissue fluid. It also means that the sodium and chloride ions which are at a high concentration in the tissue fluid, due to the action of the ascending limb, end up diffusing down the concentration gradient into the descending limb. By the time the filtrate reaches the bottom of the descending limb, the fluid in the loop has lost a lot of water and is very concentrated. And the fluid surrounding the bottom of the loop, deep in the medulla, is also very concentrated because of the accumulation of sodium and chloride ions. The fluid then moves up the ascending limb and the salt ions are actively pumped out (as previously mentioned) and so the filtrate becomes more and more dilute. Note that fluid passing down the descending limb is flowing in the opposite direction to the fluid in the ascending limb. The fluid is increasingly concentrated as it moves down and increasingly dilute as it moves upwards - this counter current flow allows concentrated urine to be produced and is known as the "countercurrent multiplier". The importance of creating this "salty medulla" is so that when the fluid flows down the collecting duct, yet more water is drawn out as it passes through the concentrated surroundings. Any filtrate which is not reabsorbed (most of the urea, some salt and some water) flows into the bladder. The amount of water reabsorbed is controlled by the quantity of water in the blood. The less water in the blood, the more that must be reabsorbed. The hormone antidiuretic hormone or ADH controls the extent to which water is reabsorbed. If blood is concentrated, more ADH is released which makes the walls of the collecting duct more permeable to water, allowing more to be reabsorbed. If blood is more dilute, less ADH is released and so less water is reabsorbed.