Explain how the body responds to a fall in water potential.

Providing there is a basic understanding of the anatomy and functioning of the kidney, we can start by stating that the homeostatic control of osmoregulation in the blood is achieved by hormone ADH (anti-diuretic hormone) that acts on the distal convoluted tubule & the collecting duct in the kidney. The water potential of the blood depends on the concentration of solutes (like glucose, proteins, sodium chloride & other mineral ions) and volume of water in the body so therefore, a decrease in blood water potential can be due to too little water being consumed, excessive sweating occurring or large amounts of ions (e.g. NaCl) being taken in. Controlling blood water potential is an example of both homeostasis and a negative feedback system within the body (an example of either may be required at some point in a further exam!). It's easiest to remember the function of this hormone in a simple flow diagram, that can be drawn out for either an increase or decrease in blood water potential. Firstly we need to remember that the water potential of the blood is monitored by the osmoreceptor cells in the hypothalamus of the brain. When the water potential decreases, i.e. the blood becomes too salty, these cells shrink (as water is lost by osmosis into the blood) and produce ADH hormone. This travels to the pituitary gland in the brain where it's released into the blood. The effect of this hormone is to increase the permeability of the tubule and collecting duct within the kidney by increasing the number of aqua-porins in the surface membrane of cells. This means more water moves back out of the tubule and duct so can be reabsorbed back into the blood stream. More concentrated urine, containing less water, is therefore produced by the kidney. Once able we're able to understand how the body copes with a fall in blood water potential, we should also be able to understand how it copes with an increase; simply by osmoreceptor cells, in this case, decreasing the release of ADH to reduce the permeability of the tubule and collecting duct. We can then predict that the urine will become less concentrated.