Explain the role of ADH in regulating the water potential of the blood.

Firstly, a description of water potential would be a good place to start. An example definition of this could be: the pressure created by water molecules. The greater the number of water molecules present, the higher (less negative) the water potential, in relation to pure water, which has a water potential of zero. If the solute concentration is higher, the water molecules can move less freely in solution, so the water potential will be lower. The blood has lots of solutes such as proteins, glucose and sodium chloride, which all influence the water potential. As water potential must be relatively stable within the body, via a process called osmoregulation (homeostasis), the body must have mechanisms in place to cope with a change in water potential. Osmoreceptors in the hypothalamus detect changes in water potential. When the water potential is low (so the blood has a high solute concentration), these osmoreceptor cells lose water by osmosis and shrink, causing the release of a peptide hormone called ADH (vasopressin/anti-diruretic hormone). ADH passes to the posterior pituitary gland and is secreted into the capillaries. ADH passes through the blood into the nephrons of the kidney, where it acts on the collecting tubules and distal convoluted tubules (DCT) to stimulate V2 receptors. Via an intracellular cascade, the end result is the insertion of aquaporins onto these cell membranes. This increases the permeability of the collecting tubules and DCT to water, so more water can be reabsorbed, down a water potential gradient, to re-enter the blood. Less urine and urine that is more concentrated is produced. The thirst centre of the brain is stimulated by the osmoreceptors so the individual is alerted to change their behaviour (ie. to drink more water). This whole system works via negative feedback, as when osmoreceptors detect a rise in water potential, they send fewer impulses to the pituitary gland, so less ADH is released and the normal water potential of the blood can be restored. ADH also affects urea concentrations in the body, as it can stimulate more urea channels to be inserted onto the membranes of the collecting ducts, also contributing to the homeostasis of blood water potential.