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

ADH (anti-diuretic hormone) helps to regulate the water content of blood by influencing how much urine is produced by the kidneys. When blood volume increases, due to increased hydration, the osmolarity decreases as salts in the blood are more diluted. This is detected by osmoreceptors in the hypothalamus, which stimulates secretion of ADH. ADH acts on the collecting ducts of the kidney nephron, making it less permeable to water so more urine is produced. Similarly, when blood volume decreases due to dehydration, this leads to more concentrated salts, a higher osmolarity, and hence less secretion of ADH by the hypothalamus. Therefore, the collecting duct membranes are more permeable, so more water can be reabsorbed and less urine is produced.