Extra Feedback

ADH is not the only way the body corrects its water balance. Several extra feedback mechanisms work alongside it.Thirst. When the blood water potential falls, the osmoreceptors in the hypothalamus also stimulate the thirst centre, producing the feeling of thirst. Drinking raises the blood water potential. When it returns to normal, the osmoreceptors are stimulated less and thirst stops. This is a behavioural negative feedback. Swallowing water also gives a quick, temporary reduction in thirst before the water is even absorbed, which helps to stop us drinking far too much.Removal of ADH. ADH is broken down continually, mainly in the liver, so it has a short half-life in the blood (roughly 10 to 20 minutes). This means the ADH concentration falls quickly when release slows, so the kidneys respond within minutes to a change in blood water potential.Blood volume and pressure. Losing a lot of fluid, for example by bleeding or heavy sweating, lowers blood volume and blood pressure. Stretch receptors in the walls of the atria, and the baroreceptors in the aorta and carotid arteries, detect this and signal to the hypothalamus, which increases ADH release. When blood volume is too high, the atria are stretched more and ADH release is reduced, so more water is excreted.Sodium balance. When the blood sodium ion concentration or blood volume is low, the adrenal cortex releases aldosterone, which increases Na⁺ reabsorption in the distal convoluted tubule and collecting duct. Water follows the ions by osmosis.These mechanisms reinforce each other, so that both the concentration and the volume of the blood are kept stable.