Heat and water challenges activate linked responses

1. A challenge can disturb body temperature and water balance at the same time.
2. Homeostatic systems use negative feedback to oppose each disturbance and return conditions towards their normal ranges.
3. The response depends on the direction and size of the change rather than on a single fixed action.
4. Temperature control changes heat production and heat loss, while osmoregulation changes thirst, ADH release and kidney water reabsorption.

Heavy sweating causes cooling and dehydration

1. In hot conditions or during exercise, sweat secretion increases and evaporation transfers thermal energy away from the skin.
2. Sweating removes water from the body, so blood volume falls and the water potential of the blood decreases.
3. The osmoregulatory centre detects the lower blood water potential.
4. The pituitary releases more ADH, making kidney tubules and collecting ducts more permeable to water.
5. More water is reabsorbed into the blood, so a smaller volume of concentrated urine is produced.
6. The thirst centre stimulates drinking, replacing water that cannot be recovered by the kidneys.
7. If dehydration becomes severe, reduced sweating limits further water loss but also reduces evaporative cooling, increasing the risk of overheating.

Excess water intake increases water excretion

1. Drinking a large volume of water raises the water potential of the blood and dilutes its dissolved substances.
2. The osmoregulatory centre detects the rise in water potential.
3. Less ADH is released, so the kidney tubules and collecting ducts become less permeable to water.
4. Less water is reabsorbed into the blood and more remains in the tubule fluid.
5. A large volume of dilute urine is produced, removing the excess water.
6. As blood water potential returns towards normal, ADH release returns towards its usual level.

High salt intake draws water from cells

1. Eating a salty meal increases the concentration of dissolved ions in the blood and lowers blood water potential.
2. Water moves from body cells into the blood by osmosis, so cells may lose water and shrink.
3. The lower blood water potential stimulates thirst and increases ADH release.
4. Drinking adds water to the blood and helps dilute the excess salt.
5. Increased ADH causes more water reabsorption, producing a smaller volume of concentrated urine.
6. The kidneys also excrete excess ions, but water is required to carry them out in urine.
7. Once water potential returns towards normal, thirst and ADH release decrease.

Extended answers must connect both systems

1. A complete response to a combined challenge identifies the initial change, the receptor or control centre, the effectors and the effect on the controlled condition.
2. For heat, name sweating, vasodilation or shivering and explain how each changes heat transfer or production.
3. For water balance, name thirst, ADH, kidney tubule permeability, water reabsorption and urine concentration.
4. Finish by showing how the response returns temperature or water potential towards the normal range.