Thirst: the behavioural response to raised plasma osmolarity
Thirst is the urge to drink, and a rise in plasma osmolarity is its most direct trigger; drinking adds free water, meaning water without dissolved particles, which dilutes the plasma.
Remember
The kidney can only partly compensate for a large rise in plasma osmolarity, so thirst takes over and the person drinks free water to dilute the plasma.
When plasma osmolarity (the concentration of dissolved particles in the plasma) rises, osmoreceptors detect the change. These are sensor cells in the hypothalamus, an area at the base of the brain. They trigger two responses: the release of antidiuretic hormone (ADH), the hormone that makes the kidney reabsorb more water, and thirst, which is the urge to drink.
ADH reduces the amount of water lost in the urine. However, ADH only conserves water that the body already has: it does not add any. Because of that limit, large changes in osmolarity need the behavioural response, meaning a response that changes what the person does.
Drinking adds free water, which dilutes the plasma and lowers osmolarity towards normal. ADH is released with about a 1% change in plasma osmolarity, but thirst takes over when the kidney cannot keep up with a large rise.
Osmolarity is the most direct trigger for thirst, but not the only one. A fall in blood volume is a second trigger, partly through angiotensin II (a hormone that helps to stimulate thirst). When enough has been drunk, the urge stops, although the exact mechanism is unknown.
Key facts
- Trigger
- a rise in plasma osmolarity, detected by osmoreceptors in the hypothalamus.
- Response
- thirst, the urge to drink, which leads to the person drinking water.
- Why drinking matters
- ADH only conserves water, whereas drinking adds free water to the body.
- Off switch
- when enough has been drunk, the urge stops, although the exact mechanism is unknown.
- Second stimulus
- a fall in blood volume also stimulates thirst, partly through angiotensin II.
Exam traps
- A fall in arterial blood pressure is sensed by baroreceptors (pressure sensors in the arteries). It can contribute to thirst, but the hypothalamic osmoreceptors do not detect it directly.
- A fall in plasma osmolarity, even when osmoreceptors in the hypothalamus detect it, does not make a person want to drink. Plasma that is already too dilute does not need extra free water, and the fall removes the stimulus to drink.
- A rise in ADH release does not make a person want to drink. The osmoreceptors that trigger thirst also trigger ADH release, but ADH acts on the kidney to conserve water as a parallel response, not as the trigger for drinking.
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