16.10 - Role of Hormones in Osmoregulation
- 1The role of the kidney in osmoregulation
- 2How ADH controls water reabsorption in the kidney
- 3Negative feedback control of ADH release
The kidney and osmoregulation
Osmoregulation is the homeostatic control of the water potential of the blood. It keeps the water potential of bodily fluids within a narrow range. This balance is essential because significant changes in water potential can harm cells.
Antidiuretic hormone (ADH) plays a crucial role in water reabsorption in the kidneys.
Key features of ADH:
- It is produced in the hypothalamus.
- It is stored in the posterior pituitary gland after production.
- Its target cells are those lining the distal convoluted tubules (DCTs) and collecting ducts in the kidneys.
Mechanism of ADH action:
- ADH attaches to receptors on the surface of cells in the DCT and collecting duct.
- This triggers the activation of the enzyme phosphorylase.
- This results in water channel proteins called aquaporins being integrated into the cell-surface membrane.
- Water then moves through aquaporins by osmosis from the DCT and collecting duct into the surrounding interstitial space.
- Water is then reabsorbed into the surrounding blood vessels.
This process effectively returns water to the bloodstream while facilitating the removal of concentrated urine.
Negative feedback control of ADH release
The release of ADH is governed by a negative feedback system. This involves osmoreceptors in the hypothalamus that respond to changes in blood water and ion levels.
For instance, a lack of water, excessive salt intake, or excessive sweating can reduce the blood water potential, triggering ADH release. By contrast, less ADH is released when there is too much water in the blood.
| Lack of water | Excess water | |
|---|---|---|
| 1. Detection of blood water potential | Water moves from osmoreceptors into the blood by osmosis and osmoreceptors shrink, detecting the decrease in the water potential of the blood. They respond by producing ADH. | Water moves into osmoreceptors from the blood by osmosis, and osmoreceptors detect an increase in the water potential of the blood. |
| 2. Nerve signals and ADH secretion | Nerve signals prompt the release of ADH from the posterior pituitary gland, and ADH is transported via the blood to the kidneys. | Nerve signals to the posterior pituitary gland decrease, reducing the release of ADH. |
| 3. Effect on target kidney cells | An increase in aquaporins in DCT and collecting duct cell membranes makes them more permeable to water. | DCT and collecting duct cell membranes become less permeable to water. |
| 4. Reabsorption of water into the blood | More water is reabsorbed into the blood. | Less water is reabsorbed into the blood. |
| 5. Urine volume and concentration | Urine becomes more concentrated and is produced in smaller volumes. | Urine becomes more dilute and is produced in larger volumes. |
The regulation of blood water potential through urine concentration adjustments is crucial for maintaining homeostasis.