Chapter 14 Homeostasis (part 3)
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:
1. ADH attaches to receptors on the surface of cells in the DCT and collecting duct.
2. This triggers the activation of cyclic AMP (cAMP), a second messenger, initiating a series of
reactions that lead to the phosphorylation of water channel proteins called aquaporins.
3. Aquaporin vesicles merge with the cell-surface membrane.
4. Water moves through aquaporins by osmosis from the DCT and collecting duct into the
surrounding interstitial space.
5. 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
Water moves from osmoreceptors into
Water moves into
the blood by osmosis and
1. Detection of osmoreceptors from the blood
osmoreceptors shrink, detecting the
blood water by osmosis, and osmoreceptors
decrease in the water potential of the
potential detect an increase in the water
blood. They respond by producing
potential of the blood.
ADH.
Nerve signals prompt the release of
2. Nerve signals Nerve signals to the posterior
ADH from the posterior pituitary
and ADH pituitary gland decrease,
gland, and ADH is transported via the
secretion reducing the release of ADH.
blood to the kidneys.
3. Effect on An increase in aquaporins in DCT and DCT and collecting duct cell
target kidney collecting duct cell membranes makes membranes become less
cells them more permeable to water. permeable to water.
4. Reabsorption
More water is reabsorbed into the Less water is reabsorbed into
of water into the
blood. the blood.
blood
5. Urine volume
Urine becomes more concentrated and Urine becomes more dilute and
and
is produced in smaller volumes. is produced in larger volumes.
concentration
The regulation of blood water potential through urine concentration adjustments is
crucial for maintaining homeostasis.
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:
1. ADH attaches to receptors on the surface of cells in the DCT and collecting duct.
2. This triggers the activation of cyclic AMP (cAMP), a second messenger, initiating a series of
reactions that lead to the phosphorylation of water channel proteins called aquaporins.
3. Aquaporin vesicles merge with the cell-surface membrane.
4. Water moves through aquaporins by osmosis from the DCT and collecting duct into the
surrounding interstitial space.
5. 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
Water moves from osmoreceptors into
Water moves into
the blood by osmosis and
1. Detection of osmoreceptors from the blood
osmoreceptors shrink, detecting the
blood water by osmosis, and osmoreceptors
decrease in the water potential of the
potential detect an increase in the water
blood. They respond by producing
potential of the blood.
ADH.
Nerve signals prompt the release of
2. Nerve signals Nerve signals to the posterior
ADH from the posterior pituitary
and ADH pituitary gland decrease,
gland, and ADH is transported via the
secretion reducing the release of ADH.
blood to the kidneys.
3. Effect on An increase in aquaporins in DCT and DCT and collecting duct cell
target kidney collecting duct cell membranes makes membranes become less
cells them more permeable to water. permeable to water.
4. Reabsorption
More water is reabsorbed into the Less water is reabsorbed into
of water into the
blood. the blood.
blood
5. Urine volume
Urine becomes more concentrated and Urine becomes more dilute and
and
is produced in smaller volumes. is produced in larger volumes.
concentration
The regulation of blood water potential through urine concentration adjustments is
crucial for maintaining homeostasis.
