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Did you know a hormone helps us fight against dehydration? The anti-diuretic hormone (ADH), also known as vasopressin, is an essential endocrine hormone for the homeostatic control of osmoregulation. This hormone increases water reabsorption and reduces the amount of water lost in the urine. ADH is released in response to a fall in blood water potential. It acts on the distal…
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Jetzt kostenlos anmeldenDid you know a hormone helps us fight against dehydration? The anti-diuretic hormone (ADH), also known as vasopressin, is an essential endocrine hormone for the homeostatic control of osmoregulation. This hormone increases water reabsorption and reduces the amount of water lost in the urine.
ADH is released in response to a fall in blood water potential. It acts on the distal convoluted tubule and the collecting duct to increase water reabsorption from the filtrate to restore the water potential in the blood.
There are specialised cells in the hypothalamus called osmoreceptors. These osmoreceptors are situated outside the blood-brain barrier and are very sensitive to blood water potential and osmolarity changes.
Osmolarity: Number of solutes per litre of solution.
Water potential and osmolarity are negatively correlated. Therefore, a fall in water potential means that the osmolarity in the blood is too high. Water moves from an area of low osmolarity (high water potential) to an area of high osmolarity (low water potential).
When the blood water potential drops:
In the case of a rise in blood water potential:
A drop in blood volume (hypovolaemia) also stimulates ADH secretion. Hypovolaemia causes low blood pressure detected by baroreceptors in the heart, aorta, and the carotid artery. The neuronal signal is then relayed to the posterior pituitary to release more ADH.
ADH is synthesised in the supraoptic and paraventricular nuclei of the hypothalamus. After synthesis, this peptide hormone is transported and stored in the posterior pituitary gland until released into circulation.
The synthesis and release of ADH are controlled by negative feedback. When the blood water potential and blood pressure are restored, the amount of ADH released returns to its normal level.
The main target of the ADH is the kidney. It acts on the distal convoluted tubule and the collecting duct to regulate the volume and osmolarity of the urine by controlling how much water is reabsorbed back into the blood.
ADH can also act on peripheral blood vessels to regulate blood pressure.
Low water potential corresponds with high osmolarity in the blood. This result can arise from various causes such as:
A fall in water potential is detected by the osmoreceptors in the hypothalamus, triggering ADH secretion. ADH then targets the kidney and acts on the distal convoluted tubule and the collecting duct.
The process:
ADH also increases the permeability of the collecting duct to urea. As more water is being reabsorbed, urea becomes highly concentrated, generating a high gradient for passive movement of urea out of the filtrate. Reabsorption of urea with water ensures that the water potential at the interstitial space is kept lower than the filtrate, so water continues to be reabsorbed.
High concentrations of ADH also causes vasoconstriction of peripheral blood vessels, ensuring that blood pressure does not drop too low.
The action of ADH on the kidney merely prevents further lowering of the blood water potential. Restoring the blood osmolarity is only fully achieved by drinking more water. The hypothalamus also activates the thirst centre in response to low blood water potential, increasing the urge to drink water. Following the restoration of the blood water potential, osmoreceptors send fewer impulses to the hypothalamus and secretion of ADH is reduced. This is an example of negative feedback.
Low plasma osmolarity means the blood water potential is high. There is either too little salt and electrolytes in the blood or too much water in the blood. This may be caused by:
The process:
Upon restoration of the blood osmolarity, impulses sent by the hypothalamus osmoreceptors return to normal and so does the ADH secretion.
ADH is produced in the hypothalamus but is released from the posterior pituitary gland.
ADH increases the reabsorption of water in the distal convoluted tubule and the collecting duct. At high concentration it also causes vasoconstriction of peripheral blood vessels to counter low blood pressure and hypovolaemia.
ADH is released from the posterior pituitary gland.
Anti-diuretic hormone
ADH is a peptide hormone produced in the hypothalamus. It is released from the posterior pituitary gland in response to low blood plasma water potential.
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