Are you aware that we have a hormone that helps us stay hydrated? It's called anti-diuretic hormone (ADH), or vasopressin, and it plays a vital role in regulating the amount of water in our body. ADH helps to increase the amount of water our body absorbs and reduces the amount of water we lose when we pee.

When there's a drop in our blood water potential, ADH is released. It works on the distal convoluted tubule and the collecting duct to increase the amount of water that's absorbed from the urine back into our bloodstream, which helps to restore the water potential in the blood. In summary, ADH is an important hormone that helps us maintain our body's water balance. With ADH, our body can absorb more water, making sure we stay hydrated and healthy.

The role of the hypothalamus and pituitary in secreting ADH

Yes, there are specialized cells in the hypothalamus called osmoreceptors that are situated outside the blood-brain barrier and are very sensitive to changes in blood water potential and osmolarity. Osmolarity is the number of solutes per litre of solution ^[1] and is used to measure the concentration of dissolved particles in a solution. It is an important factor maintaining homeostasis in the body, as it helps regulate the amount of water in the body's cells. Osmoreceptors are sensitive to changes in osmolarity and can detect even small changes in the concentration of solutes in the blood. This allows the body to respond quickly to changes in osmolarity and maintain homeostasis.

Correlation between water potential and osmolarity

Water potential and osolarity are which the water potential drops, osmolarity in the blood is too high. When the osmolarity is high, water moves from an area of low osmolarity (high water potential) to an area of high osmolarity (low water potential).

If the blood water potential drops, water moves out of the osmoreceptor cells, causing them to shrink. This triggers a neuronal response to the hypothalamus control center to increase ADH release, which raises the blood water potential.

On the other hand, if there is a rise in blood water potential, water moves down its osmotic gradient into the osmoreceptors. This causes the osmoreceptors to swell and increase in size, which triggers neuronal responses to decrease ADH release. This lowers the blood water potential.

In summary, water potential and osmolarity are negatively correlated, and changes in blood water potential can trigger a response to increase or decrease ADH release, which helps to maintain proper water balance in the body.

Hypovolaemia

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 action of ADH

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.

In response to high plasma osmolarity

Low water potential in the blood triggers the secretion of ADH, acts on the kidney by increasing the permeability of the distal convoluted tubule and collecting duct to water through the insertion of aquaporins. This leads to more water being reabsorbed from the filtrate and returned to the bloodstream, helping to restore blood water potential and maintain normal hydration levels.

Additionally, ADH increases the permeability of the collecting duct to urea, which helps to maintain the concentration gradient necessary for continued water reabsorption. This process ensures that the body conserves water during times of low water availability and prevents dehydration. Once blood water potential and pressure are restored, ADH secretion returns to normal levels through negative feedback, preventing excessive water retention.

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That's correct! The role of ADH in vasoconstriction helps to maintain blood pressure, and its action on the kidney is a temporary measure to prevent further lowering of the blood water potential. Drinking water is the most effective way to fully restore blood osmolarity. The activation of the thirst centre in response to low blood water potential is an example of how the body uses multiple mechanisms to maintain fluid balance. Once the body's fluid balance is restored, the negative feedback mechanism ensures that ADH secretion returns to normal levels, preventing overcorrection and maintaining homeostasis. 

In response to low plasma osmolarity

Great job! A drop in plasma osmolarity triggers osmoreceptors to increase in size due to the influx of water, resulting in fewer impulses being sent to the hypothalamus and less ADH being released. This leads to a decrease in water reabsorption in the kidney, resulting in the production of very dilute urine. Once the blood osmolarity is restored, the osmoreceptors in the hypothalamus send normal impulses, and ADH secretion returns to normal levels through negative feedback. This helps to maintain fluid balance and prevent overcorrection.

ADH - Key takeaways The anti-diuretic hormone (ADH) is also known as vasopressin. The osmoreceptors in the hypothalamus are situated outside the blood-brain barrier and are very sensitive to changes in the blood water potential and blood osmolarity. The ADH acts on the collecting duct and the distal convoluted tubule. It increases the aquaporins on the apical side of the epithelial cells in these two regions and increases water reabsorption. The ADH release is regulated by negative feedback.

ADH

Where is ADH produced?

ADH is produced in the hypothalamus but is released from the posterior pituitary gland. 

What does ADH do?

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.  

Where is ADH released from?

ADH is released from the posterior pituitary gland.

What does ADH stand for?

Anti-diuretic hormone 

What is ADH?

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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