ADH (Antidiuretic Hormone) and Aldosterone Roles

Antidiuretic hormone (ADH) and aldosterone play crucial roles in water and sodium ion reabsorption, excretion, and blood pressure regulation.

Kidneys are essential for maintaining water balance in the blood.
The average adult loses approximately 2 liters of water daily through sweat, urine, and exhalation.
To compensate for this loss, we need to drink at least 2 liters of water per day.
The retention and release of water are controlled by:
- Osmotic pressure: High osmotic pressure indicates a high concentration of solutes in the blood and low water levels.
- Blood pressure: Low blood pressure usually indicates low levels of water in the blood.

The hypothalamus, a structure in the brain, contains specialized cells called osmoreceptors, which monitor osmotic pressure.
If a person is dehydrated:
- Blood becomes too concentrated, leading to increased osmotic pressure.
- Osmoreceptors in the brain signal the pituitary gland to release antidiuretic hormone (ADH).
- ADH increases the permeability of the collecting duct, resulting in decreased urine output.
If a person is hydrated:
- Blood becomes dilute, leading to decreased osmotic pressure.
- Osmoreceptors stop signaling (inhibit) the release of ADH.
- The collecting duct becomes less permeable, resulting in increased urine output.

Dehydration leads to increased osmolarity of plasma.
Osmoreceptors in the hypothalamus detect this change.
The posterior pituitary gland increases ADH secretion.
This leads to increased reabsorption of water, reducing urine output and increasing water intake through thirst.
The increased water intake and reabsorption provide negative feedback, returning osmolarity to normal.

Also known as the "anti-pee hormone."
Synthesized in the hypothalamus and released by the posterior pituitary.
Targets the distal convoluted tubule and collecting duct to reabsorb water back into the blood.

Osmoreceptors in the hypothalamus sense changes in osmotic pressure and shrink or swell accordingly.
The hypothalamus shrinks if osmotic pressure is high (low water/high solute).
This stimulates the release of ADH, which travels to the kidney via the bloodstream.
ADH causes the tubules to become more permeable to water.
Consequently, more water is reabsorbed back into the blood.
This process decreases urine output.
ADH-mediated permeability can reabsorb up to 15% of the water in the nephron.
As more water is reabsorbed, the osmotic pressure decreases.
A decrease in osmotic pressure stimulates osmoreceptors, leading to less ADH production.

Too little water in the blood is detected by the hypothalamus, leading to increased ADH secretion by the pituitary gland, causing the kidneys to absorb more water, resulting in less urine and a return to normal blood water levels.
Too much water in the blood is detected by the hypothalamus, leading to decreased ADH secretion by the pituitary gland, causing the kidneys to absorb less water, resulting in lots of dilute urine and a return to normal blood water levels.

If body fluids are too dilute (osmotic pressure is too low), osmoreceptors in the hypothalamus send a signal to decrease ADH release, leading to decreased reabsorption of water and increased water in urine.
If body fluids are too concentrated (osmotic pressure is too high), osmoreceptors in the hypothalamus sense increased osmotic pressure and signal the pituitary gland to release ADH into the bloodstream, leading to increased reabsorption of water and decreased water in urine.

Dehydration increases water permeability in the distal tubule and collecting duct due to:
- Increase in aldosterone (from the adrenal gland).
- Increase in ADH (from the pituitary gland).

Diabetes insipidus: ADH activity is too low, leading to excessive urination (4L to 8L per day!) and severe dehydration.
Ethanol (alcohol): Acts as a diuretic by inhibiting ADH activity, resulting in less water reabsorption and increased urine volume.
Leads to thirst and dehydration.

Aldosterone: A hormone that stimulates the distal tubule and collecting duct to reabsorb $Na^+$ if blood concentration is too low.
This is followed by passive diffusion and osmosis of chloride ions and water.
Too much aldosterone causes water retention and high blood pressure.
Aldosterone also stimulates the secretion of $K^+$ into the distal and collecting duct if concentration is too high.

Three mechanisms maintain a blood pH of ~7.4:
- Acid-base buffer: Adding $H^+$ to bicarbonate ion to form carbonic acid (and vice versa).
- Changes in breathing rate: Increased respiration generates more $CO_2$ , lowering blood pH.
- Secretion/reabsorption of $H^+$ ions: If blood is too acidic, $H^+$ is excreted in urine, and $HCO_3^-$ is reabsorbed by the kidneys.