Osmoregulation

What is osmoregulation?

The control of the water potential of the blood — ensuring the balance between water and solutes in the body remains within normal limits.

How does the body detect changes in blood water potential?

Osmoreceptors in the hypothalamus detect changes in the water potential (osmolarity) of the blood flowing through them. When blood water potential falls (blood becomes more concentrated), the osmoreceptors shrink as water leaves by osmosis, triggering a response.

What is ADH and what triggers its release?

ADH (antidiuretic hormone) is a peptide hormone produced by neurosecretory cells in the hypothalamus and then moves down axon to be stored in vesicles and then released later from the posterior pituitary gland when an action potential arrives . It is released by exocytosis when blood water potential falls (detected by osmoreceptors shrinking ), when blood pressure drops, or when dehydrated.

How does ADH act on the kidney to increase water reabsorption?

ADH travels in the blood to the kidneys. It binds to receptors on cells of the distal convoluted tubule and collecting duct. This activates an intracellular enzyme (via a second messenger system involving cAMP), causing vesicles containing aquaporin channel proteins to fuse with the cell membrane. Aquaporins increase the permeability of the membrane to water, so more water leaves the filtrate by osmosis into the surrounding medulla tissue fluid and back into the blood. The result is a smaller volume of more concentrated urine.

What happens when blood water potential is too high (the person is overhydrated)?

Osmoreceptors in the hypothalamus detect the higher water potential. Less ADH is released from the posterior pituitary. With less ADH, fewer aquaporins are present in the collecting duct and DCT walls, so less water is reabsorbed. A larger volume of more dilute urine is produced — this is a negative feedback mechanism.

What is the effect of alcohol on ADH and urine production?

Alcohol inhibits ADH release from the posterior pituitary. This reduces aquaporin insertion into collecting duct walls, so less water is reabsorbed. The result is increased production of dilute urine, which can lead to dehydration.

Negative feedback loop of ADH

What is

→Osmoreceptors in hypothalamus shrink
→ Neurosecretory cells generate action potentials
→ ADH in vesicles released from posterior pituitary by exocytosis
→ ADH transported in blood to kidney
→ ADH binds to receptors on collecting duct cells
→ Second messenger cAMP activated
→ Vesicles containing aquaporins fuse with plasma membrane
→ Collecting duct becomes more permeable to water
→ More water reabsorbed by osmosis
→ Blood water potential rises