Excretory System: ADH & Osmoregulation | A-level Biology | OCR, AQA, Edexcel

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The key points covered of this video include:

1. Effect of ADH
2. How ADH Increases Permeability
3. Increasing Water Potential
4. Decreasing Water Potential

Effect of ADH

ADH is released from the posterior pituitary in response to low water potential. From there it travels in the blood to the kidneys. The cells lining the collecting duct (and also the distal convoluted tubule) have ADH receptors on them. The response to ADH is to increase the permeability of the collecting duct to water. This allows more water to travel from the collecting duct to the medulla via osmosis. This water is then absorbed into the blood so that less is lost in urine.

How ADH Increases Permeability

ADH after reaching the receptors on the cells of the collecting duct (and the distal convoluted tubule) takes its effect. It activates the intracellular enzyme phosphorylase. Inside the cell phosphorylase causes vesicles of aquaporin to move to the cell surface. Aquaporin is a special type of protein channel that lets lots of water through the membrane. ADH causes vesicles of aquaporin to fuse with the membrane - increasing the permeability of the collecting duct to water. Water now moves out by osmosis into the medulla and into the blood - concentrating the urine and reducing water loss. Some urea also moves out of aquaporin channels - reducing the water potential of the medulla and allowing more water to be removed.

Increasing Water Potential

The action of osmoreceptors, ADH, and the collecting duct all make up part of the negative feedback loop for osmoregulation. If water potential decreases below a set point it is detected by osmoreceptors - these are the detectors in the feedback loop. This leads to increased ADH release from the posterior pituitary gland - ADH is the co-ordinator in the feedback loop. ADH increases the permeability of the collecting duct and results in less water being lost in the urine - the collecting duct is the effector. This raises the water potential of the blood back to a normal level and results in concentrated urine.

Decreasing Water Potential

ADH also controls negative feedback for any raise in water potential. The raise above normal levels is detected by osmoreceptors which results in less ADH being released from the pituitary gland. ADH breaks down in the blood slowly - if no ADH is being released its concentration will fall. Less ADH means receptors on collecting duct cells are note being activated. Without stimulation the membrane of collecting duct cells is pulled into the cell - creating vesicles. The invagination brings aquaporins with it - resulting in reduced permeability and more water loss in the urine. As a result the water potential of the blood is decreased back to normal levels and urine becomes less concentrated.

Summary

ADH results in an increase in the permeability of the collecting duct to water
It attaches to receptors on collecting duct cells and increases the number of aquaporins in the membrane
Low water potential is detected, resulting in an increase in ADH and more permeability in the collecting duct
High water potential is detected, leading to a decrease in ADH and less permeability in the collecting duct