5.2.7: Osmoregulation Flashcards
What is osmoregulation?
The control of water potential in the body.
What is water potential?
The tendency of water to move from one place to another.
What does osmoregulation involve and why is it important?
- Controlling levels of both water and salt in the body.
- The correct water balance between cells and surrounding fluids mush be maintained to prevent water entering cells and causing lysis. Or leaving cells and causing crenation.
From which sources does the body gain water?
Food, drink and metabolism (e,g, respiration)
How is water lost from the body?
- Urine
- Sweat
- Water vapour in exhaled air
- Faeces.
How do the kidneys act as an effector to control the water content of the body and the salt concentrations in the body fluids?
- On a cool day or when you have drunk a lot of fluid, the kidneys will produce a large volume of dilute urine.
- On a hot day or when you have drunk very little, the kidneys will produce smaller volumes of more concentrated urine.
How do the kidneys alter the volume of urine produced?
-By altering the permeability of the collecting ducts.
The walls of the collecting ducts can be made more or less permeable according to the needs of the body.
What happens on a cool day or when you have drunk a lot of fluid?
- The walls of the collecting ducts become less permeable.
- Less water is reabsorbed
- Greater volume of urine produced.
The walls of the collecting ducts can be made more or less permeable according to the needs of the body.
What happens on a hot day or when you have drunk very little?
- The walls of the collecting ducts become more permeable.
- More water reabsorbed into the blood.
- Smaller volume of urine produced.
How do the cells in the walls of the collecting duct respond to the level of antidiuretic hormone (ADH) in the blood and become more permeable?
- These cells have membrane-bound receptors for ADH. -The ADH binds to these receptors and cause a chain of enzyme-controlled reactions inside the cell (an example of cell signalling).
- The end result of these reactions is to cause vesicles containing water permeable channels (aquaporins) to fuse with the cell membrane.
- This makes the walls more permeable to water.
What happens as the level of ADH in blood rises?
- More water-permeable channels are inserted.
- This allows more water to be absorbed by osmosis, into the blood.
- Less urine is produced and the urine has a lower water potential.
What happens if the levels of ADH in the blood falls?
- The cell surface membrane folds inwards (invaginates) to create new vesicles that remove the water-permeable channels from the membrane.
- This makes the cells less permeable and less water is reabsorbed, by osmosis, into the blood.
- more water passes on down the collecting duct to form a greater volume of urine which is more dilute (high water potential)
Where are osmoreceptors found?
In the hypothalamus in the brain.
What are osmoreceptors?
-They are sensory receptors that detect the stimulus- they monitor the water potential of the blood.
How do osmoreceptors respond to the effects of osmosis?
- When water potential of the blood is low…
- the osmoreceptor cells lose water by osmosis and shrink.
- As a result, they stimulate neurosecretory cells in the hypothalamus.
What are neurosecretory cells?
specialised neurones (nerve cells) that produce and release ADH.
Where in the neurotransmitter cells is ADH produced?
-The ADH is manufactured in the cell body, which lies in the hypothalamus.
What happens after ADH is produced?
-ADH moves down the axon to the terminal bulb in the posterior pituitary gland, where it is stored in vesicles.
What happens when the neurosecretory cells are stimulated by the osmoreceptors?
- They carry action potentials down their axons and cause the release of ADH by exocytosis.
- ADH enters the blood capillaries running through the posterior pituitary gland.
- It is transported around the body and acts on the cells of the collecting ducts.
What happens when the water potential of the blood rises again after ADH is released?
- Less ADH is released.
- ADH is swiftly broken down.
- Therefore ADH present in blood is broken down and the collecting ducts will receive less stimulation.
What is the half-life of ADH
-It has a half-life of about 20 minutes
