Topic 6—C: Homeostasis- 5. Controlling blood water potential Flashcards
What is water essential for?
- To keep the body functioning so the amount of water in the blood needs to be kept constant
How is water lost during excretion?
- by excreting urea
- through sweat
Osmoregulation
The regulation of the water potential of the blood
What does it mean when the water potential of the blood is low?
- the body is dehydrated
- more water is reabsorbed by osmosis into the blood from the tubules of the nephrons
- The urine is more concentrated
- Less water is lost during excretion
What does it mean if the water potential of the blood is too high ?
- the body is hydrated
- less water is reabsorbed by osmosis into the blood from the tubules of the nephrons
- urine is more dilute
- more water is lost during excretion
Where does regulation of water potential mainly take place?
- loop of henle
- DCT
- Collecting duct
Where is the loop of henle located?
- in the medulla (inner layer) of the kidneys
What is the name of the two limbs that the loop of henle are made up of?
Descending and ascending limb
What do the limbs control?
- they control the movement of sodium ions so that water can be reabsorbed by the blood
Loop of henle (system)
- near the top of the ascending limb, na+ ions are actively pumped out into the medulla
- the ascending limb is impermeable to water so the water stays inside the tubule
- this creates a low water potential in the medulla because there’s a high concentration of ions
- because there’s a lower water potential in the medulla than in the descending limb, water moves out of the descending limb (which is permeable to water) into the medulla by osmosis
- this makes the glomerular filtrate more concentrated (ions can’t diffuse out)- the descending limb isn’t permeable to them
- The water in the medulla is reabsorbed into the blood through the capillary network
- Near the bottom of the ascending limb Na+ diffuse out into the medulla further lowering the water potential in the medulla (ascending limb is impermeable to water, so it stays in the tubule)
- water moves out of the distal convoluted tubule (DCT) by osmosis and is reabsorbed into the blood
- the first 3 stages massively increase the ion concentration in the medulla, which lowers the water potential
- This causes water to move out of the collecting duct by osmosis
- As before the water in the medulla is reabsorbed into the blood through the capillary network
What limb is impermeable to water?
Ascending limb
How is the volume of water reabsorbed into the capillaries controlled?
- by changing the permeability of DCT and the collecting duct
What cells monitor the water potential of the blood?
- osmoreceptors in a part of the brain called the hypothalamus
What happens when the water potential of the blood decreases?
- water will move out of the osmoreceptor cells by osmosis
- this causes the cells to decrease in volume
- this sends a signal to other cells in the hypothalamus which sends a signal to the posterior pituitary gland
- This causes the posterior pituitary to release a hormone called antidiuretic hormone (ADH) into the blood
What do ADH molecules bind to?
- Receptors on the plasma membranes of cells in the DCT and the collecting duct
What happens when ADH binds to receptors on the plasma membranes of cells in the DCT and the collecting duct?
- Protein channels called aquaporins are inserted into the plasma membrane
- these channels allow water to pass through via osmosis
- this makes the walls of the DCT and collecting duct more permeable to water
- this means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis
- A small amount of concentrated urine is produced, which means less water is lost from the body
What does ADH do?
- Change the water content of the blood when its too low or too high
What is dehydration?
- its what happens when you lose water e.g. by sweating during exercise
- the water content of the blood needs to be increased
Dehydration-blood water content is too low
- the water content of the blood drops, so its water potential drops
- this is detected by osmoreceptors in the hypothalamus
- posterior pituitary gland is stimulated to release more ADH into the blood
- More ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed into the blood by osmosis
- A small amount of highly concentrated urine is produced and less water is lost
What is hydration?
- you’ve taken in lots of water
- water content of blood needs to be reduced
Hydration- blood water content is too high
- water content of blood rises, so the water potential rises
- this is detected by osmoreceptors in the hypothalamus
- the posterior pituitary gland releases less ADH into the blood
- Less ADH means that the DCT and collecting duct are less permeable, so less water is reabsorbed into the blood by osmosis
- a large amount of dilute urine is produced and more water is lost
What happens if the water potential of the blood is too low? (Dehydrated)
- more water is reabsorbed by osmosis into the blood from the tubules of the nephrons
- this means urine is more concentrated so less water is lost during excretion
What happens if the water potential of the blood is too high? (Hydrated)
- less water is absorbed by osmosis into the blood from the tubules of the nephrons
- this means the urine produced is more dilute, so more water is lost during excretion
