6C - Controlling blood water potential

Question 1

What do the kidneys regulate?

Answer 1

The water potential of the blood (and urine).

Question 2

How is water lost in mammals?

Answer 2

During excretion in urea, and by sweating.

Question 3

What is osmoregulation?

Answer 3

The homeostatic control of the water potential of the blood.

Question 4

What is the homeostatic control of the water potential of the blood called?

Answer 4

Osmoregulation

Question 5

What is maintained in osmoregulation?

Answer 5

Concentration of water and salts maintained to ensure a constant water potential of blood plasma and tissue fluid.

Question 6

What happens if the water potential of the blood is too low (the body is dehydrated)?

Answer 6

More water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated, so less water is lost during excretion.

Question 7

What is the concentration of the urine like when a person is dehydrated?

Answer 7

More concentrated so less water is lost during excretion.

Question 8

What happens is the water potential of the blood is too high (the body is too hydrated)?

Answer 8

Less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion.

Question 9

What is the concentration of the urine like when a person is too hydrated?

Answer 9

More dilute, so more water is lost during excretion.

Question 10

How does osmoregulation control the water potential of body fluids?

Answer 10

By controlling both the volume and concentration of urine produced.

Question 11

Where does the regulation of water potential mainly take place in the nephron?

Answer 11

In the loop of Henle, DCT and collecting duct.

Question 12

What is the volume of water reabsorbed by the DCT and collecting duct controlled by?

Answer 12

Hormones.

Question 13

What does the loop of Henle maintain?

Answer 13

A sodium ion gradient.

Question 14

Where is the loop of Henle located?

Answer 14

In the medulla (inner layer) of the kidneys.

Question 15

What is the loop of Henle made up of?

Answer 15

2 ‘limbs’ - the ascending and descending limb.

Question 16

What do the ascending and descending limbs of the loop of Henle control?

Answer 16

The movement of sodium ions so that water can be reabsorbed by the blood.

Question 17

Explain how the loop of Henle maintains a sodium ion gradient.

Answer 17

1) Near top of ascending limb, Na+ ions are pumped into medulla using AT. Ascending limb = impermeable to water so water stays inside the tubule. This creates a low water potential in the medulla as there’s a high conc. of ions. (Some Na+ ions passively diffuse into the descending limb, the majority of the Na ions accumulate in the interstitial region, lowering the WP).
2) Because there’s a lower WP in the medulla than in the descending limb, water moves out of descending limb and into the medulla by osmosis. This makes the filtrate more concentrated as ions can’t diffuse out as the limb isn’t permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network.
3) Near the bottom of the ascending limb, Na+ ions diffuse out into medulla, further lowering the WP in the medulla. Ascending limb = impermeable to water so it stays in the tubule.
4) Water moves out of the DCT by osmosis and is reabsorbed into the blood.
5) The first 3 stages massively increase the ion concentration in the medulla, which lowers the WP. 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.

Question 18

What is an adaptation of the ascending limb of the loH?

Answer 18

Impermeable to water, so the water stays inside the tubule.

Question 19

What does the ascending limb being impermeable to water create?

Answer 19

A low water potential in the medulla because there’s a high concentration of ions.

Question 20

What is an adaptation of the descending limb of the loH?

Answer 20

Permeable to water.

Question 21

What is the volume of water reabsorbed into the capillaries controlled by?

Answer 21

Changing the permeability of the DCT and the collecting duct.

Question 22

Explain the movement of sodium ions in the loop of Henle and what this does to the WP and filtrate

Answer 22

1) Na+ ions actively transported out of ascending limb.
2) Some sodium ions passively diffuse into descending limb, the majority of the sodium ions accumulate in the interstitial region, lowering the WP.
3) Water moves by osmosis from descending limb and enter blood vessels.
4) Filtrate moves along descending limb becoming more and more concentrated.
5) At bottom of ascending limb, sodium ions diffuse out,
6) As filtrate moves along ascending limb, sodium ions are actively transported out.
7) Therefore the filtrate becomes less and less concentrated and the water potential becomes higher and higher.

Question 23

Explain the countercurrent multiplier process

Answer 23

• The filtrate in the collecting duct with a low water potential meets interstitial fluid that has an even lower water potential.
• Ensures a water potential gradient exists for the entire length of the collecting duct = lots of water can be reabsorbed.

Question 24

What cells monitor the water potential of the blood?

Answer 24

Osmoreceptors.

Question 25

Where are the osmoreceptors?

Answer 25

In the hypothalamus in the brain.

Question 26

What happens to water when the water potential of the blood decreases?

Answer 26

• 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 send a signal to the posterior pituitary gland.
• This causes the posterior pituitary to release a hormone called antidiuretic hormone (ADH) into the blood.
• ADH make 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.

Question 27

What happens to the osmoreceptor cells when the water potential decreases?

Answer 27

Decrease in volume as water leaves.

Question 28

What does the posterior pituitary gland do?

Answer 28

Releases ADH.

Question 29

What does ADH stand for?

Answer 29

Antidiuretic hormone.

Question 30

What is a shorter way to write antidiuretic hormone?

Answer 30

ADH

Question 31

What does ADH do to the DCT and collecting duct?

Answer 31

Makes their walls more permeable to water so more water is reabsorbed from the tubules into the medulla and blood by osmosis.

Question 32

Where is ADH produced?

Answer 32

Hypothalamus

Question 33

Where is ADH stored and secreted?

Answer 33

Posterior pituitary gland.

Question 34

What happens to blood ADH level when you are dehydrated?

Answer 34

It rises.

Question 35

What is dehydration?

Answer 35

When you lose water, e.g. by sweating during exercise.

Question 36

When does the water content of the blood need to be increased?

Answer 36

When there is a loss of water (dehydration).

Question 37

How does ADH increase the water content of the blood?

Answer 37

1) The water content of the blood drops, so its water potential drops.
2) This is detected by osmoreceptors in the hypothalamus and causes them to shrink.
3) The posterior pituitary gland is stimulated to release more ADH into the blood.
4) More ADH means that the DCT and collecting duct become more permeable, so more water is reabsorbed into the blood by osmosis.
5) A small amount of highly concentrated urine is produced and less water is lost.

Question 38

How does ADH reduce the water content of the blood?

Answer 38

1) The water content of the blood rises, so its water potential rises.
2) This is detected by osmoreceptors in the hypothalamus and causes them to increase in volume.
3) The posterior pituitary gland is stimulated to release less ADH into the blood.
4) Less ADH means that the DCT and collecting duct become less permeable, so less water is reabsorbed into the blood by osmosis.
5) A large amount of dilute urine is produced and more water is lost.

Question 39

When do blood ADH levels rise?

Answer 39

When you are dehydrated.

Question 40

When do blood ADH levels fall?

Answer 40

When you are hydrated.

Question 41

Explain the ADH mechanism (when water content is low/there is dehydration)

Answer 41

• ADH binds to receptors on cells of DCT and collecting duct.
• Causes phosphorylase to be activated.
• Phosphorylase causes vesicles within the cell to move and fuse with cell surface membrane.
• The vesicles contain pieces of plasma membrane and many aquaporins which are inserted in the membrane.
• This increases the permeability of the collecting duct to water and urea.
• More water passes into the medulla fluid and then into the blood vessels by osmosis.
• Urea passes into the medulla fluid and lowers the water potential, allowing more water to be reabsorbed.
• Therefore a small volume of concentrated urine is produced.

Question 42

Apart from sending signals to the hypothalamus, where else do the osmoreceptors send impulses?

Answer 42

To the thirst centre of the brain, increasing the intake of water.

Question 43

Explain how controlling blood water potential is an example of negative feedback

Answer 43

• As more water is drank and reabsorbed, the water potential of the blood rises.
• The osmoreceptors increase in size (water enters).
• Fewer impulses are sent to the posterior pituitary and thirst centre of the brain.
• Less ADH to be released and less water is drank.