6C - Controlling blood water potential Flashcards

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1
Q

What do the kidneys regulate?

A

The water potential of the blood (and urine).

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2
Q

How is water lost in mammals?

A

During excretion in urea, and by sweating.

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3
Q

What is osmoregulation?

A

The homeostatic control of the water potential of the blood.

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4
Q

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

A

Osmoregulation

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5
Q

What is maintained in osmoregulation?

A

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

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6
Q

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

A

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.

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7
Q

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

A

More concentrated so less water is lost during excretion.

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8
Q

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

A

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.

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9
Q

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

A

More dilute, so more water is lost during excretion.

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10
Q

How does osmoregulation control the water potential of body fluids?

A

By controlling both the volume and concentration of urine produced.

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11
Q

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

A

In the loop of Henle, DCT and collecting duct.

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12
Q

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

A

Hormones.

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13
Q

What does the loop of Henle maintain?

A

A sodium ion gradient.

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14
Q

Where is the loop of Henle located?

A

In the medulla (inner layer) of the kidneys.

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15
Q

What is the loop of Henle made up of?

A

2 ‘limbs’ - the ascending and descending limb.

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16
Q

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

A

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

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17
Q

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

A

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.

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18
Q

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

A

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

19
Q

What does the ascending limb being impermeable to water create?

A

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

20
Q

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

A

Permeable to water.

21
Q

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

A

Changing the permeability of the DCT and the collecting duct.

22
Q

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

A

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.

23
Q

Explain the countercurrent multiplier process

A
  • 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.
24
Q

What cells monitor the water potential of the blood?

A

Osmoreceptors.

25
Q

Where are the osmoreceptors?

A

In the hypothalamus in the brain.

26
Q

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

A
  • 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.
27
Q

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

A

Decrease in volume as water leaves.

28
Q

What does the posterior pituitary gland do?

A

Releases ADH.

29
Q

What does ADH stand for?

A

Antidiuretic hormone.

30
Q

What is a shorter way to write antidiuretic hormone?

A

ADH

31
Q

What does ADH do to the DCT and collecting duct?

A

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

32
Q

Where is ADH produced?

A

Hypothalamus

33
Q

Where is ADH stored and secreted?

A

Posterior pituitary gland.

34
Q

What happens to blood ADH level when you are dehydrated?

A

It rises.

35
Q

What is dehydration?

A

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

36
Q

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

A

When there is a loss of water (dehydration).

37
Q

How does ADH increase the water content of the blood?

A

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.

38
Q

How does ADH reduce the water content of the blood?

A

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.

39
Q

When do blood ADH levels rise?

A

When you are dehydrated.

40
Q

When do blood ADH levels fall?

A

When you are hydrated.

41
Q

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

A
  • 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.
42
Q

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

A

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

43
Q

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

A
  • 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.