3.6.4.3 Control of blood water potential (Unit 6 Homeostasis)

Question 1

What are the 4 stages of water reabsorption by the kidneys

Answer 1

1. Ultrafiltration by the glomerulus
2. Selective reabsorption by the proximal convoluted tubule
3. Loop of Henle , where in the ascending limb sodium ions are actively transported out and in the descending limb water moves out by osmosis and into capillaries.
4. Collecting duct, where water moves out into the blood

Question 2

Where are osmoreceptors?

Answer 2

In the hypothalamus

Question 3

Where specifically is ADH secreted from?

Answer 3

The posterior pituitary gland

Question 4

What does ADH stand for?

Answer 4

Antidiuretic hormone

Question 5

Describe and explain ultrafiltration and selective reabsorption, including where they occur.

Answer 5

Question 6

What is osmoregulation?

Answer 6

Osmoregulation is a key part ofhomeostasis

Specialisedsensory neurones, known asosmoreceptors, monitor the water potential of the blood (these osmoreceptors are found in an area of the brain known as thehypothalamus)

If the osmoreceptors detect adecreasein the water potential of theblood, nerve impulses are sent along these sensory neurones to theposterior pituitary gland(another part of the brain just below the hypothalamus)

These nerve impulses stimulate the posterior pituitary gland to releaseantidiuretic hormone (ADH)

ADH molecules enter the blood and travel throughout the body

ADH causes thekidneystoreabsorbmore water

Thisreduces the loss of water in the urine

Question 7

How does ADH affect the reabsorption of water in the collecting duct? (5 steps)

Answer 7

Question 8

Give three ways water is removed from the body? Besides the kidneys

Answer 8

Respiration
Sweating
Blood loss

Question 9

Explain how the Loop of Henle maintains a graidient of ions which allows H20 to be reabsorbed from filtrate in the collecting duct

Answer 9

• Epithelial cells of descending tubule carry out active transport
  -transport sodium ions out of filtrate
  -against conc gradient
  -into interstitial/tissue fluid
  -created/maintains water potential gradient for water reabsorption from collecting duct
  -countercurrent multiplier

Question 10

Describe ultrafiltration

Answer 10

Blood passes through the afferent arteriole to the glomerulus
The efferent arteriole has a narrower lumen so this generates a high hydrostatic pressure
Small molecules (ions/water/glucose) are forced through gaps in the capillary endothelium
They pass through the basement membrane into the renal capsule
Large proteins and cells remain in the blood

Question 11

Describe selective reabsorption

Answer 11

Na+ is actively pumped out of the cell into the blood by the sodium potassium pump
This lowers the concentration of sodium in the Proximal convoluted tubule
Na+ moves into the cell from the lumen by facilitated diffusion and glucose/amino acids/salts are co-transported with it
Glucose/amino acids/salts are then transported into the blood by facilitated diffusion
Absorption of the solutes increased the water potential in the lumen
Water is absorbed from the proximal convoluted tubule to the blood by osmosis

Question 12

Describe the events at the loop of henle

Answer 12

Water cannot move out of the ascending limb. Na+ and Cl- are ACTIVELY TRANSPORTED out of the ascending limb of the loop of Henle.
Water moves out of the descending limb via Osmosis due to the high conc of Na+ / Cl- (lowering the water potential) in the tissue fluid. Na+ moves into the descending limb.
This loss of water means that the conc of Na+ / Cl- is increased (less dilute solution).
As the fluid moves down the descending limb and start to ascend, Na+ / Cl- diffuse out of the loop.
This causes more water to be lost from the lowest descending part of the loop via osmosis.
The longer the loop, the greater the water potential gradient so more can be absorbed.

Question 13

What happens in the brain if someone is dehydrated?

Answer 13

Water potential in the blood decreases
Osmoreceptors in the hypothalamus shrink
Stimulates the hypothalamus
Increases ADH release by posterior pituitary gland
ADH carried in the blood to the distal convoluted tubule and the collecting duct

Question 14

How does ADH regulate blood water potential?

Answer 14

ADH binds to complementary receptors on specific target cells in the collecting duct and distal convoluted tubule (DCT)
The enzyme phosphorylase is activated by this binding
Leading to aquaporins fusing with the membrane of the epithelial cells in the collecting duct (DCT)
Membrane permeability to water increases
Water moves from a high water potential in the collecting duct to a low water potential in the blood via osmosis
This leads to increased concentration of urine (so less urine is produced)