3.6.4.3 Control of blood water potential

Question 1

What is osmoregulation?

Answer 1

The control of water potential of the blood to maintain homeostasis.

Question 2

Which part of the brain detects changes in blood water potential?

Answer 2

The hypothalamus.

Question 3

What is the role of osmoreceptors in the hypothalamus?

Answer 3

To detect changes in the water potential of the blood.

Question 4

What hormone is secreted by the posterior pituitary during osmoregulation?

Answer 4

Antidiuretic hormone (ADH).

Question 5

How does ADH affect the permeability of the collecting ducts?

Answer 5

ADH increases the permeability of the collecting ducts to water by inserting aquaporins into their membranes.

Question 6

What is the function of the glomerulus in the nephron?

Answer 6

To produce glomerular filtrate through ultrafiltration of blood.

Question 7

What is filtered into the glomerular filtrate during ultrafiltration?

Answer 7

Small molecules such as water glucose amino acids urea and ions.

Question 8

Which part of the nephron reabsorbs most glucose and water?

Answer 8

The proximal convoluted tubule.

Question 9

How are glucose and sodium ions reabsorbed in the proximal convoluted tubule?

Answer 9

By co-transport through carrier proteins using facilitated diffusion and active transport.

Question 10

What structural features of the proximal convoluted tubule aid reabsorption?

Answer 10

It has microvilli for a large surface area and many mitochondria for ATP production.

Question 11

What is the function of the loop of Henle?

Answer 11

To create a sodium ion concentration gradient in the medulla for water reabsorption.

Question 12

How does the descending limb of the loop of Henle function?

Answer 12

It is permeable to water but impermeable to sodium ions leading to water loss into the medulla by osmosis.

Question 13

How does the ascending limb of the loop of Henle function?

Answer 13

It is impermeable to water but actively transports sodium ions into the medulla creating a concentration gradient.

Question 14

Why is maintaining a sodium ion gradient important in the medulla?

Answer 14

It allows water to be reabsorbed by osmosis from the collecting ducts.

Question 15

What is the role of the distal convoluted tubule?

Answer 15

To fine-tune salt and water reabsorption based on the body’s needs.

Question 16

How does ADH affect the distal convoluted tubule and collecting duct?

Answer 16

It increases their permeability to water leading to greater water reabsorption.

Question 17

Describe and explain how three features of the cells in the proximal convoluted tubule allow the rapid reabsorption of glucose into the blood.

Answer 17

1. Microvilli provide a large surface area
   OR
   Folded (cell-surface) membrane provides a large surface area;
2. Many channel/carrier proteins for facilitated diffusion;
3. Many carrier proteins for active transport;
4. Many channel/carrier proteins for co-transport;
5. Many mitochondria produce ATP OR
   Many mitochondria for active transport;
6. Many ribosomes to produce carrier/channel proteins;

Question 18

Describe the effect of ADH on the collecting ducts in kidneys.

Answer 18

1. (Stimulates) addition of channel proteins into membrane;
   Accept aquaporins for channel proteins.
   Accept movement for addition. Accept (stimulates) opening of channel proteins in membrane.
2. Increases permeability to water OR
   (More) water (re)absorbed; Accept for reabsorbed ‘enters blood’ or ‘leaves collecting duct’.
3. By osmosis;

Question 19

Describe how ultrafiltration occurs in a glomerulus.

Answer 19

1. High blood/hydrostatic pressure; Ignore references to afferent and efferent arterioles
   Ignore ‘increasing/higher blood pressure’ as does not necessarily mean high
2. Two named small substances pass out eg water, glucose, ions, urea; Accept correct named ions
   Accept mineral ions/minerals

Accept amino acids/small proteins

Ignore references to molecules not filtered

3. (Through small) gaps/pores/fenestrations in (capillary) endothelium;
   Accept epithelium for endothelium
4. (And) through (capillary) basement membrane;

Question 20

How does the inhibition of the absorption of sodium and chloride ions from the filtrate cause an increase in the volume of urine produced.

Answer 20

1. Water potential of filtrate/tubule decreased

Accept maintains low water potential.

Accept nephron for filtrate/tubule.

2. Less water (reabsorbed) by osmosis (from filtrate/tubule);

Accept nephron for filtrate/tubule. Accept no water (reabsorbed) for ‘less’

Accept (more) water (absorbed) by osmosis (into filtrate/tubule)

3. Collecting duct (is where osmosis occurs);

Accept proximal convoluted tubule or distal convoluted tubule or (descending) loop of Henle

Ignore PCT, DCT.

Question 21

Give the location of osmoreceptors in the body of a mammal.

Answer 21

Hypothalamus.

Question 22

When a person is dehydrated, the cell volume of an osmoreceptor
decreases.

Explain why

Answer 22

1. Water potential of blood will decrease;
2. Water moves from osmoreceptor into blood by osmosis

Question 23

Describe and explain how the secretion of ADH affects urine produced by
the kidneys.

Answer 23

1. Permeability of membrane / cells (to water) is increased;
2. More water absorbed from / leaves distal tubule / collecting
3. Smaller volume of urine;
4. Urine becomes more concentrated.