[3.6.4.3] Control of Blood Water Potential

Question 1

Describe the structure of a nephron.

Answer 1

• Nephron = basic structural and functional unit of the kidney.
• Associated with each nephron is a network of blood vessels.

Question 2

Summarise the role of different parts of the nephron.

Answer 2

1. Bowman’s / renal capsule.
   
   • Formation of glomerular filtrate (ultrafiltration).
2. Proximal convoluted tubule.
   
   • Reabsorption of water and glucose (selective reabsorption).
3. Loop of Henle.
   
   • Maintenance of a gradient of sodium ions in the medulla.
4. Distal convoluted tubule & collecting duct.
   
   • Reabsorption of water (permeability controlled by ADH).

Question 3

Describe the formation of glomerular filtrate.

Answer 3

1. High hydrostatic pressure in glomerulus.
   
   • As diameter of afferent arteriole (in) is wider than efferent arteriole (out).
2. Small substances e.g. water, glucose, ions, urea forced into glomerular filtrate, filtered by:
   
   • a. Pores / fenestrations between capillary endothelial cells.
   • b. Capillary basement membrane.
   • c. Podocytes.
3. Large proteins / blood cells remain in blood.

Question 4

Describe the reabsorption of glucose and water by the proximal convoluted tubule.

Answer 4

1. Na⁺ actively transported out of epithelial cells to capillary.
2. Na⁺ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient.
3. Glucose moves into capillary by facilitated diffusion down its concentration gradient.
4. Glucose etc. in capillaries lower water potential.
5. Water moves by osmosis down a water potential gradient.

Question 5

Describe and explain how features of the cells in the PCT allow the rapid reabsorption of glucose into the blood.

Answer 5

• Microvilli / folded cell-surface membrane -> provides a large surface area.
• Many channel / carrier proteins -> for faciliated diffusion / co-transport.
• Many carrier proteins -> for active transport.
• Many mitochondria -> produce ATP for active transport.
• Many ribosomes -> produce carrier / channel proteins.

Question 6

Suggest why glucose is found in the urine of an untreated diabetic person.

Answer 6

• Blood glucose concentration is too high so not all glucose is reabsorped at the PCT.
• As glucose carrier / cotransporter proteins are saturated / working at maximum rate.

Question 7

Explain the importance of maintaining gradient of sodium ions in the medulla (concentration increases further down).

Answer 7

• So water potential decreases down the medulla (compared to filtrate in collecting duct).
• So a water potential gradient is maintained between the collecting duct and medulla.
• To maximise reabsorption of water by osmosis from filtrate.

Question 8

Describe the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla.

Answer 8

1. In the ascending limb:
   
   • Na⁺ actively transported out (so filtrate concentration decreases).
   • Water remains as ascending limb is impermeable to water.
   • This increases concentration of Na⁺ in the medulla, lowering water potential.
2. In the descending limb:
   
   • Water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases).
   • Na⁺ ‘recycled’ -> diffuses back in.

Question 9

Suggest why animals needing to conserve water have long loops of Henle (thick medulla).

Answer 9

• More Na⁺ moved out -> Na⁺ gradient is maintained for longer in medulla / higher Na⁺ concentration.
• So water potential gradient is maintained for longer.
• So more water can be reabsorbed from collecting duct by osmosis.

Question 10

Describe the reabsorption of water by the distal convoluted tubule & collecting ducts.

Answer 10

• Water moves out of distal convoluted tubule & collecting duct by osmosis down a water potential gradient.
• Controlled by ADH which increases their permeability.

Question 11

What is osmoregulation?

Answer 11

• Control of water potential of the blood (by negative feedback).

Question 12

Describe the role of the hypothalamus in osmoregulation.

Answer 12

1. Contains osmoreceptors which detects increase OR decrease in blood water potential.
2. Produces more ADH when water potential is low OR less ADH when water potential is high.

Question 13

Describe the role of the posterior pituitary gland in osmoregulation.

Answer 13

• Secretes (more / less) ADH into blood due to signals from the hypothalamus.

Question 14

Describe the role of antidiuretic hormone (ADH) when osmoreceptors detect a decrease in blood water potential due to increasing sweating, reduced water intake and increased salt intake.

Answer 14

1. Hypothalamus produces more ADH.
2. Attaches to receptors on collecting duct and distal convoluted tubule.
3. Stimulating addition of channel proteins (aquaporins) into cell-surface membranes.
4. So increases permeability of cells of collecting duct and DCT to water.
5. So increases water re absorption from collecting duct / DCT back into blood by osmosis.
6. So decreases volume and increases concentration of urine produced.

Question 15

Describe the role of antidiuretic hormone (ADH) when osmoreceptors detect a increase in blood water potential.

Answer 15

1. Hypothalamus produces less ADH.
2. Less attaches to receptors on collecting duct and distal convoluted tubule.
3. Less addition of channel proteins (aquaporins) into cell-surface membranes.
4. So low permeability of cells of collecting duct and DCT remain to water.
5. So less water reabsorption from collecting duct / DCT back into blood by osmosis.
6. So increases volume and decreases concentration of urine produced.