Kidney

Question 1

Summarise the role of different parts of the nephron

Answer 1

1. Bowman’s 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 and 5. Collecting Duct: Reabsorption of water (permeability controlled by ADH)

Question 2

Describe the formation of glomerular filtrate

Answer 2

1. There is a high hydrostatic pressure in glomerulus as the diameter of afferent arteriole is wider than efferent arteriole (out)
2. Small substances eg. water, glucose, ions, urea are forced into glomerular filtrate
3. Large proteins / blood cells remain in blood

Question 3

Describe the reabsorption of glucose
by the proximal convoluted tubule

Answer 3

1. Na+ is actively transported out of epithelial cells to the capillary
2. Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient
3. Glucose moves into the capillary by facilitated diffusion down its concentration gradient

Question 4

Describe the reabsorption of water
by the proximal convoluted tubule

Answer 4

Glucose in the capillaries lower water potential, so water moves by osmosis down the 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 provides a large surface area
• Many channel / carrier proteins for facilitated diffusion / co-transport
• Many carrier proteins for active transport
• Many mitochondria, which produce ATP for active transport
• Many ribosomes to 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 reabsorbed at the PCT. Glucose carrier / cotransporter proteins are saturated / working at maximum rate

Question 7

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

Answer 7

So that 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

In the ascending limb: Na+ is actively transported out (so filtrate concentration decreases). Water remains inside as the ascending limb is impermeable to water. This increases concentration of Na+ in the medulla, lowering water potential.

In the descending limb: Water moves out by osmosis then is reabsorbed by capillaries (so filtrate concentration increases). Na+ diffuses back in.

Question 9

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

Answer 9

More Na+ moves out, so Na+ gradient is maintained for longer in medulla, so water potential gradient is maintained for longer, and more water can be reabsorbed from collecting duct by osmosis

Question 10

Describe the reabsorption of water by the distal convoluted tubule and
collecting ducts

Answer 10

Water moves out of distal convoluted tubule and 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

Contains osmoreceptors which detect increase or decrease in blood water potential. It then 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) in osmoregulation

Answer 14

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