Osmoregulation

Question 1

what is osmoregulation

Answer 1

the homeostatic control of the water potential of blood

Question 2

role of osmoregulation in stages

Answer 2

1. formation of glomerular filtrate by ultrafiltration
2. reabsorption of glucose and water by the proximal convoluted tubules
3. maintenance of a gradient of sodium ions in the medulla by the loop of henle
4. reabsorption of water by the distal convoluted tubules and collecting ducts

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 eg. 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
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

Question 5

Describe the reabsorption of water
by the proximal convoluted tubule

Answer 5

Glucose etc. in capillaries lower water potential
● Water moves by osmosis down a water potential
gradient

Question 6

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

Answer 6

Microvilli / folded cell-surface membrane → provides a large surface area
● Many channel / carrier proteins → for facilitated diffusion / co-transport
● Many carrier proteins → for active transport
● Many mitochondria → produce ATP for active transport
● Many ribosomes → produce carrier / channel proteins

Question 7

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

Answer 7

● Blood glucose concentration is too high so not all glucose is reabsorbed at the PCT
● As glucose carrier are saturated / working at maximum rate

Question 8

Explain the importance of maintaining a gradient of sodium ions in the
medulla

Answer 8

● 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 9

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

Answer 9

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 10

Suggest why animals needing to conserve water have long loops of Henle

Answer 10

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 11

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

Answer 11

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

Question 12

Describe the role of the hypothalamus in osmoregulation

Answer 12

1. Contains osmoreceptors which detect 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) in osmoregulation

Answer 14

1. Attaches to receptors on collecting duct (and
   distal convoluted tubule)
2. Stimulating addition of channel proteins
   (aquaporins) 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 it decreases volume and increases concentration of urine produced