Control of blood water potential 3.6.4.3 ( Organisms respond to changes in their internal and external environments 3.6)

Question 1

Describe the structure of a nephron

Answer 1

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

Question 2

Summarise the role of different parts of the nephron

Answer 2

1) 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 = reabsorption of water
5) Collecting duct = reabsorption of water

Question 3

Describe the formation of glomerular filtrate

Answer 3

1) high hydrostatic pressure in glomerulus
- as diameter of afferent arteriole is wider than efferent arteriole
2) small substances e.g. water , glucose , ions , urea forced into glomerular filtrate , filtered by :
a) pores 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) N+ actively transported out of epithelial cells to capillary
2) Na+ moves by facilitated diffusion into epithelial cells down a conc gradient , bringing glucose against its conc gradient
3) Glucose moves into capillary by facilitated diffusion down its conc gradient

Question 5

Describe the reabsorption of water
by the proximal convoluted tubule

Answer 5

• Glucose etc . in capillaries lowers ¥
• Water moves by osmosis down a ¥ 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 = provides a large surface area
• Many channel proteins / carrier proteins = for facilitated diffusion / co transport
• Many carrier proteins = for active transport
• Many mitochondria = to produce ATP for active transport
• Many ribosomes = produce channel / carrier 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

Question 8

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

Answer 8

• So water potential decreases down the medulla
• 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

The loop of Henle acts as a countercurrent multiplier
1) in the ascending limb :
• Na+ actively transported out
• water remains as ascending limb is impermeable to water
• This increases conc of Na+ in the medulla , lowering ¥
2) Jn the descending limb :
• water moves out by osmosis then reabsorbed by capillaries
• Na+ recycled - diffused back in

Question 10

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

Answer 10

• more Na+ moved out - Na+ gradient is maintain for longer in the medulla
• 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 the collecting ducts

Answer 11

• 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 12

What is osmoregulation

Answer 12

control of water potential of the blood by negative feedback

Question 13

Describe the role of the hypothalamus in osmoregulation

Answer 13

1) contains osmoreceptors which detect increase of decrease in blood water potential
2) produced more ADH when water potential is low or less ADH when water potential is high

Question 14

Describe the role of the posterior pituitary gland in osmoregulation

Answer 14

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

Question 15

Describe the role of ADH in osmoregulation

Answer 15

1) attaches to receptors on collecti by duct
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 the blood by osmosis
5) So decreases volume and increases concentration of urine produced