control of blood water potential

Question 1

Explain what is meant by the term Osmoregulation

Answer 1

• Osmoregulation is the control of water and salt levels in the body
• Controlled by hormones e.g. antidiuretic hormone (ADH) which affect the distal convoluted tubule and collecting duct

Question 2

Explain the roles of hypothalamus, posterior pituitary and antidiuretic
hormone (ADH) when the body responds to a decrease in water potential.

Answer 2

1. Detected by osmoreceptors in hypothalamus
2. Hypothalamus produces more ADH → posterior pituitary gland secretes more ADH into blood
3. ADH travels in blood to kidney and attaches to receptors on collecting duct / DTC of kidney
4. ADH increases permeability of cells/walls of the DTC / collecting duct (more aquaporins fuse with cell membrane) to water → more water absorbed from/leaves DCT/collecting duct by osmosis
5. (Less water lost in urine so) smaller volume of urine, more concentrated

Question 3

Explain the roles of hypothalamus, posterior pituitary and antidiuretic
hormone (ADH) when the body responds to a increase in water potential.

Answer 3

1. Detected by osmoreceptors in hypothalamus
2. Hypothalamus produces less ADH → posterior pituitary gland secretes less ADH into blood
3. Less ADH travels in blood to kidney and attaches to receptors of collecting duct / DTC of kidney
4. ADH decreases permeability of cells/walls of the DTC / collecting duct to water and urea to water
   → less water absorbed from/leaves DCT/collecting duct by osmosis
5. (More water lost in urine so) larger volume of urine, less concentrated

Question 4

Describe the role of nephron in osmoregulation:

Answer 4

Formation of glomerular filtrate → reabsorption of glucose and water by the proximal convoluted tubule → maintenance of a gradient of sodium ions in the medulla by the loop of Henle → reabsorption of water by the distal convoluted tubule and collecting duct

Question 5

Describe the formation of glomerular filtrate

Answer 5

• Diameter of efferent arteriole smaller than afferent
  arteriole
• Build-up of hydrostatic pressure in glomerulus
  Water, glucose, mineral ions squeezed out of capillary / glomerulus into the Bowman’s capsule to form glomerular filtrate
• Through pores in capillary endothelium,
  basement membrane and podocytes (act as filter)
  -Large proteins / blood cells aren’t pushed out as
  too large

Question 6

Explain the process of reabsorption of glucose and water by the proximal convoluted tubule

Answer 6

1. Sodium ions actively transported out of epithelial
   cell to capillary - Lowers concentration of Na+ in epithelial cell
2. Na+ moves via facilitated diffusion from PCT into epithelial cell down concentration gradient - Co-transporting glucose / amino acids / Cl-
   - Increases concentration of glucose etc. in epithelial
   cell
3. Glucose / amino acids / Cl- move into capillary via facilitated diffusion down concentration gradient (reabsorbed) - Lowers water potential in capillary
4. Water moves via osmosis down water potential gradient into capillary (reabsorbed)

Question 7

Explain how a gradient of sodium ions in the
medulla by the loop of Henle is maintained?

Answer 7

1. Loop of Henle acts as a counter current multiplier → maintains water potential gradient → water leaves collecting duct /DCT by osmosis
2. Na+ actively transported out of ascendinglimb and ascending limb is impermeable to water so water remains - Increases conc of Na+ in medulla → lowers water potential
3. Water moves out of descending limbs / collecting duct by osmosis into medulla - Water reabsorbed by capillaries
   - Filtrate more concentrated as move down the ‘hairpin’
   .
4. Na+ diffuse into descending limb - Recycles Na+ in loop of Henle
   - Reduces water potential further

Question 8

Explain the process of reabsorption of water by the distal convoluted tubule and collecting ducts

Answer 8

• Water moves out of the DCT and collecting duct by osmosis down a water
  potential gradient
• Controlled by ADH which changes their permeability