6.4.3 Osmoregulation

Question 1

what is osmoregulation

Answer 1

control of the water potential of the blood

Question 2

what happens when there is a fall in blood water potential

Answer 2

• osmoreceptors in the hypothalamus detect the fall in WP
• so hypothalamus increases ADH production
• ADH passes to the posterior pituitary gland where it is released into the bloodstream
• in kidneys, ADH binds to receptors on the cells lining in the collecting duct
• causes vesicles containing many aquaporins to fuse with the cell membrane
• the additions of aquaporins to the cell membrane increases the permeability to water
• more water is reabsorbed by osmosis and small volume of concentrated urine produced

Question 3

what occurs when there is a rise in water potential

Answer 3

• Osmoreceptors in the hypothalamus detect the rise in water potential
• In response, the hypothalamus decreases ADH production
• Less ADH passes to the posterior pituitary gland, where it is released into the bloodstream
• In the kidneys, less ADH binds to receptors on the cells lining the collecting duct
• Cells are less permeable to water, as fewer aquaporins in the cell membrane
• Less water reabsorbed by osmosis and large volume of dilute urine produced

Question 4

how is glomerular filtrate formed

Answer 4

• high hydrostatic pressure is generated in the glomerular capillaries
• ultrafiltration occurs where small molecules forced out of capillary into Bowman’s capsule
• basement membrane prevents large proteins and cells from entering the filtrate

Question 5

which arteriole is wider

Answer 5

afferent is wider than efferent

Question 6

what is filtrate formation resisted by

Answer 6

• capillary endothelial cells and connective tissue (basement membrane)
• epithelial cells of Bowman’s capsule
• osmotic potential of the blood

Question 7

what is filtrate resistance decreased by

Answer 7

• gaps between podocytes - cells forming the inner lining of the Bowman’s capsule
• fenestrated capillaries in glomerulus

Question 8

where does the reabsorption of glucose and water occur

Answer 8

proximal convoluted tubule

Question 9

how does the reabsorption of glucose and water occur

Answer 9

• sodium is actively transported out of tubule epithelial cells into blood by sodium-potassium pump (using ATP)
• produces Na+ conc gradient
• Glucose/AA co transported into epithelial cells with Na+
  as Na+ moves down its conc. gradient and G/AA moves against
• G/AA move by facilitated diffusion from tubule cell into blood
• loss of solutes increases water potential of tubule filtrate, so water moves by osmosis from filtrate into cells, and into blood

Question 10

what is the concentration gradient of sodium in PCT

Answer 10

higher concentration of sodium in tubule filtrate to lower in epithelial cells

Question 11

what are specializations that increase rate of reabsorption

Answer 11

• microvilli on inner surface of tubule cells to increase surface area
• infoldings on basal surface of tubule cells next to capillary to increase surface area
• many mitochondria to increase ATP supply for active transport

Question 12

how is a sodium gradient in the medulla maintained in the ascending limb of the loop of Henle

Answer 12

• sodium ions actively transported out of filtrate into interstitial fluid
• creates a low water potential in the interstitial fluid
• ascending limb is impermeable to water
• water potential of the filtrate increases up the ascending limb

Question 13

what is interstitial fluid

Answer 13

fluid between the descending and ascending limb of the loop of Henle

Question 14

why does the water potential increase up the ascending limb

Answer 14

loss of Na+

Question 15

how is a sodium gradient in the medulla maintained in the descending limb of the loop of Henle

Answer 15

• the descending is impermeable to water
• water moves out of the descending limb by osmosis, down the water potential gradient into interstitial fluid
• water then moves by osmosis into the capillaries
• filtrate progressivley loses water as it moves down the descending limb

Question 16

what occurs at the base of the loop henle

Answer 16

the filtrate is highly concentrated so sodium ions move out of the bottom of the ascending limb by facilitated diffusion

Question 17

what is the water potential gradient in the interstitial fluid

Answer 17

higher salt concentration and lower water potential deeper in the medulla

Question 18

what is the countercurrent multiplier effect

Answer 18

fluid flowing in opposing directions up and down the ascending limb

Question 19

why is the countercurrent multiplier good

Answer 19

• concentration difference in maintained along the whole length of the loop of Henle
• interstitial fluid always has a slightly lower water potential than filtrate in the descending limb
• maximises reabsorption of water by osmosis

Question 20

where does reabsorption of water occur

Answer 20

collecting duct

Question 21

how does the reabsorption of water occur

Answer 21

• water potential is always lower in the interstitial fluid surrounding the collecting duct than inside the duct (due to countercurrent effect)
• so water diffuses by osmosis from collecting duct into fluid, then capillaries, along the length of the collecting duct

Question 22

what is amount of water reabsorbed dependent on

Answer 22

the permeability of the collecting duct (aquaporin density) which is controlled by ADH

Question 23

what is the efferent arteriole

Answer 23

has a small lumen so blood is squeezed out
- creating a high hydrostatic pressure