Control of blood water potential

Question 1

What is osmoregulation?

Answer 1

The homeostatic control of water potential in the blood

Question 2

What is the structure of the kidney?

Answer 2

• fibrous capsule = outer membrane that protects the kidney
• cortex = Lighter reigon of the kidney made of the bowmans capsule, convulated tubules and blood vessels
• medulla = darker coloured reigon made of the loops of henle, collecting ducts and blood vessels

Question 3

State and explain the role of each blood vessel?

Answer 3

afferent arteriole = tiny blood vessel from the renal artery that supplies the nephron with blood. The afferent arteriole enters the renal capsule and the glomerulus

glomerulus = a tiny branched knot capillaries from which fluid is forced out of the blood. The glomerular capillaries then recombine and form the efferent arteriole

efferent arteriole = leaves the renal capsule. Has a smaller diameter and causes an increase in blood pressure within the glomerulus

Question 4

How does ultrafiltration occur?

Answer 4

• blood enters the glomerulus
• the endothelium of the glomerular capillaries has pores, allowing small molecules to diffuse through
• the molecules then pass through the basement layer, allowing filtering
• finally, the inner layer of the renal capsule is made of podocytes which have spaces beneath them, this allows filtrate to pass beneath them and gaps between their branches
  -The diameter of the afferent arteriole is greater than the efferent causing a build up of hydrostatic pressure within the glomerulus and forcing small molecules such as glucose, water, urea and minerals out of the blood and large molecules remain in the blood

Question 5

How was the proximal convulated tubule adapted for selective reabsorption?

Answer 5

• microvilli to provide a large SA
• inner foldings at their base to give a large SA to transport reabsorbed substances into blood capillaries
• a high density of mitochondria to provide ATP for active transport

Question 6

What is the process of selective reabsorption?

Answer 6

• sodium ions are actively transported out of the cells lining the proximal convulated tubule into blood capillaries which carry them away
• this creates a concentration gradient for sodium ions as their concentration is lowered
• sodium ions now diffuse down a concentration gradient from the lumen of the proximal convulated tubule into epithelial lining cells but through special carrier proteins by facillitated diffusion
• these carrier proteins also co transport glucose
• glucose then diffuses into the blood, this reduces the water potential of the blood, resulting in the osmosis of water back into the blood

Question 7

What are the reigons of the loop of henle?

Answer 7

• the descending limb = narrow, has thin walls that are highly permeable to water
• the ascending limb = thick walls, wider and walls are impermeable to water

Question 8

What is the process of the reabsorption of water?

Answer 8

• sodium ions are actively transported out of the ascending limb out of the loop of henle using ATP provided by mitochondria
• This lowers the water potential of the interstitial reigon, however water does not leave the ascending limb as it is impermeable to water
• The walls of the descending limb are very permeable to water so it passes out of the filtrate into the interstitial space. Water in the interstitial space is then reabsorbed into blood capillaries
• the filtrate progressively looses water as it moves down the descending limb lowering its water potential. the lowest water potential is at the hairline
• At the base of the ascending limb, sodium ions diffuse out of the filtrate as it moves up the ascending limb. They are also actively transported out
• Therefore the filtrate develops a higher water potential
• in the interstitial space there is a gradient of water potential with the highest being in the cortex and lowest in the medulla
• as the collecting duct is permeable to water, as the filtrate moves down the collecting duct water passes out of it by osmosis due to the low water potential of the cortex
• As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space so water continues to move out by osmosis down the length of the collecting duct

Question 9

What is the role of the DCT?

Answer 9

• the walls of the DCT have many mitochondria and microvilli to reabsorb materials from the filtrate
• control ph of blood by selecting what materials to reabsorb

Question 10

What is the counter current multiplier?

Answer 10

• the filtrate in the collecting duct has a lower water potential than the interstitial fluid
• this means there is a water potential for the whole length of the collecting duct and a steady flow of water into the interstitial fluid to be reabsorbed

Question 11

What happens when the W.P of the blood falls?

Answer 11

• This is detected by osmoreceptors in the hypothalamus
• in a lower W.P, water is lost from osmoreceptors by osmosis which causes them to shrink.
• there is an increase of impulses to the posterior pituitary gland
• ADH is then released by the posterior pituitary gland
• ADH then passes to the kidney where it binds to receptors on the cell surface membrane, activating phosphorylate
• this results in vesciles within the cell fusing with the cell surface membrane
• these vesicles display and increase the number of water channels, making the cell surface membrane more permeable to water
• This increases the permeability of the collecting duct

Question 12

What happens when the W.P of the blood rises?

Answer 12

• This is detected by osmoreceptors
• Increases the frequency of impulses to the posterior pituitary gland, releasing less ADH
• therefore this decreases the permeabiliy of the collecting duct
• less water is reabsorbed into the blood
• more dilute urine is produced