Reabsorption and Secretion

Question 1

What is the difference between the pressure in the glomerular capillaries and in the peritubular capillaries?

Answer 1

Pressure in glomerular capillaries is much higher than the pressure in the peritubular capillaries

Question 2

Why is the pressure in the peritubular capillaries low?

Answer 2

The hydrostatic pressure is overcoming the frictional resistance in the efferent arterioles

Question 3

Why are the oncotic forces in the peritubular capillaries higher?

Answer 3

Some (20%) of the plasma is filtered into Bowman’s capsule so the blood remaining in the efferent arteriole and then peritubular capillaries has a higher concentration of plasma proteins so oncotic forces are higher

Question 4

What factors favour reabsorption in the peritubular capillaries?

Answer 4

Low pressure and high oncotic forces

Question 5

How are substances such as glucose and amino acids reabsopred in the peritubular capillaries?

Answer 5

Through carrier mediated transport systems

Question 6

What happens when the maximum transport capacity of a carrier is reached?

Answer 6

The carriers are saturated with substrates and the excess substrate enters the urine

Question 7

What is the renal threshold of a molecule?

Answer 7

The plasma concentration at which saturation occurs

Question 8

What is the renal threshold of glucose?

Answer 8

10mmol/L

Question 9

What are the differences between filtration and reabsorption of glucose?

Answer 9

Flitration of glucose is free so whatever the plasma concentration, all glucose will be filtered
Reabsorption is carrier mediated and so has a renal threshold, which for glucose is around 10mmol/L

Question 10

How do the kidneys regulate the concentrations of some particles?

Answer 10

If the renal threshold is set at a level where the normal plasma volume causes reabsorption then any increase above this level will be excreted

Question 11

How are sodium ions reabsorped?

Answer 11

Reabsorped by active transport which establishes a gradient for Na+ across the cell wall. Sodium pumps decrease the [Na+] in the epithelial cells which increases the gradient for sodium ions to move into the cells passively across the luminal membrane

Question 12

Why does the brush border have a higher permeability to sodium ions?

Answer 12

Large surface area created by microvilli allows for large number of sodium channels that facilitate the passive diffusion of sodium

Question 13

How are negative ions reabsorped?

Answer 13

They diffuse passively across the proximal tubular membrane, down the electrical gradient created by the active transport of sodium

Question 14

Describe the role of water in reabsorption

Answer 14

The active transport of sodium out followed by negative ions creates an osmotic force that draws water out of the tubules. This concentrates the substances left in the tubules and creates outgoing concentration gradients

Question 15

What does the rate of reabsorption of non-actively reabsorped solutes depend on?

Answer 15

The amount of water removed, which will determine the extent of the concentration gradient and the permeability of the membrane to any given solute

Question 16

What is the function of tubular secretion?

Answer 16

Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen and so provide a second route into the tubule

Question 17

How is potassium dealt with in the kidneys?

Answer 17

Potassium that is filtered at the glomerulus is reabsopred, primarily at the proximal tubule. Changes in potassium secretion are due to changes in its secretion at the distal parts of the tubule

Question 18

What hormone regulates potassium secretion?

Answer 18

Aldosterone

Question 19

How does aldosterone regulate potassium secretion?

Answer 19

An increase in the [K+] bathing the aldosterone secreting cells stimulates release of aldosterone, which circulates to the kidneys to stimulate an increase in renal tubule cell potassium secretion