Reabsorption and Secretion

Question 1

what is reabsorbed?

Answer 1

> glucose
> sulphate
> amino acids
> organic acids
> phosphate ions

Question 2

do transport carriers have a maximum capacity?

Answer 2

yes due to the saturation of those carriers

Question 3

if the maximum transport capacity is exceeded in reabsorption where does the excess substrate enter?

Answer 3

the urine

Question 4

what is the renal threshold?

Answer 4

the plasma threshold that saturation occurs in reabsorption

Question 5

true or false:

glucose is not freely filtered, it depends on its plasma concentration

Answer 5

false

glucose is freely filtered no matter the plasma concentration

Question 6

why can only up to 10 mmoles/l of glucose be reabsorbed?

Answer 6

because beyond this level is the renal threshold for glucose and so it is excreted in the urine

Question 7

appearance of glucose in the urine is a sign of kidney or insulin failure?

Answer 7

insulin failure

Question 8

where does the majority of sodium ions get reabsorbed?

Answer 8

the proximal tubule

Question 9

describe the reabsorption of Na+ ions through a cell in the proximal tubule

Answer 9

> active Na+ pumps in the basolateral surface of the cell transport Na+ from the cell into the interstitial fluid
this decreases the conc. of Na+ in the cell increasing the gradient for Na+ to move into the cell passively
(> these Na+ are then transported into the interstitial fluid)

Question 10

what drives Na+ reabsorption?

Answer 10

the Na+ pump

Question 11

how does the brush border of the proximal tubules have increased permeability to Na+ ions?

Answer 11

> an enormous surface area due to the microvilli

> a large number of Na+ ion channels

Question 12

how does Na+ reabsorption drive the reabsorption of other filtrate components?

Answer 12

> Na+ creates an electrochemical gradient that drives Cl- (anions) across the proximal tubule membrane

> this creates an osmotic force, drawing H20 out of the tubule

> this concentrates the remaining substances (urea, K+, Ca2+) in the tubule creating an outgoing conc. gradient so they are reabsorbed by diffusion

Question 13

what does the rate of reabsorption of passively absorbed solutes depend on?

Answer 13

> amount of H20 removed (extent of the gradient)

> permeability of the membrane to a particular solute

Question 14

True or false:

the tubular membrane is impermeable to urea and moderately permeable to insulin/mannitol

Answer 14

false

it is moderately permeable to urea and impermeable to insulin/mannitol

Question 15

True or False:

Anything that decreases active transport disrupts renal function

Answer 15

true

Question 16

what is the effect of decreased Na+ concentration on glucose transport?

Answer 16

it inhibits glucose transport

Question 17

for what substances is secretion important?

Answer 17

> protein bound substances

> potentially harmful substances (they can be excreted rapidly)

Question 18

true or false:

carrier mediated secretory mechanisms are very specific to the substances they secrete

Answer 18

false they are not very specific:

organic acid mechanisms can also secrete lactic and uric acid, and penicillin

Question 19

what problems occur in hyperkalaemia?

Answer 19

> decreased resting membrane potential of excitable cells

> ventricular fibrillation and death

Question 20

what problems occur with hypokalaemia?

Answer 20

> increased resting membrane potential

> cardiac arrhythmias and death

Question 21

where is K+ primarily reabsorbed?

Answer 21

the proximal tubule

Question 22

what are changes in K+ excretion due to?

Answer 22

changes in its secretion in distal parts of the tubule

Question 23

what hormone controls K+ concentration?

Answer 23

aldosterone

Question 24

how does aldosterone increase K+ excretion?

Answer 24

it stimulates K+ secretion in the renal tubule cells