Reabsorption and Secretion Flashcards
what is reabsorbed?
> glucose > sulphate > amino acids > organic acids > phosphate ions
do transport carriers have a maximum capacity?
yes due to the saturation of those carriers
if the maximum transport capacity is exceeded in reabsorption where does the excess substrate enter?
the urine
what is the renal threshold?
the plasma threshold that saturation occurs in reabsorption
true or false:
glucose is not freely filtered, it depends on its plasma concentration
false
glucose is freely filtered no matter the plasma concentration
why can only up to 10 mmoles/l of glucose be reabsorbed?
because beyond this level is the renal threshold for glucose and so it is excreted in the urine
appearance of glucose in the urine is a sign of kidney or insulin failure?
insulin failure
where does the majority of sodium ions get reabsorbed?
the proximal tubule
describe the reabsorption of Na+ ions through a cell in the proximal tubule
> 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)
what drives Na+ reabsorption?
the Na+ pump
how does the brush border of the proximal tubules have increased permeability to Na+ ions?
> an enormous surface area due to the microvilli
> a large number of Na+ ion channels
how does Na+ reabsorption drive the reabsorption of other filtrate components?
> 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
what does the rate of reabsorption of passively absorbed solutes depend on?
> amount of H20 removed (extent of the gradient)
> permeability of the membrane to a particular solute
True or false:
the tubular membrane is impermeable to urea and moderately permeable to insulin/mannitol
false
it is moderately permeable to urea and impermeable to insulin/mannitol
True or False:
Anything that decreases active transport disrupts renal function
true
what is the effect of decreased Na+ concentration on glucose transport?
it inhibits glucose transport
for what substances is secretion important?
> protein bound substances
> potentially harmful substances (they can be excreted rapidly)
true or false:
carrier mediated secretory mechanisms are very specific to the substances they secrete
false they are not very specific:
organic acid mechanisms can also secrete lactic and uric acid, and penicillin
what problems occur in hyperkalaemia?
> decreased resting membrane potential of excitable cells
> ventricular fibrillation and death
what problems occur with hypokalaemia?
> increased resting membrane potential
> cardiac arrhythmias and death
where is K+ primarily reabsorbed?
the proximal tubule
what are changes in K+ excretion due to?
changes in its secretion in distal parts of the tubule
what hormone controls K+ concentration?
aldosterone
how does aldosterone increase K+ excretion?
it stimulates K+ secretion in the renal tubule cells
