reabsorption + secretion

Question 1

pressure in peritubular capillaries

Answer 1

pressure is low as hydrostatic pressure is overcoming frictional resistance in efferent arteriole

oncotic pressure high compared to normal as loss of 20% plasma concentrates plasma proteins

Question 2

starling’s forces in peritubular capilaries

Answer 2

oncotic > hydrostatic

reabsorption only

Question 3

mecanisms of reabsorption

Answer 3

carrier mediated transport

Na+ reabsorption

Question 4

Tm

Answer 4

carriers have a maximum transport capacity Tm which is due to saturation of the carriers

Question 5

what happens if Tm is exceeded

Answer 5

excess substrate enters urine

Question 6

glucose reabsoprtion

Answer 6

glucose is freely filtered, so whatever [plasma] that will be filtered

in man plasma glucose up to 10mmolel/l will be reabsorbed

beyond this level it appears in urine - renal plasma threshold for glucose

Question 7

how can kidneys regulate some substances

Answer 7

Tm mechanism - Tm set at level whereby normal [plasma] causes saturation
anything above will be excreted therefor acheiving plasma regulation

e.g. sulphate, phosphate ions

Question 8

reabsorption of Na+ ions

Answer 8

active transport which establishes gradient across tubule wall

Na+ enters cell through membrane proteins down electrochemical gradient
Na+ is pumped out basolateral side by NaK+ATPase

Question 9

how is reabsorption of Na+ key to reabsorption of other components of filtrate

Answer 9

negative ions like Cl- diffuse passively across the proximal tubual membrane down electical gradient established by transport Na+

active transport of Na+ followed by Cl- created osmotic forec drawing water out the tubules

H20 removed concentrates all the substances left in the tubule creating outgoing concentration gradients

Question 10

rate of reabsorpion of non-actively reabsorbed solute depends on

Answer 10

a) amount of H20 removed - determines extent of concentration gradient
b) permeabiliy of membrane to any particular solute

Question 11

tubule membrane urea permeabiliy

Answer 11

tubule membrane is only moderately permeable to urea

50% reabsorbed, remainder stays in tubule

Question 12

insulin tubule permeability

Answer 12

tubule membrane is impermeable to insulin

(despite concentration gradient favouring reabsorption, cannot gauin access through tubule membrane so all that is filtered stays in tubule and passes out in urine_

Question 13

how is active transport of Na+ also important for carrier mediated transport systems

Answer 13

substances such as glucose, amino acid etc. share same carrier molecule as Na+
(symport)

Question 14

how does [Na+] affect glucose reabsorption

Answer 14

high [Na+] in tubulel facilitates and low [Na+] inhibits glucose transport

Question 15

SGLT

Answer 15

sodium dependant glucose transporter

Question 16

tubular secretion

Answer 16

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

Question 17

what is tubular secretion important for

Answer 17

substances that are protein bound since filtration at glomerulus is very restricted

also, for potentially harmful substances, means can be eliminated more rapidly

Question 18

Tm- limited carrier mediated secretory mechanisms

Answer 18

known for large number of endogenous and exogenous substances (drugs)

not very specific, can be used for different things

Question 19

where does tubular secretion occur

Answer 19

secreted at proximal tubule

Question 20

why is K+ handling important

Answer 20

K+ is the major cation in the cells of our body and maintenance of K+ balance is essential for life

Question 21

normal ECF[K+]

Answer 21

approx 4mmoles/l

Question 22

hyperkalamia

Answer 22

5.5mmoles/l

dec resting membrane potential of excitable cells and evenually ventricullar fibrillation and death

Question 23

hypokalaemia

Answer 23

<3.5mmoles/l

inc resting potential i.e. hyperpolarises muscle, cadiac cells -> cardiac arrythmias and death

Question 24

what determines K+ excretion

Answer 24

changes in K+ secretion in distal parts of tubule

Question 25

effect of [K+] intracellular conc on secretion

Answer 25

inc in renal tubule cell [K+] due to inc ingestion will inc K+ secretion

dec in intracellular K+ will dec secretion

Question 26

what else is K+ secretion regulated by

Answer 26

adrenal cortical hormone - aldosterone

Question 27

how does aldosteone affect K+ secretion

Answer 27

inc in K+ in ECF bathing the aldosterone secreting cells stimulate aldosterone release which circulates to the kidneys to stimulate inc renal tubule cell K+ secretion

Question 28

H+ secretion

Answer 28

H+ ions are actively secreted from tubule cells (not the peritubular capillaries) into the lumen A/B balance