reabsorption + secretion Flashcards
pressure in peritubular capillaries
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
starling’s forces in peritubular capilaries
oncotic > hydrostatic
reabsorption only
mecanisms of reabsorption
carrier mediated transport
Na+ reabsorption
Tm
carriers have a maximum transport capacity Tm which is due to saturation of the carriers
what happens if Tm is exceeded
excess substrate enters urine
glucose reabsoprtion
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
how can kidneys regulate some substances
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
reabsorption of Na+ ions
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
how is reabsorption of Na+ key to reabsorption of other components of filtrate
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
rate of reabsorpion of non-actively reabsorbed solute depends on
a) amount of H20 removed - determines extent of concentration gradient
b) permeabiliy of membrane to any particular solute
tubule membrane urea permeabiliy
tubule membrane is only moderately permeable to urea
50% reabsorbed, remainder stays in tubule
insulin tubule permeability
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_
how is active transport of Na+ also important for carrier mediated transport systems
substances such as glucose, amino acid etc. share same carrier molecule as Na+
(symport)
how does [Na+] affect glucose reabsorption
high [Na+] in tubulel facilitates and low [Na+] inhibits glucose transport
SGLT
sodium dependant glucose transporter
tubular secretion
secretory mechanisms transport substances from the peritubular capillaries into tubule lumen and so provide a second route into tubule
what is tubular secretion important for
substances that are protein bound since filtration at glomerulus is very restricted
also, for potentially harmful substances, means can be eliminated more rapidly
Tm- limited carrier mediated secretory mechanisms
known for large number of endogenous and exogenous substances (drugs)
not very specific, can be used for different things
where does tubular secretion occur
secreted at proximal tubule
why is K+ handling important
K+ is the major cation in the cells of our body and maintenance of K+ balance is essential for life
normal ECF[K+]
approx 4mmoles/l
hyperkalamia
5.5mmoles/l
dec resting membrane potential of excitable cells and evenually ventricullar fibrillation and death
hypokalaemia
<3.5mmoles/l
inc resting potential i.e. hyperpolarises muscle, cadiac cells -> cardiac arrythmias and death
what determines K+ excretion
changes in K+ secretion in distal parts of tubule
effect of [K+] intracellular conc on secretion
inc in renal tubule cell [K+] due to inc ingestion will inc K+ secretion
dec in intracellular K+ will dec secretion
what else is K+ secretion regulated by
adrenal cortical hormone - aldosterone
how does aldosteone affect K+ secretion
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
H+ secretion
H+ ions are actively secreted from tubule cells (not the peritubular capillaries) into the lumen A/B balance
