obligatory resorption and secretion in the PCT Flashcards
what is specific gravity
weight of a set volume of solution / weight of same volume of pure water
dissolved solutes make the specific gravity greater
what is the function of the proximal tubule
most of the recovery of ion, sugar, aa, peptides and water
also active secretion of compounds for excretion with urine
also metabolises some aa
what are the two ways that solutes might cross the tubule
transcellular
paracellular (through tight junctions between cell bodies)
how is an electrochemical gradient set up across the basolateral membrane
active tranport 3 Na out and 2 K in
this creates -3mV in the tubule lumen
what does the electrochemical gradient do
causes paracellular efflux of cations (-) because the tubule lumen is negative (-3mV)
how is water resorbed
water moves along the paracellular path due to osmotic pressure. this drags solutes with it. Solvent drag.
what does the sodium pump do
sets up sodium concentration gradient
pumps sodium from the tubule to the cell
this is used in secondary active transport
what happens to protons
exchanged with sodium so is pumped out of the cell into the tubule
hydrogen ions are used to reclaim bicarbonate ions
what happens on the basolateral membrane
chloride and potassium leave down their conc gradients
calcium is exchanged for sodium so calcium leaves. calcium is reabsorbed through the apical membrane
what is Tmax
limit as to how much can be moved (rate) mg/min or mmol/ min
if blood glucose is high then T max can be exceeded. there is more glucose in the lumen than is able to be reabsorbed
how is the amount of solute filtered determined
plasma concentration x GFR
therefore if either of these parameters increase the amount filtered into the lumen increases
this means it might pass the threshold for reabsorption
what is the threshold
the point at which the amount filtered is equivalent to Tmax
above the threshold the substance is in the urine
how is tmax related to secretion
amount filtered is proportional to the amount present in the plasma
on top of that there is secretion from the blood to the tubule
how is urea reabsorbed
simple diffusion 50% reabsorbed
how are lipid soluble substances reabsorbed
simple diffusion
how is phosphate reabsorbed
sodium linked transport
activity changed by PTH
how is protein reaborbed
some protein is digested to amino acids within the tubule cells
what is clearance
a theoretical volume of plasma cleared per minute
what is the filtration fraction
glomerular filtration rate / renal plasma flow
plasma flow tells you what percentage of renal plasma flow is filterable
what is required of a substance used to measure GFR
It must be freely filtered
it must not be secreted or reabsorbed
it must not be metabolised or toxic
what does it mean if the clearance ratio is greater than 1
substance is secreted
what does it mean if the clearance ratio is less than 1
substance is reabsorbed
what are clearance ratios used to determine
the renal transport mechanism
what proportion of bicarbonate is reabsorbed
90%
what proportion of sodium and water is reabsorbed
65%
how are lipids reabsorbed
simple diffusion
how is phosphate reabsorbed
sodium linked transport
what is Tm
tubule maximum
the maximum amount that can be excreted
what is the threshold
the point at which the amount filtered is equivalent to Tmax so no more can be reabsorbed/ secreted
how is the osmotic gradient set up
pumping sodium out of the cell into the interstitium
what is co transport and cotransport
co transport- is moved in the same direction as sodium (into the cell from the lumen) counter transport (is exchanged for sodium eg calcium from the cell into the interstitium)
how is bicarb reabsorbed
H ions transported out of the cell into the lumen by sodium
combines with bicarb to form carbonic acid which forms water and CO2 which diffuse into the cell
water and co2 then form bicarb and hydrogen in the cell
bicarb diffuses out of the cell into the interstitium
what substances can move by solvent drag
water drags potassium , sodium and calcium
how much bicarb is reabsorbed
90%
therefore if there is a problem with the kidney it might cause acidosis due to not enough bicarb reabsorption
how can we work out how much substance is excreted
amount filtered - amount reabsorbed + amount secreted
