obligatory resorption and secretion in the PCT

Question 1

what is specific gravity

Answer 1

weight of a set volume of solution / weight of same volume of pure water

dissolved solutes make the specific gravity greater

Question 2

what is the function of the proximal tubule

Answer 2

most of the recovery of ion, sugar, aa, peptides and water

also active secretion of compounds for excretion with urine

also metabolises some aa

Question 3

what are the two ways that solutes might cross the tubule

Answer 3

transcellular

paracellular (through tight junctions between cell bodies)

Question 4

how is an electrochemical gradient set up across the basolateral membrane

Answer 4

active tranport 3 Na out and 2 K in

this creates -3mV in the tubule lumen

Question 5

what does the electrochemical gradient do

Answer 5

causes paracellular efflux of cations (-) because the tubule lumen is negative (-3mV)

Question 6

how is water resorbed

Answer 6

water moves along the paracellular path due to osmotic pressure. this drags solutes with it. Solvent drag.

Question 7

what does the sodium pump do

Answer 7

sets up sodium concentration gradient

pumps sodium from the tubule to the cell

this is used in secondary active transport

Question 8

what happens to protons

Answer 8

exchanged with sodium so is pumped out of the cell into the tubule

hydrogen ions are used to reclaim bicarbonate ions

Question 9

what happens on the basolateral membrane

Answer 9

chloride and potassium leave down their conc gradients

calcium is exchanged for sodium so calcium leaves. calcium is reabsorbed through the apical membrane

Question 10

what is Tmax

Answer 10

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

Question 11

how is the amount of solute filtered determined

Answer 11

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

Question 12

what is the threshold

Answer 12

the point at which the amount filtered is equivalent to Tmax

above the threshold the substance is in the urine

Question 13

how is tmax related to secretion

Answer 13

amount filtered is proportional to the amount present in the plasma
on top of that there is secretion from the blood to the tubule

Question 14

how is urea reabsorbed

Answer 14

simple diffusion 50% reabsorbed

Question 15

how are lipid soluble substances reabsorbed

Answer 15

simple diffusion

Question 16

how is phosphate reabsorbed

Answer 16

sodium linked transport

activity changed by PTH

Question 17

how is protein reaborbed

Answer 17

some protein is digested to amino acids within the tubule cells

Question 18

what is clearance

Answer 18

a theoretical volume of plasma cleared per minute

Question 19

what is the filtration fraction

Answer 19

glomerular filtration rate / renal plasma flow

plasma flow tells you what percentage of renal plasma flow is filterable

Question 20

what is required of a substance used to measure GFR

Answer 20

It must be freely filtered
it must not be secreted or reabsorbed
it must not be metabolised or toxic

Question 21

what does it mean if the clearance ratio is greater than 1

Answer 21

substance is secreted

Question 22

what does it mean if the clearance ratio is less than 1

Answer 22

substance is reabsorbed

Question 23

what are clearance ratios used to determine

Answer 23

the renal transport mechanism

Question 24

what proportion of bicarbonate is reabsorbed

Answer 24

90%

Question 25

what proportion of sodium and water is reabsorbed

Answer 25

65%

Question 26

how are lipids reabsorbed

Answer 26

simple diffusion

Question 27

how is phosphate reabsorbed

Answer 27

sodium linked transport

Question 28

what is Tm

Answer 28

tubule maximum

the maximum amount that can be excreted

Question 29

what is the threshold

Answer 29

the point at which the amount filtered is equivalent to Tmax so no more can be reabsorbed/ secreted

Question 30

how is the osmotic gradient set up

Answer 30

pumping sodium out of the cell into the interstitium

Question 31

what is co transport and cotransport

Answer 31

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)

Question 32

how is bicarb reabsorbed

Answer 32

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

Question 33

what substances can move by solvent drag

Answer 33

water drags potassium , sodium and calcium

Question 34

how much bicarb is reabsorbed

Answer 34

90%

therefore if there is a problem with the kidney it might cause acidosis due to not enough bicarb reabsorption

Question 35

how can we work out how much substance is excreted

Answer 35

amount filtered - amount reabsorbed + amount secreted