kidney

Question 1

what are the 2 categories of epithelia?

Answer 1

absorptive: active Na+ transport drives solute and water reabsorption
secretory: active Cl- transport drives fluid secretion

Question 2

what is the structure of epithelia?

Answer 2

sheets of cells
each cell separated by lateral intercellular spaces but held together by tight junctions at the luminal (apical) edge
creates the distinct apical vs basolateral

Question 3

what are the cardinal properties of epithelia/

Answer 3

1. ability to translocate ions from 1 side to other (unidirectional/vectorial transport)
2. asymmetry: underlies capacity for vectorial transport of ions/solutes/water

Question 4

how is the Na+/K+ ATPase distributed in the kidney?

Answer 4

asymmetrically

localised to basolateral membrane

Question 5

what do tight junctions do?

Answer 5

provide a barrier to varying degrees to solutes + H2O
separate BL and A membranes

Question 6

what does the Na/K ATPase do?

Answer 6

pumps 2 K+ in and 3Na+ out using ATP on the cytosol-side binding site

Question 7

what does ouabain do?

Answer 7

inhibit Na/K ATPase by binding to the external K+ binding site, inhibiting the pump

Question 8

where is the basolateral membrane?

Answer 8

anything on the non-luminal side of the tight junctions

Question 9

what are tight junctions?

Answer 9

prevent any significant movement of molecules from one side of the membrane to the other in the gaps between cells

things only move transcellularly

Question 10

what are leaky junctions?

Answer 10

junctions form imperfect seals and are low-resistance leak pathway for ions and water

things more paracellularly and transcellularly

Question 11

what are leaky epithelia specialised for and where are they found?

Answer 11

‘valves’
bulk handling of isosmotic solutions
found at the start of systems: PCT and small intestine for bulk reabsorption

Question 12

what are tight epithelia specialised for and where are they found?

Answer 12

can withstand large osmotic gradients and are more highly regulated due to the fact they only allow transcellular pathway that is determined by the proteins expressed in cells (can be hormonally regulated)

found distally (CD, colon), regulating Na+ and H2O by hormones

Question 13

what is the advantage of placing both kinds of epithelia in series?

Answer 13

can achieve bulk absorption followed by fine control

Question 14

what does the direction of transport depend on?

Answer 14

electrical (charged) and chemical (uncharged) gradients for ions

osmotic pressure gradients for water

Question 15

what determines the magnitude of the p.d. across an epithelium?

Answer 15

whether epithelium is leaky (5mV) or tight (30mV) (maintains charge separation, so can have larger p.d.)

Question 16

what is the underlying process in absorptive epithelia?

Answer 16

active Na+ transport based on differential permeability of membranes
(Ussing model)

Question 17

describe the Ussing model

Answer 17

high Na+ permeability due to Na+ channels (A): inward gradient of Na+ from lumen to cell

Na/K ATPase on BL, in parallel with K+ channel exporting K+ to the interstitial fluid

Question 18

how is a sodium gradient established to allow absorptive processes to occur?

Answer 18

Na/K ATPase in the BL membrane

Question 19

what causes plasma alkalosis?

Answer 19

ascent to altitude

Question 20

what can happen as a result of plasma acidosis?

Answer 20

hyperkalaemia