(11) - Epithelial Transport

Question 1

(Paracellular Transport)

1-2. for paracellular transport to occur… what two things must be?

(voltage gradients also play a role)

3. early PT lumen slightly (+) or (-) vs. interstitium?
4. Late PT lumen?
5. answer the question in the pic

Answer 1

1. must be chemical gradient across tight junctions
2. tight junctions must be permeable to the substance (leaky)
3. (-)
4. (+)
5. Late

Question 2

(Paracellular Transport)

(one thing common amongst all of these is that they are small)

1. answer the question

Answer 2

1. no (transcellular)

Question 3

(Key Points)

1. Paracellular transport can occur only for substances that can do what?
2. What drive the movement across the tight junctions?

Answer 3

1. can move across the tight junctions
2. concentration or voltage gradients (or both)

Question 4

(Non-Ionic Diffusion)

1. passive or active process?
2. by which impermeant ions derived from the dissociatino of weak acids or bases can cross cell membranes

Answer 4

1. passive

(just read the rest…)

Question 5

Answer 5

an example of this is ammonia and ammonium

ammonia can move easily - if we add H to ammonia becomes ammonium - is trapped - will get trapped in urin and will get excreted

Question 6

Answer 6

A

Question 7

Answer 7

Question 8

(Limits on Trasport Rate)

1. Transport capacity of the PT huge, bout not unliimited - there are limits!

2-3. What are the two transport mechanisms classified by these limits?

Answer 8

2. tubular maximum-limited systems (Tm)
3. gradient-limited systems

Question 9

cats are better at moving glucose

why is this more important from clinical standpoint

so when you see glucose in urine - means this maxiumum has been exceeded which means the glucose conc in the blood is too high (diabetes)

the other possibilty is that tubular maximum isn’t at correct level (even with a normal blood glucose)

so glucose in urine doesn’ always mean diabetes

Question 10

Answer 10

(look at next slide too)

Question 11

what it looks like in patient is a little messier - cause each nephrons maximum is a little different

Question 12

(Gradient Limited Transport)

(Epithelium has a finite passive permeability)

1. usually through what?
2. large concentration gradient between interstitium and tubular lumen –> ?
3. As Na transport increases, interstitial [Na] increases until what occurs?

Answer 12

1. tight junctions
2. passive “back leak” into tubule
3. movement into capillaries and lumen are equal

Question 13

(Gradient-Limited transport)

1. Net reabsoprtion = ?

limit reached when what and what are equal?

when they are equal, net reabsporption is what?

2. the leakier the tight junctions, the lower or higher the gradient limit?

Answer 13

1. transcellular transport less reverse paracellular transport

transcellular and paracellular transport

0

2. lower

Question 14

(PAH - Secreted Organic Acid)

(PAH filtered and secreted)

1. what percent in plasma is protein bound?
2. how does this affect filtration?

(Secretory transporters present in peritubular membranes of PT cells)

1. finite capacity
2. when low plasma conc, ~ all transported
3. carrier saturated at Tm

Answer 14

1. 90%
2. alot of it doesn’t get filtered or secreted at all

Question 15

Answer 15

Question 16

(Key points)

1-2. Limits on transport systems may be what two things?

Answer 16

1. tubular maximum limited
2. gradient limited

(just read the rest - you should already know all of this…)

Question 17

should already know this too

Answer 17

Question 18

Question 19

if the blood level exceeds that Tm then the patient probably has diabetes - although stressed cats can elevate their blood glucose a bunch which can be confusing - so there are other possible explanations - for most animals diabetes found by persistently high glucose levels in urine

renal glucosuria has normal blood level glucose - problem here that Tm of the tubules of the kidney has been reduced due to damge/genetics

Question 20

Answer 20

if you increase level in blood - you increase level in the filtrate - the problem is… the filtrate…. we’re already pulling out as much bicarbonate as we can - so what happens is it spills over - and most of the bicarbonate that we give doesn’t get reabsordeb - beacuse all the mecahnism for resoabrding it is right there in th proximal tubule and the disease process here loss of function oin th e proximal tubule

bicarb is difficult needs to be resorbed in proximal - which is difficult cause most is just expelled in urine - so you need to use a ton of bicarbonate which isn’t ideal