Lecture #37 - Tubular function

Question 1

1. What 6 things are only re-absorbed?
2. What are the 2 things (with 2 e.g’s each) that are only secreted?
3. What 5 things are secreted and re-aborbed?
4. What can you say about K+, NH3 and Urea?

Answer 1

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Question 2

Tubular epithelia:

1. What are the cells held together by?
2. What increases SA?

Answer 2

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Question 3

Tight junctions - for what two things?

Answer 3

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Question 4

Two pathways through which water and substances can move:

1. Between cells - _______
   - leaky = bulk ______ e.g. _____
2. Through cells - ______
   - “____” - channels/energy
   - more _____ - _____ control

These two work together. Trans is actually in all parts of the tubule but paracellular isn’t

Answer 4

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Question 5

Paracellular route:

1. How many barriers?
2. Connects what two things?
3. No requirement for what so limited _____
4. Permeability depends on……(e.g. PCT and DCT)

Transcellular route:

1. How many barriers? What are they?
2. Connects what?
3. Usually involves what therefore ______ but often _____ dependent

Answer 5

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Question 6

Na reabsorption:

1. Where is it reabsorbed and how much percentage?

Answer 6

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Question 7

Proximal tubule:

1. Site where ______ reabs occurs (“bulk _____”)
2. 65% of ____, ____ and _____ reabs
3. All go the filtered ____ and ____ ______ reabs here
4. Most of ___ (90%), ____ and _____
5. Half of _____
6. What does PCT secrete?
7. Reabs driven by what reabs?
8. Explain the movement of Na+ into the cell and out of the cell and talk about its gradient
9. What are two examples of solutes that are coupled to Na+ reabs
10. What is it called?

Answer 7

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Question 8

1. Is the membrane between the lumen of the tubule and the interstitium permeable without transporters or channels?
2. With the Na/K+ pump - we invest ____ and form a ______ _____ and this gradient can be used by other substances = coupled _____
3. Even with the sodium gradient, can sodium or glucose pass the luminal membrane without any transporters or channels?
4. Okay so to get Na+ across the luminal membrane, you need transports (mostly in…..) or channels (mostly in…..)
5. With sodium as the driving force, other substances such as glucose can be absorbed if there is a…….
6. So what moves up and what moves down its grad in 5.?
7. Then once glucose has reached a certain cocentration, it is released into the _____ via ______ _____ (is it dependent on sodium now? - what are the transporters called?)

Question 9

Glucose:

1. At normal filtered loads, ___ glucose is reabsorbed - none in urine
2. High plasma glucose (diabetes) leads to what exceeding what? This leads to glycosuria - what is that?

Answer 9

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Question 10

Water movement:

1. In ___ epithelium, the consequence of sodium absorption is a huge ____ ___ over the epithelium; this drives _____ and _____ cellular absorption of water.
2. So Na+ reabs builds up ___ ____ and use facilitators called _____. So if 66% Na+ reabs in PCT, how much reabs of water?
3. Na+ reabs facilitates H2O reabs - also facilitated by what? Use ___ ___ to reabs Na+ (since “sodium and water cannot pass the luminal membrane without any transports or channels) and H2O. Same principle in intestine

PCT vs CT

• leaky/tight
• high/low water permeability
• trans and/or para

Answer 10

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Question 11

Counter-current multiplier system

1. The thin descending limb has ____ epithelium facilitating what?
2. The thick ascending limb is ___-___ and reabsorbs ____ into the interstitium via what? Is this active?
3. Explain this whole process

4.

Answer 11

3. Okay so, urine comes iso-osmotic. The ascending limb actively pumps sodium and chloride out of the tubule fluid and into the interstitial fluid. These two don’t diffuse right back because the TAL prevents the diffusion of these ions so they’re trapped in that medulla part. You would expect the water from the TAL to flow from inside to out as well (following the solutes) but it doesn’t because TAL walls are impermeable to water. So the interstitial fluid builds a high solute concentration (high osmotic pressure).

So because the interstitial fluid has a high solute concentration (created by ion pumps in the ascending limb, the fluid in the descending limb loses water osmotically. So the solute concentration of the tubule fluid becomes increasingly higher. When the loop “rounds the bend” - Na+ and Cl- are removed and it becomes increasingly lower in solute concentration. Urea, a solute that’s highly conc in renal medulla flows into the tDL

This allows water to be reabs in collecting duct as required. This depends on hydration. The urine at the end of the loop is hypo-osmotic

Question 12

DCT and CD:

1. Fine tune what 3 things?
2. Reabs most of what here?
3. Secrete what two things?
4. Under what control?
   - sodium reabs by ____
   - water reabs by _____

Answer 12

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Question 13

CD: water reabs

1. ADH alters what? Through what?
2. But need what for water to move? Need to do what to kidney around CD?
3. How is 2. achieved?
4. Water reabs from CD lumen down ____ ___ depending on ____ (ADH)

Answer 13

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