Renal Physiology 3 - Checked and Complete

Question 1

Describe Paracellular Transport

Answer 1

Substance travels between cells, crossing tight junctions

Examples include Water, Urea, Chloride

Tight junctions have variable permeability - some allow solute crossing

Also varies with concentration gradient, electric potential

This process is passive and non-selective

Question 2

Describe Transcellular Transport

Answer 2

Substance travels through cell -both apical and basolateral membranes

**Examples include H2O, Na+ , Ca2+ and glucose. **

Often use channels, carriers, pumps which differ in apical membrane and basolateral membrane

Varies with concentration gradient, membrane potential, membrane permeability and particular transporters

Question 3

Describe how sodium is transported from the nephron lumen into peritubular capillaries.

Answer 3

1) Active transport of sodium up its concentration gradient across basolateral membrane into interstitium

**2) Transport of sodium down its concentration gradient across apical membrane (often passive symport to bring in other molecules such as glucose) **

Anions travel with sodium into cell and interstitium to balance charge

3) Water follows osmotic gradient into interstitium

4) Travel from interstitium into peritubular capillaries

Question 4

What is the difference between simple and facilitated diffusion (passive transport)?

Answer 4

Facilitated diffusion uses transport protein, channel, etc… to help solute move down its gradient

Question 5

Define the difference between primary and secondary active transport.

Answer 5

Primary active transport directly uses ATP in transporting molecule

Secondary active transport uses a gradient established by a different ATP-dependent transport process

Question 6

What can affect the rate at which a solute is transported?

Answer 6

Can reach max T_M where transporters are saturated with solute

Or may have backleak of substance (perhaps with leaky tight junctions or other condition)

Question 7

Describe glucose filtration

Answer 7

Glucose is all filtered and normally, it is all reabsorbed in the PCT via transcellular transport. Steps as follow:

1) active sodium potasssium pump in basolateral membrane

2) sodium/glucose symporter in apical membrane

3) glucose uniporter in basolateral membrane

Rate-limiting step is Gluc-Na symporter

Question 8

At what point do nephron sodium-glucose symporters reach their max? (and sugar is not reasbsorbed, instead secreted in urine)

Answer 8

at 300 mg/dL plasma glucose

Question 9

Describe peptide/protein filtration.

Answer 9

Small proteins like angiotensin and insulin need to be reabsorbed by the kidney in the PCT

They are endocytosed by the apical membrane, transported to the lysosome, and released as amino acids into peritubular capillaries

Some peptides are catabolized by enzymes on apical border of the PCT and use Na-dependent amino acid symporters to get through the apical membrane

Question 10

Describe filtration of organic anions

Answer 10

The primary pathway to their eventual excretion is via secretion (of course filtration also helps) using nonselective transcellular transporters

Active transporters bring anions INTO the cell from basolateral membrane

Then passive transport excretes into lumen with uniporters or antiporters

If saturated, not all of it will be excreted (which is great for something like penicillin)

Question 11

Describe PAH filtration. Describe Urate filtration (both organic anions)

Answer 11

PAH is secreted from the blood into the urine at about 90% and thus is a good estimate of RPF

Urate is complex. It is filtered, reabsorbed, secreted and then reabsorbed again. This can be regulated but usually about 10% is excreted.

Question 12

Describe filtration of organic cations

Answer 12

Secreted into the renal tubules analogous to organic anions

Question 13

Why is it so important that cations and anions are excreted into the urine via secretionand not just filitration?

Answer 13

These ions are often bound to large unfiltered proteins so would never have a chance to be excreted without secretion.

Question 14

Describe urea filtration.

Answer 14

Urea filtered, reabsorbed in PCT, secreted at loop of Henle, reabsorbed again at collecting duct.

50% of filtered urea ends up in urine.

Urea ends up 25 times more concentrated than plasma because water is removed while much urea is actively secreted

Question 15

Describe urea filtration again in detail.

Answer 15

(Remember: other solutes also influencing water flow, parts of nephron impermeable to urea and/or water)

1) Water is absorbed from the PCT into the blood, concentrating urea in the PCT. Absorption of water in blood eventually (active transport) makes urea concentration in plasma less than PCT - Urea diffuses from PCT into blood via paracellular transport
2) After this, tight junctions in nephron no longer permeable to urea. Uniporters mediate facilated diffusion of urea transcellularly
3) Because of active transport (from ascending loop of Henle) urea concentration in the kidney medulla ISF is much higher than that in the cortex or plasma. Thus, urea is secreted back into the loop of Henle from medullary ISF
4) In the collecting duct, more water is absorbed into blood. Urea becomes so concentrated that some diffuses back into medullary ISF. **Urea reabsorbed at collecting duct into Medullary ISF. **However, urea is still VERY concentrated

Question 16

Where is urea recycled?

Answer 16

Between the loop of Henle (secreted) and the collecting duct in the kidney medulla (absorbed)

Question 17

Draw cell diagrams to show filtration of glucose, sodium, anions/cations, urea

Answer 17

Woohoo!