Renal Physiology 4 - Checked and Complete

Question 1

Describe sodium absorption generally

Answer 1

Sodium is absorbed actively transcellularly via the sodium/potassium pump in the basolateral membrane

Question 2

Describe chloride absorption generally

Answer 2

May be passive paracellularly

May be active transcellularly

Follows sodium flow

Question 3

Describe water absorption generally

Answer 3

By osmosis - follows solutes, especially Sodium

Question 4

List 3 ways sodium output is accomplished

Answer 4

Sweat

Feces

MOSTLY Urine

Question 5

List places in nephron where sodium is reabsorbed and what percent.

Answer 5

PCT = 65%

Thick Ascending Loop of Henle = 25%

DCT = 5%

Collecting Duct = 4-5%

Question 6

What are some examples of sodium transporters found in the apical membrane of the PCT?

Answer 6

Glucose or Amino Acid/Sodium Symporter

Phosphate/Sodium Symporter

Hydrogen/Sodium Antiporter

Sodium channel

Question 7

Describe Chloride flow in the PCT

Answer 7

MOST chloride moves via passive paracellular route through leaky tight junctions

Chloride may use Hydrogen/Chloride Symporter in Apical Membrane

Chloride may use Potassium/Chloride Symporter in Basolateral Membrane to follow sodium back into blood

Question 8

List places in nephron where water is reabsorbed and what percent. (Compare with sodium reabsorption)

Answer 8

PCT = 65%

Thin loop of Henle = 10%

Collecting duct = 5-24%

Question 9

Compare water reabsorption in nephron with sodium reabsorption.

Answer 9

PCT - approximately equal Na vs H₂O

Loop of Henle - locations are different; More Na than H₂​O reabsorbed

DCT - Only Na reabosrped

Collecting Duct - Variable but more H₂​O than Na

Question 10

What regulates H₂O permeability along the nephron?

Answer 10

**“Tightness” of tight junctions. **

Auqaporins are throughout the nephron at the basolateral membrane so they don’t change water regulation generally.

Question 11

Which areas of the Nephron are NOT H₂O permeable?

Answer 11

Ascending limb of loop and DCT

Question 12

Define obligatory water loss.

Answer 12

The minimum amount of urine the body must produce to rid itself of this solute. About .43 L/day

Question 13

Why can’t you drink sea water when dehydrated?

Answer 13

The obligatory water loss would be almost twice as much as the volume of sea water ingested because sea water is so solute dense.

Question 14

Define maximum/minimal/average urine osmolarity (such as during conditions of extreme dehydration or water overload)

Answer 14

Maximum = 1400 mmole/L (dehydration)

Minimum = 50 mmole/L (well-watered)

Average = 500-800 mmole/L

Question 15

What is the major anion reabsorbed with Na in the PCT?

Answer 15

Bicarb - formed in the cell by epelling H+ via H+/Na+ antiporter

PCT level of bicarb is low because it gets absorbed well

Question 16

What happens to chloride concentration in the PCT?

Answer 16

Chloride concentration in the PCT starts equal with plasma, but rises gradually as water is reabsorbed.

Bicarb is the intial anion of choice to follow Na+ absorption so Chloride concentration in nephron goes up

When bicarb availability diminishes further down in the PCT, Chloride is used so its concentration increases. A gradient forms and some of it leaves paracellularly.

Question 17

Define iso-osmotic volume reabsorption in regards to the PCT.

Answer 17

Water follows sodium equally out of the PCT

Absorption occurs as Na+ is actively reabsorbed into the interstitium and water follows. However, peritubular capillaries whisk away sodium and water with a countercurrent circulation, restoring original osmolarity of the interstitium.

Thus, the volume of the tubular fluid decreases but its overall osmolarity remains relatively constant.

Question 18

Define osmotic diuresis

Answer 18

**Active Na+ absorption is disrupted in the PCT so water is left behind and you pee a lot. **

Question 19

Describe concentrations of molecules/ions throughout the PCT. (replicate graph verbally)

Answer 19

Chloride increases in concentration

Sodium stays the same in concentration (osmolarity thus stays the same since this is most abundant solute)

Bicarb decreases in concentration

Glucose and AAs decrease in concentration

Question 20

Why isnephron fluid after the Loop of Henle always more dilute?

Answer 20

Loop of Henle absorbs more sodium than water

Question 21

Which part of the loop of Henle absorbs water? Which part absorbs sodium?

Answer 21

** Descending limb of the loop only reabsorbs H2O**

**Ascending limb of the loop (thick & thin) only reabsorbs Na+ **

Question 22

Is sodium reabsorption in the ascending loop of Henle passive or active?

Answer 22

Trick question - both

The Na+ reabsorption is passive in the thin region and active in the thick region

Passive thin

Active thick

“Need to be active if you are thick!”

Question 23

Describe apical transporters of sodium in the loop of Henle.

Answer 23

** Na-K-2Cl symporter used**

also sodium/hydrogen antiporter

Question 24

Why is the Na-K-2Cl symporter significant?

Answer 24

It is a target of many diuretics known as loop diuretcs

Example = furosemide (Lasix)

Question 25

How does the Na-K-2Cl symporter keep working on the apical membrane?

Answer 25

Apical Potassium channel allows potassium to flow back into nephron

Chlorine and Sodium pumped into interstial space

Question 26

Describe reabsorption in the distal tubules

Answer 26

Water NOT absorbed

Sodium IS absorbed

Question 27

What type of apical transporters might you find in the DCT?

Answer 27

Na-Cl symporter

**Calcium channels **

Question 28

What drugs could block sodium absorption (and thus water absorption later on) in the DCT?

Answer 28

Sodium/Chloride symporter inhibitors

** thiazide diuretics (hydrochlorthiazide)**

Question 29

What other ion is regulated in the DCT?

Answer 29

Calcium absorption by parathyroid hormone

Question 30

What type of fluid arrives in the collecting duct in terms of osmolarity?

Answer 30

Very hypo-osmotic dilute fluid

Question 31

What types of cells handle Na+/H₂O?

What cell type handles Cl-?

Answer 31

principle cells handle Na+ and H2O

** intercalated cells handle Cl- and acid/base balance**

Question 32

What type of apical transporters can you find in the collecting duct?

Answer 32

Sodium Channels (principle cell)

Chloride/Bicarb antiporter (Intercalated cell)

Question 33

What drug can block sodium channels in the collecting duct?

Answer 33

Amirilide

Also regulated by aldosterone

Question 34

What is the permeability of H2O in the collecting duct?

Answer 34

Normally VERY low

However, in the presence of ADH it increases

Question 35

What portion of the kidney contains collecting ducts with higher H₂O permeability?

Answer 35

Inner Medulla holds collecting ducts which let some water through without ADH

Question 36

Describe the function of ADH

Answer 36

Dose-dependent function

Signals migration of intracellular aquaporin-containing vesicles to fuse with the apical membrane

Lack of ADH signals apical membrane to endocytose aquaporins (lose them)

Question 37

Describe the kidney interstitium in renal cortex vs medulla in terms of osmolarity.

Answer 37

Kidney cortex is iso-osmolar to plasma

Medulla is hyperosmotic to plasma

Question 38

Describe how dilute urine is produced.

Answer 38

** The loop of Henle and distal tubule reabsorb more sodium than water. **

Without ADH, collecting duct remains impermeable to water and filled with hypo-osmotic fluid

Travels through ureters to bladder, etc….

Question 39

Describe how concentrated urine is produced.

Answer 39

The loop of Henle and distal tubule reabsorb more sodium than water, fluid is initially hypoosmotic.

ADH signals for aquaporin insertion in apical membranes of collecting ducts. Water is drained out of collecting duct because of hyper-osmotic interstitium of the renal medulla.

Concentrated urine created

Question 40

What 3 elements contribute to the hyperosmotic state of the kidney medulla necessary to concentrate urine (if allowedby ADH)?

Answer 40

1. Active Na+ reabsorption by the thick ascending limb of loop of Henle.

**2. Vasa Recta Capillaries **

**3. Recycling of urea between collecting duct and loop of Henle. **

Question 41

How does the thick ascending limb contribute to a hyperosmotic medulla?

Answer 41

It is impermeable to water and actively secretes sodium into the interstitium via the Na-K-2Cl symporter on the apical membrane (and Na+/K+ pump on basolateral membrane)

The cortex has thick ascending limbs but the blood supply there is much greater and whisks away solutes before they can accumulate

Question 42

How do the vasa recta contribute to a hyperosmotic medulla?

Answer 42

1) Blood flow is much less so solutes tend to build up in the medulla interstitium

2) Countercurrent exchange with hairpin loops in capillaries allows osmolarity of medulla to be left alone since blood goes back up into the cortex in order to leave

Question 43

How does urea cycling contribute to a hyperosmotic medulla?

Answer 43

So much water is absorbed by the collecting duct that urea will be more concentrated in the nephron than in the medulla and urea will diffuse into the medulla.

**(Roughly half the high medullary osmolarity is contributed by urea and half is contributed by NaCl during dehydration) **

Question 44

What can contribute to a less than normal osmolarity in the kidney medulla?

Answer 44

Lack of protein (thus lack of urea production) in diet

Chronic over-hydration