Session 4 - Renal control of Volume (Na)

Question 1

Which main ion is the major osmotically effective solute in the ECF?

Answer 1

Na+

Question 2

What happens with changes of Na and Cl in ECF?

Answer 2

Change in the volume of water (as water follows)

Thus change in Blood Pressure.

Question 3

How does the body prevent changes in blood pressure when salt intake is varied?

Answer 3

The kidneys can differ their excretion to match ingestion.

Question 4

Where is most sodium, and water absorbed in the kidney tubules?

Answer 4

• Most sodium (67%) and water (65%) absorbed in the proximal convoluted tubule.
• 10-15% water absorbed in descending limb (LoH),
• 25% of sodium absorbed in thin/thick ascending limg of LoH, (5% DCT, 3% Collectind duct)

Question 5

What controls the amount of Na+ reabsorption in the kidney?

Answer 5

• Changes in osmotic pressure and hydrostatic pressure.
• Stimulated by RAAS
• Principle cells of DCT and CD targets by aldosterone.

Question 6

What is pressure natriuresis and diuresis?

Answer 6

Both occure together

When renal artery BP increases.

• Reduced number of Na-H antiporter and reduced Na-K ATPase activity in proximal tubule.
• Causes reduction in Na and water resorption in proximal tubule.

Therefore, > Na and Water excretion.

Question 7

What drives sodium reabsorption in the tubule cells?

Answer 7

Actively driven.
Main driver is 3Na-2K-ATPase on basolateral membrane.
(Cl reabsorption transcellular AND paracellular, follows Na)

Question 8

Name the different transporters and locations for Na reabsorption in the nephron.

Answer 8

Proximal Tubule

• Na-H antiporter
• Na-glucose symporter
• Na-AA co-transporter
• Na-Pi

Loop of Henle
- NaKCC

Early DCT
- NaCl symporter

Late DT + CD
- ENac

Question 9

What is the characteristic of the fluid in the PCT?

Answer 9

It is isosmotic.

Question 10

How is the proximal convoluted tubule divided?

Answer 10

Into three segments - S1, S2, S3

Question 11

What happens in S1?

Answer 11

Sodium and glucose reabsorbed into capillary.
Cl- and Urea remain in lumen increasing in concentration.
(compensate for loss of glucose)

Question 12

What happens in S2?

Answer 12

• Paracellular Cl- reabsorption (+ transcellular), due to high gradient (passive).
• Aquaporin - water reabsorbed, following solutes.

Question 13

What is bulk transport and what are the driving forces?

Answer 13

The bulk movement/ reabsorption of water in the PCT.

• > Osmotic gradient from solute reabsorption
• > Hydrostatic force in interstitium
• > Oncotic force in peritubular capillaries due to loss of 20% of filtrate at glomerulus, but cells + protein remain.

Question 14

What are the characteristics of the loop of henle?

Answer 14

Descending Loop

• Loose junctions
• Permeable to water
• Passive, no mitochondria
• Thinner walls
• Lots of aquaporin channels

Ascending loop

• Lots of active mitochondria
• Tight junctions
• No aquaporins (impermeable to water)

Question 15

How does the loop of henle ensure efficient absorption of ions in the ascending limb?

Answer 15

• Descending limb permeable to water, but not ions, so ions become concentrated.
• Increased concentration of Na and Cl for reabsorption in thin ascending and thick ascending limb.

Question 16

How is sodium reabsorbed in the thin ascending limb?

Answer 16

Passively via paracellular route (loss of water previously has created gradient)

Question 17

How is sodium/ ions reabsorbed in the thick ascending limb?

Answer 17

• Mainly NKCC2 transporter.

- Na/K ATPase driver.

Question 18

Why is ROMK important in the thick ascending limb?

Answer 18

There are fewer K+ ions in the filtrate, so to maintain the activity of NKCC2 transporter they must diffuse back into the filtrate.

Question 19

What is the thick ascending limb sensitive to?

Answer 19

Sensitive to hypoxia, due to using the most energy than any other part of the nephron.

Question 20

How is sodium reabsorbed in the distal convoluted tubule?

Answer 20

DCT 1 and DCT 2

DCT1 = NCC transporter (Na/Cl enter) this is sensititve to thiazide diuretics.

DCT 2= Enters via NCC and ENaC.

ENaC is amiloride diuretic sensitive.

Question 21

What is the tonicity of the fluid leaving the distal convoluted tubule?

Answer 21

The fluid is hypo-osmotic.

Question 22

How is calcium reabsorbed in the DCT?

Answer 22

Apical calcium transport.

• Cytosolic Ca binds to calbindin, which shuttles it to basolateral membrane.
• Transported out by NCX (sodium calcium exchanger)

Question 23

How is calcium reabsorption regulated?

Answer 23

By hormones, e.g. Parathyroid hormone, and 1,25-dihydroxyvitamin D.
(see MEH)

Question 24

What are the types of cells in the cortical collecting duct system?

Answer 24

Principle cells (70%)
Intercalated cells - A-IC and B-IC

Question 25

What is the function of principle cells?

Answer 25

Na+ Reabsorption via ENaC in CD.
ENaC, and Na/K-ATPase.
Drive Cl- via paracellular.
(- charge in lumen important for potassium secretion)
Variable H20 uptake via AQP dependent on ADH.

Question 26

What is the function of Intercalated cells?

Answer 26

• Active reabsorption of chloride
• A-IC = secrete H+ ions (acids)
• B-IC = secrete HCO3- ions. (bicarbonate)

Question 27

How do A-IC cells secrete H+?

Answer 27

Express H+/ATPase and H+/K+ ATPase on apical membrane.

Express Cl-/HCO3- exchanger at basolateral membrane.

Question 28

How do B-IC cells secrete bicarbonate?

Answer 28

Express pendrin (Cl-/HCO3-) exchanger on apical membrane (secrete HCO3-).
?