S4.1 Contorl Of Volume

Question 1

What is the major osmotically effective solute in the ECF?

Answer 1

Na+

Thus water in the ECF compartment depends on the Na+ content. Changes in Na affect ECV which can affect BP.

Question 2

What occurs in ECF expansion?

Answer 2

Na excretion is less than intake, it is retained in the body, so water is drawn out of the nephron and blood volume and pressure increases

Question 3

What occurs in ECF contraction?

Answer 3

If Na+ excretion is greater than intake, the Na+ content of the ECF decreases. Less water is drawn out of the nephron, so ECF volume decreases, as does blood volume and pressure.

Question 4

How is plasma (ECF) volume controlled?

Answer 4

Add isosmotic solution to increase volume/remove to reduce, without changing the osmolarity.

Question 5

What is the effect of changes in Na+ excretion?

Answer 5

Peritubular capillary osmotic and hydrostatic pressure:
Increase inhibits Na+ reabsorption
Decrease promotes Na+ reabsorption

Reabsorption stimulated by RAAS in PT
Principle cells of DT and CD targets for aldosterone

Question 6

Describe the mechanisms to reduce an initial raised renal artery blood pressure

Answer 6

Decreased Na-H antiporters and Na-K-ATPase activity in PT

Less Na and H2O reabsorption in PT, so more sodium excretion (pressure natriuresis) and more water excretion (pressure diuresis). This reduces ECF volume.

Question 7

Describe Cl- reabsorption

Answer 7

Transcellular (active) and some paracellular (passive, between nephrons) processes, coupled to 3Na-2K-ATPase therefore depends on Na+ reabsorption.

Question 8

What are aquaproin channels and where are they found?

Answer 8

Aquaporin channels in the kidney allow water to move down the concentration gradient. It is a hole in the membrane.
There are 2 in the descending LoH, none in the ascending LoH.
2 in the proximal tubule, 3 in the collecting duct.

Question 9

Na+ reabsorption in the nephron is mainly active, what drives it?

Answer 9

3Na-2K-ATPase pumps on the basolateral membrane.

These pumps also control Na movement into the interstium across basolateral membrane

Question 10

Describe Na transport on the apical membrane in the proximal tubule

Answer 10

Occurs via Na-H antiporter, Na-glucose symporter, Na-aa cotransporter

Question 11

Describe Na transport on the apical membrane in the LoH

Answer 11

Descending limbs of LoH reabsorb water but not Na.
Thin ascending limb reabsorbs Na passively.
Thick ascending limb reabsorbs Na via NaK2Cl. ROMK helps maintain a gradient for NaKATPase to work.

Question 12

Describe Na transport on the apical membrane in the distal tubule

Answer 12

Early DCT - Na enter cell via NaCl transporter

Late DCT - Na enter cell via NaCl and ENac

Question 13

Describe Ca transport on the apical membrane in the distal tubule

Answer 13

Ca2+ enters via TRPV5, binds to calbindin

Ca2+ moved out by sodium calcium exchanger, NCX

Question 14

Describe Na transport on the apical membrane in the collecting duct

Answer 14

Principle cells - via ENac

There are also acid-IC which secrete H+, or base-IC which secrete HCO3-

Question 15

What is the function of tight junction between tubule cells?

Answer 15

They form a barrier and prevent diffusion of transporter, channel and pump proteins between the cells. They maintain the polarity of the tubule cells.