156 - Kidney Function 4

Question 1

Area of the kidney where paracellular pathway takes place at the greatest rate

Answer 1

Proximal tubule (tight junctions are leakiest here)

Question 2

How are cations absorbed in the tubule?

Answer 2

Solvent drag (as water is absorbed, drags solutes with it).
Transporters.

Question 3

Solutes particularly absorbed by early proximal tubule

Answer 3

Bicarbonate, amino acids, glucose

Question 4

Solute particularly absorbed by late proximal tubule

Answer 4

Chloride

Question 5

Why is bicarbonate preferentially absorbed at the early proximal tubule?

Answer 5

Sodium-bicarbonate transport.

NHE (sodium hydrogen exchanger) very active.
Leads to formation of HCO3- (from acid-base pathway, splitting of CO2 to bicarbonate and H+) through carbonic anhydrase pathway.

Cl- is left behind in the lumen after Na+ is reabsorbed

Question 6

What in the late proximal tubule leads to greater Na+ reabsorption?

Answer 6

Cl- absorption leads to a +3mV luminal charge, which pushes Na+ into the epithelium

Question 7

Inefficiency in Na+ reabsorption in the proximal tubule

Answer 7

Na+ can leak back into the lumen through the paracellular pathway.

Question 8

Voltage of the early proximal tubule lumen

Answer 8

Negative

Question 9

Why is Cl- absorbed at a greater rate than Na+ in the late proximal tubule?

Answer 9

Na+ has been absorbed in the early tubule and Cl- has been left behind, leading to a greater Cl- gradient in the late tubule.

Question 10

Effect of positive voltage in lumen of late proximal tubule

Answer 10

50% of Na+ reabsorption is paracellular, 50% is transcellular, because positive Na+ follows electric gradient (goes towards relatively negative epithelium).

Question 11

How is Cl- reabsorbed in the late proximal tubule?

Answer 11

Cl-/base exchanger on apical membrane, Cl-/K+ cotransporter and Cl- channel on basal membrane.

Paracellular pathway.

Question 12

Na+ in thin ascending limb

Answer 12

Na+ is reabsorbed via passive diffusion in response to concentration gradient.
Paracellular pathway.

Question 13

Thick ascending limb reabsorption

Answer 13

Continues reabsorbing NaCl.

Reabsorbs 25% of filtered NaCl

Question 14

Concentration of filtrate in the distal convoluted tubule

Answer 14

100mOsM

Question 15

Part of nephron that all diuretics work on

Answer 15

Na+ transporter on apical membrane

Question 16

Part of nephron that loop diuretics act on

Answer 16

Na/K/2Cl cotransporter (electrically neutral)

Question 17

Part of nephron that Thiazides work on

Answer 17

NaCl cotransporter in distal tubule (electrically neutral)

Question 18

Part of nephron that Amiloride works on

Answer 18

Epithelial sodium channel in the collecting duct

Question 19

Why does the concentration of filtrate drop in the thick ascending limb?

Answer 19

Tight junctions are permeable to Na+, but not to water.

Na/H exchange is still occurring apically.

Question 20

What helps balance charge of reabsorbed Na+ in the thick ascending limb?

Answer 20

K+ channel on apical membrane allows K+ to leak into lumen

Question 21

Distal tubule ion exchange

Answer 21

Further NaCl transport (cotransporter).

Question 22

Why does Frusemide need to be given with K+ supplements?

Answer 22

Inhibits Na/K/2Cl transporter in ascending limb of loop of Henle.

Not reabsorbing as much K+.

Question 23

Na movement in distal tubule

Answer 23

Entirely transcellular

Question 24

Cortical collecting tubule ion exchange

Answer 24

ENaC is only Na+ channel.

K+ channel lets K+ diffuse into the lumen.

Question 25

What drives all Na+ (and therefore ion) diffusion?

Answer 25

Active 3Na+/2K+ pump on the basolateral surface

Question 26

Endogenous organic acids

Answer 26

Want to retain these in the kidney.

Question 27

Mono-carboxylates (organic acids) reabsorption

Answer 27

Apical membrane Na-dependent cotransporter.

Basolateral membrane H+-dependent cotransporter (MCP1)

Question 28

Di- and tri-carboxylate (organic acids) reabsorption

Answer 28

Apical membrane: Na-dependent cotransporter NaDC1

Basolateral membrane: Organic Anion Exchanger eg ROAT1

Question 29

How can xenobiotics be reabsorbed?

Answer 29

All use ROAT1 to enter late proximal tubule cells (secretion).
Cross apical membrane with another organic anion exchanger.

Question 30

Countercurrent exchange system of the kidney

Answer 30

At any time, descending limb concentration is ~200mOsM higher than ascending limb. Equilibrates ECF with descending limb.

Descending limb is only ion-permeable, so loses ions into the ECF, which go into the vasa recta (which flow antiparallel to loop of Henle fluid).

This concentrates blood in vasa recta, which pull water out of descending limb when they run beside the descending limb.

Question 31

What is absorbed in descending limb?

Answer 31

Only water

Question 32

What is reabsorbed in the thick ascending limb?

Answer 32

Only ions

Question 33

How is hypo-osmotic filtrate made in the ascending limb?

Answer 33

Wall is water-impermeable.

Active reabsorption of ions.

Question 34

Concentration of urine in collecting duct

Answer 34

Duct travels through very hyperosmolar ECF of kidney.
Vasopressin increases water-permeability of wall.
Water is carried away by vasa recta capillaries.

Question 35

Effect of absend vasopressin on collecting duct

Answer 35

Collecting duct is impermeable to water, urine remains dilute (100mOsM, same as in distal tubule)

Question 36

Stimuli for vasopressin release

Answer 36

Osmolarity.

ECF volume is an even more powerful stimulus.

Question 37

Effects of plasma angiotensin II
1
2
3
4
5
6
7

Answer 37

1) Arteriolar vasoconstriction
2) Increased cardiovascular response, from stimulation of cardiovascular control centre in the medulla oblongata
3) Preferential constriction of efferent arteriole
4) Increased proximal tubule Na+ reabsorption (stimulates NHe)
5) Acts on glomerular mesangial cells to regulate glomerular filtration.
6) Hypothalamus releases more vasopressin, increases thirst.
7) Adrenal cortex releases aldosterone, which increases Na+ reabsorption