Tubular function 2

Question 1

What is the permeability of the descending thin limb

Answer 1

Highly permeable to water

Much less to urea and NaCl (movement largely passive)

Question 2

What is the permeability of the ascending limb

Answer 2

Impermeable to water
Some Na+ reabsorbed
Cl- and K+ reabsorbed

Question 3

What are symporter proteins and which drugs inhibit these proteins

Answer 3

Loop diuretics inhibit 
Transport these across the apical membrane into the epithelial membrane:
2x Cl-
K+
Na+

Question 4

How are Cl- moved across the basolateral membrane

Answer 4

Diffuse through chloride channels

Question 5

How are Na+ moved across the base;ateral membrane

Answer 5

Na+ pump

Question 6

Where do K+ go once inside epithelial cells

Answer 6

some diffuse through K+ channels into peritubular fluid but some go back into the tubular lumen which creates a positive charge within tubular fluid. This is driving force for paracellular transport (ions move between cells. This is how 50% of these get reabsorbed)

Question 7

How does the distal tubule further decrease osmolality

Answer 7

IT is impermeable to water and continues active dilution using the sodium potassium pump which creates the gradient for other ions to continue to move.

Question 8

Where do thiazide like diuretics act on

Answer 8

Symporter proteins on the early distal tubule

Question 9

What are the two different types of epithelial cells in late distal tubule and collecting duct

Answer 9

Principal cells- reabsorb sodium ad water and secrete potassium

Intercalated cells- Important role in regulation of acid base balance by secreting new bicarbonate ions. Secrete H+ into tubular fluid which can be excreted in urine as free H+ or can be buffered with hydrogen phosphate to form dihydrogen phosphate or can be buffered with ammonia to form ammonium

Question 10

How does aldosterone affect Na+ reabsorption in principal cells

Answer 10

Enhances Na+ reabsorption

• Increases number and activity of apical Na+ channels
• Increases activity of basolateral Na+ pump

Question 11

How does aldosterone affect K+ secretion in principal cells

Answer 11

Enhances K+

• INcreases number and activity of apical K+ channels
• Increases activity of basolateral Na+ pump
• Na+ reabsorption

Question 12

HOw does aldosterone affect H+ secretion in intercalated cells

Answer 12

Stimulates H+- ATpase pump

Question 13

How does excess aldosterone affect the body

Answer 13

Causes metabolic alkalosis, hypokalaemia, hypertension and oedema (due to Na+ and water retention)

Question 14

How does hypoaldosteronism affect the body

Answer 14

Hyperkalaemia

Question 15

What does ADH have an effect on

Answer 15

Late distal tubule and collecting duct

Question 16

What does ADH do

Answer 16

Facilitates the reabsorption of water. Increases water permeability in the late distal tubule and collecting duct through stimulation of insertion of aquaporins into apical membrane because of principal cells.

Question 17

What is antidiuresis

Answer 17

When ADH is present and causes concentrated urine

Question 18

What is diuresis

Answer 18

Increased urine volume

Question 19

How does ADH affect permeability

Answer 19

Increases aquaporins on the surface of the membrane

Increases permeability of inner medullary collecting duct (IMCD) to urea

Question 20

How does ADH increase permeability of IMCD to urea

Answer 20

IN absence of ADH, urea permeability of nephron is restricted to proximal tubule and inner medulla

UT1 on apical membrane in IMCD. In presence of ADH, water reabsorption concentrates urea which diffuses out of tubule lumen into medullary interstitial

Urea reabsorption helps to maintain osmotic gradient between interstitial and collecting duct lumen

Urea diffuses into ascending and descending limbs of loop of hence

Question 21

What is diabetes insipidus

Answer 21

Inadequate secretion or response to ADH so a large volume of dilute urine is excreted

Question 22

What is SIADH (syndrome of inappropriate secretion of ADH)

Answer 22

Usually at night, ADH secretion increases so we don’t have to urinate at night. However, in this disorder, there is increased secretion of ADH and so this leads to bed wetting

Question 23

What is urea recycling and what is its effect

Answer 23

ADH increases the urea channels in DCT and so it goes to the ascending limb of loop of henle. This increases the concentration of the fluid inside the tubule which helps increase water reabsorption into the body.

Question 24

What does ADH increase permeability of

Answer 24

Late distal tubule and collecting duct