nephron function 2

Question 1

describe the similarities and differences between Na+ reabsorption in the proximal tubules compared to the short loop nephrons

Answer 1

similarily, Na+ moves out of the cell by a basolateral Na/K/ATPase transporter creating a higher conc in ECF

In contrast, Na+ moves in by Na/2Cl/K channel
K+ is recycled by ROMK channel
tight junctions so no flow of water passively = hypertonic ECF

Question 2

describe the importance of the Na/2Cl/K channel

Answer 2

this transporter is the target for the diuretic furosemide

Question 3

describe the short loop nephron

Answer 3

• only within the outer medulla
• NaCl gradient formed by countercurrent multiplication mechanism
• Allows interstitium to reach a concentration of 600mosm/L

Question 4

how does the water leave the short loop and where does it go

Answer 4

• leaves via osmosis

- enters vasa recta

Question 5

what is the significance of the blood flow being counter to the direction of tubular flow

Answer 5

countercurrent exchange system prevents wash out of the gradient

• slow BF favours optimal exchange
• increasing flow can cause a loss of the gradient

Question 6

describe the concentration and transporters within the early distal convoluted tubule

Answer 6

• tubular fluid leaving the TAL is dilute
• further dilution occurs as the tubular fluid is impermeable to water
• Na/Cl transporter on apical surface (Na/K/ATPase basolateral)

Question 7

what is the importance of the Na/Cl transporter in the early distal convoluted tubule

Answer 7

this transporter is blocked by thiazide diuretics

- used to treat hypertension & heart failure

Question 8

name the two cell types present in the late distal convoluted tubule, connecting tubule, and collecting duct

Answer 8

principal cells

intercalated cells

Question 9

describe the role of principal cells and how they carry this out

Answer 9

reabsorb Na+ via ENaC channel

secrete K+ via ROMK (moves to counterbalance electronegativity created by movement of Na+ out)

Question 10

why are thiazide diuretic importance in terms of K+

Answer 10

thiazide diuretics block Na/Cl transport causing K+ loss (hypokalemia) as more Na+ stays within the late distal tubule
hypokalemia can cause ventricular arrhythmias

Question 11

describe the role of aldosterone on principal cells

Answer 11

aldosterone released by adrenal cortex and diffuses into principal cell by binding to a receptor. this receptor goes into the nucleus, binds to DNA and changes targets. Causes ENaC up-regulation, induce more ENaC channels, and more Na/K/ATPases
= more Na+ reabsorbed, more K+ excreted

Question 12

what drug blocks the affect of aldosterone

Answer 12

Spirolactone (is a weak K+ sparing diuretic)

Question 13

describe the role of intercalated cells

Answer 13

important for acid-base balance and K+ reabsorption
secrete H+ via H+/ATPase & H+/K/ATPase
H+ secreted is used to reabsorb HCO3- and generate new HCO3-

Question 14

why do we need to make new bicarbonate

Answer 14

bicarbonate is used to buffer out the acidity of H+ ions however in doing so bicarbonate sources get depleted. To maintain pH more bicarbonate is needed to be reabsorbed and made

Question 15

describe diffusion trapping and why it occurs

Answer 15

• occurs in the collecting ducts
• as a result of excess H+ (after all HCO3- is reabsorbed) it combines with NH3
  because NH3 has no charge it can diffuse freely
  NH3 + H+ = NH4+
  therefore NH4+ excreted, maximum urine pH 4.5

Question 16

describe water reabsorption in the collecting ducts

Answer 16

depends on ADH (released from post. pituitary)

• inserts aquaporin channels on apical surface allowing water reabsorption
• high levels of ADH = high water reabsorption & conc urine

Question 17

describe the passive hypothesis in the inner medulla

Answer 17

1. high levels of ADH causing urea conc to increase in corticol collecting ducts (NOT medulla collecting ducts)
2. ADH down the collecting duct moves water and urea into interstitium (base of LOH)
3. salt conc in the inner medulla (interstitium) drops due to presence of water
4. therefore NaCl moves out of the base of the ascending limb (long loop) into the interstitium via osmosis to counter this

(base of nephron within inner medulla now very concentrated)

Question 18

describe the role of the vasa recta countercurrent

Answer 18

preserves osmotic gradient and carries away water

Question 19

what happens in the long loop nephrons when ADH is low

Answer 19

• water isn’t reabsorbed so urea doesn’t increase
• inner medulla CD still permeable to urea when ADH is low
• if urea interstitium > urea CD then urea will diffuse into CD and be excreted = washout
• ADH low = urea in inner medulla low

Question 20

Describe the drugs acetazolamide, thiazide, spirolactone, and furosemide?

Answer 20

acetazolamide = Carbonic anhydrase inhibitor (relieves metabolic alkalosis)

furosemide = Na/2Cl/K channel blocker (diuretic)

spirolactone = blocks effect of aldosterone

thiazide = Na/Cl in distal convoluted tubule (diuretic)