PCT, Loop, DCT

Question 1

What is the role of the PCT?

Answer 1

Reabsorb majority of filtrate

Question 2

How similar is Na conc in plasma and absorbed filtrate?

Answer 2

The same

Question 3

What is the percentage of reabsorbed molecules from the PCT?

Answer 3

70% Na + H2O
99% HCO3 & Glucose

Question 4

How is Na reabsorbed in the PCT?

Answer 4

1) Na/H antiporter (needs ATP) in tubular cells
- Na into cell where excess HCO3
- Na & HCO3 co-transported into interstitial fluid and then blood
2) Na/Glucose symporter (needs ATP) into tubular cells
- Na/K pump (needs ATP) on basement membrane pulls Na through into interstitial fluid & then blood

Question 5

Where is most (if not all) glucose absorbed?

Answer 5

PCT
System can become saturated (max 2mmol/min)

Question 6

How is bicarb reabsorbed in the PCT?

Answer 6

• Fitered bicarb in glomerular filtrate
• Combines w/H+ ions to make H2CO3
• Broken down into H2O + CO2 by carbonic anhydrase
• CO2 absorbed into cell and recombined with H2O
• Bicarb is then moved out with Na into interstitial fluid & blood

Question 7

What is the glomerulotubular balance?

Answer 7

COnstant proportion of the filtered Na & H2O being reabsorbed irrespective of GFR

Question 8

What are the mechanisms of glomerulotubular balance?

Answer 8

• Glucose load: As GFR increases so does filtered glucose which pulls Na & H2O across via symporter
• Oncotic pressure in peritubular capillaries: As GFR increases so does protein content in capillaries and oncotic pressure. Favours H2O movement into capillaries

Question 9

What does the PCT secrete?

Answer 9

Organic anions: Urate, bile salts, fatty acids
- Organic cations: Ach, Catecholamines, creatinine, histamine
- Drugs: Aspirin, Penicillin, Morphine

Question 10

What is the role of the LOH?

Answer 10

• Produce hypertonic, hyperosmolar interstitial fluid in medulla
• Hypotonic tubular fluid
• Some reabsorption of Na, K, Cl

Question 11

What are the osmolarities of tubular fluid, medullary interstitial & resulting tubular fluid?

Answer 11

TF: 300mOsm/L
M: 1200mOsm/L
RTF: 100mOsm/L

Question 12

In the LoH where is Na/K/Cl absorbed?

Answer 12

Ascending loop

Question 13

What is osmolality & osmolarity?

Answer 13

LAL: No. of active particles per kg of solvent (mOsm/kg)
LAR:No. of active particles per L of solution (mOsm/L)

Question 14

How does the LoH create a hypertonic environment?

Answer 14

Descending: Permeable to H2O, impermeable to ions
Thick ascending: Permeable to ions, impermeable to H2O

Question 15

How are ions reabsorbed in the LoH?

Answer 15

• Co-transporter, requires energy
• K/Na/2Cl into tubular cell
• K freely diffuses into interstitial fluid
• Na/K pumps Na into interstitial fluid & K back into cell

Question 16

What is the countercurrent mechanism?

Answer 16

• Fluid leaves descending LoH, filtrate becomes more concentrated (400mosm/l)
• Ions leave ascending LoH, filtrate more dilute (300mosml/l)
• Vasa recta runs in countercurrent contains ‘normal plasma & electrolytes’
• H2O & electrolytes pass freely into vessels

Question 17

What are the functions of the DCT?

Answer 17

• Buffering of H+ in filtrate
• Fine tuning of H+/K/Na excreted in urine

Question 18

What are the types of cells found in the DCT?

Answer 18

• Principal cells: Reabsorb Na, secrete K, Aldosterone controlled
• Intercalated cells: Secrete H+, Reabsorb K, ATP driven
• Both impermeable to H2O

Question 19

How is Na reabsorbed in the DCT?

Answer 19

• NA/K ATP pump in principle cell
• K freely pumped into interstitial fluid
• In tubular lumen ENaC channels allow Na reabsorption

Question 20

How are ENaC channels triggered?

Answer 20

• Aldosterone stimulated: Inc K+, Alkalosis, Inc tubular flow
• Causes gene expression & inc synthesis of channels
• Also inc activity of Na/K pump

Question 21

What is Aldosterone?

Answer 21

• Mineralocorticoid
• Secreted by zona glomerulosa of cortex
• Released in response to: inc K, ACTH, angiotensin II
  -Stimulates Na reabsorption, K excretion
• Inc Na in gut/sweat/salivary glands

Question 22

How is K+ reabsorbed?

Answer 22

• K/H+ ATP pump secretes H+ into tubular fluid
• Stimulated by acidosis & low K
• K freely moves from cell into interstitial fluid & blood

Question 23

How is H+ buffered in the urine?

Answer 23

1) Phosphate remains in tubular lumen
- Combines w/H+ to make H2PO4 = excreted
2) Ammonia made in intercalated cells by glutamine
- Pumped into lumen & combine w/H+ = NH4+

Question 24

What is the role of the collecting duct?

Answer 24

• Regulation of H2O
• Normally impermeable to all ions & H2O
• Osmolarity- 60-1400
• regulated by ADH

Question 25

How can the CD be made permeable to water?

Answer 25

• Aquaporins inserted into luminal membrane allows H2O permeability by osmosis
• Trans-membrane proteins with narrow helical peptides
• Positively charged walls allow H2O passage

Question 26

Where are the different aquaporins found?

Answer 26

• AQP1: PCT & descending LoH
• AQP2: CD in response to ADH
• AQP3&4: CD (always present)

Question 27

How do aquaporins work?

Answer 27

• ADH V2 receptor binding leads to AQP2 insertion
• Allows H2O movement into cell
• AQP3&4 on tubular cells allows H2O to pass through cell into interstitial fluid