PCT, Loop, DCT Flashcards
What is the role of the PCT?
Reabsorb majority of filtrate
How similar is Na conc in plasma and absorbed filtrate?
The same
What is the percentage of reabsorbed molecules from the PCT?
70% Na + H2O
99% HCO3 & Glucose
How is Na reabsorbed in the PCT?
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
Where is most (if not all) glucose absorbed?
PCT
System can become saturated (max 2mmol/min)
How is bicarb reabsorbed in the PCT?
- 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
What is the glomerulotubular balance?
COnstant proportion of the filtered Na & H2O being reabsorbed irrespective of GFR
What are the mechanisms of glomerulotubular balance?
- 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
What does the PCT secrete?
Organic anions: Urate, bile salts, fatty acids
- Organic cations: Ach, Catecholamines, creatinine, histamine
- Drugs: Aspirin, Penicillin, Morphine
What is the role of the LOH?
- Produce hypertonic, hyperosmolar interstitial fluid in medulla
- Hypotonic tubular fluid
- Some reabsorption of Na, K, Cl
What are the osmolarities of tubular fluid, medullary interstitial & resulting tubular fluid?
TF: 300mOsm/L
M: 1200mOsm/L
RTF: 100mOsm/L
In the LoH where is Na/K/Cl absorbed?
Ascending loop
What is osmolality & osmolarity?
LAL: No. of active particles per kg of solvent (mOsm/kg)
LAR:No. of active particles per L of solution (mOsm/L)
How does the LoH create a hypertonic environment?
Descending: Permeable to H2O, impermeable to ions
Thick ascending: Permeable to ions, impermeable to H2O
How are ions reabsorbed in the LoH?
- 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
What is the countercurrent mechanism?
- 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
What are the functions of the DCT?
- Buffering of H+ in filtrate
- Fine tuning of H+/K/Na excreted in urine
What are the types of cells found in the DCT?
- Principal cells: Reabsorb Na, secrete K, Aldosterone controlled
- Intercalated cells: Secrete H+, Reabsorb K, ATP driven
- Both impermeable to H2O
How is Na reabsorbed in the DCT?
- NA/K ATP pump in principle cell
- K freely pumped into interstitial fluid
- In tubular lumen ENaC channels allow Na reabsorption
How are ENaC channels triggered?
- Aldosterone stimulated: Inc K+, Alkalosis, Inc tubular flow
- Causes gene expression & inc synthesis of channels
- Also inc activity of Na/K pump
What is Aldosterone?
- 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
How is K+ reabsorbed?
- K/H+ ATP pump secretes H+ into tubular fluid
- Stimulated by acidosis & low K
- K freely moves from cell into interstitial fluid & blood
How is H+ buffered in the urine?
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+
What is the role of the collecting duct?
- Regulation of H2O
- Normally impermeable to all ions & H2O
- Osmolarity- 60-1400
- regulated by ADH
How can the CD be made permeable to water?
- Aquaporins inserted into luminal membrane allows H2O permeability by osmosis
- Trans-membrane proteins with narrow helical peptides
- Positively charged walls allow H2O passage
Where are the different aquaporins found?
- AQP1: PCT & descending LoH
- AQP2: CD in response to ADH
- AQP3&4: CD (always present)
How do aquaporins work?
- 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
