Proximal Tubular Function

What is the role of the proximal tubule?

"Bulk of reabsorption", 60-70% of filtered load of Na/H20/Cl/K/other solutes & nearly all filtered glucose and amino acids. Reabsorption of majority coupled with Na reabsorption

1 of 16

What are the terms for transport across & between cells?

Across cells = transcellular/transepithelial transport. Between cells = paracellular transport

2 of 16

Where in the cell is the Na/K-ATPase (Na+ pump)? What is its function?

Located exclusively in the basolateral membrane. It is the primary active transport mechanism of the nephron. Reabsorption of every substance, including water, is linked to operation of Na/K-ATPase

3 of 16

What is the role of peritubular capillaries? What helps this process?

Reabsorption from peritubular/interstitial fluid into capillaries, facilitated by low hydrostatic pressure that exits along the entire length of peritubular capillaries. Colloid osmotic pressure favours movement of fluid into capillaries

4 of 16

What is the important about tight junctions in proximal tubule?

Leaky! Proximal tubule is highly water permeable due to leaky tight junctions & aquaporin-1 (AQP1), prevents build up of significant osmotic gradients -> tubular fluid isosmotic with plasma

5 of 16

How are sodium ions transported in the proximal tubule?

Na+ enter epithelial cells across apical membrane down electrical/chemical gradient created by Na/KATPase. Mechanisms: Na/H exchanger, Na entry coupled to other solutes by symporter, Na enters cells alone & Na moves passively through tight junctions

6 of 16

How much sodium & glucose is reabsorbed?

65-70% Na+ reabsorbed, all glucose reabsorbed

7 of 16

How are bicarbonate (HCO3-) ions transported in proximal tubule?

90% of HCO3- in filtrate reabsorbed from proximal tubule, indirect method involving carbonic anhydrase (CA) as apical membrane is impermeable to HCO3-, reabsorption depends on active H+ secretion in exchange for Na+ ions

8 of 16

Bicarbonate transport: what happens inside the cell?

Hydration of CO2 to form carbonic acid (H2CO3) catalysed by intracellular CA. The H2CO3 dissociates to H+ & HCO3- (this then leaves via basolateral membrane) and H+ is transported into tubular lumen by Na/H exchanger (NHE3)

9 of 16

Bicarbonate transport: what happens within the lumen? Net result?

H+ combines with HCO3- to form H2CO3. The H2CO3 dissociates to CO2 + H2O, catalysed by CA at apical membrane. CO2 + H2O diffuse across epithelial cells. Net result: reabsorption of HCO3- from lumen

10 of 16

What is the importance of bicarbonate transport in acidosis & alkalosis? Why?

For each filtered HCO3- removed from tubular lumen, one HCO3- appears in peritubular blood. Thus reabsorption of filtered HCO3- from proximal tubule doesn't correct acidosis as the amount returned to plasma is same that left. But can reduce alkalosis

11 of 16

How is H2O transported in the proximal tubule?

Proximal tubule is highly permeable to water. Water reabsorption: paracellularly across leaky tight junctions & transcellularly via water channels (AQP1) on apical and basolateral membranes

12 of 16

What factors cause water reabsorption? (Reabsorbed fluid ~ isosmotic to filtrate)

Osmotic pressure gradient, generated by Na+ reabsorption. Increased oncotic pressure in peritubular capillaries (unfiltered proteins) & low hydrostatic pressure. H2O moves passively down gradient, osmotic flow of water results in solvent drag

13 of 16

How are K+ ions transported in proximal tubule?

~67% of filtered K+ reabsorbed, mainly by paracellular transport. Also by active transport of K+ inwards (NaK-ATPase) & passive diffusion of K+ outwards

14 of 16

How are Cl- ions transported in proximal tubule?

~80% reabsorbed. Overall electrical & concentration gradient favours Cl- reabsorption, largely passive by paracellular transport & trasncellular transport due to parallel operations of Na/H & Cl/Base anitporters (base = anions such as OH-, formate)

15 of 16

How is urea transported in proximal tubule?

~50% reabsorbed. Reabsorption of Na+ & thus water concentrates urea in luminal fluid and moves it passively down concentration gradient through paracellular & transcellular transport

16 of 16

Get Revising

Proximal Tubular Function

Other cards in this set

Card 2

Front

Back

Card 3

Front

Back

Card 4

Front

Back

Card 5

Front

Back

Comments

Similar Medicine resources:

Other cards in this set

Card 2

Front

Back

Card 3

Front

Back

Card 4

Front

Back

Card 5

Front

Back

Comments

Related discussions on The Student Room

Similar Medicine resources: