proximal tubule
Main site of reabsorption
gross morphology
- brush border on luminal side = ↑↑ surface area
sodium handling
absorption down concentration gradient via facilitated diffusion
- 80% reabsorbed via Na/H antiporter on apical surface (lumen to cell)
- moves into interstitial fluid via Na/K ATPase
- regulated by angiotensin II
this Na/K ATPase we keep hearing about
loads of them on the proximal tubule
used to generate a Na gradient - used to co-transport everything else
copied shamelessly from deranged physiology:
- SGLT2 glucose and sodium co-transporter is the target of SGLT2 inhibitors such as empagliflozin
- Phosphate transport in the kidneys: the phosphate and sodium co-transporters (that's the only place where phosphate reabsorption is regulated in the nephron)
- Ammonia transport: the proximal tubule exchanges sodium for ammonia at the apical membrane; the resulting ammonia secretion is an important part of maintaining acid-base balance (ammonia also being an important urinary buffer)
- Sodium-hydrogen ion exchange: NHE-3 proton antiporter is also essential for acid-base homeostasis.
water handling
reabsorbed by osmosis
70% of water reabsorption in the kidneys happens in PCT
water reabsorption → ↑ tubular concentration of Cl, K, Ca, urea → reabsorbed to peritubular space passively
glucose handling
co-transport with Na
Glucose threshold
acid-base → see Renal acid-base balance
secreted H+ exchanged for HCO3