Bicarbonate Reabsorption and Kidney Regulation of Blood pH (Proximal Tubule)

Flashcards cover the renal handling of bicarbonate, key enzymes and transporters in the proximal tubule, nephron anatomy basics, and the link between kidney function and blood pH regulation.

What are the two main determinants of blood pH discussed in the lecture, and what compensatory mechanism adjusts them?

Carbon dioxide (CO2) and bicarbonate (HCO3-); respiratory compensation adjusts CO2 by changing breathing to shift the CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- equilibrium.

If blood becomes acidic (acidemia), which component would clinicians look to increase to normalize pH?

Bicarbonate (HCO3-) concentration; increasing it buffers excess H+ and helps restore pH.

Which organ is primarily responsible for replenishing bicarbonate in the blood to maintain normal pH?

The kidney, specifically the proximal tubule's bicarbonate reabsorption and regeneration.

What is the functional unit of the kidney and what is the first tubular segment involved in filtration and reabsorption described in the lecture?

The nephron; the proximal convoluted tubule (PCT) is the initial segment discussed for bicarbonate handling.

Name the carbonic anhydrase isoforms mentioned as important in kidney bicarbonate handling and their locations.

CA IV on the luminal (apical) surface of proximal tubule cells; CA II in the cytoplasm of tubular cells; RBC carbonic anhydrase (often CA I/II) is also present in blood.

What is the role of Na+/K+-ATPase in proximal tubule basolateral membranes?

Maintains the basolateral Na+ gradient by pumping 3 Na+ out and 2 K+ in, using ATP, which drives downstream transport processes.

Which transporter reabsorbs sodium in exchange for proton secretion at the luminal membrane of proximal tubule cells?

Sodium–proton exchanger (NHE3); it brings Na+ into the cell from the lumen in exchange for H+ being secreted into the lumen.

How is bicarbonate generated inside the proximal tubule cell for reabsorption?

CO2 and H2O diffused into the cell are converted to H2CO3 by cytosolic carbonic anhydrase II; H2CO3 dissociates to HCO3- and H+ inside the cell.

Which transporter moves bicarbonate from the proximal tubule cell into the blood, and what ion often accompanies it?

Bicarbonate exits across the basolateral membrane via a bicarbonate transporter (basolateral Na+/HCO3- cotransporter, NBC1); it often moves with Na+ into blood.

What happens to the H+ produced inside the proximal tubule cell?

H+ is secreted into the tubular lumen via NHE3, where it can combine with HCO3- to form CO2 and H2O in the lumen via luminal carbonic anhydrase, continuing the cycle.

Why is proximal tubule bicarbonate reabsorption described as an efficient 'engineering' solution in biology?

It reclaims bicarbonate efficiently through coordinated transporter activity (NHE3, Na+/K+-ATPase, NBC) and concomitant water reabsorption, maintaining blood pH with high efficiency.

What is the approximate rate of urine production mentioned in the notes?

About 1 mL of urine per minute.