21 - Acid-Base Physiology and Nephron Buffering (Vocabulary)

A set of vocabulary flashcards covering pH, bicarbonate buffering, renal acid-base handling, buffering systems, key transporters, and common acid-base disorders from the notes.

pH

The negative logarithm of hydrogen ion concentration; pH = -log10[H+].

Bicarbonate (HCO3−)

Hydrogen carbonate; major extracellular base buffer that reacts with H+ to form CO2 and H2O.

Hydrogen carbonate (IUPAC name for HCO3−)

IUPAC name for bicarbonate.

Henderson–Hasselbalch equation

pH = pK + log10([base]/[acid]); for the bicarbonate system: pH = pK + log10([HCO3−]/(0.03×PCO2)).

H2CO3 (carbonic acid)

Carbonic acid in solution, in rapid equilibrium with CO2 and H2O; [H2CO3] ≈ 0.03×PCO2.

CO2 (carbon dioxide)

Gas dissolved in blood; participates in the bicarbonate buffering system and is exhaled.

Carbonic anhydrase (CA)

Enzyme that converts CO2 and H2O to H2CO3 (and back), a key step in bicarbonate reabsorption and buffer handling.

Arterial pH

Normal arterial blood pH about 7.40.

Venous pH

Normal venous blood pH about 7.35; slightly lower due to tissue metabolism.

Partial pressure of CO2 (PCO2)

POCO2; arterial ~40 mmHg, venous ~46 mmHg; indicates CO2 levels in blood.

Proximal tubule bicarbonate reabsorption

Most filtered HCO3− is reabsorbed in the proximal tubule; driven by Na+/H+ exchange and carbonic anhydrase; reabsorption is transport-limited.

Na+/H+ exchanger (NHE)

Apical transporter that exchanges Na+ for H+, facilitating HCO3− reclamation in the proximal tubule.

Na+/HCO3− cotransporter (NBC)

Basolateral transporter that moves Na+ and HCO3− toward blood; part of bicarbonate handling.

Distal tubule H+ secretion

Active secretion of H+ in the distal nephron via apical proton pumps, chiefly H+-ATPases and H+-K+-ATPases.

H+-K+-ATPase

Apical proton pump exchanging H+ for K+ in the distal nephron.

H+-ATPase

Proton pump that secretes H+ across the apical membrane; contributes to acid secretion.

α-intercalated cells

Distal tubule cells specialized for acid secretion via H+-transporters.

Phosphate buffering (HPO4^2−/H2PO4−)

Buffering system in filtrate/urine using hydrogen phosphate species; pKa ≈ 6.8.

Hydrogen phosphate (HPO4^2−)

Base form of the phosphate buffer; partner in buffer with H2PO4−.

Dihydrogen phosphate (H2PO4−)

Acid form of the phosphate buffer; partner in buffer with HPO4^2−.

pKa of phosphate buffer

Approximately 6.8 (at 36°C); pH where HPO4^2− and H2PO4− are in equal amounts.

Urinary phosphate buffering

In acidic urine, H2PO4− predominates and buffers urine pH.

Ammonia secretion (NH3/NH4+)

NH4+ produced in proximal tubule from glutamine; NH3 is membrane-permeable and NH4+ serves as an H+ reservoir in filtrate.

Glutamine metabolism to ammonium

Glutamine is converted to glutamate and α-ketoglutarate, yielding NH4+ in the proximal tubule.

NHE, NBC, NKCC2

Nephron transporters: NHE (Na+/H+ exchanger), NBC (Na+/HCO3− cotransporter), NKCC2 (Na+/K+/Cl− cotransporter in thick ascending limb).

pH along the nephron

Proximal tubule pH about 6.9; final urine pH can be highly variable, down to ~4.5 depending on acid load.

Davenport diagram

Graphical tool showing lines of constant CO2; illustrates buffering and pH changes with varying HCO3− and CO2.

Anion gap

Difference between measured cations and anions: [Na+] − [Cl−] − [HCO3−]; helps detect unmeasured anions in acidosis.

Normal anion gap

Typically 3–11 mmol/L (lab-dependent).

Increased anion gap causes

Lactate, ketones, sulfates, phosphates, urate, hippurate, aspirin overdose; indicate unmeasured anions in acidosis.

Respiratory acidosis

Hypoventilation raises CO2, lowers pH; kidneys compensate by increasing HCO3− production.

Respiratory alkalosis

Hyperventilation lowers CO2, raises pH; kidneys compensate by decreasing HCO3− production.

Metabolic acidosis

Increased H+ or decreased HCO3−; lowers pH; compensation via increased ventilation to reduce CO2.

Metabolic alkalosis

Increased HCO3− or decreased H+; raises pH; compensation via reduced ventilation to retain CO2.

Transport maximum (Tm)

Maximum reabsorption rate for a substrate (e.g., HCO3−) in the proximal tubule; excess is excreted.

De novo HCO3− production

Formation of new bicarbonate, often via CO2 hydration with buffering (phosphate or ammonium) to replace losses.