Body Fluid pH Regulation

Respiratory Regulation (Minutes–Hours)

• Core reaction: \mathrm{CO2 + H2O \leftrightarrow H2CO3 \leftrightarrow H^+ + HCO_3^-}
• ↓ pH (↑ \mathrm{[H^+]}):
  • ↑ breathing depth/rate → ↑ \mathrm{CO_2} exhalation
  • Drives reaction ← (left) → ↓ \mathrm{[H^+]} → pH rises
• ↑ pH (↓ \mathrm{[H^+]}):
  • ↓ breathing depth/rate → ↓ \mathrm{CO_2} exhalation
  • Drives reaction → (right) → ↑ \mathrm{[H^+]} → pH falls
• Control loop: medullary & peripheral chemoreceptors sense \mathrm{pH/\,P{CO2}} → alter DRG output to respiratory muscles

Renal Regulation (Hours–Days)

• Removes non-volatile acids (lactic, sulfuric, phosphoric)
• Type A intercalated cells (acidosis, ↓ pH):
  • Apical \mathrm{H^+} ATPase pumps \mathrm{H^+} into urine
  • Basolateral \mathrm{Cl^-}/\,HCO3^- antiporter returns \mathrm{HCO3^-} to blood
• Type B intercalated cells (alkalosis, ↑ pH):
  • Reverse transporter positions
  • Secrete \mathrm{HCO_3^-} into urine, return \mathrm{H^+} to blood

Chemical Buffer Systems (Seconds)

• Protein buffers (most abundant overall; includes hemoglobin)
• Carbonic-acid–bicarbonate buffer (primary ECF buffer)
• Phosphate buffer (important in ICF & urine)
• Act as immediate stop-gap; do not remove \mathrm{H^+}, merely bind/release

Time-Scale Overview

• Chemical buffers: seconds (fastest)
• Respiratory adjustment: minutes; max effect in hours
• Renal compensation: starts in minutes; max effect hours–days

Key Terms

• Volatile acid: carbonic acid (removable via lungs)
• Non-volatile acids: metabolic; removed only by kidneys