Integration
Friday, 5 December 2025 13:35
How the body maintains pH
• The body must keep pH in a very narrow range (between 7.35 and 7.45).
→ To prevent this, the body uses three lines of defense.
• Molecules present in blood, cells, and urine that act immediately to resist pH changes.
→ Do not require organs to act in the moment; they work instantly by binding or releasing H⁺.
→ Lowest survivable pH ≈ 6.8
→ Highest survivable pH ≈ 7.8
Buffer systems
1) Chemical buffers
→ First line of defense.
→ Fastest response (almost instant).
→ How buffers work:
→ They bind free H⁺ when pH falls (acidosis)
→ They release H⁺ when pH rises (alkalosis).
→ Main chemical buffer systems:
► Bicarbonate buffer (HCO₃⁻ / H₂CO₃)
• Most important in extracellular fluid buffer.
• Controlled by lungs (CO₂) and kidneys (HCO₃⁻).
• Especially effective at acidic pH (≈6.8).
• Components:
§ Carbonic acid (H₂CO₃) — weak acid
§ Bicarbonate ion (HCO₃⁻) — weak base
§ Bicarbonate salts: NaHCO₃ in plasma; KHCO₃ and MgHCO₃ in cells.
• H₂CO₃ supply is almost limitless because it comes from CO₂ (respiratory control).
• HCO₃⁻ concentration is regulated by the kidneys.
• Buffering power depends on the amount of alkaline reserve (available HCO₃⁻).
§ If all HCO₃⁻ is used up, buffering stops.
• Mechanism:
→ When a strong acid is added (e.g. HCl):
→ The bicarbonate (HCO₃⁻) acts as a weak base and binds the added H⁺:
HCl + NaHCO₃ → H₂CO₃ + NaCl
→ Goes from strong acid to weak acid.
→ Thus, pH decreases only slightly.
→ When a strong base is added (e.g., NaOH):
§ Carbonic acid (H₂CO₃) donates H⁺ to neutralize OH⁻:
NaOH + H₂CO₃ → NaHCO₃ + H₂O
→ Goes from strong base to weak base.
→ Thus, pH increases only slightly.
► Protein buffer
• Found in intracellualr fluid, blood plasma and RBC's.
• Protein buffer systems:
→ Plasma protein buffers
→ Albumin is a plasma buffer.
→ Intracellular protein buffers
→ Hemoglobin buffer system
• Proteins are good buffers as they contain amino acids with:
→ Carboxyl groups (–COOH) - weak acids
→ Amino groups (–NH₂) - weak bases
• Mechanism:
→ As a weak acid (when pH rises):
→ Releases H⁺ to counteract alkalosis
R–COOH ↔ R–COO⁻ + H⁺
, Intracellular protein buffers
→ Hemoglobin buffer system
• Proteins are good buffers as they contain amino acids with:
→ Carboxyl groups (–COOH) - weak acids
→ Amino groups (–NH₂) - weak bases
• Mechanism:
→ As a weak acid (when pH rises):
→ Releases H⁺ to counteract alkalosis
R–COOH ↔ R–COO⁻ + H⁺
→ As a weak base (when pH falls):
→ Binds H⁺ to prevent acidosis.
R–NH₂ + H⁺ ↔ R–NH₃⁺
• Hemoglobin buffer system:
→ Inside RBCs:
→ CO₂ enters RBC and forms H₂CO₃, which can dissociate into H⁺ + HCO₃⁻.
→ Hemoglobin (Hb⁻ when deoxygenated) binds H⁺:
H⁺ + Hb⁻ → HHb
→ This prevents large pH changes in venous blood.
→ Hemoglobin acts as an amphoteric molecule so it can accept or donate H⁺.
► Phosphate buffer
• Strong buffer in intracellular fluid and kidneys.
→ Concentration of phosphate is higher inside cells.
→ In kidney tubules, phosphate helps excrete H⁺.
• Components:
→ Dihydrogen phosphate (H₂PO₄⁻) — weak acid
→ Monohydrogen phosphate (HPO₄²⁻) — weak base
• Mechanism:
→ When a strong acid is added:
HPO₄²⁻ binds H⁺ → H₂PO₄⁻
→ When a strong base is added:
H₂PO₄⁻ donates H⁺ → HPO₄²⁻
Buffer System Components Main Location Strengths
Bicarbonate H₂CO₃ / HCO₃⁻ ECF Most important ECF buffer; linked to lungs & kidneys
Phosphate H₂PO₄⁻ / HPO₄²⁻ ICF and urine Strong in ICF; major role in renal H⁺ excretion
Protein (incl. Hb) R–COOH/R–NH₂ groups ICF & plasma Most buffering capacity; hemoglobin very powerful
→ Buffers prevent sudden changes in pH when acid is added or removed.
→ They buy time until the lungs and kidneys act.
Chemical Buffer Organ Involved How They Interact
Bicarbonate buffer Lungs Regulate CO₂ (H₂CO₃)
Kidneys Regulate HCO₃⁻ reabsorption & H⁺ excretion
GI tract Secretes & absorbs HCO₃⁻; vomiting/diarrhea alter HCO₃⁻
Phosphate buffer Kidneys Used as major urinary buffer to excrete H⁺
Protein buffer Lungs Hb binds H⁺ during CO₂ transport
Kidneys Maintain protein levels & acid–base environment
GI tract Provides amino acids for plasma proteins
---------------------------------------------------------------------------------------------------------------------------------------
2) Physiological buffers
→ Organ-level systems that restore acid–base balance after chemical buffers act.
→ These systems do not buffer chemically.
→ They adjust the components of chemical buffers over minutes to days.
→ Chemical buffers temporarily bind to H+ but cannot remove acid.
→ Only physiological buffers (lungs and kidneys) can eliminate acid from the body.
► Respiratory buffer system (ventilation)
→ Second line of defense (acts in minutes).
→ Ventilation corrects 75% of acute pH disturbances.
→ This is why ventilation is considered a powerful, fast-acting physiological buffer.
→ The lungs regulate CO₂, which is in equilibrium with carbonic acid.
CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻
→ If pH drops (acidosis):
→ Chemoreceptors sense increased H⁺ or CO₂
→ Ventilation increases (hyperventilation)
Friday, 5 December 2025 13:35
How the body maintains pH
• The body must keep pH in a very narrow range (between 7.35 and 7.45).
→ To prevent this, the body uses three lines of defense.
• Molecules present in blood, cells, and urine that act immediately to resist pH changes.
→ Do not require organs to act in the moment; they work instantly by binding or releasing H⁺.
→ Lowest survivable pH ≈ 6.8
→ Highest survivable pH ≈ 7.8
Buffer systems
1) Chemical buffers
→ First line of defense.
→ Fastest response (almost instant).
→ How buffers work:
→ They bind free H⁺ when pH falls (acidosis)
→ They release H⁺ when pH rises (alkalosis).
→ Main chemical buffer systems:
► Bicarbonate buffer (HCO₃⁻ / H₂CO₃)
• Most important in extracellular fluid buffer.
• Controlled by lungs (CO₂) and kidneys (HCO₃⁻).
• Especially effective at acidic pH (≈6.8).
• Components:
§ Carbonic acid (H₂CO₃) — weak acid
§ Bicarbonate ion (HCO₃⁻) — weak base
§ Bicarbonate salts: NaHCO₃ in plasma; KHCO₃ and MgHCO₃ in cells.
• H₂CO₃ supply is almost limitless because it comes from CO₂ (respiratory control).
• HCO₃⁻ concentration is regulated by the kidneys.
• Buffering power depends on the amount of alkaline reserve (available HCO₃⁻).
§ If all HCO₃⁻ is used up, buffering stops.
• Mechanism:
→ When a strong acid is added (e.g. HCl):
→ The bicarbonate (HCO₃⁻) acts as a weak base and binds the added H⁺:
HCl + NaHCO₃ → H₂CO₃ + NaCl
→ Goes from strong acid to weak acid.
→ Thus, pH decreases only slightly.
→ When a strong base is added (e.g., NaOH):
§ Carbonic acid (H₂CO₃) donates H⁺ to neutralize OH⁻:
NaOH + H₂CO₃ → NaHCO₃ + H₂O
→ Goes from strong base to weak base.
→ Thus, pH increases only slightly.
► Protein buffer
• Found in intracellualr fluid, blood plasma and RBC's.
• Protein buffer systems:
→ Plasma protein buffers
→ Albumin is a plasma buffer.
→ Intracellular protein buffers
→ Hemoglobin buffer system
• Proteins are good buffers as they contain amino acids with:
→ Carboxyl groups (–COOH) - weak acids
→ Amino groups (–NH₂) - weak bases
• Mechanism:
→ As a weak acid (when pH rises):
→ Releases H⁺ to counteract alkalosis
R–COOH ↔ R–COO⁻ + H⁺
, Intracellular protein buffers
→ Hemoglobin buffer system
• Proteins are good buffers as they contain amino acids with:
→ Carboxyl groups (–COOH) - weak acids
→ Amino groups (–NH₂) - weak bases
• Mechanism:
→ As a weak acid (when pH rises):
→ Releases H⁺ to counteract alkalosis
R–COOH ↔ R–COO⁻ + H⁺
→ As a weak base (when pH falls):
→ Binds H⁺ to prevent acidosis.
R–NH₂ + H⁺ ↔ R–NH₃⁺
• Hemoglobin buffer system:
→ Inside RBCs:
→ CO₂ enters RBC and forms H₂CO₃, which can dissociate into H⁺ + HCO₃⁻.
→ Hemoglobin (Hb⁻ when deoxygenated) binds H⁺:
H⁺ + Hb⁻ → HHb
→ This prevents large pH changes in venous blood.
→ Hemoglobin acts as an amphoteric molecule so it can accept or donate H⁺.
► Phosphate buffer
• Strong buffer in intracellular fluid and kidneys.
→ Concentration of phosphate is higher inside cells.
→ In kidney tubules, phosphate helps excrete H⁺.
• Components:
→ Dihydrogen phosphate (H₂PO₄⁻) — weak acid
→ Monohydrogen phosphate (HPO₄²⁻) — weak base
• Mechanism:
→ When a strong acid is added:
HPO₄²⁻ binds H⁺ → H₂PO₄⁻
→ When a strong base is added:
H₂PO₄⁻ donates H⁺ → HPO₄²⁻
Buffer System Components Main Location Strengths
Bicarbonate H₂CO₃ / HCO₃⁻ ECF Most important ECF buffer; linked to lungs & kidneys
Phosphate H₂PO₄⁻ / HPO₄²⁻ ICF and urine Strong in ICF; major role in renal H⁺ excretion
Protein (incl. Hb) R–COOH/R–NH₂ groups ICF & plasma Most buffering capacity; hemoglobin very powerful
→ Buffers prevent sudden changes in pH when acid is added or removed.
→ They buy time until the lungs and kidneys act.
Chemical Buffer Organ Involved How They Interact
Bicarbonate buffer Lungs Regulate CO₂ (H₂CO₃)
Kidneys Regulate HCO₃⁻ reabsorption & H⁺ excretion
GI tract Secretes & absorbs HCO₃⁻; vomiting/diarrhea alter HCO₃⁻
Phosphate buffer Kidneys Used as major urinary buffer to excrete H⁺
Protein buffer Lungs Hb binds H⁺ during CO₂ transport
Kidneys Maintain protein levels & acid–base environment
GI tract Provides amino acids for plasma proteins
---------------------------------------------------------------------------------------------------------------------------------------
2) Physiological buffers
→ Organ-level systems that restore acid–base balance after chemical buffers act.
→ These systems do not buffer chemically.
→ They adjust the components of chemical buffers over minutes to days.
→ Chemical buffers temporarily bind to H+ but cannot remove acid.
→ Only physiological buffers (lungs and kidneys) can eliminate acid from the body.
► Respiratory buffer system (ventilation)
→ Second line of defense (acts in minutes).
→ Ventilation corrects 75% of acute pH disturbances.
→ This is why ventilation is considered a powerful, fast-acting physiological buffer.
→ The lungs regulate CO₂, which is in equilibrium with carbonic acid.
CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻
→ If pH drops (acidosis):
→ Chemoreceptors sense increased H⁺ or CO₂
→ Ventilation increases (hyperventilation)
