26.4 Acid-Base Balance

The body maintains a delicate pH balance crucial for proper functioning. The bicarbonate-carbonic acid buffer system plays a key role, keeping blood pH between 7.35-7.45. This system, along with respiratory and renal mechanisms, works to counteract acidosis and alkalosis.

Acid-base disorders can arise from metabolic or respiratory issues. The body responds through various buffer systems, including bicarbonate-carbonic acid, phosphate, and protein buffers. These systems act quickly, while the respiratory and renal systems provide longer-term regulation of acid-base balance.

Acid-Base Balance

Bicarbonate-carbonic acid buffer system

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• Primary buffer system in the blood maintains pH within normal range of 7.35-7.45
  • Consists of carbonic acid ($H_2CO_3$) and bicarbonate ion ($HCO_3^-$) in a 1:20 ratio
  • Ratio of $HCO_3^-$ to $H_2CO_3$ determines blood pH (normal ratio 20:1 results in pH 7.4)
  • This relationship is described by the Henderson-Hasselbalch equation
• Bicarbonate ion accepts hydrogen ions ($H^+$) when blood pH drops, preventing acidosis
  • $HCO_3^- + H^+ \rightarrow H_2CO_3$ removes excess $H^+$ from the blood
  • Examples: lactic acid buildup during exercise, ketoacidosis in diabetes
• Carbonic acid releases hydrogen ions when blood pH rises, preventing alkalosis
  • $H_2CO_3 \rightarrow HCO_3^- + H^+$ releases $H^+$ into the blood
  • Examples: hyperventilation, vomiting leading to loss of stomach acid

Respiratory system in acid-base balance

• Regulates blood pH by controlling elimination of carbon dioxide ($CO_2$)
  • $CO_2$ is a byproduct of cellular respiration converted to carbonic acid in the blood ($CO_2 + H_2O \rightleftharpoons H_2CO_3$)
• Hyperventilation (increased breathing rate and depth) occurs during acidosis
  • Eliminates more $CO_2$ from the blood, reducing carbonic acid formation and raising pH
  • Examples: exercise, anxiety, aspirin overdose
• Hypoventilation (decreased breathing rate and depth) occurs during alkalosis
  • Retains more $CO_2$ in the blood, increasing carbonic acid formation and lowering pH
  • Examples: sleep apnea, narcotic overdose, severe lung disease
• Provides rapid response to changes in blood pH within minutes

Speed and mechanisms of buffer systems

• Bicarbonate-carbonic acid buffer system
  • Primary blood buffer, acts within seconds to minutes
  • Maintains pH by adjusting ratio of bicarbonate ion to carbonic acid
• Phosphate buffer system
  • Major buffer in intracellular fluid and urine, acts within minutes to hours
  • Consists of hydrogen phosphate ion ($HPO_4^{2-}$) and dihydrogen phosphate ion ($H_2PO_4^-$)
  • Maintains pH by accepting or releasing hydrogen ions
• Protein buffer system
  • Present in blood plasma and intracellular fluid, acts within minutes to hours
  • Proteins (hemoglobin, albumin) contain amino acids that accept or release hydrogen ions
  • Helps maintain pH in respective compartments
• Respiratory system
  • Regulates blood pH by controlling $CO_2$ elimination, acts within minutes
  • Provides rapid response to changes in blood pH
• Renal system
  • Regulates blood pH by adjusting excretion of $H^+$ and reabsorption of $HCO_3^-$, acts within hours to days
  • Provides slow but long-term regulation of acid-base balance
  • Calculates anion gap to assess acid-base disturbances

Acid-Base Disorders

• Metabolic acidosis: decreased blood pH due to increased acid production or decreased base
• Metabolic alkalosis: increased blood pH due to decreased acid or increased base
• Respiratory acidosis: decreased blood pH due to decreased CO2 elimination
• Respiratory alkalosis: increased blood pH due to increased CO2 elimination