Acid-Base Balance and Regulation

Acid-Base Balance

• pH affects all proteins and biochemical reactions, closely regulated
• Normal arterial blood pH: 7.4
• Venous blood and interstitial fluid pH: 7.35
• Intracellular fluid pH: 7.0
• Alkalosis (alkemia): arterial pH > 7.45
• Acidosis (acidemia): arterial pH < 7.35
• Normal ECF pH range: 7.35-7.45

Acid-Base Regulation

• Regulated sequentially by:
  • Chemical buffer systems (rapid, first line of defense)
  • Respiratory buffer system (acts within 1-3 minutes)
  • Renal buffer system (most powerful, takes hours to days)

Chemical Buffer Systems

• Systems of one or more compounds that resist pH changes when strong acid or base is added
• Bicarbonate buffer system
• Phosphate buffer system
• Protein buffer system

Physiological Buffer Systems

• Respiratory and renal systems regulate acid/base amounts in body

• Respiratory system:

  • Eliminates CO2 (an acid)
  • CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
  • CO2 unloading: shifts equilibrium to left
  • CO2 loading: shifts equilibrium to right
  • Acidosis: increased respiratory rate
  • Alkalosis: decreased respiratory rate

• Renal system:

  • Eliminates non-volatile acids (phosphoric, uric, and lactic acids)
  • Regulates blood alkaline levels
  • Renews chemical buffers

Acid-Base Balance Disturbances

• Respiratory or metabolic

• Metabolic acidosis or alkalosis: blood pH and bicarbonate levels are abnormal

  • Caused by malfunctioning of physiological buffer systems (other systems try to compensate)
  • Respiratory compensation: Increased/decreased respiratory rate
  • Renal compensation: H+ secretion, HCO3 secretion

• Respiratory acidosis or alkalosis: blood pH is abnormal due to blood PCO2

  • Respiratory compensation:Increased/ Decreased respiratory rate
  • Renal compensation: H+ secretion, HCO3 secretion

• Compensation:

  • Compensated: pH is normal but PCO2 or HCO3- is abnormal
  • Partially compensated: pH, PCO2, and HCO3- are all abnormal
  • Uncompensated: pH, PCO2, or HCO3- is abnormal

Acid and Base Balance

• Anion Gap: calculation of major cations and anions in plasma. Indication of acid-base balance
• Anion Gap = Sodium - (Chloride + Bicarbonate)

Determining Acidosis or Alkalosis

• Step 1: Check pH

• Step 2: Check pCO2

• Step 3: Check bicarbonate level

• Match pCO2 or HCO3 with the pH (Remember RO-ME).

Normal Ranges

• pH: 7.35-7.45
• pCO2: 35-45 mmHg
• HCO3-: 22-26 mEq/L