Acid-Base Balance: pH Regulation

• Module 6 discusses acid-base balance, including factors and parameters to maintain pH homeostasis

Learning Outcomes

• Trace acid-base regulation by lungs and kidneys
• Explain blood buffers and the importance of Henderson-Hasselbalch Equation
• Calculate blood pH using Henderson-Hasselbalch equation to determine acidosis and alkalosis
• Distinguish respiratory from metabolic imbalances
• Identify compensated or uncompensated acid-base imbalance

Definition

• Acid yields H+, H3O; associated with ↑CO2
• Base yields OH-, associated with ↑HCO3-
• pKa is the negative log of the ionization constant; protonated and unprotonated forms are present in equal concentrations
• Buffer is a combination of a weak acid or weak base and its salt; resists changes in pH
• Acidemia means arterial blood pH is <7.35
• Alkalemia means arterial blood pH >7.45

pH regulation in the body

• CO2 combines with H2O to form H2CO3, which quickly dissociates into H+ and HCO3-
• This process is accelerated by carbonic anhydrase
• Dissociation of H2CO3 caused HCO3- concentration to increase in the red cells and diffuse into the plasma
• Through mechanisms involving the lungs and kidneys, the body controls and excretes H+ in order to maintain pH homeostasis

Organs involved in acid-base balance

• Lungs provide respiratory control of CO2 elimination for rapid adjustment of blood pH
• H2CO3 dissociates into CO2 and H2O, allowing CO2 to be eliminated by the lungs and H+ as water through ventilation
• Elimination of CO2 resists accumulation of H+
• Kidneys provide metabolic reclamation of HCO3 from the glomerular filtrate and excretion of H+
• H+ is excreted as NH4+ (indirect disposal) and titratable acid (direct disposal)
• Kidneys excrete considerable amounts of acid and base for acid-base regulation
• Bicarbonate reabsorption occurs in the kidneys

Buffer systems

• Bicarbonate-carbonic acid are principal buffers with pKa of 6.1
• Plasma proteins
• Hemoglobin
• Phosphate buffer is an important IC buffer

Assessment of acid-base balance

• Arterial Blood Gas (ABG) analysis measures the amounts of arterial gases, such as oxygen and carbon dioxide
• Used mainly in pulmonology and critical-care medicine to determine gas exchange across the alveolar-capillary membrane
• Specimen is arterial blood anticoagulated with heparin (0.05mL/mL of blood)
• Methods include Gasometer (Van Slyke, Natelson) and Electrodes for pH (potentiometry)
  • Ag-AgCl is the reference electrode
  • Calomel is the reference electrode
  • Severinghaus measures pCO2
  • Clark measures pO2
  • Glass is commonly used electrode

Parameters

• pH evaluates blood pH
• pCO2 evaluates the ventilation (lungs), efficiency of gas exchange
• HCO3 evaluates the metabolic process (kidneys)
• pO2 evaluates the degree of oxygenation and availability of gas in the blood

Ranges

• pH normal range: 7.35-7.45
• pCO2 normal range: 35-45 mmHg
• pO2 normal range: 80-110 mmHg
• HCO3 normal range: 22-26 mmol/L
• TCO2 normal range: 23-27 mmol/L
• H2CO3 normal range: 1.05-1.35 mmol/L
• O2 Saturation normal range: >95%

Acid-base balance calculation

• The pH of plasma is dependent on the partial CO2 (Respiratory) and HCO3- concentration (Metabolic)
• The acid-base balance relationship uses these components and is calculated using the Henderson-Hasselbalch equation
• Normal HCO3- to pCO2 ratio: 20:1

Acid-Base Imbalances

• Respiratory acidosis is an increase in CO2 due to ineffective respiration (hypoventilation)
  • e.g., chronic obstructive pulmonary, ingestion of narcotics and barbiturates, meningitis
• Respiratory alkalosis is a decrease in CO2 due to excessive respiration (Hyperventilation)
  • e.g., hypoxia, anxiety, nervousness, excessive crying, pulmonary embolism, pneumonia, congestive heart failure, salicylate overdoses
• Metabolic acidosis due to HCO3- deficiency
  • e.g., organic acid production when ingestion exceeds excretion rate, diabetic ketoacidosis, chemical poisoning, renal failure
• Metabolic alkalosis is HCO3- excess
  • e.g., ingestion of excess base, decreased elimination of base, or loss of acidic fluids
• Mixed Acid-base disorders: two or more primary ABI coexist; due to excessive compensation; pCO2 and HCO3- is in opposite direction

Compensatory mechanism

• The body alters the factor not primarily affected by the pathologic process
• If the problem is of respiratory origin, metabolic functions will compensate, and vice versa
• Fully compensated means pH has returned to normal range (20:1 ratio is restored)
• Partially compensated means pH is approaching to normal; parameters are trying to compensate with the origin of imbalance