Acid-Base Balance and pH Regulation

Acid-Base Balance

• To achieve homeostasis, there must be a balance between the intake/production of H+ and the net removal of H+ from the body.
• Precise regulation of H+ is needed because almost all enzyme systems in the body are influenced by H+ concentration.
• H+ concentrations are kept at low levels of 0.00004 mEq/L.

pH

• Hydrogen concentration is expressed as pH units.
• There is an inverse relationship between pH and H+ concentration.
• Normal pH of arterial blood is 7.4.
• Normal pH of venous blood is 7.35, due to extra carbon dioxide released from tissues forming H2CO3.

Hydrogen Ion

• A single free proton is released from a hydrogen atom.

Regulation Systems

• The systems that regulate H+ concentration include:
  • Chemical Acid-Base buffer systems
  • Respiratory Center
  • The Kidneys

Buffer System Mechanisms

• Buffers limit pH changes when strong acids/bases are introduced.
• A buffer is any substance that can reversibly bind H+.
• Bicarbonate, proteins, and phosphate buffers are examples of buffers.
• The Bicarbonate-Carbonic Acid Buffer System is the main extracellular buffer.
  • This system is affected by the lungs and kidneys.
  • Carbonic Anhydrase accelerates it.
  • CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

Respiratory Regulation

• Control of extracellular fluid CO2 is the second line of defense against acid-base disturbances.
• Alveolar ventilation provides oxygen for oxidative metabolism and eliminates CO2 produced by the metabolic process.
• Hyperventilation eliminates CO2 from the ECF, reducing H+ concentration.
• Hypoventilation increases CO2 in the ECF, raising H+ concentration.

Renal Regulation

• Variations in body fluid hydrogen ion concentration stimulate appropriate regulatory processes in the kidneys.
• The main functions of renal regulation are:
  • Excretion of acid
  • Retention of bicarbonates

Clinical Abnormalities

• Clinical abnormalities in acid-base balance include:
  • Metabolic Acidosis
  • Metabolic Alkalosis
  • Respiratory Acidosis
  • Respiratory Alkalosis

Metabolic Acidosis

• Metabolic acidosis is an abnormal physiological process, characterized by a primary gain of strong acid or primary loss of bicarbonate from the ECF.
• Situations which can cause metabolic acidosis:
  • Ketoacidosis
  • Lactic Acidosis (Shock, Hypoxia, Drugs, Toxic Substance)
  • Poisoning
  • Reduced acid excretion (renal insufficiency, Tubular acidosis)
  • Loss of bases (Diarrhea)

Metabolic Alkalosis

• Metabolic alkalosis is an abnormal physiological process characterized by primary gain of strong base or primary gain of bicarbonate by ECF.
• It can be associated with H depletion, such as from diuretic therapy or hypoparathyroidism.

Respiratory Acidosis

• Respiratory acidosis is an abnormal physiological process in which there is a primary reduction in alveolar ventilation relative to the rate of CO2 production.
• Alterations of alveolarcapillary diffusion and perfusion can cause respiratory acidosis.

Respiratory Alkalosis

• Respiratory alkalosis is an abnormal physiological process in which there is a primary increase in the rate of alveolar ventilation relative to the rate of CO2 production.
• Hypermetabolic states cause it.
• Conditions which can cause this:
  • Hypoxia
  • Septicemia
  • Pregnancy
  • Pain

Compensation

• Compensation is the secondary physiological process that responds to a primary disturbance in one component, which causes changes in a direction that restores blood pH toward normal.
• The compensatory changes are brought about by the action of certain organs or organ systems.
• Metabolic disorders of acid-base balance are modified by respiratory compensations.
• Respiratory disorders of acid-base balance are modified by metabolic compensations.

ABG Analysis

• Key parameters of interest in ABG analysis are used to figure out underlying causes of clinical abnormalities:
  • pH: Evaluates the pH itself (acidic or alkaline)
  • pCO2: Evaluates ventilation
  • HCO3: Evaluates metabolic processes
  • pO2: Evaluates the degree of oxygenation

Specimen Collection

• The specimen used is arterial blood.
• Lithium Heparin is used as the anticoagulant.
• Use anaerobic blood collection techniques.
• Process immediately, within less than 30 minutes after collection.
• Transport in a chilled condition.

Levels

• pH: 7.34-7.44 indicates acid (lower) or alkaline (higher)
• pO2: 81-100 mmHg indicates oxygen levels
• pCO2: 35-45 mmHg indicates carbon dioxide levels (acid)
• HCO3: 21-28 mmHg indicates bicarbonate levels (base)
• Basic approach to assessing blood gas levels:
  • Is the pH acidosis or alkalosis?
  • What is the primary disturbance?
  • Is the compensation appropriate?

Clinical Examples

• Case 1: 24-year-old female with drug abuse, brought to ER
  • pH: 7.08
  • pCO2: 80
  • pO2: 37
  • HCO3: 26
• Case 2: 42 year old, Female, IDDM, unwell for already 4 days
  • pH: 7.23
  • pCO2: 27
  • pO2: 118
  • HCO3: 12
• Case 3:
  • A 50 y/o patient has the following results
    • pH: 7.34
    • PCO2: 48
    • HCO3: 31

Measurement

• Spectrophotometer
• Electrodes