Physiology Quiz: Electrolyte Balance and Acid-Base Regulation

Electrolyte Balance and Acid-Base Disorders

• Hypercalcaemia is related to alkalosis and hypokalaemia
• Acidosis is associated with decreased cardiac contractility, while alkalosis is associated with decreased oxygen delivery due to increased Hb affinity to oxygen

Sources of Hydrogen Ions (H+)

• Metabolic processes: aerobic metabolism, triglyceride breakdown, ketoacids, and metabolism of phosphoric, sulphuric, and amino acids
• Anaerobic metabolism: lactic acid
• Phosphoric, sulphuric, and hydrochloric acids

Maintaining [H+]

• Buffers: bicarbonate, phosphate, haemoglobin, and protein buffers
• Respiratory system: seconds to minutes
• Kidneys: hours to days

Buffers

• Definition: a mixture of a weak acid and its conjugate base that lessens a change in [H+] when a strong acid/base is added
• Types: bicarbonate, phosphate, haemoglobin, and protein buffers

Bicarbonate Buffer

• Equation: H2CO3 ⇌ H+ + HCO3-
• pKa = 6.1
• pH = 7.4
• Role: volatile acid, reclaimed/regenerated, and excreted as H+

Henderson-Hasselbalch Equation

• Equation: pH = 6.1 + log ([HCO3-]/(0.03 x PaCO2))
• Used to calculate pH

Phosphate Buffer

• Equation: H2PO4- ⇌ H+ + HPO42-
• pKa = 6.8
• Role: intracellular and in urine

Other Buffers

• Haemoglobin buffer: in red blood cells, plasma, and tissues
• Protein buffers: in intracellular fluid and other proteins
• Bone buffer: in bone water, exchange of protons on bone surface, and release of carbonate and phosphate proton buffers

Respiratory Regulation

• Role: regulates pH of cerebrospinal fluid (CSF)
• Equation: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
• Crosses blood-brain barrier (BBB)
• Central chemoreceptors: increase ventilation in response to decreased pH and decreased PCO2
• Peripheral chemoreceptors: increase ventilation in response to increased PCO2 and decreased PO2

Renal Regulation

• Metabolic component: HCO3- and H+
• HCO3- reabsorption and regeneration: 80% in proximal tubules, 10% in distal tubules, and 4% in collecting ducts
• H+ excretion: 4% in proximal tubules, 10% in distal tubules, and 80% in collecting ducts

Bicarbonate Reabsorption

• Process: HCO3- reabsorption, regeneration, and conservation
• Steps:
  1. Filtered HCO3- and Na+ in proximal tubules
  2. H+ and HCO3- in tubular lumen
  3. H+ excretion in urine
  4. Net HCO3- reabsorption in blood

Bicarbonate Regeneration

• Process: linked to H+ excretion and titratable acidity
• Steps:
  1. Filtered HPO42- and H+ in proximal tubules
  2. H+ excretion in urine
  3. New HCO3- absorbed in blood

Approach to Acid-Base Disorders

• Traditional approach: Henderson-Hasselbalch equation
• Stewart's approach: physiochemical approach, independent variables: ATOT, SID, and PaCO2

Interpreting pH, HCO3, PCO2

• Primary disorder: respiratory or metabolic
• Compensation: response by non-primary system to bring pH back within range
• Correction: response by the body to bring all parameters within range

Laboratory Parameters

• Blood gas: pH, PCO2, and HCO3-
• U&E: electrolyte levels, including potassium, sodium, and chloride

Terminology

• Acid: substance that donates H+ ions

• Base: substance that accepts H+ ions

• Acidosis: excess acid

• Alkalosis: excess base

• Acidaemia: pH < 7.35

• Alkalaemia: pH > 7.45### Arterial/Capillary Blood

• Electrodes are used for blood gas analysis

• Anticoagulated, sealed syringe, and on ice are necessary for blood sample collection

• Minimal delays are essential to ensure accurate results

Blood Parameters

• Actual bicarbonate can be calculated using the H-H equation, which requires pH and PaCO2 values
• Standard bicarbonate is a better indicator, as it is corrected for abnormal PaCO2
• Standard base excess is corrected for CO2, temperature, and the effect of Hb as a buffer

Laboratory Parameters

• U&E (Urea and Electrolytes) require a serum sample with minimal delays
• Measured TCO2 (Total CO2) is calculated using 4 parameters: Na+, K+, CL-, and HCO3-

Arterial Blood Gas

• Measured pH is calculated using the equation: pH = 6.1 + log [0.03 x PaCO2]
• Calculated base excess and standard bicarbonate are also derived from arterial blood gas analysis
• Measured HCO3- (bicarbonate) is an important parameter in acid-base disorders

Acid Base Disorders

• pH < 7.35 indicates acidosis, while pH > 7.45 indicates alkalosis
• Respiratory acid-base disorders can be classified into respiratory acidosis and respiratory alkalosis

Respiratory Acid Base Disorders

• Respiratory acidosis is caused by inadequate mechanical ventilation, CNS depression, hypoventilation, airway obstruction, lung defects, and increased CO2 intake

• Primary disorder: hypoventilation, which leads to increased CO2 and decreased pH

• Compensation: metabolic compensation, which increases HCO3- to buffer excess H+

• Compensation in acute CO2: ventilation, buffering of H+ by Hb and proteins, and small HCO3- increase

• Compensation in chronic CO2: greater HCO3- increase, H+ excretion, and HCO3- reabsorption/regeneration

Respiratory Alkalosis

• Causes: excessive mechanical ventilation, CNS stimulation, hyperventilation, and lung defects

• Primary disorder: hyperventilation, which leads to decreased CO2 and increased pH

• Compensation: metabolic compensation, which decreases HCO3- to buffer excess H+

• Compensation in acute CO2: equilibrium shifts, intracellular buffering, small HCO3- decrease, and chronic CO2: greater HCO3- decrease, H+ excretion, and HCO3- reabsorption/regeneration