Acid-Base Balance Summary

Acid-Base Balance

• Maintaining a constant blood pH is crucial for normal enzyme function, metabolite uptake, oxygen delivery, and overall cellular function. Blood pH is tightly regulated within a narrow range to ensure optimal physiological processes.

• Normal arterial blood pH range: 7.40 \pm 0.04 (36-44 nmol/L H+). This range is essential for proper bodily function. Variations outside this range can indicate serious health issues.

• Acidosis: pH below reference range. This condition can result from various factors, including metabolic and respiratory disturbances.

• Alkalosis: pH above reference range. Alkalosis can also stem from metabolic or respiratory issues, leading to a variety of symptoms and complications.

• pH controlled by buffers, respiratory system, and kidneys. These three systems work in concert to maintain acid-base balance; each plays a critical role in responding to pH changes.

Buffering Systems

• Hemoglobin: Binds CO_2, hydrogen, and oxygen; participates in chloride shift; exhibits Bohr effect. Hemoglobin's role as a buffer is vital, especially in transporting gases and maintaining pH within red blood cells.

• Phosphate: Major role in H+ elimination via kidneys; assists in Na+ for H+ exchange; important in urine. The phosphate buffer system is particularly important in renal regulation of acid-base balance.

• Bicarbonate/Carbonic Acid: Most important buffer in the body (>60% blood buffering capacity).

  • Maintained by the lungs (CO2 expulsion). The respiratory system adjusts ventilation to alter CO2 levels, thereby affecting pH.

  • Extracellular ratio: [HCO3^-]:[CO2] = 20:1. This ratio is critical for maintaining the proper pH balance in the extracellular fluid.

Disturbances of Hydrogen Ion Homeostasis

• Acidosis: Fall in the ratio of [HCO3^-]:pCO2 in the ECF.

  • Metabolic: Decreased HCO_3^-. Various conditions such as diabetic ketoacidosis or renal failure can lead to a decrease in bicarbonate levels.

  • Respiratory: Increased pCO_2. Hypercapnia if >20kPA. Respiratory acidosis often results from impaired ventilation, leading to carbon dioxide retention.

• Alkalosis: Rise in the ratio of [HCO3^-]:pCO2 in the ECF.

  • Metabolic: Increased HCO_3^-. Excessive vomiting or overuse of antacids can cause metabolic alkalosis by increasing bicarbonate levels.

  • Respiratory: Decreased pCO_2. Hypocapnia if <4.5kPA. Hyperventilation is a common cause of respiratory alkalosis, leading to excessive carbon dioxide elimination.

Base Excess

• Assesses the metabolic component of acid-base disturbance. Base excess (BE) indicates the amount of excess or deficit of base in the blood.

• Positive BE: Excess bicarbonate. A positive BE suggests an alkaline state due to an excess of bicarbonate.

• Negative BE: Bicarbonate deficiency. A negative BE indicates an acidic state due to a deficiency in bicarbonate.

Preanalytical Considerations

• Collection, handling, and transport are critical for accurate blood gas analysis. Proper technique is essential to avoid introducing errors.

• Arterial specimens preferred. Arterial blood provides a more accurate representation of systemic acid-base status compared to venous blood.

• Syringe should be heparinized and mixed well; minimize air bubbles. Heparin prevents clotting, while minimizing air bubbles prevents alterations in gas partial pressures.

• Assay ASAP to avoid bias from RBC metabolism. Red blood cell metabolism can alter blood gas values if the sample is not analyzed promptly.

Blood Gas Analyzers

• Use electrodes to measure pO2, pCO2, and pH. Modern blood gas analyzers provide rapid and accurate measurements.

• pO_2: Amperometric measurement. Amperometry measures the current generated by the reduction or oxidation of a substance.

• pCO_2 and pH: Potentiometric measurement. Potentiometry measures the potential difference between two electrodes to determine ion concentrations.

Interpretation of Results

1. Assess pH: Acidic (<7.35), Normal (7.35-7.45), Alkalotic (>7.45). pH provides an initial indication of acid-base status.

2. Examine pCO_2: Normal (35-45mmHg).

   • Respiratory alkalosis: < 35mmHg. Low pCO_2 indicates hyperventilation.

   • Respiratory acidosis: > 45mmHg. High pCO_2 indicates hypoventilation.

3. Determine HCO_3^- level: Normal (24 \pm 2 mmol/L).

   • Metabolic acidosis: < 22mmol/L. Low bicarbonate levels suggest a metabolic acidosis.

   • Metabolic alkalosis: > 26mmol/L. High bicarbonate levels suggest a metabolic alkalosis.

4. Determine Compensation

   • Respiratory Acidosis (\uparrow CO2) = HCO3 values should be N/high. Kidneys retain bicarbonate to compensate for respiratory acidosis.

   • Respiratory alkalosis (\downarrow CO2) = HCO3 values should be N/ low. Kidneys excrete bicarbonate to compensate for respiratory alkalosis.

   • Metabolic acidosis (\downarrow HCO3) = pCO2 levels should be