Acid-Base balance

Introduction

  • Title: BEST AND TAYLOR'S Physiological Basis of Medical Practice (TWELFTH EDITION)

  • Edited by: John B. West, M.D., Ph.D., D.Sc.

  • Location: University of California, San Diego School of Medicine, La Jolla, California.

  • Publisher: Williams & Wilkins

Chapter 32: Acid-Base Balance and Regulation of Excretion

Roles of the Kidneys

  • Major role in regulating carbonic acid (H2CO3).

  • Responsible for maintaining the pH of extracellular fluid (ECF) within a narrow range, normal values:

    • Arterial Plasma: 7.40 ± 0.02

    • Mixed Venous Plasma: 7.38 ± 0.02

Evaluating Acid-Base Status

  • Clinical evaluation often based on laboratory studies of arterial samples.

  • Acidosis defined as arterial pH < 7.38.

  • Alkalosis defined as arterial pH > 7.42.

Effects of pH Changes

  • Precise control of pH is essential due to:

    • Impact on protein conformation.

    • Effects on enzymatic reactions and CNS function.

  • For example, 10,000 mmol of H+ added to a 60-kg man with 36 liters of body water without compensation would drastically increase H+ concentration, demonstrating the critical nature of pH regulation.

Fixed Acids and Metabolic Processes

  1. Volatile Acid: Carbon Dioxide (CO2)

    • CO2 produced from oxidation of carbohydrates, fats, and amino acids.

    • Reacts with water to form H2CO3, then dissociates to form H+ ions and bicarbonate (HCO3).

  2. Fixed Acids: Sulfuric and Phosphoric Acid

    • Sulfuric acid formed from oxidation of sulfur-containing amino acids.

    • Phosphoric acid arises from the metabolism of certain biological molecules.

  3. Organic Acids

    • Include lactic acid, acetoacetic acid, and B-OH butyric acid.

    • Typically further oxidized to CO2 and water, with pathological conditions causing accumulation and acidosis.

Buffer Systems

  • Acid-Base Buffer Systems: Critical for immediate pH defense.

    • Include weak acids that can exist in protonated and unprotonated forms.

    • Weak acids buffer added strong acid (HX) by accepting H+.

Importance of Protein Buffers

  • Proteins, including hemoglobin, contain multiple ionizable sites contributing to their buffering ability, particularly effective near physiological pH.

  • Hemoglobin acts as a major buffer in blood, aiding in CO2 transport from tissues.

Bicarbonate Buffer System

  • Primary buffer in ECF, reacts to regulate pH effectively.

pH Regulation

Renal Regulation

  • Kidneys regulate plasma HCO3 concentration, must recover filtered HCO3 and generate new HCO3 to replace losses from metabolic acidosis.

Respiratory Regulation

  • Lungs help regulate acid-base balance by controlling arterial CO2 levels through ventilation adjustments.

Summary of Acid-Base Disturbances

  1. Primary Disturbances:

    • Respiratory acidosis (increased arterial Pco2).

    • Respiratory alkalosis (decreased Pco2).

    • Metabolic acidosis (decreased [HCO3]).

    • Metabolic alkalosis (increased [HCO3]).

Compensation Mechanisms

  • Compensatory responses may not fully restore normal conditions and vary between acute and chronic states.

  • Respiratory compensatory mechanisms tend to initiate faster than renal adaptations.