Acid-Base Balance in Human Physiology

Acid-Base Balance

• Refers to the steady state of the pH of the body
• Normal metabolic processes continually generate acids
• Normal pH range is 7.35 to 7.45
• If pH falls out of this range, proteins are diminished or destroyed
• Below 7.3 = acidosis (excess H+) = causes acidemia
• Above 7.4 = alkalosis (low H+) = causes alkalemia

Importance of pH

• Normal blood pH is between 7.35 and 7.45
• Blood pH below 7.35 is known as acidemia
• Blood pH above 7.45 is known as alkalemia
• Values less than 6.8 or greater than 7.8 often result in death
• The body's pH influences the function of enzymes, speed of cellular reactions, cell permeability, and the integrity of cell structure

Effects of pH Changes on the Body

• Effect of Acidosis
  • ↓ blood pH (acidemia)
  • Depression of the CNS - disoriented/comatose
• Effect of Alkalosis
  • ↑ blood pH (alkaemia)
  • Hyperexcitability of the nervous system - able to generate impulses without normal stimuli
  • Spasms/tetanic contractions
  • Death with severe alkalemia is due to spasms of muscles of respiration

Regulatory Systems for Acid-Base Balance

• Chemical buffers
  • 1st line of defense
  • React in seconds
  • 3 main systems: bicarbonate, protein, phosphate
• Respiratory system (lungs)
  • 2nd line of defense
  • React in minutes
• Renal system (kidneys)
  • 3rd line of defense
  • React in hours to days
  • Most powerful and lasts the longest

Chemical Buffers: Review

• Acid
  • Substance that gives up/donates a proton (hydrogen ion, H⁺)
• Base
  • Substance that accepts/binds a proton (hydrogen ion, H⁺)
• Buffer
  • Compound that can accept or donate a proton (H⁺)
  • Buffers are usually weak acids with their corresponding salts.

Chemical Buffer Systems

• Work to counteract H⁺ imbalance created by metabolic processes
• Buffers
  • Substances that alter the H⁺ concentration
  • If H⁺ are added, the buffer combines with the extra H⁺ ions to help maintain the pH
  • If H⁺ are lost, the buffer releases H⁺ ions to combine with the base to help maintain the pH
  • Work both intracellularly and extracellularly
• 3 main systems
  • Bicarbonate Buffer
  • Protein Buffer
  • Phosphate Buffer

Bicarbonate (HCO₃⁻) Buffer

• Remember: CO₂ is produced as a waste product during aerobic cellular respiration
• CO₂ combines with H₂O to form carbonic acid (H₂CO₃)
• Most of which rapidly dissociates to form H⁺ and bicarbonate (HCO₃⁻)
• CO₂ acts like an acid because it combines with water to form carbonic acid
• H⁺ acts as an acid, and HCO₃⁻ acts as a base
• Any disturbance of the system will be compensated by a shift in the chemical equilibrium by the law of mass action.

Importance of Blood Gases

• Allow for assessment of patient's oxygenation, ventilation, and acid-base status
• BG, electrolytes, iCa²⁺, and lactate levels
• Can help in diagnosis, monitoring, and treatment of disease processes directly related to metabolic or respiratory dysfunction

4 Basic Types of Acid-Base Disturbances

• Metabolic acidosis
  • Primary gain in acid or loss of base
• Metabolic alkalosis
  • Primary gain in base or loss of acid
• Respiratory acidosis
  • Retention of CO₂ due to CO₂ production outpacing alveolar ventilation
• Respiratory alkalosis
  • Removal of CO₂ by ventilation which outpaces CO₂ production

Clarification of Terms

• PaO₂
  • Partial pressure of oxygen dissolved in arterial blood.
  • A measure of oxygenation not ventilation
• PaCO₂
  • Partial pressure of carbon dioxide dissolved in arterial blood.
  • Provided the best measurement of patients ability to ventilate
  • Determines whether resp. acidosis or resp. alkalosis is present.
• PCO₂
  • Partial pressure of carbon dioxide dissolved in venous blood.
  • When obtained properly it is a measure of the patients ability to ventilate.
• Base Excess/Deficit (BE)
  • Calculated value that estimates how much base needs to be added/subtracted to achieve a normal pH @ a normal temperature
  • Reflects the metabolic portion of acid balance
  • Evaluates for metabolic acidosis or alkalosis

Potential Sampling Errors

• Peripheral vein sampling in patients with poor perfusion
  • Sample may reflect acid-base status of limb and not of whole body
• Prolonged occlusion of sampled limb vein
  • Sample may reflect lactic acidosis specific only to that limb
• Sample not immediately evaluated or placed on ice
  • Continued cellular metabolism by RBCs will continue to use O₂ and produce CO₂
• Exposing sample to air
  • Oxygen from atmosphere will diffuse into sample and CO₂ will diffuse out altering PaO₂, PaCO₂, and pH
  • Thus causing error in calculated values (HCO₃ & BE)

The Four Primary Acid-Base Disorders and Their Compensatory Changes

• Conditions / Primary Disorder / Compensation
  • ↓pH and ↓HCO₃ (↓BEecf) / Metabolic acidosis/ ↓PCO₂
  • ↑pH and ↑HCO3 (↑BEecf) / Metabolic alkalosis/ ↑PCO₂
  • ↓pH and ↑Pco₂ / Respiratory acidosis/ ↑HCO₃ (↑BEecf)
  • ↑pH and ↓Pco₂ / Respiratory alkalosis/ ↓HCO₃ (↓BEecf)

Steps in Interpreting Blood Gas Results

• Venous or Arterial Sample?
• Is there an acidemia or alkalemia present?
  • Acidemia = decrease in blood pH
  • Alkalemia = increase in blood pH
• Determine Primary Disturbance
  • ROME
    • Respiratory Opposite
    • Metabolic Equal

Steps in Interpreting Blood Gas Results Continued

• Assess Oxygenation if arterial sample
  • F₁O₂ of room air is 21%
  • PaO₂ - should equal 5x F₁O₂
• Determine if compensatory changes have occurred
  • A change in the resp. or metabolic component of the acid-base status normally induces an opposite compensatory response in effort to normalize pH.
  • Absence or degree of compensation can provide some insight into chronicity of the disturbance.
  • Overcompensation does not occur.

Case Studies (Examples)

Example 1

• pH: 7.22
• PaCO₂: 65 mm Hg
• PaO₂: 45 mm Hg
• HCO₃⁻: 26 mEq/L
• BE(ecf): +4 mEq/L

Example 2

• pH: 7.48
• PaO₂: 63 mmHg
• PaCO₂: 25.9 mmHg
• HCO₃⁻: 18.8 mEq/L
• BE: -4.8

###Metabolic Acidosis

• Rise in H⁺ due to either an increase in production or a decrease in excretion
• Starvation
• DKA; Lactic acidosis; Depletion of HCO3- reserve either due to decreased resorption or excess loss
• Renal failure
• Diarrhea

###Metabolic Alkalosis

• Caused by a decrease or loss of metabolic acids or an increase in bicarbonate concentration not due to respiratory problems
• Loss of H⁺ (vomiting)
• Gain of HCO₃⁻ (abnormal retention of HCO₃⁻ by kidneys in response to dehydration due to vomiting/gastric retention of fluids)
• Excessive ingestion of antacids (increase in -HCO₃⁻)

This quiz explores the critical concepts surrounding acid-base balance in the human body. You will learn about the importance of pH levels, what defines acidemia and alkalemia, and the effects of pH changes on bodily functions, including the nervous system and overall health. Test your understanding of these essential physiological processes.