Chapter 44: Acid-Base Balance
I. Introduction
Acid-Base Balance: One of the most crucial concepts in physiology.
Regulation of the H+ ion concentration in body fluids.
Importance of Acid-Base Balance:
Must be controlled within a very narrow range for optimal functioning of enzymes and other significant body proteins.
A drop in pH can reduce enzyme effectiveness.
II. Meaning of the Term pH
Definition of pH:
Negative logarithm of the hydrogen ion concentration in one liter of solution:
pH = - ext{log}[H^+]
pH Scale:
pH of 0: Very acidic.
pH of 7: Neutral (equal concentrations of OH- and H+ ions).
pH of 14: Very alkaline.
All aqueous solutions contain both H+ and OH- ions.
Blood pH Range:
Arterial blood: 7.35 – 7.45
Venous blood is more acidic than arterial blood due to higher CO2 concentration.
III. Everyday Sources of Acids and Bases
A. Acid Sources:
Carbonic Acid: Generated from the aerobic metabolism of glucose.
Lactic Acid: Produced during anaerobic metabolism of glucose.
Sulfuric Acid: Resulting from the oxidation of sulfur-containing amino acids.
Phosphoric Acid: Derived from the breakdown of phosphoproteins and ribonucleotides.
Acidic Ketone Bodies: Accumulate due to incomplete fat breakdown.
B. Food Sources of Acids and Bases:
Acid-Forming Foods: High in protein.
Base-Forming Foods: High in fruits and vegetables.
IV. Mechanisms to Control Blood pH
A. Regulatory Mechanisms:
In a healthy individual, several mechanisms help maintain systemic arterial blood pH.
B. Quick Mechanism:
Temporarily binds H+ ions, raising the pH of body fluids without removing H+.
Buffer System Types:
Protein Buffer System: Most abundant buffer in intracellular fluid (ICF) and blood plasma.
Example:
H^+ + ext{Hb}^-
ightarrow ext{HbH} ext{ Acid + protein (hemoglobin) = temporary acid neutralizer}
Bicarbonate Buffer System: Key regulator of blood pH, most abundant in extracellular fluid (ECF).
Example:
H+ + ext{HCO}3^- ightleftharpoons ext{H}2 ext{CO}3 ightleftharpoons ext{H}2 ext{O} + ext{CO}_2Carbon dioxide is typically carried in the blood and exhaled via the lungs.
Take-home Messages:
ext{if} riangle H^+ = ext{increased} ext{ acid}
ightarrow ext{decreased} ext{ pH}ext{if} riangle CO_2 = ext{increased} ext{ acid}
ightarrow ext{decreased} ext{ pH}ext{if} riangle HCO_3^- = ext{decreased acid}
ightarrow ext{decreased} ext{ pH}
Phosphate Buffer System: Significant buffer in ICF and urine (kidneys) for both H+ and OH-.
Examples:
ext{OH}^- + ext{H}2 ext{PO}4^-
ightarrow ext{H}2 ext{O} + ext{HPO}4^{2-}H^+ + ext{HPO}4^{2-} ightarrow ext{H}2 ext{PO}_4^-
Take-home Messages:
ext{if} riangle H^+ = ext{increased} ext{ acid}
ightarrow ext{decreased} ext{ pH}ext{if} riangle OH^- = ext{decreased acid}
ightarrow ext{decreased} ext{ pH}$$
C. Kidneys:
Excrete H+ ions (a slow process).
Meant to eliminate a substantial load of metabolic acids.
This process occurs in the proximal and distal convoluted tubules.
Involves secretion of H+ and reabsorption of Na+.
D. Lungs:
Functionality: Exhalation of CO2.
Increase in CO2 concentration lowers the pH of body fluids, while a decrease raises pH.
Changes in breathing rate and depth can modify pH within minutes.
Hyperventilation: Raises blood pH towards 7.45-7.6 (alkalosis).
Hypoventilation: Lowers blood pH towards 7.35-7.0 (acidosis).
Negative feedback loops involving respiratory and cardiac centers help restore blood pH to normal levels.
The respiratory system is a powerful acid eliminator, but primarily manages carbonic acid.
V. Important Terms
A. Acidosis:
Condition where blood pH falls below 7.35.
Physiological Effect: Depression of central nervous system (CNS).
Symptoms include disorientation, potential coma, and may lead to death.
B. Alkalosis:
Condition where blood pH rises above 7.45.
Physiological Effect: Over-excitability of CNS and peripheral nerves.
Symptoms include nervousness, muscle spasms, convulsions, and may also lead to death.
C. Compensation:
Physiological responses to acid-base imbalance that seek to normalize arterial blood pH.
1. Respiratory Compensation:
Hyper/hypoventilation adjusts pH due to metabolic causes, which occurs in minutes and reaches maximum effect within hours.
2. Metabolic (Renal) Compensation:
Changes in the secretion of H+ and HCO3- in response to respiratory system imbalances, takes minutes to days to take effect.
VI. Acid/Base Disorders
a. Respiratory Acidosis:
Definition: Increased pCO2 (above 45 mmHg) with decreased pH (below 7.35).
Common Causes: Hypoventilation due to conditions like emphysema, pulmonary edema, trauma to the respiratory center, airway obstructions, or muscle dysfunction.
Renal Compensatory Mechanism.
Symptoms: Rapid, shallow respirations, dyspnea, disorientation, muscle weakness.
b. Respiratory Alkalosis:
Definition: Decreased pCO2 (below 35 mmHg) with increased pH (above 7.45).
Common Causes: Hyperventilation due to oxygen deficiency (e.g., high altitude or pulmonary disease), cerebrovascular accidents (CVA), or severe anxiety.
Renal Compensatory Mechanism.
Symptoms: Tingling in extremities, confusion, deep rapid breathing, potential seizures.
c. Metabolic Acidosis:
Definition: Decreased HCO3- (below 22 mEq/liter) with decreased pH (below 7.35).
Common Causes: Loss of bicarbonate due to diarrhea or renal dysfunction, acid accumulation (but not carbonic acid typical in ketosis), failure of kidneys to excrete H+ from dietary protein metabolism.
Respiratory Compensatory Mechanism.
Symptoms: Disorientation, Kussmaul breathing (deep, labored, gasping), altered level of consciousness (LOC).
d. Metabolic Alkalosis:
Definition: Increased HCO3- (above 26 mEq/liter) with increased pH (above 7.45).
Common Causes: Loss of acid from vomiting, gastric suctioning, use of specific diuretics, excessive intake of alkaline drugs (antacids), severe dehydration.
Respiratory Compensatory Mechanism.
Symptoms: Nausea, vomiting, diarrhea, restlessness, slow respiration, arrhythmias.
VII. Laboratory Evaluation
How to Determine Blood Condition:
Analyze Arterial Blood Gases (ABG’s).
Involves taking a small sample from an artery.
Measures pH, bicarbonate, PCO2, and PO2 values.
Results are provided via an analyzer.
VIII. How to Determine Disorder
Steps Involved:
Assess if the pH is high (alkalosis) or low (acidosis).
Identify which value (pCO2 or HCO3-) is outside the normal range and could be responsible for the pH change.
Example: Elevated pH could be a result of low pCO2 or high HCO3-.
Determine the cause:
If pCO2 changes, issue is respiratory.
If HCO3- changes, issue is metabolic.
Assess the value not corresponding with the pH change:
If within normal range, compensation is occurring and partially correcting the pH imbalance.
IX. Acid and Base-Forming Potential of Foods
See Box 44-1 on page 1018 for details.
X. Health Matters
Refer to Box 44-2 on page 1021 discussing metabolic alkalosis caused by vomiting.
Mechanism: Bicarbonate excess from massive chloride loss as hydrochloric acid from the stomach; compensatory increase in bicarbonate.
Therapy: IV administration of chloride-containing solutions (e.g., normal saline, 0.9% NaCl in water).
XI. Mechanisms for pH Control
Respiratory Mechanism: Visual representation in Figure 44-8 (page 1023).
Urinary Mechanism: Maintains blood pH homeostasis; see Figure 44-11 (page 1).
XII. Mechanisms of Disease
A. Metabolic Acidosis (Bicarbonate Deficit):
Lowered pH stimulates the respiratory center, leading to hyperventilation to expel carbon dioxide; this may become a significant clinical sign of acidosis.
B. Metabolic Alkalosis (Bicarbonate Excess):
Can stem from ingesting excess alkaline substances (e.g., baking soda) with suppressed breathing; kidneys compensate by excreting bicarbonate ions, potentially restoring pH.
C. Respiratory Acidosis (Carbonic Acid Excess):
May be related to pneumonia or emphysema with suppressed breathing; compensation increases bicarbonate fraction to normalize pH.
D. Respiratory Alkalosis (Carbonic Acid Deficit):
Often caused by hyperventilation due to fever or psychological conditions (hysteria); compensation may restore normal pH.