Acid Base Balance

Acid-Base Balance

Introduction

• Acid-base balance refers to the precise regulation of free hydrogen-ion concentration ([H^+]) in body fluids.

Acids and Bases

• Acids: Hydrogen-containing substances that dissociate in solution to liberate free H^+ and anions.
• Carbohydrates contain hydrogen but are not acids because hydrogen is tightly bound within their molecular structure.
• Strong Acid: Greater tendency to dissociate in solution, yielding a higher percentage of free H^+ and anions.
  • Example: Hydrochloric acid (HCl) dissociates completely into H^+ and Cl^- in water.
    HCl \rightarrow H^+ + Cl^-
• Weak Acid: Only a portion of molecules dissociate in solution.
  • Example: Carbonic acid (H2CO3) dissociates into H^+ and HCO3^-. H2CO3 \rightleftharpoons H^+ + HCO3^-
• Only free hydrogen ions contribute to the acidity of a solution.
• H2CO3 is weaker than HCl because it yields fewer free hydrogen ions per molecule in solution.
• Base: A substance that can combine with a free H^+ and remove it from solution.
• A strong base binds H^+ more readily than a weak base.

pH Concept

• pH expresses [H^+] more conveniently.
• pH = log \frac{1}{[H^+]} or pH = -log[H^+]
• pH of pure water is 7.0, considered chemically neutral.
• Solutions with pH < 7.0 are acidic; [H^+] is higher than in pure water.
• Solutions with pH > 7.0 are basic or alkaline; [H^+] is lower than in pure water.

Blood pH

• Arterial blood pH is normally 7.45, and venous blood pH is 7.35, with an average of 7.4.
• Acidosis: Blood pH falls below 7.35.
• Alkalosis: Blood pH is above 7.45.
• The reference point for acid-base status is the normal plasma pH of 7.4, not the chemically neutral pH of 7.0.
• Arterial pH outside the range of 6.8 to 8.0 is not compatible with life.
• A narrow pH range is essential for normal cell function; even small [H^+] changes have dramatic effects.

Consequences of [H^+] Fluctuations

1. Changes in excitability of nerve and muscle cells.
2. Marked influence on enzyme activity.
3. Changes in [H^+] influence K^+ levels in the body; renal tubular cells secrete either K^+ or H^+ in exchange for Na^+ reabsorption.

H+ Input and Output

• Maintaining constant [H^+] requires balanced input and output.
• Small amount of acid is ingested with food (e.g., citric acid in oranges).
• Most H^+ is generated internally from metabolic activities.

Sources of H+ in the Body

1. Carbonic Acid Formation: Major source via H2CO3 formation from metabolically produced CO_2.
2. Breakdown of Nutrients: Dietary proteins (meat) contain sulfur and phosphorus, producing sulfuric acid and phosphoric acid as by-products.
3. Intermediary Metabolism: Lactic acid is produced by muscles during heavy exercise and partially dissociates to yield free H^+.

Maintaining Alkalinity

• Key to H^+ balance: maintaining normal ECF alkalinity (pH 7.4).
• Generated free H^+ must be removed and eliminated to maintain pH within a narrow range.

Three Lines of Defense Against Changes in [H^+]

1. Chemical Buffer Systems
2. Respiratory Mechanism of pH Control
3. Renal Mechanism of pH Control

Chemical Buffer Systems

• Mixtures of chemical compounds that minimize pH changes.
• Consist of a pair of substances involved in a reversible reaction.
  • One yields free H^+ as [H^+] falls.
  • The other binds with free H^+ when [H^+] rises.

Four Buffer Systems in the Body

1. H2CO3 : HCO_3^- buffer system
2. Protein buffer system
3. Hemoglobin buffer system
4. Phosphate buffer system

1. H2CO3 : HCO_3^- Buffer System

• Most important in ECF for buffering pH changes not caused by CO2-generated H2CO_3.
• Kidneys regulate HCO3^-, and the respiratory system regulates CO2, which generates H2CO3.

2. Protein Buffer System

• Proteins are excellent buffers due to acidic and basic groups that can give up or take up H^+.
• Most important in buffering [H^+] changes in the ICF due to abundant intracellular proteins.
• Plasma proteins reinforce the H2CO3 : HCO_3^- system in extracellular buffering.

3. Hemoglobin Buffer System

• Hemoglobin (Hb) buffers H^+ generated from metabolically produced CO_2 between tissues and lungs.
• Most H^+ generated from CO_2 at the tissue level binds to reduced Hb.
• Venous blood is only slightly more acidic than arterial blood due to Hb's buffering capacity.

4. Phosphate Buffer System

• Consists of an acid phosphate salt that can donate a free H^+ when [H^+] falls and a basic phosphate salt that can accept a free H^+ when [H^+] rises.
• Significant contributor to intracellular buffering.

Major Functions of Buffer Systems

• Buffer systems are the first line of defense.
• Chemical buffer systems do not eliminate H^+ from the body; they merely remove it from solution by incorporating it within one member of the buffer pair.
• If H^+ were not eventually eliminated, all body fluid buffers would become bound with H^+, eliminating further buffering ability.
• Respiratory and renal mechanisms eliminate acid from the body but respond more slowly than chemical buffer systems.

Respiratory Mechanism of pH Control

• Alters pulmonary ventilation to alter excretion of H^+-generating CO_2.
• Respiratory activity is partly governed by arterial [H^+].
• Increased arterial [H^+] (nonrespiratory/metabolic cause) stimulates the respiratory center to increase pulmonary ventilation.
• Conversely, decreased arterial [H^+] reduces pulmonary ventilation.
• The respiratory system serves as the second line of defense against changes in [H^+].
• It comes into action a few minutes later if the buffer systems do not swiftly and completely correct a deviation in [H^+].

Respiratory Adjustments to Acidosis and Alkalosis Induced by Nonrespiratory Causes

• Impaired lungs cannot compensate for acidosis caused by CO2 accumulation from lung disease by increasing CO2 removal.
• Buffer systems (other than H2CO3 : HCO_3^- pair) plus renal regulation are the only mechanisms available for defending against respiratory-induced acid-base abnormalities.

Renal Mechanism of pH Control

• Kidneys control pH of body fluids by adjusting:
  1. H^+ excretion
  2. HCO_3^- excretion
  3. Ammonia (NH_3) secretion
• Kidneys take hours to days to compensate for changes in body fluid pH, compared to buffer systems (immediate) and respiratory system (few minutes delay).
• The kidneys are the third line of defense against [H^+] changes.
• Kidneys are the most potent acid-base regulatory mechanism; they can vary removal of H^+ from any source and conserve or eliminate HCO_3^- based on the body's acid-base status.

Intercalated Cells

• Type A intercalated cells: H^+-secreting, HCO_3^--reabsorbing, K^+-reabsorbing cells.
• Type B intercalated cells: HCO_3^--secreting, H^+-reabsorbing, K^+-secreting cells (opposite of Type A cells).
• Type A cells are more active normally and increase activity during acidosis.
• Type B cells become more active during alkalosis.

Renal Regulation of Plasma [HCO_3^-]

1. Variable reabsorption of filtered HCO_3^- back into the plasma in conjunction with H^+ secretion.
2. Variable addition of new HCO_3^- to the plasma in conjunction with H^+ secretion.
3. Variable secretion of HCO_3^- in conjunction with H^+ reabsorption.

Renal Responses to Acidosis and Alkalosis

Causes of Acid-Base Imbalances

Respiratory Acidosis

• Lung disease
• Depression of the respiratory center by drugs or disease
• Nerve or muscle disorders that reduce respiratory muscle ability
• Holding one's breath.

Respiratory Alkalosis

• Fever
• Anxiety
• Aspirin poisoning
• Physiological mechanisms at high altitude.

Metabolic Acidosis

• Severe diarrhea
• Diabetes mellitus
• Strenuous exercise
• Uremic acidosis

Metabolic Alkalosis

1. Vomiting
2. Ingestion of alkaline drugs