Physiological Acid Base Balance

Physiological Acid Base Balance: Physiological acid–base balance refers to the mechanisms by which the body maintains the concentration of hydrogen ions (H⁺) within a very narrow range to ensure normal cellular and metabolic functions. The balance between acids and bases is essential for maintaining the pH of body fluids, particularly blood. Even slight deviations in blood pH can significantly affect enzyme activity, cellular metabolism, oxygen transport, electrolyte balance, and organ function. Pharmaacademias.com

The normal pH of arterial blood is maintained between 7.35 and 7.45, which is slightly alkaline. A pH below 7.35 is termed acidosis, while a pH above 7.45 is termed alkalosis. The body continuously produces acids through metabolic processes; therefore, sophisticated regulatory mechanisms are required to maintain acid–base homeostasis.

Physiological Acid Base Balance

Concept of Acids, Bases, and pH

Acid: An acid is a substance that releases hydrogen ions (H⁺) in solution.

Examples:

• Hydrochloric acid (HCl)
• Carbonic acid (H₂CO₃)
• Lactic acid

Base: A base is a substance that accepts hydrogen ions or releases hydroxyl ions (OH⁻).

Examples:

• Bicarbonate (HCO₃⁻)
• Ammonia (NH₃)

pH: pH is a measure of hydrogen ion concentration and is expressed as:

pH = −log[H⁺]

A lower pH indicates increased acidity, while a higher pH indicates increased alkalinity.

Importance of Acid–Base Balance

Maintaining a stable pH is critical because:

Enzyme Activity: Most enzymes function optimally within a narrow pH range. Changes in pH can alter enzyme structure and reduce activity.

Cellular Metabolism: Metabolic reactions depend on normal hydrogen ion concentrations.

Oxygen Transport: Hemoglobin’s affinity for oxygen is influenced by blood pH.

Electrolyte Balance: Acid-base disturbances affect the distribution of potassium, calcium, and other electrolytes.

Neuromuscular Function: Abnormal pH can alter nerve excitability and muscle contraction.

Cardiovascular Stability: Severe acid-base imbalance may cause arrhythmias and impaired cardiac function.

Sources of Acids in the Body

The body continuously produces acids through metabolic processes.

1. Volatile Acids

Carbonic Acid (H₂CO₃)

Produced from carbon dioxide generated during cellular respiration.

CO₂​ + H₂​O ↔ H₂​CO₃​

Carbonic acid is considered a volatile acid because carbon dioxide can be excreted through the lungs.

2. Non-Volatile (Fixed) Acids

These acids cannot be removed by the lungs and must be excreted by the kidneys.

Examples

• Sulfuric acid (from protein metabolism)
• Phosphoric acid
• Uric acid
• Lactic acid
• Ketoacids

Mechanisms Maintaining Acid–Base Balance

The body maintains acid-base homeostasis through three major defense mechanisms:

1. Buffer systems
2. Respiratory regulation
3. Renal regulation

1. Buffer Systems

Buffers act immediately and constitute the first line of defense against pH changes.

A buffer is a substance that resists changes in pH when small amounts of acid or base are added.

A. Bicarbonate Buffer System

Most Important Extracellular Buffer

The bicarbonate buffer system consists of:

• Carbonic acid (H₂CO₃)
• Sodium bicarbonate (NaHCO₃)

Reaction

H⁺ + HCO₃⁻ ​↔ H₂​CO₃ ​↔ CO₂ ​+ H₂​O

Function

• Neutralizes excess acids and bases.
• Maintains blood pH near 7.4.
• Works closely with respiratory and renal systems.

B. Phosphate Buffer System

Consists of:

• Dihydrogen phosphate (H₂PO₄⁻)
• Monohydrogen phosphate (HPO₄²⁻)

Function

• Important intracellular buffer.
• Plays a major role in renal tubular fluid.

Reaction

H⁺ + HPO₄²⁻ ​↔ H₂​PO₄^−​

C. Protein Buffer System

Proteins contain amino and carboxyl groups that can accept or donate hydrogen ions.

Hemoglobin Buffer

Hemoglobin is the most important intracellular protein buffer.

Functions

• Buffers hydrogen ions in red blood cells.
• Facilitates carbon dioxide transport.

2. Respiratory Regulation

The lungs provide the second line of defense and respond within minutes.

Mechanism

Carbon dioxide is continuously produced during metabolism.

CO₂​ + H₂​O ↔ H_2​CO₃ ​↔ H⁺ + HCO₃^−​

Changes in ventilation alter carbon dioxide concentration and consequently blood pH.

Response to Acidosis

Increased hydrogen ion concentration stimulates respiration.

Result

• Increased breathing rate (hyperventilation)
• Increased CO₂ elimination
• Reduced carbonic acid concentration
• Rise in blood pH

Response to Alkalosis

Reduced hydrogen ion concentration decreases respiratory activity.

Result

• Hypoventilation
• CO₂ retention
• Increased carbonic acid formation
• Lowering of blood pH

3. Renal Regulation

The kidneys provide the most powerful but slowest mechanism for acid-base control.

Renal compensation occurs over hours to days.

Functions of Kidneys in Acid–Base Balance

Reabsorption of Bicarbonate: Almost all filtered bicarbonate is reabsorbed to prevent loss of buffering capacity.

Excretion of Hydrogen Ions: Kidneys actively secrete hydrogen ions into urine.

Formation of New Bicarbonate: New bicarbonate ions are generated and returned to circulation.

Ammonia Buffer System

Ammonia combines with hydrogen ions to form ammonium ions that are excreted in urine.

NH₃​ + H⁺ → NH₄⁺​

Acid–Base Disorders

Disturbances in acid-base balance are classified as respiratory or metabolic.

Acidosis

Blood pH less than 7.35.

A. Respiratory Acidosis

Caused by carbon dioxide retention.

Causes

• Chronic obstructive pulmonary disease (COPD)
• Asthma
• Respiratory depression
• Airway obstruction

Effects

• Increased carbonic acid
• Decreased blood pH

B. Metabolic Acidosis

Caused by excess acid production or bicarbonate loss.

Causes

• Diabetic ketoacidosis
• Severe diarrhea
• Renal failure
• Lactic acidosis

Effects

• Decreased bicarbonate concentration
• Reduced blood pH

Alkalosis

Blood pH greater than 7.45.

A. Respiratory Alkalosis

Caused by excessive carbon dioxide loss.

Causes

• Hyperventilation
• Anxiety
• High altitude exposure

Effects

• Reduced carbonic acid concentration
• Increased blood pH

B. Metabolic Alkalosis

Caused by excess bicarbonate or hydrogen ion loss.

Causes

• Persistent vomiting
• Excessive antacid use
• Diuretic therapy

Effects

• Increased bicarbonate concentration
• Elevated blood pH

Normal Arterial Blood Values

Parameter	Normal Range
pH	7.35–7.45
PaCO₂	35–45 mmHg
HCO₃⁻	22–26 mEq/L
Hydrogen ion concentration	35–45 nmol/L

Clinical Significance

Assessment of acid-base status is essential in diagnosing and managing:

• Respiratory diseases
• Kidney disorders
• Diabetic ketoacidosis
• Shock and sepsis
• Electrolyte imbalances
• Poisoning and drug overdose

Arterial blood gas (ABG) analysis is commonly used to evaluate acid-base disturbances.

Conclusion

Physiological acid–base balance is the process by which the body maintains blood pH within the narrow range of 7.35–7.45 despite continuous production of acids during metabolism. This balance is achieved through the coordinated actions of buffer systems, the respiratory system, and the kidneys. The bicarbonate, phosphate, and protein buffer systems provide immediate protection against pH changes, while the lungs regulate carbon dioxide levels and the kidneys control hydrogen ion excretion and bicarbonate conservation. Proper acid-base balance is essential for enzyme activity, cellular metabolism, nerve function, muscle contraction, and overall physiological homeostasis.