Acid-Base Physiology: Homeostatic Regulation of Body Fluid pH

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Acid-Base Physiology

Introduction to Acid-Base Balance

The maintenance of acid-base balance is essential for normal cellular function and overall homeostasis. The body employs several mechanisms to regulate the concentration of hydrogen ions (H+) in body fluids, thereby stabilizing the pH within a narrow, optimal range.

Chemistry of Acids, Bases, and Buffers

Acid: A substance that donates hydrogen ions (H+) to a solution. Example: Hydrochloric acid (HCl).
Base: A substance that accepts hydrogen ions or donates hydroxide ions (OH-). Example: Sodium hydroxide (NaOH).
pH: A measure of hydrogen ion concentration, defined as . The normal arterial blood pH is 7.35–7.45.
Chemical Buffer: A system of molecules that minimizes changes in pH by reversibly binding or releasing H+. Buffers act quickly to stabilize pH.
Acidosis: A condition in which blood pH falls below 7.35.
Alkalosis: A condition in which blood pH rises above 7.45.

Lines of Defense Against pH Changes

The body utilizes three primary mechanisms to defend against changes in arterial blood pH, ranked by response time:

Chemical Buffers (seconds): Immediate response to pH changes.
Respiratory System (minutes): Adjusts CO2 exhalation to influence blood pH.
Renal System (hours to days): Modifies excretion or reabsorption of H+ and HCO3-.

Major Chemical Buffers in Body Fluids

Buffers are distributed throughout body fluids:

Buffer System	Location	Main Components
Bicarbonate Buffer	Extracellular fluid (plasma)	HCO3- / H2CO3
Phosphate Buffer	Intracellular fluid, renal tubules	H2PO4- / HPO42-
Protein Buffer	Intracellular fluid, plasma	Hemoglobin, plasma proteins

Daily Acid Inputs and Outputs

Inputs: Metabolic production of acids (e.g., CO2 from cellular respiration, lactic acid from exercise, dietary acids).
Outputs: Removal of CO2 via lungs, excretion of H+ and reabsorption of HCO3- by kidneys.

Chemical Buffer Compensation

Buffers bind excess H+ when pH drops (acidosis) or release H+ when pH rises (alkalosis).
Example: The bicarbonate buffer system operates according to the equation:
Buffers provide immediate, but limited, compensation for pH disturbances.

Respiratory Compensation

The respiratory system regulates blood pH by altering the rate and depth of breathing.
Increased ventilation removes more CO2, reducing H+ and raising pH (compensating for acidosis).
Decreased ventilation retains CO2, increasing H+ and lowering pH (compensating for alkalosis).
Respiratory compensation acts within minutes.

Renal Compensation

The kidneys regulate acid-base balance by excreting or reabsorbing H+ and HCO3-.
In acidosis, kidneys excrete more H+ and reabsorb more HCO3-.
In alkalosis, kidneys excrete more HCO3- and reabsorb more H+.
Renal compensation is slower (hours to days) but provides long-term regulation.

Respiratory Disturbances and Acid-Base Balance

Respiratory Acidosis: Caused by hypoventilation (e.g., COPD), leading to CO2 retention and decreased pH.
Respiratory Alkalosis: Caused by hyperventilation, leading to excessive CO2 loss and increased pH.

Metabolic Disturbances and Acid-Base Balance

Metabolic Acidosis: Results from increased acid production (e.g., lactic acidosis, ketoacidosis) or loss of HCO3- (e.g., diarrhea).
Metabolic Alkalosis: Results from excessive loss of acids (e.g., vomiting) or increased HCO3- intake.

Predicting Responses to Specific Situations

Situation	Primary Disturbance	Respiratory Response	Renal Response
COPD	Respiratory acidosis (CO2 retention)	Limited (chronic hypoventilation)	Increase HCO3- reabsorption, excrete H+
High protein diet	Metabolic acidosis (increased acid load)	Increase ventilation	Increase H+ excretion, reabsorb HCO3-
High fat diet	Metabolic acidosis (ketone production)	Increase ventilation	Increase H+ excretion, reabsorb HCO3-
Heavy exercise	Metabolic acidosis (lactic acid)	Increase ventilation	Increase H+ excretion, reabsorb HCO3-
Excessive vomiting	Metabolic alkalosis (loss of gastric acid)	Decrease ventilation	Excrete HCO3-, reabsorb H+
Severe diarrhea	Metabolic acidosis (loss of HCO3-)	Increase ventilation	Increase H+ excretion, reabsorb HCO3-

Summary

Acid-base homeostasis is maintained by chemical buffers, the respiratory system, and the renal system.
Each system compensates for disturbances in blood pH, with varying speed and capacity.
Understanding the mechanisms and responses to acid-base disturbances is essential for clinical assessment and intervention.