Acid/Base Physiology: Homeostatic Regulation of Body Fluid pH

Acid/Base Physiology

Introduction

The regulation of acid/base balance is essential for maintaining homeostasis in body fluids. The body employs multiple mechanisms to control pH, ensuring optimal conditions for cellular function and metabolic processes.

Chemistry of Acids, Bases, and Buffers

• Acid: A substance that releases hydrogen ions (H+) into solution. Acids increase the concentration of H+ and lower pH.

• Base: A substance that accepts hydrogen ions or releases hydroxide ions (OH-). Bases decrease the concentration of H+ and raise pH.

• pH: A measure of hydrogen ion concentration, defined as $pH = -\log_{10}[H^+]$.

• Chemical Buffer: A system of molecules that resists changes in pH by absorbing or releasing H+.

• Acidosis: A condition where blood pH falls below 7.35.

• Alkalosis: A condition where blood pH rises above 7.45.

Lines of Defense Against Changes in Arterial Blood pH

The body uses three main mechanisms to defend against pH changes, ranked by response time:

1. Chemical Buffers (Immediate response, seconds): Rapidly neutralize excess acids or bases.

2. Respiratory System (Minutes): Adjusts CO2 exhalation to influence blood pH.

3. Renal System (Hours to days): Modifies excretion or reabsorption of H+ and HCO3-.

Major Chemical Buffers in Body Fluids

• Bicarbonate Buffer System (Extracellular fluid, especially plasma)

• Phosphate Buffer System (Intracellular fluid and urine)

• Protein Buffer System (Both intracellular and plasma, e.g., hemoglobin in RBCs)

Normal Daily Inputs and Outputs of Acid

• Inputs: Metabolic production of acids (carbonic acid from CO2, lactic acid from exercise, dietary acids).

• Outputs: Removal via lungs (CO2 exhalation), kidneys (excretion of H+ and reabsorption of HCO3-).

Compensation Mechanisms for Acid/Base Disturbances

Chemical Buffers

• React immediately to neutralize excess H+ or OH-.

• Example: Bicarbonate buffer system reacts with H+ to form carbonic acid, which can be converted to CO2 and exhaled.

Respiratory System

• Alters rate and depth of breathing to regulate CO2 (and thus carbonic acid) levels.

• Increased ventilation removes CO2, reducing acidity; decreased ventilation retains CO2, increasing acidity.

• Equation: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

Renal System

• Excretes or reabsorbs H+ and HCO3- to adjust blood pH.

• Slowest but most powerful mechanism for long-term pH regulation.

• Example: Kidneys increase H+ excretion and HCO3- reabsorption during acidosis.

Respiratory and Metabolic Disturbances

Respiratory Disturbances

• Caused by changes in CO2 elimination (e.g., hypoventilation leads to respiratory acidosis; hyperventilation leads to respiratory alkalosis).

Metabolic Disturbances

• Result from changes in acid or base production or loss (e.g., excessive vomiting causes loss of gastric acid, leading to metabolic alkalosis; severe diarrhea causes loss of bicarbonate, leading to metabolic acidosis).

Predicting Responses to Specific Situations

• COPD: Chronic hypoventilation causes respiratory acidosis; kidneys compensate by increasing HCO3- reabsorption.

• High Protein Diet: Increases acid load; kidneys excrete more H+ and reabsorb more HCO3-.

• High Fat Diet: May increase production of ketone acids; similar renal compensation as above.

• Heavy Exercise: Produces lactic acid; respiratory rate increases to remove CO2, kidneys excrete H+.

• Excessive Vomiting: Loss of gastric acid leads to metabolic alkalosis; respiratory system compensates by hypoventilation, kidneys excrete HCO3-.

• Severe Diarrhea: Loss of bicarbonate leads to metabolic acidosis; respiratory system compensates by hyperventilation, kidneys increase HCO3- reabsorption.

Summary Table: Lines of Defense Against pH Changes

Defense Mechanism	Location	Response Time	Example
Chemical Buffers	Blood, cells, urine	Seconds	Bicarbonate buffer neutralizes excess H+
Respiratory System	Lungs	Minutes	Increased breathing removes CO2
Renal System	Kidneys	Hours to days	Excretion of H+, reabsorption of HCO3-

Additional info: The notes expand on brief points by providing definitions, examples, and context for each mechanism. The summary table is inferred from the learning objectives and standard physiology content.