Acid-Base Balance in Human Anatomy & Physiology

Acid-Base Balance

Introduction to Acid-Base Balance

Acid-base balance refers to the mechanisms the body uses to maintain the proper pH of its fluids, which is essential for normal cellular function and metabolic processes. The body's pH is tightly regulated, primarily through chemical buffers, respiratory activity, and renal function.

• pH Definition: pH is a measure of hydrogen ion concentration; lower pH indicates higher acidity, while higher pH indicates alkalinity.

• Normal Blood pH: The normal range for arterial blood pH is 7.35–7.45.

• Importance: Enzyme activity, oxygen transport, and cellular metabolism are all pH-dependent.

• Disorders: Acidosis (pH < 7.35) and alkalosis (pH > 7.45) can disrupt physiological functions.

pH Scale and Its Biological Relevance

The pH scale ranges from 0 (most acidic) to 14 (most alkaline), with 7 being neutral. Biological fluids have specific pH ranges necessary for proper function.

• Acids: Substances that release H+ ions in solution (e.g., HCl).

• Bases: Substances that accept H+ ions or release OH- (e.g., NaOH).

• Buffer Systems: Help resist changes in pH by neutralizing excess acids or bases.

Equation:

$\text{pH} = -\log_{10}[H^+]$

Major Buffer Systems in the Body

Buffers are mixtures of weak acids and their conjugate bases that minimize pH changes. The body uses several buffer systems to maintain acid-base homeostasis.

• Bicarbonate Buffer System: The primary buffer in extracellular fluid.

• Phosphate Buffer System: Important in intracellular fluid and urine.

• Protein Buffer System: Proteins, including hemoglobin, act as buffers in both plasma and cells.

Buffer System	Location	Key Components
Bicarbonate	Extracellular fluid	HCO3- / H2CO3
Phosphate	Intracellular fluid, urine	HPO42- / H2PO4-
Protein	Plasma, cells	Amino acid side chains

Bicarbonate Buffer System

The bicarbonate buffer system is the most important buffer for maintaining blood pH. It involves the equilibrium between carbonic acid (H2CO3) and bicarbonate ion (HCO3-).

• Reaction: $\mathrm{CO_2} + \mathrm{H_2O} \leftrightarrow \mathrm{H_2CO_3} \leftrightarrow \mathrm{HCO_3^-} + \mathrm{H^+}$

• Regulation: Lungs regulate CO2 (acid component); kidneys regulate HCO3- (base component).

Henderson-Hasselbalch Equation:

$\text{pH} = \text{p}K_a + \log \left( \frac{[\text{HCO}_3^-]}{0.03 \times \text{P}_{\text{CO}_2}} \right)$

Respiratory Regulation of Acid-Base Balance

The respiratory system helps control blood pH by regulating the amount of CO2 exhaled. CO2 is converted to carbonic acid in the blood, affecting pH.

• Hyperventilation: Decreases CO2, raises pH (respiratory alkalosis).

• Hypoventilation: Increases CO2, lowers pH (respiratory acidosis).

• Feedback Mechanisms: Chemoreceptors in the brainstem detect changes in blood pH and adjust breathing rate accordingly.

Renal Regulation of Acid-Base Balance

The kidneys maintain acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate from urine. This process is slower than respiratory regulation but more powerful for long-term pH control.

• H+ Secretion: Tubular cells secrete H+ into the filtrate.

• Bicarbonate Reabsorption: Bicarbonate is reabsorbed into the blood.

• Ammonium Buffering: Ammonia (NH3) combines with H+ to form ammonium (NH4+), which is excreted.

Disorders of Acid-Base Balance

Imbalances in acid-base homeostasis can lead to clinical conditions that require medical intervention.

Disorder	Cause	Compensation
Respiratory Acidosis	Hypoventilation, lung disease	Renal compensation: increased HCO3- reabsorption
Respiratory Alkalosis	Hyperventilation, anxiety	Renal compensation: decreased HCO3- reabsorption
Metabolic Acidosis	Diabetes, renal failure, diarrhea	Respiratory compensation: increased ventilation
Metabolic Alkalosis	Vomiting, diuretics	Respiratory compensation: decreased ventilation

Integration of Buffer, Respiratory, and Renal Systems

Acid-base homeostasis is achieved through the coordinated actions of chemical buffers, the respiratory system, and the renal system. Each system compensates for disturbances in the other to maintain pH within the narrow physiological range.

• Immediate Response: Chemical buffers act within seconds.

• Short-Term Response: Respiratory system acts within minutes.

• Long-Term Response: Renal system acts over hours to days.

Summary Table: Acid-Base Homeostasis Mechanisms

System	Speed	Mechanism	Example
Chemical Buffers	Seconds	Bind/release H+	Bicarbonate buffer
Respiratory	Minutes	Alter CO2 exhalation	Hyperventilation
Renal	Hours–days	Excrete H+, reabsorb HCO3-	Acidic urine

Clinical Application Example

Example: In diabetic ketoacidosis, excess ketone bodies lower blood pH (metabolic acidosis). The body compensates by increasing respiratory rate to expel CO2 and by renal excretion of H+.

Key Terms

• Acidosis: Condition of increased acidity (low pH).

• Alkalosis: Condition of increased alkalinity (high pH).

• Buffer: Substance that minimizes pH changes.

• Compensation: Physiological response to restore pH balance.

Additional info: The notes integrate content from Ch. 26 Fluids & Electrolytes and renal/respiratory physiology, as well as clinical applications relevant to acid-base disorders.