Respiratory Physiology: Gas Exchange, Transport, and Regulation

Gas Exchange Regulation in the Lungs

Ventilation-Perfusion Coupling

Ventilation-perfusion coupling refers to the physiological mechanisms that match the amount of air reaching the alveoli (ventilation) with the blood flow in pulmonary capillaries (perfusion). This ensures efficient gas exchange.

• Partial Pressure of Oxygen (O2): Controls perfusion by altering the diameter of pulmonary arterioles.

• Partial Pressure of Carbon Dioxide (CO2): Controls ventilation by altering the diameter of bronchioles.

• High alveolar O2: Arterioles dilate, increasing blood flow to well-ventilated alveoli.

• Low alveolar O2: Arterioles constrict, diverting blood away from poorly ventilated alveoli.

• High alveolar CO2: Bronchioles dilate, allowing rapid elimination of CO2.

• Low alveolar CO2: Bronchioles constrict.

• Systemic vs. Pulmonary Arterioles: In systemic circulation, arterioles dilate when O2 is low and constrict when high (opposite of pulmonary arterioles).

Additional info: This synchronization of ventilation and perfusion maximizes gas exchange efficiency.

Partial Pressures and Diffusion Gradients

External vs. Internal Respiration

Gas exchange occurs due to differences in partial pressures between alveoli and blood (external respiration) and between blood and tissues (internal respiration).

• External Respiration: O2 diffuses from alveoli (high pO2) to blood (low pO2); CO2 diffuses from blood (high pCO2) to alveoli (low pCO2).

• Internal Respiration: O2 diffuses from blood (high pO2) to tissues (low pO2); CO2 diffuses from tissues (high pCO2) to blood (low pCO2).

• Diffusion Gradients: The direction of O2 and CO2 movement is reversed in external vs. internal respiration.

Oxygen Transport in Blood

Hemoglobin Saturation and Influencing Factors

Oxygen is primarily transported bound to hemoglobin in red blood cells. The degree of hemoglobin saturation is influenced by several factors.

• Partial Pressure of Oxygen (pO2): Higher pO2 increases hemoglobin saturation.

• Temperature: Increased temperature decreases hemoglobin's affinity for O2 (curve shifts right).

• Blood pH: Lower pH (more acidic) decreases affinity (curve shifts right).

• Partial Pressure of CO2: Higher pCO2 decreases affinity (curve shifts right).

• Concentration of BPG (2,3-bisphosphoglycerate): Increased BPG decreases affinity (curve shifts right).

Oxygen-Hemoglobin Dissociation Curve:

• Right shift: Decreased affinity, more O2 released to tissues.

• Left shift: Increased affinity, less O2 released.

Equation:

$\text{Hb} + \text{O}_2 \rightleftharpoons \text{HbO}_2$

The Bohr Effect

The Bohr effect describes how increased CO2 and decreased pH reduce hemoglobin's affinity for oxygen, facilitating oxygen release in metabolically active tissues.

• Cause: Increased CO2 and H+ ions (lower pH).

• Effect: Oxygen is released more readily from hemoglobin.

Carbon Dioxide Transport in Blood

Mechanisms of CO2 Transport

Carbon dioxide is transported in the blood in three main forms:

• Dissolved in plasma: ~7% of CO2.

• Bound to hemoglobin (carbaminohemoglobin): ~23%.

• As bicarbonate ions (HCO3-): ~70%.

Equation:

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

The Chloride Shift

The chloride shift maintains electrical neutrality as bicarbonate ions move out of red blood cells into plasma.

• Systemic Capillaries: HCO3- exits RBCs, Cl- enters.

• Pulmonary Capillaries: HCO3- enters RBCs, Cl- exits.

The Haldane Effect

The Haldane effect describes how deoxygenated hemoglobin increases the capacity of blood to carry CO2.

• Deoxygenated blood: Hemoglobin binds CO2 more readily.

• Oxygenated blood: Hemoglobin releases CO2.

Carbon Dioxide and Blood pH

Carbonic Acid-Bicarbonate Buffer System

This buffer system helps maintain blood pH by reversible conversion of CO2 and water to carbonic acid and bicarbonate.

• Increased CO2: More H+ produced, lowering pH (acidosis).

• Decreased CO2: Less H+ produced, raising pH (alkalosis).

Equation:

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

Neural Control of Respiration

Brain Regions Involved

Respiratory rhythm and rate are controlled by centers in the brainstem.

• Medulla oblongata: Sets basic rhythm of breathing (ventral and dorsal respiratory groups).

• Pons: Modifies and smooths respiratory rhythm (pontine respiratory group).

Factors Influencing Breathing Rate and Depth

Chemical and Neural Regulation

Breathing is regulated by chemical factors, higher brain centers, and reflexes.

• Chemical Factors: Monitored by chemoreceptors (central in medulla, peripheral in carotid and aortic bodies).

• Most Closely Controlled: CO2 levels are most tightly regulated because small changes can significantly affect pH.

• Higher Brain Centers: Voluntary control (cerebral cortex), emotional responses (limbic system, hypothalamus).

• Pulmonary Irritant Reflexes: Respond to irritants (e.g., dust, smoke) by triggering coughing or sneezing.

• Inflation Reflex (Hering-Breuer Reflex): Prevents over-inflation of lungs by inhibiting inspiratory neurons when stretch receptors are activated.

Summary Table: Factors Affecting Hemoglobin Saturation

Factor	Effect on O2 Affinity	Curve Shift
Increased pO2	Increases affinity	Left
Increased Temperature	Decreases affinity	Right
Decreased pH	Decreases affinity	Right
Increased pCO2	Decreases affinity	Right
Increased BPG	Decreases affinity	Right

Additional info: The Hering-Breuer Reflex is a protective mechanism that prevents lung overexpansion during deep inspiration.