Respiratory Physiology: Mechanisms of Gas Exchange and Transport

Respiratory Processes and Anatomy

Overview of Respiration

Respiration involves the exchange of gases (O2 and CO2) between the atmosphere, blood, and tissues. It consists of several coordinated steps:

• Pulmonary Ventilation: Movement of air into and out of the lungs (inspiration and expiration).

• External Respiration: Exchange of O2 and CO2 between alveoli and blood.

• Transport of Respiratory Gases: Movement of O2 from lungs to tissues and CO2 from tissues to lungs via the blood.

• Internal Respiration: Exchange of gases between blood and tissue cells.

Respiratory Zone: Site of gas exchange; includes respiratory bronchioles, alveolar ducts, and alveoli.

Conducting Zone: Conducts air to the exchange region; includes nose, trachea, bronchi, and bronchioles.

Respiratory Mucosa and Alveolar Structure

• Respiratory Mucosa: Pseudostratified ciliated columnar epithelium with mucous-secreting goblet cells; traps and removes particles.

• Alveoli: Main site of gas exchange; each alveolus is lined by a single layer of squamous epithelium (type I alveolar cells).

• Type II Alveolar Cells: Secrete surfactant to reduce surface tension and prevent alveolar collapse.

• Alveolar Macrophages: Remove debris and pathogens from alveolar spaces.

Additional info: Surfactant is essential for maintaining alveolar stability, especially during exhalation.

Mechanics of Breathing

Pressure Relationships in the Thorax

Breathing is driven by pressure differences between the atmosphere and the lungs:

• Intrapulmonary Pressure: Pressure within alveoli; fluctuates with breathing.

• Intrapleural Pressure: Pressure within pleural cavity; always negative relative to intrapulmonary pressure (approx. -4 mm Hg).

• Transpulmonary Pressure: Difference between intrapulmonary and intrapleural pressures; keeps lungs inflated.

Inspiration and Expiration

• Inspiration: Diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow into the lungs.

• Expiration: Muscles relax, thoracic volume decreases, intrapulmonary pressure increases, and air flows out of the lungs.

Gas Exchange and Transport

Oxygen Transport

Oxygen is transported in the blood in two forms:

• Dissolved in Plasma: About 1.5% of O2 is carried this way.

• Bound to Hemoglobin (Hb): About 98.5% of O2 is transported bound to hemoglobin in red blood cells.

Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen.

Deoxyhemoglobin (HHb): Hemoglobin after releasing oxygen.

As O2 binds, Hb affinity for O2 increases; as O2 is released, affinity decreases.

Oxygen-Hemoglobin Dissociation Curve

• Shows the relationship between partial pressure of oxygen (PO2) and hemoglobin saturation.

• At high PO2 (lungs), Hb is almost fully saturated; at low PO2 (tissues), Hb releases O2.

• Factors shifting the curve: Temperature, pH, PCO2, and BPG (2,3-bisphosphoglycerate).

Equation:

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

Carbon Dioxide Transport

CO2 is transported in the blood in three forms:

• Dissolved in Plasma: 7-10% of CO2.

• Bound to Hemoglobin (Carbaminohemoglobin): 20% of CO2 binds to globin portion of Hb.

• Bicarbonate Ions (HCO3-): 70% of CO2 is transported as bicarbonate ions.

Equation:

$\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{carbonic anhydrase}} \text{H}_2\text{CO}_3 \rightarrow \text{HCO}_3^- + \text{H}^+$

In tissues, CO2 diffuses into RBCs, is converted to HCO3-, and transported in plasma. In lungs, the process reverses, and CO2 is exhaled.

Regulation of Respiration

Neural Control

• Medullary Respiratory Centers: Control the basic rhythm of breathing.

• Chemoreceptors: Detect changes in blood levels of CO2, O2, and pH; influence respiratory rate and depth.

Influence of CO2, O2, and pH

• Increased CO2 (hypercapnia) or decreased pH stimulates increased ventilation.

• Decreased O2 (hypoxemia) stimulates ventilation, but is a weaker stimulus than CO2.

Respiratory Pathologies

Chronic Obstructive Pulmonary Disease (COPD)

COPD is a group of lung diseases characterized by chronic airflow obstruction:

• Emphysema: Destruction of alveolar walls, loss of elasticity, and impaired gas exchange.

• Chronic Bronchitis: Inflammation of bronchi, increased mucus production, and frequent infections.

Major risk factor: Smoking. COPD leads to progressive respiratory failure and is a leading cause of death.

Additional info: Cilia paralysis in COPD impairs removal of debris and pathogens, increasing infection risk.

Summary Table: Forms of Gas Transport in Blood

Gas	Transport Form	Percentage	Key Features
Oxygen (O2)	Dissolved in plasma	1.5%	Low solubility; minor transport form
Oxygen (O2)	Bound to hemoglobin	98.5%	Major transport form; forms oxyhemoglobin
Carbon dioxide (CO2)	Dissolved in plasma	7-10%	Directly soluble; minor transport form
Carbon dioxide (CO2)	Bound to hemoglobin (carbaminohemoglobin)	20%	Binds to globin portion of Hb
Carbon dioxide (CO2)	Bicarbonate ions (HCO3-)	70%	Formed via carbonic anhydrase in RBCs