Respiratory Physiology: Mechanics, Volumes, and Gas Exchange

The Respiratory System: Physiology

Mechanics of Breathing

The process of pulmonary ventilation involves the movement of air into and out of the lungs, consisting of two phases: inspiration and expiration. These phases are governed by pressure changes within the thoracic cavity.

• Inspiration: Gases flow into the lungs as thoracic volume increases.

• Expiration: Gases exit the lungs as thoracic volume decreases.

Pressure Relationships in the Thoracic Cavity

• Atmospheric Pressure (Patm): The pressure exerted by air surrounding the body, typically 760 mm Hg at sea level.

• Intrapulmonary Pressure (Ppul): Pressure within the alveoli, fluctuates with breathing, and eventually equalizes with Patm.

• Intrapleural Pressure (Pip): Pressure within the pleural cavity, always negative relative to Patm and Ppul, usually 4 mm Hg less than Ppul. Maintained by the adhesive force between parietal and visceral pleurae.

• Transpulmonary Pressure: The difference between Ppul and Pip ($P_{tp} = P_{pul} - P_{ip}$), keeps lung spaces open and prevents collapse.

Pneumothorax and Atelectasis

• Pneumothorax: Occurs when air enters the pleural cavity, causing Pip to equalize with Patm or Ppul, resulting in lung collapse.

• Atelectasis: Collapse of lung tissue due to plugged bronchioles or pneumothorax. Treatment involves removing air with chest tubes.

Pulmonary Ventilation: Volume and Pressure Changes

Pulmonary ventilation is a mechanical process dependent on volume changes in the thoracic cavity, which lead to pressure changes and subsequent airflow.

• Boyle’s Law: The relationship between pressure and volume of a gas is inversely proportional: $P_1V_1 = P_2V_2$.

Inspiration

• Active Process: Involves contraction of the diaphragm and external intercostals, increasing thoracic volume.

• Accessory Muscles: During forced inspiration, muscles such as scalenes, sternocleidomastoid, and pectoralis minor are activated.

• Pressure Gradient: As thoracic volume increases, intrapulmonary pressure drops below atmospheric pressure, causing air to flow into the lungs.

Expiration

• Passive Process: Inspiratory muscles relax, thoracic volume decreases, and lungs recoil, increasing intrapulmonary pressure above atmospheric pressure, causing air to flow out.

• Forced Expiration: An active process involving abdominal and internal intercostal muscles.

Physical Factors Influencing Pulmonary Ventilation

Three main physical factors affect the ease of air passage and the energy required for ventilation:

• Airway Resistance: Friction in airways is the major nonelastic source of resistance. Resistance is usually insignificant due to large airway diameters and extensive branching. Resistance is highest in medium-sized bronchi and disappears at terminal bronchioles.

• Alveolar Surface Tension: The attraction of liquid molecules at the gas-liquid interface tends to collapse alveoli. Surfactant, produced by type II alveolar cells, reduces surface tension and prevents collapse.

• Lung Compliance: The measure of lung stretchability, normally high due to distensible tissue and surfactant. Compliance can be diminished by fibrosis, reduced surfactant, or decreased thoracic flexibility.

Assessing Ventilation: Respiratory Volumes and Capacities

Respiratory volumes and capacities are used to assess respiratory status and are measured using spirometry.

• Tidal Volume (TV): Amount of air moved in and out with each breath (~500 ml).

• Inspiratory Reserve Volume (IRV): Air inspired forcibly beyond TV (2100–3200 ml).

• Expiratory Reserve Volume (ERV): Air expelled forcibly beyond TV (1000–1200 ml).

• Residual Volume (RV): Air remaining in lungs after forced expiration, keeps alveoli open.

• Inspiratory Capacity (IC): TV + IRV

• Functional Residual Capacity (FRC): RV + ERV

• Vital Capacity (VC): TV + IRV + ERV

• Total Lung Capacity (TLC): TV + IRV + ERV + RV

Dead Space

• Anatomical Dead Space: Air in passageways not involved in gas exchange (~150 ml).

• Alveolar Dead Space: Nonfunctional alveoli due to collapse or obstruction.

• Total Dead Space: Sum of anatomical and alveolar dead space.

Pulmonary Function Tests

• Obstructive Pulmonary Disease: Increased airway resistance (e.g., bronchitis), TLC, FRC, RV may increase.

• Restrictive Disease: Reduced TLC due to disease or environmental exposure (e.g., fibrosis), VC, TLC, FRC, RV decline.

• Forced Vital Capacity (FVC): Gas expelled after deep breath.

• Forced Expiratory Volume (FEV): Gas expelled during specific interval of FVC.

Alveolar Ventilation

• Minute Ventilation: Total gas flow in/out of respiratory tract per minute (~6 L/min at rest).

• Alveolar Ventilation Rate (AVR): Flow of gases into/out of alveoli per time, calculated as: $AVR = (TV - Dead\ Space) \times Frequency$

• Increasing TV is more effective for increasing AVR than increasing frequency.

Gas Exchange: External and Internal Respiration

Gas exchange occurs between lungs and blood (external respiration) and between blood and tissues (internal respiration), governed by partial pressure gradients and gas solubilities.

Basic Properties of Gases

• Dalton’s Law: Total pressure of a gas mixture equals the sum of partial pressures of individual gases.

• Henry’s Law: Each gas dissolves in liquid in proportion to its partial pressure and solubility.

External Respiration

• Partial Pressure Gradients: Steep gradient for O2 (drives oxygen into blood), less steep for CO2 (but CO2 is more soluble).

• Respiratory Membrane: Thin and large surface area facilitates rapid gas exchange.

• Ventilation-Perfusion Coupling: Matching of alveolar ventilation with pulmonary blood perfusion for optimal gas exchange.

Clinical Considerations

• Pneumonia/Edema: Increased membrane thickness impairs gas exchange.

• Emphysema: Reduced alveolar surface area impairs gas exchange.

Ventilation-Perfusion Coupling

• Perfusion: Blood flow reaching alveoli.

• Ventilation: Gas reaching alveoli.

• Local PO2 and PCO2 regulate arteriolar and bronchiolar diameters to optimize gas exchange.

Internal Respiration

• Partial pressures and diffusion gradients are reversed compared to external respiration.

• Oxygen moves from blood to tissues; CO2 moves from tissues to blood.

Summary Table: Respiratory Volumes and Capacities

Measurement	Adult Male Avg.	Adult Female Avg.	Description
Tidal Volume (TV)	500 ml	500 ml	Air inhaled/exhaled with each breath
Inspiratory Reserve Volume (IRV)	3100 ml	1900 ml	Air forcibly inspired beyond TV
Expiratory Reserve Volume (ERV)	1200 ml	700 ml	Air forcibly expired beyond TV
Residual Volume (RV)	1200 ml	1100 ml	Air remaining after forced expiration
Total Lung Capacity (TLC)	6000 ml	4200 ml	Maximum air in lungs after maximum inspiration
Vital Capacity (VC)	4800 ml	3100 ml	Maximum air expired after maximum inspiration
Inspiratory Capacity (IC)	3600 ml	2400 ml	Maximum air inspired after normal expiration
Functional Residual Capacity (FRC)	2400 ml	1800 ml	Air remaining after normal expiration

Summary Table: Gas Partial Pressures in Atmosphere and Alveoli

Gas	Atmosphere %	Atmosphere Partial Pressure (mm Hg)	Alveoli %	Alveoli Partial Pressure (mm Hg)
N2	78.6	597	74.9	569
O2	20.9	159	14.0	104
CO2	0.04	0.3	5.2	40
H2O	0.5	3.7	6.2	47
Total	100.0	760	100.0	760

Summary Table: Effects of Breathing Rate and Depth on Alveolar Ventilation

Breathing Rate	Dead Space Volume (DSV)	Total Volume (TV)	Respiratory Rate	Minute Ventilation	Alveolar Ventilation	% Effective Ventilation
Normal rate and depth	150 ml	500 ml	12/min	6000 ml/min	4200 ml/min	70%
Slow, deep breathing	150 ml	1000 ml	6/min	6000 ml/min	5100 ml/min	85%
Rapid, shallow breathing	150 ml	250 ml	24/min	6000 ml/min	2400 ml/min	40%

Key Equations

• Boyle’s Law: $P_1V_1 = P_2V_2$

• Transpulmonary Pressure: $P_{tp} = P_{pul} - P_{ip}$

• Alveolar Ventilation Rate: $AVR = (TV - Dead\ Space) \times Frequency$

• Lung Compliance: $C_L = \frac{\Delta V_L}{\Delta (P_{pul} - P_{ip})}$

Clinical Applications

• Asthma: Increased airway resistance, treated with epinephrine.

• Infant Respiratory Distress Syndrome (IRDS): Insufficient surfactant in premature infants, treated with surfactant therapy.

• Fibrosis: Nonelastic scar tissue reduces lung compliance.

• Emphysema: Reduced alveolar surface area impairs gas exchange.

Additional info: These notes provide a comprehensive overview of respiratory physiology, including mechanics of breathing, physical factors influencing ventilation, assessment of respiratory volumes and capacities, and the principles of gas exchange. Clinical correlations are included to highlight the relevance of these concepts in health and disease.