Mechanics of Breathing and Pulmonary Ventilation: The Respiratory System (Chapter 22)

The Respiratory System: Mechanics of Breathing

Overview

The mechanics of breathing involve the movement of air into and out of the lungs, driven by pressure changes within the thoracic cavity. Understanding these pressure relationships is essential for grasping how pulmonary ventilation occurs and what physical factors influence it.

Pressure Relationships in the Thoracic Cavity

Breathing depends on pressure differences between the atmosphere and the thoracic cavity. These pressures determine the direction of airflow and the expansion or contraction of the lungs.

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

• Respiratory Pressure: Pressure within the respiratory tract, which can be less than, greater than, or equal to atmospheric pressure.

• Pulmonary Pressure (Ppul): The pressure within the alveoli (intra-alveolar pressure). This pressure fluctuates with breathing and eventually equalizes with atmospheric pressure.

• Intrapleural Pressure (Pip): The pressure within the pleural cavity, which is always slightly negative (typically 4 mm Hg less than Ppul) to keep the lungs expanded.

• Transpulmonary Pressure: The difference between intra-alveolar and intrapleural pressure. It keeps the lung spaces open and prevents lung collapse.

Key Equation:

• $\text{Transpulmonary Pressure} = P_{pul} - P_{ip}$

Example: If intra-alveolar pressure is 760 mm Hg and intrapleural pressure is 756 mm Hg, then transpulmonary pressure is 4 mm Hg.

Pulmonary Ventilation

Pulmonary ventilation is the process of moving air into and out of the lungs. It relies on changes in thoracic cavity volume, which create pressure gradients that drive airflow.

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

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

Key Equation (Boyle's Law):

• $P_1 V_1 = P_2 V_2$

Example: When the volume of the thoracic cavity increases, the pressure inside decreases, allowing air to enter the lungs.

Physical Factors Affecting Pulmonary Ventilation

Several physical factors influence the ease of air passage and the amount of energy required for ventilation.

• Airway Resistance: Resistance to airflow is usually insignificant in healthy individuals but can increase in conditions like asthma or bronchitis.

• Surface Tension: The force exerted by liquid molecules at the alveolar surface. Surfactant, produced by type II alveolar cells, reduces surface tension and prevents alveolar collapse.

• Lung Compliance: The ability of the lungs to stretch and expand. High compliance means the lungs expand easily; low compliance (e.g., fibrosis) makes breathing more difficult.

Example: Premature infants may lack sufficient surfactant, leading to infant respiratory distress syndrome due to increased surface tension and reduced compliance.

Summary Table: Pressure Relationships in the Thoracic Cavity

Pressure Type	Location	Normal Value (mm Hg)	Role
Atmospheric Pressure (Patm)	Outside body	760	Reference for other pressures
Pulmonary Pressure (Ppul)	Alveoli	Varies (equalizes with Patm)	Drives airflow in/out of lungs
Intrapleural Pressure (Pip)	Pleural cavity	~756	Keeps lungs expanded
Transpulmonary Pressure	Difference (Ppul - Pip)	~4	Prevents lung collapse

Additional info:

• Pressure changes in the thoracic cavity are essential for normal breathing and are influenced by muscle contractions and the elasticity of lung tissue.

• Disorders affecting pressure relationships (e.g., pneumothorax, fibrosis) can compromise ventilation and gas exchange.