Mechanics of Respiration: Ventilation, Pressures, and Lung Volumes

Mechanics of Respiration

Overview

The mechanics of respiration involve the processes that enable air movement into and out of the lungs, the pressures that drive this movement, and the volumes and capacities that characterize lung function. These topics are essential for understanding how the respiratory system supports cellular respiration and maintains homeostasis.

The Respiratory System

Structure and Function

• Major Function: Associated with cellular respiration—supplying cells with O2 and disposing of CO2 (a waste product).

• Other Functions: Olfaction (smell) and speech.

• Main Structures: Nasal cavity, nostril, oral cavity, pharynx, larynx, trachea, right and left main (primary) bronchi, lungs, diaphragm.

Example: The trachea conducts air to the bronchi, which branch into smaller bronchioles leading to the alveoli where gas exchange occurs.

Gas Exchange at the Alveoli

Alveolar Structure and Function

• Alveoli: Tiny, balloon-like sacs composed of elastin protein, providing high stretchability.

• Surface Area: Alveolar surface area is 40–50 times greater than skin, maximizing gas exchange efficiency.

• Airway Pathway: Bronchi (1°, 2°, 3°) → bronchioles → terminal bronchiole → respiratory bronchiole → alveolar duct → alveolar sac → alveoli.

Example: Oxygen diffuses from alveolar air into pulmonary capillaries, while carbon dioxide diffuses in the opposite direction.

Pulmonary Ventilation

Principles of Airflow

• Definition: Exchange of air between atmosphere and alveoli.

• Airflow Direction: Air moves from regions of high pressure to low pressure.

• Key Equation:

$\text{Air Flow} = \frac{\Delta \text{Pressure}}{\text{Resistance}}$

$\text{Air Flow} = \frac{P_{\text{alveolar}} - P_{\text{atmospheric}}}{\text{Resistance}}$

• Factors Hindering Ventilation:

  • Airway resistance

  • Alveolar surface tension

  • Lung compliance (ability to deform in shape)

Airway Resistance

Determinants and Clinical Relevance

• Equation:

$\text{Resistance} = \frac{8L\eta}{\pi r^4}$

• Resistance is usually insignificant in large airways (large diameters).

• Greatest resistance occurs in medium-sized bronchi.

• Progressive branching increases total cross-sectional area, reducing resistance at terminal bronchioles.

Example: Asthma attacks cause constriction of bronchioles, increasing resistance and making breathing difficult.

Airway Resistance Imbalance

• Radius (diameter) is the most important factor affecting resistance.

• Bronchoconstriction (e.g., during asthma) increases resistance and reduces ventilation.

• Adrenergic agonists (e.g., epinephrine) relax smooth muscle and reduce resistance.

• Acetylcholine constricts bronchioles; cholinergic antagonists inhibit acetylcholine signaling, relaxing airways.

Example: Inhalers for asthma often contain adrenergic agonists to open airways.

Alveolar Surface Tension

Role in Lung Function

• Surface tension: Attraction of liquid molecules at the gas-liquid interface (alveolar surface).

• Water on alveolar walls attracts them together, risking alveolar collapse.

• Even small volumes of water can cause drowning by collapsing alveoli.

Surfactant

• Surfactant: Detergent-like lipoprotein (amphiphile) produced by type II alveolar cells.

• Reduces surface tension, discourages alveolar collapse, and normalizes pressures between different-sized alveoli.

• Produced sufficiently by 35–40 weeks gestation; deficiency in premature infants causes infant respiratory distress syndrome.

Example: Premature infants may require surfactant therapy to prevent alveolar collapse.

Lung Compliance

Definition and Factors Affecting Compliance

• Compliance: Measure of the lung's ability to stretch and expand.

• Higher compliance means easier lung expansion.

• Normally high due to flexibility of thoracic cavity, tissue distensibility, and low alveolar surface tension.

• Diminished by decreased flexibility (scar tissue, aging, bronchitis) and reduced surfactant production.

Example: Pulmonary fibrosis reduces compliance, making breathing more difficult.

Pulmonary and Pleural Pressures

Negative Pressure and Boyle's Law

• Atmospheric pressure: ~760 mmHg.

• Negative pressure: Pressure below atmospheric, created by expanding thoracic cavity.

• Boyle's Law: Pressure varies inversely with volume.

$P \propto \frac{1}{V}$

$P_1 V_1 = P_2 V_2$

Example: Increasing thoracic volume during inspiration decreases pressure, drawing air into lungs.

Pulmonary Ventilation Phases

• Inspiration: Air flows into lungs (intrapulmonary pressure < atmospheric pressure).

• Expiration: Gases exit lungs (intrapulmonary pressure > atmospheric pressure).

• Mechanical processes: Volume changes cause pressure changes (Boyle's law), driving airflow.

Inspiration (Inhale)

• Inspiratory muscles contract (diaphragm descends, rib cage rises).

• Thoracic cavity volume increases.

• Lungs stretch; intrapulmonary volume increases.

• Intrapulmonary pressure drops (to ~-1 mmHg).

• Air flows into lungs down pressure gradient.

Expiration (Exhale)

• Inspiratory muscles relax (diaphragm rises, rib cage descends).

• Thoracic cavity volume decreases.

• Elastic lungs recoil; intrapulmonary volume decreases.

• Intrapulmonary pressure rises (to ~+1 mmHg).

• Air flows out of lungs down pressure gradient.

Intrapulmonary and Intrapleural Pressure (at rest)

• Atmospheric pressure (Patm): 0 mmHg (760 mmHg absolute).

• Intrapulmonary pressure (Ppul): 0 mmHg (760 mmHg absolute).

• Intrapleural pressure (Pip): -4 mmHg (756 mmHg absolute).

• Transpulmonary pressure: 4 mmHg (difference between Ppul and Pip).

Intrapleural Pressure Keeps Lungs Inflated

• Negative Pip results from opposing forces:

  • Inward: Elastic recoil of lungs and alveolar surface tension (promote collapse).

  • Outward: Elasticity of chest wall (promotes expansion).

• Surface tension of serous fluid between pleurae keeps lungs inflated.

Pressure Relationships and Lung Collapse

• Pip must remain lower than Ppul or Patm to prevent lung collapse.

• If Pip increases to Ppul or Patm, lungs collapse (atelectasis).

• Pneumothorax: Air in pleural cavity from trauma or rupture causes collapse; treated by removing air with chest tubes.

Summary of Lung Pressures during Inspiration and Expiration

• Intrapulmonary pressure: Decreases during inspiration, increases during expiration.

• Intrapleural pressure: Always negative, must remain lower than intrapulmonary pressure.

• Transpulmonary pressure: Must remain positive for lung inflation.

• Volume of breath: Pressure gradients move ~0.5 L of air in and out (tidal volume).

Respiratory Volumes and Capacities

Spirogram and Definitions

Volume/Capacity	Definition	Typical Value (ml)
Tidal Volume (TV)	Volume of air inhaled or exhaled in a normal breath	500
Inspiratory Reserve Volume (IRV)	Max volume inhaled after normal inspiration	3100
Expiratory Reserve Volume (ERV)	Max volume exhaled after normal expiration	1200
Residual Volume (RV)	Volume remaining in lungs after maximal exhalation	1200
Inspiratory Capacity (IC)	TV + IRV	3600
Functional Residual Capacity (FRC)	ERV + RV	2400
Vital Capacity (VC)	TV + IRV + ERV	4800
Total Lung Capacity (TLC)	TV + IRV + ERV + RV	6000

Example: Spirometry is used to measure these volumes and diagnose respiratory diseases.

Dead Space

• Anatomical dead space: Air in conducting airways (no gas exchange), ~150 ml.

• Alveolar dead space: Non-functional alveoli due to collapse or obstruction.

• Total dead space: Sum of anatomical and alveolar dead space; normally constant and difficult to measure.

Example: Increased dead space is seen in certain lung diseases, reducing effective ventilation.

Additional info: These notes expand on the provided lecture slides with definitions, equations, and clinical examples for clarity and completeness.