Mechanics of Mechanics of BreathingBreathing

• Mayank Mayank AgarwalAgarwal

• Arshida Arshida HusainHusain

Respiratory Respiratory musclesmuscles

Rib cage movement during breathingRib cage movement during breathing

Pressure Pressure differences differences involved in involved in ventilationventilation

Palv - Patm is the driving pressure gradient for airflow into and out of the lungs

Transpulmonary pressure (Ptp=Palv - Pip) is a determinant of lung size

Pip=Intrapleural pressure

Patm=atmospheric pressure

Pcw (=Pip- Patm) is the transmural pressure across the chest wall

Pulmonary Pulmonary pressures at rest pressures at rest

Values shown are pressures relative

to atmospheric pressure (760 mm Hg at sea level)

Ptp exactly opposes the elastic recoil of the lung, and the lung volume remains stable

Pcw is balanced by the outward elastic recoil of the chest wall Pip at rest is a balance between the tendency of the lung to

collapse and the tendency of the chest wall to expand

Sub-atmospheric pressure in the Sub-atmospheric pressure in the pleural cavity helps keep the lungs pleural cavity helps keep the lungs

inflated inflated

Puncture in the chest wall permits air from the atmosphere to flow down its pressure gradient and enter the pleural cavity, abolishing the transmural pressure gradient

Hole in the lung wall permits air to move down its pressure gradient and enter the pleural cavity from the lungs, abolishing the transmural pressure gradient

During During inspirationinspiration

During During expirationexpiration

Lung ComplianceLung ComplianceLung compliance is defined as the magnitude of the

change in lung volume produced by a given change in the transpulmonary pressure

Two major determinants of lung compliance:

1. Stretchability of the lung tissues, particularly their elastic connective tissues. Therefore, a thickening of the lung tissues decreases lung compliance.

2. Surface tension at the air–water interfaces within the alveoli

Elastic Properties Elastic Properties of Respiratory of Respiratory System: Lung System: Lung and Chest Walland Chest Wall

Stabilizing effect of surfactantStabilizing effect of surfactant

Surfactant production is decreased in infant respiratory distress syndrome

Physical, neural, and chemical factors affect airway radii

Transpulmonary pressure exerts a distending force on the airways, keeping the smaller airways—those without cartilage to support them—from collapsing

Airway resistanceAirway resistance

Elastic connective-tissue fibers (that link the outside of the airways to the surrounding alveolar tissue) are pulled upon as the lungs expand during inspiration

pull the airways open in between breathslateral traction

Both transpulmonary pressure and lateral traction act in the same direction, decreasing airway resistance during inspiration

The increase in intrapleural pressure compresses the small conducting airways radii airway resistance airflow rate cannot be increased beyond a limit during forced expiration irrespective of the effort intensity

The harder one pushes, the greater the compression of the airways, further limiting expiratory airflow: COPD and Asthma

Work Of BreathingWork Of BreathingCompliance

(elastic resistance) Tissue resistance Airway resistance

65% of total work 7% of total work 28% of total work

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