RESPIRATION PHYSIOLOGY: VENTILATION -- Sequence for Air Movement

SEQUENCE FOR AIR MOVEMENT

A. Pressure Development

1. Respiratory muscle contraction (or relaxation)

2. Transmission of pressure change through the intrapleural space

DPalv = DPpl

Note: In pneumothorax (admission of air into the intrapleural space), pressure transmission is attenuated or ineffective (also lung collapse)

External pneumothorax: admission of air from the body surface (e.g. chest puncture)
Internal pneumothorax: admission of air through a defect in alveoli or airways (e.g. lung degenerative disease)

Note: unilateral pneumothorax is serious but not usually fatal; bilateral pneumothorax can be fatal

3. Change in alveolar pressure

Negative pressure (below atmospheric): Inspire
Positive pressure (above atmospheric): Expire

B. Air Movement, which depends on

1. Force of muscle contraction and P_alv

2. Elastic recoil of the lung and chest wall (compliance, C)

C = DV/ DP or DV = C x DP

3. Resistance to rate of lung volume change (viscosity)

V' (flow or dV/dt) = P_alv / R_aw (similar to Q = DP / R in the circulation)

Note: Notation

R_aw = resistance of the airways to air movement and resistance of tissue to rapid shape change

P_alv = alveolar pressure; pressure within the alveoli; also called intrapulmonic or intrapulmonary pressure; measured by measuring pressure at mouth level when airflow is briefly occluded

P_pl = intrapleural pressure; pressure within the pleural space; also called intrathoracic pressure; measured by injecting a small volume of air into the intrapleural space or swallowing an esophageal balloon

Note: Usual pressure units in respiration: cm H₂O (because pressures in respiration are low, few cmH₂O)

1 mmHg = 1.36 cm H₂O

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