1. We begin at rest, when neither inspiration nor expiration is occurring. The intrapleural pressure is (and will always be) subatmospheric. There is no movement of air into or out of the lungs because the alveolar and atmospheric pressures are the same.
2. The respiratory centre in the brainstem generates action potentials which stimulate…
3. The diaphragm contracts to increase the vertical diameter of the thoracic cavity. Remember that the parietal pleura is adhered to the superior aspect of the diaphragm and the visceral pleura is adhered to the lung - so when the diaphragm flattens, the volume of the pleural cavity increases and consequently the pressure in the pleural cavity decreases. Note that in forced inspiration, the external intercostal muscles contract to increase the transverse and anteroposterior diameters of the pleural cavity, moving the ribs upwards and outwards. Yet again there is an increase in the volume (and decrease in the pressure) of the pleural cavity.
4. The result? The decreased intrapleural pressure (sometimes called intrathoracic pressure) pulls the visceral pleura (and the lungs) downwards, to sit beside the parietal pleura. This action increases the volume of the alveoli and consequently decreases the alveolar pressure. In fact, the alveolar pressure also becomes subatmospheric. Atmospheric air then rushes into the alveoli, moving down its pressure gradient.
'Intrapleural’ and ‘intrathoracic’ pressures are the same thing, even though they sound like they should be different things!