THORAX ABDOMEN AND PELVIS

Detailed explanation-1: -Inspiration can occur if the pressure within the lungs (intrapulmonary pressure ) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. Similarly, expiration takes place when the intrapulmonary pressure is higher than the atmospheric pressure.

Detailed explanation-2: -When the air pressure within the alveolar spaces falls below atmospheric pressure, air enters the lungs (inspiration), provided the larynx is open; when the air pressure within the alveoli exceeds atmospheric pressure, air is blown from the lungs (expiration).

Detailed explanation-3: -Alveolar pressure (PA or Palv) [ intrapulmonary pressure or lung pressure] Alveolar pressure normally changes as the intrapleural pressure changes. During spontaneous inspiration, PA is about −1 cm H2O, and during exhalation it is about +1 cm H2O.

Detailed explanation-4: -The alveolar pressure drops from 760 mm Hg to 758 mm Hg, and the pressure gradient created between the inside of the lungs and the atmosphere draws air in. During deep breathing, caused by exertion or lung disease other muscles become involved in inspiration and these are known as the accessory muscles.