Answer:
(B) the internal pressure is less than atmospheric pressure.
Explanation:
The process of respiration is divided into two distinct phases, inspiration (inhalation) and expiration (exhalation). During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs.
Hence, as the thoracic cavity increases in volume the lungs are pulled from all sides to expand, causing a drop in the pressure (a partial vacuum) within the lung itself. As such the internal pressure is less than atmospheric pressure.
During expiration, diaphragm contracts inferiorly and thoracic muscles pull the chest wall outwardly (the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases). As a result, the lungs contract and air is forced out.
