Final answer:
The contraction of the diaphragm during breathing leads to inhalation as it expands the chest cavity and decreases the internal lung pressure, allowing air to enter. Exhalation occurs when the diaphragm relaxes, increasing lung pressure and expelling air.
Step-by-step explanation:
When the diaphragm contracts (is pulled downward), inhalation occurs. This is because the contraction of the diaphragm and the external intercostal muscles expand the chest cavity, including the thoracic cavity. The downward movement of the diaphragm creates a lower pressure inside the lungs compared to the outside atmosphere, leading to air being drawn into the lungs. This process is an active phase where the thoracic volume increases, resulting in negative gauge pressure in the lungs which facilitates the entry of air.
In contrast, during exhalation, the diaphragm relaxes which causes the muscle to move upwards, decreasing the volume of the thoracic cavity. The increase in pressure within the lungs then causes air to flow out. Exhalation is typically a passive process, aided by the elasticity of the lungs and, when forced, assisted by the internal intercostal and abdominal muscles.