Final answer:
Inhalation is the process involving the expansion of the thoracic cavity through the contraction of the diaphragm and external intercostal muscles, which leads to a decrease in air pressure inside the lungs and results in air being drawn in from the atmosphere.
Step-by-step explanation:
Inhalation, or inspiration, is the process of moving air into the lungs, which occurs when the size of the thoracic cavity increases. This is primarily due to the contraction of a muscle known as the diaphragm, a dome-shaped structure situated below the lungs.
When the diaphragm contracts, it moves downward and the thoracic cavity expands. Additionally, the elevation of the sternum and rib cage, mainly by the external intercostal muscles, further increases thoracic volume.
This expansion creates a lower air pressure inside the lungs compared to the outside atmosphere, causing air to flow into the lungs through the respiratory tract until the pressure is equalized.
During this active movement of inhalation, other muscles can assist the diaphragm and intercostals, especially when taking a deeper breath. The increase in the thoracic cavity's volume, as explained by Boyle's Law, leads to a decrease in pressure within the cavity.
Therefore, the air is driven into the lungs due to the resultant pressure gradient between the atmospheric pressure and the lower pressure within the lungs.