Final answer:
During inspiration, air flows into the body because intrapleural pressure is greater than atmospheric pressure.
Step-by-step explanation:
The reason air flows into the body during inspiration is that intrapleural pressure is greater than atmospheric pressure. Intrapleural pressure refers to the pressure of the air within the pleural cavity, which is lower or negative to the intra-alveolar pressure (pressure within the alveoli). When the diaphragm and intercostal muscles contract during inspiration, the thoracic cavity expands, causing a decrease in intrapleural pressure. This creates a pressure gradient where atmospheric pressure is greater, and as a result, air flows into the lungs.