What mechanism is primarily responsible for the inflation of the lungs in mammals?

The mechanism primarily responsible for the inflation of the lungs in mammals is the negative pressure pumping action. In the respiratory process, when the diaphragm contracts and moves downward, along with the intercostal muscles contracting to expand the rib cage, the volume of the thoracic cavity increases. This increase in volume decreases the pressure within the thoracic cavity compared to the atmospheric pressure, creating a pressure gradient. As a result, air flows into the lungs to equalize the pressure difference, effectively inflating the lungs. This process illustrates the principle of negative pressure breathing, which is distinct from mechanisms such as positive pressure breathing, where air is pushed into the lungs rather than drawn in.

Active muscle contraction does play a crucial role in this process, but it is the resulting negative pressure that directly facilitates lung inflation. Similarly, while direct oxygen diffusion is vital for gas exchange, it does not primarily contribute to the physical mechanism of lung inflation itself.