What effect does the contraction of the diaphragm have on intrapleural pressure (IPP) and breathing?

When the diaphragm contracts, it moves downward into the thoracic cavity, increasing the volume of the chest cavity. According to Boyle's Law, an increase in volume leads to a decrease in pressure. As the volume of the thoracic cavity increases due to diaphragm contraction, the intrapleural pressure (IPP) becomes more negative. This negative IPP is crucial because it enhances the pressure gradient between the atmosphere and the alveoli in the lungs, allowing air to flow into the lungs—a process known as inspiration.

As the IPP becomes more negative, the lungs expand, further decreasing the pressure within the alveoli compared to atmospheric pressure. This difference in pressure facilitates the movement of air from areas of higher pressure (the atmosphere) to areas of lower pressure (the alveoli), promoting inhalation.

This relationship between diaphragm contraction, intrapleural pressure, and breathing is fundamental to normal respiratory physiology. The diaphragm's contraction is a primary driver of inhalation, while relaxation of the diaphragm would allow for expiration, causing IPP to become less negative or potentially more positive during that phase.