During inspiration when the diaphragm contracts, the intrapleural pressure becomes:
**Core Concept**
During inspiration, the diaphragm contracts, causing the volume of the thoracic cavity to increase. This increase in volume leads to a decrease in intrapleural pressure, allowing air to enter the lungs.
**Why the Correct Answer is Right**
When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity. According to Boyle's law, as the volume of a gas increases at constant temperature, the pressure decreases. This decrease in pressure in the thoracic cavity is transmitted to the intrapleural space, resulting in a decrease in intrapleural pressure. The resulting negative pressure allows the lungs to expand and draw air into the lungs. The diaphragm's contraction also increases the rib cage's volume, further contributing to the decrease in intrapleural pressure.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the diaphragm's contraction increases the volume of the thoracic cavity, not decreases it.
**Option B:** This option is incorrect because the diaphragm's contraction does not increase intrapleural pressure; it actually decreases it.
**Option D:** This option is incorrect because the diaphragm's contraction does not cause a significant increase in intrathoracic pressure; instead, it decreases the intrapleural pressure.
**Clinical Pearl / High-Yield Fact**
During inspiration, the diaphragm's contraction is the primary muscle action responsible for decreasing intrapleural pressure, allowing air to enter the lungs. This process is essential for maintaining normal respiratory function.
**Correct Answer:** C. Decreases.