Questions: Question 29 For inspiration of air, which of the following happens first? thoracic cavity volume decreases diaphragm descends, thoracic volume begins to increase, and rib cage rises air (gases) flows into lungs intrapulmonary pressure drops

The answer is the second one: diaphragm descends, thoracic volume begins to increase, and rib cage rises.

Explanation for each option:

1. Thoracic cavity volume decreases: This is incorrect. During inspiration, the thoracic cavity volume actually increases as the diaphragm contracts and moves downward, and the rib cage rises.

2. Diaphragm descends, thoracic volume begins to increase, and rib cage rises: This is correct. The first step in the process of inspiration is the contraction of the diaphragm, which causes it to descend. This increases the volume of the thoracic cavity and causes the rib cage to rise, setting the stage for air to flow into the lungs.

3. Air (gases) flows into lungs: This is incorrect as the first step. Air flows into the lungs as a result of the pressure changes that occur after the thoracic cavity volume increases and intrapulmonary pressure drops.

4. Intrapulmonary pressure drops: This is incorrect as the first step. The drop in intrapulmonary pressure occurs after the thoracic cavity volume increases, allowing air to flow into the lungs.

In summary, the process of inspiration begins with the diaphragm descending, which increases the thoracic volume and causes the rib cage to rise.