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Step-by-Step Explanation
Step 1: Contraction of the Diaphragm
During inspiration, the dome-shaped diaphragm muscle contracts and flattens. This increases the volume of the thoracic (chest) cavity in the anteroposterior axis—essentially making the chest space larger from front to back.
Step 2: Contraction of External Intercostal Muscles
The external intercostal muscles (located between the ribs) contract, causing the ribs and sternum (breastbone) to move upward and outward. This further expands the thoracic cavity volume, especially in the lateral or dorso-ventral directions.
Step 3: Increase in Thoracic (Pulmonary) Volume
When the diaphragm and external intercostals contract together, the overall thoracic cavity enlarges. This means the lungs, which are elastic and closely follow the expansion of the chest wall, also increase in volume.
Step 4: Decrease in Intrapulmonary Pressure
According to the mechanism of breathing, as lung volume increases, the pressure inside the lungs—called the intrapulmonary pressure—decreases (below atmospheric pressure). This drop in pressure is crucial for drawing air into the lungs.
Step 5: Entry of Air into the Lungs
Air flows from the atmosphere into the lungs to equalize the pressure difference; it moves from a region of higher pressure (outside) to a region of lower pressure (inside the lungs). This completes the inspiration process.
Conclusion
The correct events that occur during inspiration are contraction of the diaphragm and contraction of the external intercostal muscles. These actions increase the thoracic cavity volume and decrease intrapulmonary pressure, which ultimately draws air into the lungs. Hence, among the given options, (i) and (ii) are correct.
