Question

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what allows the thoracic cavity to increase in volume during normal inspiration?
contraction of the diaphragm upward and the external intercostal muscles lifting the ribs downward.
relaxing of the diaphragm downward and the external intercostal muscles lifting the ribs upward.
relaxing of the diaphragm upward and the external intercostal muscles lifting the ribs downward.
contraction of the diaphragm downward and the external intercostal muscles lifting the ribs upward.

Explanation:

During normal inspiration, the diaphragm contracts and moves downward. This increases the vertical dimension of the thoracic cavity. At the same time, the external intercostal muscles contract, lifting the ribs upward. This increases the anterior - posterior and lateral dimensions of the thoracic cavity. The combination of these movements increases the volume of the thoracic cavity.

Answer:

Contraction of the diaphragm downward and the external intercostal muscles lifting the ribs upward.