Which of the following best describes why thoracic volume decreases during normal exhalation?
1. elastic recoil of the lungs
2. increased airway resistance
3. contraction of rectus abdominis
4. contraction of the diaphragm

The correct answer is option 1. elastic recoil of the lungs.

Explanation:

Lungs recoil to push the air to the outside of the lungs. It causes the intercostal muscle to relax position and returning chest wall or thoracic cavity volume decreases during normal exhalation.

The diaphragm also relaxes during exhalation. This results in higher pressure within the thoracic cavity to the comparison of the environment. It is called a passive process or event as there is no contraction takes place in any muscles during exhalation.

Thus, the correct answer is option 1.

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