Question

inspiration occurs because ________.

a creation of a partial vacuum in the alveoli causes air to enter the lungs
the diaphragm relaxes and moves up
the rib cage moves down and in
a creation of a partial vacuum in the bronchioles causes air to enter the lungs

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Explanation:

During inspiration (inhalation), the diaphragm contracts and moves down, and the rib cage moves up and out. This increases the volume of the thoracic cavity, creating a partial vacuum (lower pressure) in the alveoli (the tiny air - sacs in the lungs). According to Boyle's law ($PV = k$, where $P$ is pressure, $V$ is volume, and $k$ is a constant), when the volume of a space (in this case, the alveoli) increases, the pressure decreases. Since the pressure inside the alveoli is now lower than the atmospheric pressure, air flows into the lungs (from an area of higher pressure, the atmosphere, to an area of lower pressure, the alveoli).

The diaphragm relaxing and moving up (option 2) and the rib cage moving down and in (option 3) are events that occur during expiration (exhalation). The partial vacuum is created in the alveoli, not the bronchioles (option 4 is incorrect as bronchioles are just air - conducting passages and the main gas - exchange site is the alveoli).

Answer:

a creation of a partial vacuum in the alveoli causes air to enter the lungs