The substance that reduces surface tension in the alveoli is:

A. Active transport requires energy and is used to move substances against their concentration gradient. Oxygen moves along its concentration gradient, so active transport is not involved.
B. Diffusion is the correct mechanism. Oxygen moves from an area of higher concentration in the alveoli to a lower concentration in the pulmonary capillaries through the alveolar-capillary membrane. This passive process does not require energy.
C. Endocytosis is a process cells use to engulf large particles or substances, not how gases like oxygen are transported.
D. Pinocytosis is a form of endocytosis specifically for fluid uptake and is not involved in the respiratory gas exchange process

A. Quiet inspiration involves the diaphragm and external intercostal muscles without significant involvement of the neck and abdominal muscles.
B. Forceful exhalation (such as during coughing or strenuous activities) engages the abdominal muscles and sometimes the neck muscles (such as the sternocleidomastoid), as they help to push air out of the lungs by increasing intra-abdominal and intrathoracic pressure.
C. None of the answers are correct is incorrect because forceful exhalation involves these muscles.
D. Quiet exhalation primarily relies on passive recoil of the lungs and relaxation of the diaphragm, with minimal involvement of the neck or abdominal muscles.