The diaphragm contracts during the phase of respiration called:

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. Effusion refers to the accumulation of fluid in the pleural space, not a substance that reduces surface tension in the alveoli.
B. Bicarbonate is an important component in the buffering system of the blood, but it does not reduce surface tension in the lungs.
C. Exudate is fluid that leaks out of blood vessels due to inflammation or injury, often seen in conditions like pneumonia or pleuritis, but it does not affect surface tension in the alveoli.
D. Surfactant is a lipid-protein mixture produced by type II alveolar cells in the lungs. It reduces surface tension in the alveoli, preventing the lungs from collapsing and making it easier to breathe.