Final answer:
The contraction of the diaphragm during normal breathing in infants helps to draw air into the lungs by expanding the chest cavity, and its relaxation helps to expel air during exhalation.
Step-by-step explanation:
The role of the diaphragm contraction during normal breathing in infants is to expand the chest cavity to draw in air. Specifically, the contraction of the diaphragm moves it downward, which increases the volume of the thoracic cavity. As the diaphragm contracts and the thoracic cavity expands, the pressure inside the lungs decreases, creating a pressure gradient that allows air to flow into the lungs. This is an active process essential for inhalation.
Conversely, during exhalation, the diaphragm relaxes, and the chest cavity contracts as the diaphragm moves upwards. This causes the pressure in the thoracic cavity to increase, resulting in air being pushed out of the lungs. The movement of air during exhalation is generally a passive process and does not require muscular contraction to propel the air out.