What occurs when the diaphragm contracts during respiration?

When the diaphragm contracts during respiration, it moves downward, creating a vacuum effect in the thoracic cavity. This action causes the volume of the chest cavity to increase, which results in a decrease in pressure inside the lungs compared to the atmospheric pressure. As a consequence, air is drawn into the lungs to equalize the pressure. This process is fundamental to inhalation and is vital for efficient breathing.

The other options do not describe the physiological changes associated with diaphragm contraction. The collapse of the chest, mentioned in the first option, would inhibit breathing rather than facilitate it. The pumping of blood into the pulmonary arteries and increases in heart rate are responses related to overall cardiovascular function and not directly tied to diaphragm movement during inhalation.