When the diaphragm and external intercostal muscles contract, it

The correct answer is: lets the air get inside the lungs.

Explanation:

The process of ventilation, or breathing, consists of two parts: inhaling and exhaling.

Inhaling is the name of the process that is responsible for the air entering the body by the respiratory tract. Exhaling is the opposite, it lets the air out. Both are equally important since they allow the gas exchange necessary for the oxygenation of the blood.

When inhaling, the diaphragm and the external intercostal muscles contract. This pulls the ribcage upwards and outwards, which increases lung volume while decreasing the air pressure inside.

These are the mechanism of inspiration which increases the capacity of the thoracic cavity, for maximum volume of air to be inhaled during inspiration.

The contraction makes the diaphragm to move towards the abdominal cavity inferiorily creating the space for the distention of lungs during filling with air. When the external inter coastal muscles contracts, the rib cage is pushed upwards and outwards with the sternum remain fixed. This causes expansion of the rib-cage, with further widening of the thoracic cavity for maximum lung capacity.