THORAX ABDOMEN AND PELVIS

Detailed explanation-1: -When you breathe in, or inhale, your diaphragm contracts and moves downward. This increases the space in your chest cavity, and your lungs expand into it. The muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.

Detailed explanation-2: -The diaphragm contracts and flattens during inspiration to increase the volume of the thoracic cavity. The increase in volume causes a decrease in intra-alveolar pressure, pressure lower than the atmospheric pressure, and creates a pressure gradient.

Detailed explanation-3: -As the diaphragm contracts (moves downward), the volume of the thoracic cavity increases and the intra-alveolar pressure decreases from an atmospheric pressure of 760mmHg to   756mmHg. A pressure gradient is established and air enters the lungs. The external intercostal muscles also assist inspiration.