When you breathe in, or inhale, your diaphragm contracts and moves downward, which increases the volume of your chest cavity. This movement creates a vacuum that causes your lungs to expand. Simultaneously, the intercostal muscles between your ribs contract, aiding in the expansion of the chest cavity by lifting the rib cage both upward and outward. As the lungs expand, air is drawn into the respiratory system through the nose or mouth, and travels down the trachea, through the bronchi, and into the lungs where it fills the alveoli, small sacs where oxygen is exchanged for carbon dioxide. read more >>

When you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. This increases the space in your chest cavity, into which your lungs expand. The intercostal muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.read more >>