How much of earth's atmosphere is made up of nitrogen and oxygen? a. between 25 and 75 percent b. 100 percent c. less than 1 percent d. close to 99 percent

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 enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.