I am fairly small and I am in animal cells. I help the animal cells break down molecules by emptying out my enzymes to speed up reactions. I am very helpful. What am I?

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.