However i did not pass the test. fix the sentence by adding a comma.

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.

sodium i think the chemical element of atomic number 11, a soft silver-white reactive metal of the alkali metal group.