Given what you know about the human body, what do you think accounts for the high percentage of oxygen (65.0%)? given what you know about the human body, what do you think accounts for the high percentage of oxygen (65.0%)? the human body is made up mainly of water, and there are two oxygen atoms in every water molecule. the human body is made up mainly of water, and there is one oxygen atom in every water molecule. the human body breathes in oxygen constantly, and because oxygen is a heavy element, it makes up a large percentage of body mass. the human body breathes in oxygen constantly, and oxygen in the blood and tissues makes up a large percentage of body mass.