According to About.com, water makes up between 65 and 90 percent of all cells in the human body. The remaining elements by mass include oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus, potassium, sulfur, chlorine, sodium, magnesium and trace amounts of iron, cobalt, copper, zinc, iodine, selenium and fluorine.
The composition of the different elements within the body plays a role in the formation if different types of bodily materials, such as fats, protein and DNA. About.com explains that water, composed of two hydrogen atoms and a single oxygen atom, primarily derives its mass from the heavy oxygen molecule. This explains how the human body can simultaneously be mostly water and most densely oxygen. The human body, like all organic compounds, contains a significant amount of carbon. Oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus are the elements that make up 99 percent of the human body's mass.
Not all of the numerous other elements found in the human body actually play a significant role in human life. Oxygen and carbon are the most significant in terms of cellular activity, while some trace elements are believed to have little if anything to do with cellular function; however, as many as 12 trace elements are believed to play some role in overall health and well being.