The main elements that compose the human body, by mass percent, are shown from most abundant to least abundant element. The atomic fractions (fractions of atoms) are different due to differing atomic weights. Water composes over 70% of the bodies bulk mass. Proteins fulfill multiple roles including essential structural and enzymatic functions, and are composed primarily of Carbon, Hydrogen, Oxygen and Nitrogen and, depending on the specific protein, tiny amounts of very few others. There are over 100,000 different proteins in the human body and none are interchangeable. Hydrogen is the most common atom and is the most common element in the universe. A single Oxygen atom has roughly 16 times the mass of a single Hydrogen.