Living things are composed of elements, however, certain elements are more critical than others to the structure and function of living things. The human body is composed of mostly oxygen, nitrogen, carbon, and hydrogen.
The bulk of the human body is mostly water. Water is essential to both the composition and the function of the human body. The elements found in water are Hydrogen and Oxygen.
The carbon element in the human body plays an important role as the backbone of the sugars used for energy and the amino acids used as the components of organic molecules.
Nitrogen in the body is an essential part of the structure of proteins and DNA.
The percentage of the human body that is made up by the four essential elements:
Source: Reference: H. A. Harper, V. W. Rodwell, P. A. Mayes, Review of Physiological Chemistry, 16th ed., Lange Medical Publications, Los Altos, California 1977.