Here's a look at the chemical composition of the human body, including element abundance and how each element is used. Elements are listed in order of decreasing abundance, with the most common element (by mass) listed first. Approximately 96% of body weight consists of only four elements: oxygen, carbon, hydrogen, and nitrogen. Calcium, phosphorus, magnesium, sodium, potassium, chlorine, and sulfur are macronutrients or elements the body needs in a significant amount.
By mass, oxygen is the most abundant element in the human body. If you think about it, this makes sense, since most of the body consists of water or H2O. Oxygen accounts for 61-65% of the mass of the human body. Even though there are many more atoms of hydrogen in your body than oxygen, each oxygen atom is 16 times more massive than a hydrogen atom.
UsesOxygen is used for cellular respiration.
All living organisms contain carbon, which forms the basis for all of the organic molecules in the body. Carbon is the second most abundant element in the human body, accounting for 18% of body weight.
UsesAll organic molecules (fats, proteins, carbohydrates, nucleic acids) contain carbon. Carbon also is found as carbon dioxide or CO2. You inhale air that contains about 20% oxygen. Air you exhale contains much less oxygen, but is rich in carbon dioxide.
Hydrogen accounts for 10% of the mass of the human body.
UsesSince around 60% of your body weight is water, much of the hydrogen exists in water, which functions to transport nutrients, remove wastes, lubricate organs and joints, and regulate body temperature. Hydrogen is also important in energy production and use. The H+ ion can be used as a hydrogen ion or proton pump to produce ATP and regulate numerous chemical reactions. All organic molecules contain hydrogen in addition to carbon.
The metal magnesium comprises about 0.05% of human body weight.