There are several ways to consider the composition of the human body, including the elements, type of molecule, or type of cells. Most of the human body is made up of water, H 2 O, with bone cells being comprised of 31% water and the lungs 83%.﻿﻿ Therefore, it isn't surprising that most of a human body's mass is oxygen. Carbon, the basic unit for organic molecules, comes in second. 96.2% of the mass of the human body is made up of just four elements: oxygen, carbon, hydrogen, and nitrogen.

Oxygen (O) - 65% - Oxygen together with hydrogen form water, which is the primary solvent found in the body and is used to regulate temperature and osmotic pressure. Oxygen is found in many key organic compounds. Carbon (C) - 18.5% - Carbon has four bonding sites for other atoms, which makes it the key atom for organic chemistry. Carbon chains are used to build carbohydrates, fats, nucleic acids, and proteins. Breaking bonds with carbon is an energy source. Hydrogen (H) - 9.5% - Hydrogen is found in water and in all organic molecules. Nitrogen (N) - 3.2% - Nitrogen is found in proteins and in the nucleic acids that make up the genetic code. Calcium (Ca) - 1.5% - Calcium is the most abundant mineral in the body. It's used as a structural material in bones, but it is essential for protein regulation and muscle contraction. Phosphorus (P) - 1.0% - Phosphorus is found in the molecule ATP, which is the primary energy carrier in cells. It's also found in bone. Potassium (K) - 0.4% - Potassium is an important electrolyte. It's used to transmit nerve impulses and heartbeat regulation. Sodium (Na) - 0.2% - Sodium is an important electrolyte. Like potassium, it is used for nerve signaling. Sodium is one of the electrolytes that helps regulate the amount of water in the body. Chlorine (Cl) - 0.2% - Chlorine is an important negatively-charged ion (anion) used to maintain fluid balance. Magnesium (Mg) - 0.1% - Magnesium is involved in over 300 metabolic reactions. It's used to build the structure of muscles and bones and is an important cofactor in enzymatic reactions. Sulfur (S) - 0.04% - Two amino acids include sulfur. The bonds sulfur forms help give proteins the shape they need to perform their functions. ﻿ ﻿

Many other elements may be found in extremely small quantities (less than 0.01%). For example, the human body often contains trace amounts of thorium, uranium, samarium, tungsten, beryllium, and radium. Trace elements considered essential in humans include zinc, selenium, nickel, chromium, manganese, cobalt, and lead.

Not all of the elements found within the body are essential for life. Some are considered contaminants that appear to do no harm but serve no known function. Examples include cesium and titanium. Others are actively toxic, including mercury, cadmium, and radioactive elements. Arsenic is considered to be toxic to humans, but serves a function in other mammals (goats, rats, hamsters) in trace amounts. Aluminum is interesting because it is the third most common element in the Earth's crust, but its role in the human body is unknown. While fluorine is used by plants to produce protective toxins and has "apparent beneficial intake" in humans.

You may also wish to view the elemental composition of an average human body by mass.

Additional References