Oxygen is present in water and necessary for respiration, found in the lungs where 20% of breathed air is oxygen. Carbon provides structure and is used for our daily functions, anything with carbon is considered organic. Hydrogen bonds with oxygen in our bodies to make water, while nitrogen makes up 80% of the atmosphere and is used to make nucleic acids and proteins. Calcium is needed for strong bones and teeth, movement and blood clots.

1. Elements and the Human Body Inspired by about.com