Humans breathe approximately 432 liters of oxygen per day, and that oxygen helps the tissues in the human body function properly. The body needs approximately 352.8 liters of oxygen per day when the body is at rest.
Humans need oxygen to provide nutrients to all of the cells in their bodies. If tissues and cells go without oxygen, then they begin to die quickly. For example, brain cells can only go without oxygen for three minutes before they begin to die. After three minutes of no oxygen, permanent brain damage begins to spread throughout the brain's tissues.
The body also needs energy in order to function properly. The human body uses the compound ATP for chemical energy for all of its cells and tissues. Cells must synthesize the ATP that they need in order to function properly, and they do so through a variety of biochemical pathways inside the cell. Each cell in the human body has the ability to create ATP during aerobic periods and during anaerobic periods of time. During both of these periods in the body, cells utilize glucose to create molecules of ATP. One side reaction that occurs during the synthesis of ATP provides the body with excess hydrogen ions. These ions go to the mitochrondria in the cell and utilize oxygen from the air humans breathe to form electron transport chains that power the synthesis of ATP. Without oxygen, humans cannot synthesize enough ATP to keep their cells alive.