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We need oxygen to live,we breathe in oxygen and exhale out Carbon dioxide.

In order for humans to survive, our bodies have figured out various ways to extract energy from out surroundings. 

One of the ways is with food, which provides out cells with raw materials to make, repair and replace out cells as needed. 

A different kind of energy we need is that which allows us to make use of these raw materials.  The body has figured out a way to extract this from oxygen.  When you inhale, the oxygen in the air is absorbed into your bloodstream through tiny sacs in your lungs, called alveoli. As soon as the oxygen enters your blood, a passing protein molecule called hemoglobin picks it up.  Each molecule of hemoglobin can transport four molecules of oxygen to almost anywhere in the body.

The hemoglobin transport the oxygen to your cells where another protein, called cytochrome C oxidase makes two molecules of water out of every molecules of oxygen delivered to it.