When you inhale (breathe in), air enters your lungs, and [[1]] from that air moves to your blood. 

When you inhale, air enters your lungs via the trachea and bronchi and travels to the tiny air sacs known as alveoli. 

Through the alveolar walls and the capillaries that surround the alveoli, oxygen from the inhaled air diffuses into the bloodstream. 

The oxygen-rich blood is then carried throughout the body by the cardiovascular system, where it is used by cells for cellular respiration, a process that releases energy from glucose and other organic molecules to fuel cellular functions.

Waste products such as carbon dioxide are produced by cellular respiration and diffuse from the bloodstream into the alveoli, where they are exhaled during breathing out.