The respiratory system is the organ system responsible for bringing oxygen to cells. Here's how it works:

1. Breathing: The lungs, the main organs of the respiratory system, take in oxygen from the air.

2. Gas Exchange: In the tiny air sacs of the lungs (alveoli), oxygen passes into the blood and carbon dioxide (a waste product from cells) passes out of the blood.

3. Circulation: The circulatory system, specifically the heart and blood vessels, carries the oxygen-rich blood throughout the body.

4. Cellular Respiration: At the cellular level, cells use the oxygen delivered by the blood to produce energy through a process called cellular respiration.

In short: The respiratory system takes in oxygen, the circulatory system delivers it, and cells use it to function.