Right now, without thinking about it, your body is taking in oxygen and pushing out carbon dioxide. This is the job of your respiratory system. Every cell in your body needs oxygen to release energy from food. The waste gas it makes — carbon dioxide — must be removed before it builds up to dangerous levels.

When you breathe in, air enters through your nose or mouth and travels down your throat, called the pharynx. It then passes through the larynx, also known as the voice box. From there, air moves down the trachea — the windpipe. The trachea splits into two tubes called bronchi, which lead to the right and left lungs. Inside each lung, the bronchi divide into smaller and smaller tubes called bronchioles.

At the end of the smallest bronchioles are tiny air sacs called alveoli. The alveoli are surrounded by a net of capillaries — the smallest blood vessels in the body. This is where the magic happens. Oxygen passes from the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be exhaled. This trade is called gas exchange.

None of this could happen without the diaphragm — a strong, dome-shaped muscle below your lungs. When you inhale, the diaphragm contracts and flattens, making your chest cavity bigger and pulling air into the lungs. When you exhale, the diaphragm relaxes and pushes back up, forcing air out. The respiratory system works closely with the circulatory system: your lungs deliver oxygen to the blood, and your heart pumps that blood to every cell in your body.

Background: Your breathing rate is the number of breaths you take in one minute. At rest, most 7th graders breathe between 14 and 20 breaths per minute. When you exercise, your muscles need more oxygen, so your brain signals your diaphragm to work faster. Your breathing rate goes up. Once you stop exercising, it slowly returns to normal as your body recovers.