The respiratory system delivers oxygen to the cells in our body and removes carbon dioxide, the gas our cells constantly produce.

Every time you take a breath, you take air in through the nose (nasal cavity) and sometimes the mouth (oral cavity). There are guard hairs inside your nostrils that help cleanse the air and trap particles. Mucus lines the nasal and oral cavities and helps humidify and warm the air.

Air travels past the pharynx—an organ shared between the respiratory and digestive systems—to enter the larynx, also known as the voice box. Besides producing sounds, the larynx helps keep food out of your airway.

As air passes through an opening in the vocal cords called the glottis, it enters a tube supported by C-shaped rings of cartilage. This is your trachea or windpipe. The trachea divides into two smaller tubes—called the right and left primary bronchi—that enter the lungs. The primary bronchi split off into smaller and smaller bronchi called bronchioles. At the ends of the bronchioles are tiny sacs with very thin walls called alveoli.

As you can see, that large part of our respiratory system serves as a system of pipes delivering air to and from the lungs. Respiration, also called gas exchange, occurs in the alveoli and their surrounding capillaries. The exchange of oxygen and carbon dioxide between the air you breathe in and your bloodstream takes place across the walls of the alveoli. You have about 300 million alveoli in each lung. This gives you a vast surface area—about 80 square meters, or as large as a tennis court—available for gas exchange.

The same pathway the air has to travel to reach your lungs is used when air exits your body, too. This is called bidirectional airflow. Because the path goes both ways, the air in our lungs has less oxygen in it than atmospheric air (the air you breathe in), about 13.7% versus 20.9%. You also have more carbon dioxide in your lungs than the atmospheric air (5.3% versus 0.04%). Keep in mind that as you stay indoors, you breath the air in your room repeatedly, depleting the available oxygen and increasing your carbon dioxide levels. Spending time outdoors can help you enjoy a deep breath of fresh and crisp air!

Natalya Zinkevich, PhD, is an assistant professor at the University of Illinois at Springfield. She teaches courses related to human anatomy and physiology, health and disease, and vertebrate zoology. Her research primarily focuses on the cardiovascular system.