In this topic you will learn about how oxygen gets to your cells.

Your cells can't live more than a few minutes without oxygen. They use it to turn food into energy. The process of obtaining and using oxygen in the blood is respiration. Oxygen comes from the air you breathe into your lungs. The lungs are soft, spongy tissues that can't pull in air on their own. They need muscles to do the work of breathing.

Two main muscles control breathing. One is located between your ribs. The other is a dome-shaped sheet of muscle called the diaphragm. The diaphragm stretches below your chest cavity, under your lungs and above your abdomen. To inhale, the diaphragm contracts and pulls down. Other muscles pull you ribs up and out. This creates room in your chest. Air rushes in and fills the space.

There are other parts in your respiratory system. When you breathe, air is pulled into your nose or mouth. Air travels down the trachea, a stiff tube lined with cartilage that transports air between the throat and the lungs. Your nose, throat and trachea are lined with mucus and cilia. Mucus is a moist, sticky fluid that traps small particles. Cilia are small hairlike structures that move particles and mucus along your throat. The mucus is swallowed and your stomach acids destroy the particles it has collected.

In your chest the trachea divides into two bronchial tubes. One tube enters each of your lungs. Each tube branches into smaller tubes called bronchioles. At the end of each bronchiole are tiny air sacs called alveoli. Capillaries carrying blood from the heart surround them. Your alveoli exchange carbon dioxide for oxygen by diffusion. In diffusion a substance, such as a gas, moves from areas of high concentration to areas of low concentration. In the lungs, oxygen diffuses from the alveoli to the blood. Carbon dioxide diffuses from the blood to the alveoli and is exhaled from there.

Sometimes bacteria or viruses enter your respiratory system. Your nose may run as it makes extra mucus to trap germs. You cough and sneeze to expel germs. Some people find breathing difficult even when they are not sick. Allergies and asthma can cause airways to narrow. By using medicines, people with asthma or allergies can breathe more easily.

To keep your respiratory system healthy, exercise regularly, and avoid smoking. Exercise causes your heart and lungs to work hard for short periods of time. This work strengthens your heart and breathing muscles. A stronger heart circulates more blood when it pumps. Stronger breathing muscles take in more oxygen and release more carbon dioxide.