Breathe In, Breathe Out: How Your Respiratory System Works

Place one hand flat on your chest and take a slow, deep breath in. Hold it for two seconds, then breathe out. Do this three times. You can feel your chest rising and falling — but what is actually happening inside your body? Air is travelling through a remarkable set of organs right now, delivering the oxygen your cells need to stay alive. Every minute without breathing would put your body in serious danger. Why is oxygen so urgent — and where does it actually go once it enters your body? Keep that question in mind as you work through this lesson.

The respiratory system is the group of organs that moves air into and out of your body and transfers oxygen into your bloodstream. Here is the route air takes on every single breath. **1. Nasal cavity and mouth** Air enters through your nose or mouth. The nose is preferred because tiny hairs called cilia and a moist lining trap dust and warm the air before it goes deeper. **2. Pharynx and larynx** Air passes through the pharynx (the back of your throat) and the larynx (your voice box). The larynx contains your vocal cords and sits at the top of the trachea. **3. Trachea** The trachea — often called the windpipe — is a tube about 12 cm long reinforced with C-shaped rings of cartilage. Cartilage is a firm but flexible tissue that keeps the tube open so air can always pass through. **4. Bronchi and bronchioles** The trachea splits into two bronchi (one for each lung). Inside the lungs, each bronchus branches into thousands of smaller tubes called bronchioles, like the branches of an upside-down tree. **5. Alveoli — where the magic happens** At the end of every bronchiole are clusters of tiny, balloon-like sacs called alveoli (singular: alveolus). An adult has about 300 million alveoli. Their walls are just one cell thick and are surrounded by a dense network of tiny blood vessels called capillaries. This is where gas exchange occurs. **Gas exchange explained** Blood arriving at the alveoli from the rest of the body is low in oxygen and high in carbon dioxide — a waste gas produced by your cells. Because there is more oxygen inside the alveolus than in the blood, oxygen moves from the air sac into the capillary. At the same time, carbon dioxide moves from the blood into the alveolus because it is more concentrated in the blood than in the air. This movement from a higher concentration to a lower one is called diffusion. The oxygen-rich blood then travels to the heart, which pumps it to every cell in the body. The carbon dioxide travels back to the lungs and is breathed out. **The mechanics of breathing** Breathing is powered by the diaphragm — a dome-shaped muscle beneath your lungs. When it contracts and flattens, the chest cavity gets bigger, air pressure inside drops, and air rushes in (inhalation). When the diaphragm relaxes and rises, the cavity shrinks, pressure rises, and air is pushed out (exhalation). **Exercise and breathing rate** During exercise your muscles work harder and need more oxygen. They also produce more carbon dioxide. Your brain detects the rise in carbon dioxide in the blood and sends a signal to your diaphragm and chest muscles to contract more often. Your breathing rate increases — sometimes from about 12–20 breaths per minute at rest to over 40 breaths per minute during intense exercise.