All humans must breathe. Our bodies, and all the organs in them, mostly consist of cells. And cells need oxygen in order to work. If they are without oxygen for too long they break. That's why both humans and animals must breathe.
Every year you breathe in about four million litres of air. The air that enters the nose and mouth... ... passes the nasal cavity... ... and moves on through the pharynx... ... through the trachea... ...
which splits into two pipes, the bronchi... ... which in turn split into smaller pipes, the bronchioles, inside the lungs. You might imagine the lungs are two empty bags that you fill with air when you breathe in. But the lungs aren't hollow like that. They are more like sponge ...
or maybe sourdough bread, inside! They're full of tiny sacs. These are called alveoli. The alveoli sit like tiny bunches of grapes, at the end of the thinnest bronchioles. The alveoli are very small - just half a millimetre in diameter - and they are covered with tiny capillaries.
It's here, where the alveoli touch the blood vessels, that the body picks up oxygen, and releases carbon dioxide. Here the gaseous exchange takes place. The oxygen in the air that you have inhaled, passes through the walls of the alveoli into the blood, which carries it to all the cells in the body. At the same time, the blood drops off carbon dioxide, which passes into each alveolus, and is then ventilated out with exhalation. The alveoli in your lungs are very small, but there are many of them, several hundred millions!
And there's good reason there are so many. It's through the walls of the alveoli that the blood takes up its oxygen... ... and the combined area of the walls of all the hundreds of millions of alveoli, is large. If all the alveoli were flattened out, they'd take up about as much space as a tennis court. That's way more than the surface area would be of two bags of air.
And with a larger surface area, we can absorb more oxygen. That's why we have so many alveoli in our lungs. But to be able to breathe you need more than lungs. You need help from your muscles as well. The most important breathing muscle is in your midriff: the diaphragm.
The diaphragm sits below your thorax, which encases the lungs. When you inhale, you tense the diaphragm, it contracts, and is lowered. The space in the thorax gets bigger, and the lungs get more space. They get larger, and air is sucked in: an inhalation. When you breathe out, your diaphragm relaxes again.
The space in the thorax shrinks, and air is pushed out. Your lungs are sensitive. When you breathe in, the air goes through the nasal cavity, The oral cavity, and the sinuses. Here, the air is heated up, and moistened, because the lungs need to be protected from air that is too cold, or dry. The inside of your nose is covered in mucus and nose hair, that collectively work as a filter, trapping dust and dirt.
But some of the dust makes it past the nose hairs and continues downward. The wind pipes are covered in tiny hairs, cilia. The cilia catch dust and particles. And by moving in waves upward, the cilia transfer the dust - together with mucus - upward, and out from the trachea. Then it runs down the esophagus, and we swallow it.