Respiratory system: gas exchange

Every cell in your body needs oxygen to release energy from food, and every cell makes carbon dioxide as waste. Gas exchange is the swap that brings oxygen in and sends carbon dioxide out. It happens in your lungs, in tiny air sacs called alveoli.

Air travels down the trachea and through branching tubes that end in clusters of alveoli. You have hundreds of millions of them. Together their surface covers tens of square meters, many times the area of your skin, which gives gases a huge area to cross.

The alveoli are built for speed. Each alveolus has a wall only one cell thick, wrapped in a net of capillaries, the smallest blood vessels, whose walls are also one cell thick. Gases only have to cross two very thin layers to get from air to blood.

Gases move by diffusion: particles spread from where there are many of them to where there are few. The blood arriving at your lungs has given up oxygen to body cells, so it holds little oxygen, while the air in the alveoli holds a lot. Oxygen diffuses from the alveoli into the blood, where most of it is carried by hemoglobin, a protein inside red blood cells. Carbon dioxide goes the other way. The arriving blood is full of it and the alveoli have little, so carbon dioxide diffuses from the blood into the alveoli and you breathe it out. Diffusion needs no pump. Breathing simply keeps fresh air arriving so the difference stays large. The oxygen-rich blood then returns to the heart, which pumps it to the body.

Your body does not use up all the oxygen you breathe in. Inhaled air is about 21 percent oxygen and 0.04 percent carbon dioxide. Exhaled air is still about 16 percent oxygen but about 4 percent carbon dioxide. Your lungs take only a fraction of the oxygen and add a large amount of carbon dioxide, which is why a rescuer's breaths can still help someone who has stopped breathing.

Breathing is the movement of air in and out. Gas exchange is what happens to the air once it reaches the alveoli. Keep the two ideas separate.