Breathing, gas exchange and respiration: three different jobs

People often use the words breathing and respiration as if they mean the same thing. Scientists don't. They are three connected jobs, and each happens in a different place.

Job 1: breathing moves air

Under your lungs is a dome-shaped sheet of muscle called the diaphragm. When it tightens it flattens and pulls down, and muscles between your ribs lift your ribs up and out. The space inside your chest gets bigger, so the air pressure inside drops and air rushes in through your nose or mouth, down your trachea (windpipe) and into your lungs. When the muscles relax, the space shrinks and air is pushed out.

Job 2: gas exchange swaps gases

Inside your lungs the airways split again and again, from bronchi into thin bronchioles, ending in about 300 to 500 million tiny air sacs called alveoli. Each is wrapped in tiny blood vessels called capillaries. The walls are only one cell thick. Oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air to be breathed out. Spread flat, your alveoli would cover roughly the floor of a classroom.

Job 3: respiration releases energy

Your blood carries oxygen and sugar from digestion to every cell. In each cell's mitochondria, cellular respiration uses the oxygen to break down sugar and release energy for moving, growing and repairing. Carbon dioxide and water are left over. When you run, your muscle cells respire faster, so you breathe faster and your heart pumps faster to keep up.

Plants swap gases too

Leaves don't have lungs, but they do exchange gases. Tiny holes called stomata, mostly on the underside of leaves, let carbon dioxide in and oxygen out. Pairs of guard cells swell to open each hole and go floppy to close it, which helps the plant avoid losing too much water.