The inside of your lungs has enough surface area to cover a tennis court.

The human respiratory system uses a branching structure to fit a large surface area into a small space. The windpipe splits into smaller tubes that end in 300 to 500 million microscopic sacs called alveoli. These sacs are where oxygen enters your blood and carbon dioxide leaves it.Each sac is wrapped in tiny blood vessels called capillaries. The wall between the air and your blood is very thin. It measures only 0.2 to 0.5 micrometers thick. This thinness allows gases to pass through quickly.An average adult has enough of these sacs to cover 70 to 100 square meters. This is roughly the size of one side of a tennis court. A large surface area is necessary because oxygen moves into the blood through a process called diffusion. Diffusion works best when there is more space for molecules to move.Your lungs process about 11,000 liters of air every day. This provides oxygen to the 37 trillion cells in your body. Smoking or diseases can destroy the walls of these air sacs. This reduces the surface area and makes it harder to breathe.