The breathing system and ventilation

Last time

Before we start: last time, in Inspired and expired air, exercise and lung protection, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Twenty thousand breaths

You take about twenty thousand breaths every day without thinking about it. Each breath brings oxygen into your lungs and carries carbon dioxide out. Today: the parts of the breathing system, how the alveoli are adapted, and the muscles that move air in and out.

The breathing system

Here is the human breathing system. Air enters through the nose and mouth, passes the larynx, goes down the trachea, which splits into two bronchi, then into smaller bronchioles, ending in tiny air sacs called alveoli. The lungs sit in the ribcage, with the diaphragm below.

The airways

The trachea is the windpipe. It has C-shaped rings of cartilage that keep it open. It divides into two bronchi, one to each lung, which also contain cartilage. The bronchi branch into many bronchioles, which end in clusters of alveoli.

Alveoli

Alveoli give a very large surface area, about the size of a tennis court. Their walls, and the capillary walls, are only one cell thick, so the diffusion distance is short. A good blood supply carries oxygen away, and breathing brings fresh air, keeping a steep concentration gradient. The surface is moist, so gases dissolve before they diffuse.

Gas exchange

Oxygen diffuses from the air in the alveoli into the blood in the capillaries. Carbon dioxide diffuses from the blood into the alveoli, to be breathed out. Both move by diffusion, down their concentration gradients.

The muscles of breathing

The diaphragm is a sheet of muscle below the lungs. The external intercostal muscles, between the ribs, lift the ribcage up and out. The internal intercostal muscles pull the ribs down and in, during forced breathing out.

Breathing in

Breathing in. The diaphragm contracts and flattens. The external intercostals contract, lifting the ribs up and out. The volume of the chest increases, the pressure falls below air pressure, and air rushes in.

Breathing out

Breathing out is the reverse. The diaphragm relaxes and moves up. The external intercostals relax, and the ribs move down and in. The volume decreases, the pressure rises, and air is pushed out.