Breathing

Air goes in through your nose or mouth, down the trachea (windpipe), into two bronchi, along smaller and smaller bronchioles, and ends in hundreds of millions of tiny air sacs called alveoli. Mucus traps dust and microbes, and tiny hairs called cilia sweep it up and out.

Gas exchange happens in the alveoli. Oxygen diffuses from the air into the blood in the capillaries; carbon dioxide diffuses from the blood into the air, to be breathed out.

The alveoli have…	…which helps because
Walls one cell thick	Gases only have a short way to diffuse
A huge surface area (hundreds of millions of them)	Lots of gas can cross at once
A moist lining	Gases dissolve before they diffuse across
A rich supply of capillaries	Blood keeps carrying the oxygen away, so diffusion keeps going

	Breathing in	Breathing out
Intercostal muscles	Contract: the ribs move up and out	Relax: the ribs move down and in
Diaphragm	Contracts and flattens	Relaxes and domes upwards
Volume of the chest	Increases	Decreases
Pressure inside	Falls below the air outside, so air moves in	Rises above the air outside, so air is pushed out

The bell jar model. The rubber sheet is the diaphragm, the balloons are the lungs, the Y-tube is the trachea and bronchi, and the jar is the chest. Pull the sheet down and the balloons fill. But the glass cannot move like real ribs, and real lungs are spongy, not hollow balloons.

Gas	Air breathed in	Air breathed out
Oxygen	About 21%	About 16%
Carbon dioxide	About 0.04%	About 4%
Nitrogen	About 78%	About 78%

Measuring lung volume: breathe out fully through a tube into an upside-down bottle full of water. The water pushed out equals the air breathed out. A spirometer does it more accurately. Exercise makes you breathe faster and deeper, and regular exercise strengthens the breathing muscles. In asthma, the airways swell, tighten and fill with mucus; a reliever inhaler relaxes the muscles around them. Smoking damages the cilia and the alveoli: see Drugs and health.

Breathing is not respiration. Breathing moves air in and out of the lungs. Respiration is the reaction in every cell that releases energy from glucose.