Breathing and gas exchange.

Step 1 of 7

A breath in

Every cell in the body needs oxygen to release energy from food, and makes carbon dioxide that has to be got rid of. The breathing system brings air to a place where the gases can cross into the blood and back.

In the Scene the chest is breathing at rest: 12 breaths a minute, about 500 mL each. Watch what moves as the air comes in, and what stays still.

Step 2 of 7

Air a minute

The minute volume is all the air breathed in a minute: the number of breaths times the air in each. Breathing rate and Breath depth are now fixed. Work out the minute volume yourself.

Step 3 of 7

The dead space

Air comes in through the windpipe, which branches into bronchi and smaller and smaller tubes, until it reaches the air sacs (alveoli): tiny bags, hundreds of millions of them, with walls one cell thin wrapped in a net of capillaries. Only there do the gases pass between the air and the blood.

But the tubes leading to the air sacs are full of air too, and it meets no blood. At the end of each breath out they are full of air that was already in the lungs, and on the next breath in it goes back in first. About 150 mL of every breath only refills these tubes, called the dead space.

Step 4 of 7

Panting

The dead space does not depend on how deeply you breathe: it is the same volume of tubes. So it takes a small share of a deep breath and a large share of a shallow one. What happens with many shallow breaths?

Step 5 of 7

Deeper, not faster

When the body needs more air, it can add breaths or deepen them. Every millilitre that deepens a breath goes straight to the air sacs, because the dead space is already filled. Breathing rate is now fixed, and Breath depth is yours.

Step 6 of 7

Inside the air sac

Zoom to an air sac is at Air sac. Oxygen diffuses from the air in the sac into the blood in the capillaries, and binds to the haemoglobin in the red blood cells. Carbon dioxide goes the other way. Each gas moves from where its pressure is high to where it is low.

The chart shows haemoglobin’s loading curve: how much of it carries oxygen at each oxygen pressure. At rest, at sea level, the blood leaving the air sac is nearly full.

Step 7 of 7

Thin air

The higher you go, the lower the air pressure. The air still has the same share of oxygen, about 21%, but each breath takes in fewer molecules, and the pressure pushing oxygen into the blood is smaller. Altitude is yours; the breathing itself stays as it is.