The human respiratory system

The way in

Air takes this path into your body.

1. Nostrils. Fine hairs and sticky mucus in the nasal passage trap dust and germs. The air is warmed and made moist.
2. Pharynx, the throat. Food passes through it too.
3. Larynx, the voice box.
4. Trachea, the windpipe. Rings of cartilage in its wall keep it open, so it does not collapse.
5. Two bronchi, one into each lung.
6. Smaller and smaller tubes called bronchioles.
7. Tiny balloon-like sacs called alveoli, at the ends of the bronchioles.

The alveoli: where gases are exchanged

• Each alveolus has a very thin wall, with a network of blood capillaries around it.
• Oxygen diffuses from the air in the alveoli into the blood.
• Carbon dioxide diffuses from the blood into the alveoli, and it is breathed out.
• There are millions of alveoli. If their walls were spread out flat, they would cover about $80$ m². That is about the floor of a badminton court.

The alveoli have every feature of a good surface for the exchange of gases. They are huge in area, thin, moist and full of blood.

Breathing in and breathing out

• Breathing in. The rib muscles lift the ribs up and out. The diaphragm, a sheet of muscle under the lungs, contracts and flattens. The chest gets bigger, and air rushes into the lungs.
• Breathing out. The ribs move down and in. The diaphragm relaxes and curves up. The chest gets smaller, and air is pushed out.
• The lungs never empty fully. About $1$ to $1.5$ L of residual air always stays in them. So oxygen can be taken in, and carbon dioxide given out, even between two breaths.

You should find that $14$ breaths of $0.5$ L give $7$ L of air in a minute. During exercise you take more breaths, and each one is deeper, so the air in a minute rises sharply.

Carrying the gases

In a large body, diffusion alone would be far too slow. It has been estimated that an oxygen molecule would take about three years to diffuse from the lungs to the toes. So the blood carries the gases.

• Haemoglobin is a red pigment in the red blood cells. It has a very high affinity for oxygen. It picks up oxygen in the lungs and gives it up in the tissues, where oxygen is low.
• Carbon dioxide dissolves in water much more easily than oxygen. So most of it is carried dissolved in the blood plasma.
• Small animals need no pigment of this kind, because diffusion is fast enough over short distances.

Try it in Python

The program adds up the air you breathe in a whole day, activity by activity.

You should see that walking uses the most air, $6000$ L, and the whole day about $18\,180$ L. One hour of running uses almost as much air as five hours of walking.

Practise with new numbers

Common mistakes

• Thinking the lungs pull the air in by themselves. The ribs and the diaphragm make the chest bigger, and the air rushes in.
• Thinking we breathe out all the air. Residual air always stays in the lungs.
• Thinking haemoglobin carries all the carbon dioxide. Most carbon dioxide is carried dissolved in the plasma.
• Calling the alveoli tubes. They are tiny sacs at the ends of the bronchioles.

Remember these

• The air passes through the nostrils, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli.
• The alveoli give a huge, thin, moist surface with a rich blood supply, about $80$ m² in all.
• Breathing in: the ribs lift and the diaphragm flattens. Breathing out: the reverse.
• Residual air stays in the lungs, so the exchange goes on between breaths.
• Haemoglobin carries oxygen. Carbon dioxide travels mostly dissolved in the plasma.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What stops the trachea from collapsing?

2. Where in the lungs are gases exchanged, and why is that place good for it?

3. Your chest gets bigger. Does air go into the lungs or out of them? Why?

4. Why is it useful that some air always stays in the lungs?

5. Why do large animals need a pigment like haemoglobin?