The anatomy of the human gas exchange system

1Where does a breath go?

Sit still, close your mouth and count your breaths for one minute. An adult at rest takes about 12 to 15 breaths a minute, so in one day you breathe roughly 20 000 times without thinking about it. Each breath travels through a chain of tubes, and ends in about 600 million tiny air sacs, around 300 million in each lung.

In Unit 5 you met the mitochondrion, where aerobic respiration uses oxygen to release energy from glucose and gives off carbon dioxide as waste. Every living cell in your body does this, all the time. So two questions drive this unit: how does oxygen get from the air into your blood, and how does carbon dioxide get out? The organs that do this job make up the gas exchange system.

2Observation: the lungs of a goat

A goat's lungs are built on the same plan as yours, and a butcher can supply a set, still joined to the trachea (a "pluck"). Your teacher will lay them on a dissecting tray.

Safety. Wear disposable gloves and cover any cut on your hands. Use only fresh lungs from a licensed butcher. Never eat or drink in the laboratory. Air may be pushed into the lungs only with a clean foot pump or bicycle pump, never by mouth. Afterwards, wrap the lungs for disposal, disinfect the tray and wash your hands with soap and water.

What to do and what to look for:

1. Run a gloved finger down the trachea. Feel the firm ridges: these are rings of cartilage.
2. Follow the trachea to where it divides into two tubes, one entering each lung.
3. Press the lung gently. It feels soft and spongy, and springs back when you let go.
4. Look at the surface: it is smooth and shiny, because each lung is covered by a thin membrane.
5. If your teacher inflates the lungs with the pump, watch them swell, then shrink back on their own as the air escapes: lung tissue is elastic.
6. Your teacher may cut off a small piece of lung and drop it into a beaker of water. It floats, because it is full of air.

What this tells you: the lungs are not hollow bags. They are a sponge made of millions of air spaces, reached by branching tubes that are held open by cartilage.

3The parts of the gas exchange system

Figure 1 shows the whole system. The head is drawn from the side, so that you can see the passages inside it; the chest is drawn from the front, cut open.

Figure 1: The human gas exchange system. The person's right lung is on your left.

The path of the air. When you breathe in, air passes through these structures in order:

1. the nostrils and the nasal cavity behind them (or the mouth, when you breathe through it);
2. the pharynx (throat), which also carries food;
3. the larynx (voice box), at the top of the trachea;
4. the trachea (windpipe), about 10 to 12 cm long and 2 cm wide in an adult, held open by rings of cartilage;
5. two bronchi, one into each lung;
6. thousands of narrower bronchioles, which branch again and again through the lung;
7. the alveoli, tiny air sacs at the ends of the finest bronchioles, where gas exchange takes place.

Breathing out follows the same path in reverse.

4The chest around the lungs

The lungs lie in the thorax (chest cavity). The ribs form a bony cage around them, and between neighbouring ribs run the intercostal muscles. Below the lungs lies the diaphragm, a dome-shaped sheet of muscle that separates the thorax from the abdomen. Each lung is wrapped in two pleural membranes: one covers the lung, the other lines the inside of the chest wall, and a thin layer of pleural fluid lies between them. In Lesson 3 you will see how the ribs, intercostal muscles and diaphragm work together to move air.

5Zooming in: one alveolus

Every alveolus is wrapped in a net of the smallest blood vessels, the capillaries. Figure 2 shows one alveolus, greatly magnified.

Figure 2: One alveolus and its capillary. Blue arrows: oxygen in. Orange arrows: carbon dioxide out.

The air inside the alveolus has a lot of oxygen; the blood arriving from the heart has little, because the body's cells have used it. So oxygen diffuses from the air, through the wall of the alveolus and the wall of the capillary, into the red blood cells. Carbon dioxide does the opposite: there is more of it in the blood than in the air, so it diffuses from the blood into the alveolus, and is breathed out. The blood then carries the oxygen to the heart, which pumps it to every cell in the body.

6Where you meet this anatomy

At a health centre. When a nurse listens to your chest with a stethoscope, she moves it over the front and back of the thorax, in the positions of the lungs in Figure 1, listening to air moving through the bronchi and alveoli. Crackling sounds can mean that fluid has collected in the alveoli, as happens in pneumonia.

In first aid. In Unit 1 you learned the checks D, R, S, A, B, C. The "A" step, tilting the head back and lifting the chin, works because of the anatomy in Figure 1: in an unconscious person the tongue can fall back against the pharynx and block the way to the larynx. Lifting the chin pulls the tongue forward and opens the airway, so air can reach the trachea.