How each part is built for its job

1Why does the air arrive clean?

A moto passes you on a murram road in the dry season and leaves a cloud of red dust. You cough, and later the mucus you blow from your nose is brown. Yet the air that reaches your alveoli, deep in the lungs, is almost free of dust, warm and moist, whatever the weather outside. Something on the way cleaned, warmed and wetted it.

In Lesson 1 you named the parts of the gas exchange system. In this lesson you will see that each part has a structure that suits its job. Biologists call this the link between structure and function.

2Observation: feel it, then look at it

A. On your own neck. Rest two fingertips lightly on the front of your neck, just below the bump of the larynx (the "Adam's apple"). Move them slowly downwards: you can feel firm ridges, the rings of cartilage of the trachea. Keep your fingers on the larynx and swallow: the larynx moves up. As it rises, a flap called the epiglottis closes over it, so that food and drink go down the oesophagus and not into the trachea.

B. Under the microscope. Look at a prepared slide of lung tissue, first with the low-power objective ($\times 10\backslash times\; 10$) and then with the high-power objective ($\times 40\backslash times\; 40$). Find one tube with a thick, folded lining: a bronchiole. Around it, find many empty spaces separated by very thin walls: the alveoli.

Safety. Press gently on your own neck only, never on another person's. Carry the microscope with two hands, and focus with the coarse focus only at low power, as you learned in Unit 4.

3Structure fits function, part by part

Table 1: The parts of the gas exchange system and how each is built for its job

4Why the rings are C-shaped

Figure 1 shows the trachea cut across. The cartilage is not a complete circle: it is a letter C, open at the back, where the trachea lies against the oesophagus.

Figure 1: The trachea cut across, with the front of the neck at the top.

The C shape does two jobs at once. The stiff front and sides stop the airway being squashed flat when you breathe in hard. The soft gap at the back lets the oesophagus bulge forward when a mouthful of food passes down it.

5The alveoli: built for fast diffusion

Gas exchange is fast when four things are true, and the alveoli have all four:

1. A large surface area. About 300 million alveoli in each lung give a total surface of about $75{\mathrm{m}}^{2}75\backslash ,\backslash mathrm\{m^2\}$. The more surface, the more ${\mathrm{O}}_2\backslash mathrm\{O\_2\}$ can cross it at the same time.
2. A thin surface. The wall of an alveolus and the wall of a capillary are each only one cell thick, so gases have a very short distance to diffuse.
3. A moist surface. A thin film of water lines each alveolus. ${\mathrm{O}}_2\backslash mathrm\{O\_2\}$ dissolves in it before it diffuses through the wall.
4. A rich blood supply. A dense net of capillaries surrounds every alveolus. The blood flows on all the time, carrying the ${\mathrm{O}}_2\backslash mathrm\{O\_2\}$ away and bringing more $\mathrm{C}{\mathrm{O}}_2\backslash mathrm\{CO\_2\}$. So the blood next to the alveolus always has less ${\mathrm{O}}_2\backslash mathrm\{O\_2\}$ than the air, and diffusion never stops.

6Dust, mucus and cilia in daily life

In Rwanda's dry seasons, unpaved roads, building sites and brick-making sites send fine dust into the air. Most of the dust you breathe in is caught by the hairs and mucus in your nose, or by the mucus in your trachea and bronchi, and the cilia carry it up to your throat to be swallowed or coughed out. That is why mucus looks darker after a dusty journey.

Three habits help this cleaning system: breathe through your nose rather than your mouth; sprinkle the classroom floor with water before sweeping, so the dust stays down; and wear a dust mask for dusty work such as sanding, quarrying or brick-making.