Inspired and expired air, protection and exercise

Last time

Before we start: last time, in Pathogens, transmission and cholera, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

What you breathe out

Breathe onto a cold mirror and it mists up. Breathe through limewater and it turns cloudy. Your breath is different from the air you breathe in. Today: what changes, how the airways are protected, and why you breathe harder when you run.

Inspired and expired air

Here are the approximate percentages. Expired air has less oxygen, about a hundred times more carbon dioxide, and more water vapour. Nitrogen is about the same, because the body does not use it.

Why the air changes

Less oxygen: oxygen has diffused into the blood, to be used in respiration. More carbon dioxide: it is made by respiration and diffuses out of the blood. More water vapour: water evaporates from the moist alveolar surfaces. Expired air is also warmer, because it has been warmed in the body.

Limewater test

Limewater turns milky or cloudy when carbon dioxide bubbles through it. Breathe out through one tube of limewater, and draw air in through another. The expired air tube turns milky much faster: it contains more carbon dioxide.

Protecting the airways

Goblet cells in the lining of the trachea and bronchi make mucus. Mucus traps dust, pollen and pathogens. Ciliated cells beat their cilia to sweep the mucus up to the throat, where it is swallowed.

Smoking

Tar in cigarette smoke damages the cilia, so mucus builds up and causes a smoker's cough. Carbon monoxide combines with haemoglobin, so less oxygen is carried. Smoking can cause lung cancer, bronchitis and emphysema, where alveoli walls break down.

Breathing and exercise

During exercise, both the rate and the depth of breathing increase. Muscles respire faster and make more carbon dioxide, which enters the blood. Receptors in the brain detect the higher carbon dioxide concentration. The brain sends impulses to the breathing muscles, so you breathe faster and deeper.

After exercise

Breathing stays fast for a while after exercise stops. This extra oxygen is used to break down lactic acid made in anaerobic respiration. The rate falls back to normal once the lactic acid is removed.