Respiration

Food gives energy only when it is broken down in respiration. Every kind begins the same way: in the cytoplasm, glucose (6 carbons) is broken into pyruvate (3 carbons). What happens next depends on oxygen.

During a sudden burst of hard exercise, your muscles run short of oxygen and make lactic acid. Its build-up causes cramps.

The energy released is stored in a molecule called ATP, the cell's energy currency. It is spent on muscle contraction, making proteins, sending nerve impulses and more.

Fish take in water through the mouth and pass it over the gills. Water holds much less dissolved oxygen than air, so fish breathe much faster than land animals, a bit like writing faster in the last five minutes of an exam. Plants exchange gases through their stomata.

In humans, air goes in through the nostrils (fine hairs and mucus trap dust), down the throat, which is kept open by rings of cartilage, and into the lungs. There it ends in millions of tiny balloon-like sacs, the alveoli. Their walls are thin and full of blood vessels, so oxygen passes into the blood easily. Spread out, the alveoli would cover about 80 m².

We breathe in by pulling the ribs out and the diaphragm down: the chest gets bigger and air rushes in. In the blood, haemoglobin in red blood cells carries oxygen; carbon dioxide is carried mostly dissolved in the plasma.

Smoking is injurious to health. The book warns that smoking destroys the tiny hairs that keep germs and dust out of the lungs, and that smoking is a common cause of lung cancer. Smokeless tobacco is also a major risk for heart attacks and strokes.