Human body systems

The skull

提示The bony helmet that sits just above your eyes.

为什么Protection is one of the skeleton's jobs, alongside support, movement and making blood cells. The skull is a rigid case of fused bone, and the ribs do the same job for the heart and lungs.

Relaxing

提示Its partner has shortened, so this one must give way and lengthen.

为什么Muscles can only pull, never push, so they are arranged in antagonistic pairs. One contracts to move a bone one way; the other contracts to move it back.

A hinge joint

提示It swings in one direction only, like a door on its frame.

为什么A hinge allows movement in a single plane, so the elbow can fold and straighten but not rotate. The shoulder is a ball-and-socket joint, which is why your arm can circle.

The breakdown of large food molecules into small soluble ones

提示Your gut wall has tiny gaps — anything too big to fit through must be dealt with first.

为什么Starch and protein are far too big to cross the gut wall. Enzymes cut them into sugars and amino acids, which dissolve and pass into the blood.

Amylase

提示Chew a piece of bread for a minute and it begins to taste sweet — thank the substance in your saliva.

为什么Amylase is made in the salivary glands and the pancreas, and it cuts starch into sugar. Each enzyme has its own shape and works on one type of food molecule only.

The small intestine

提示A long, folded tube lined with millions of finger-like projections.

为什么Those projections are villi. They give a huge surface area, are only one cell thick and each has its own blood supply, so absorbed food is carried away at once.

In the alveoli

提示At the very end of the smallest airways sit millions of tiny air sacs.

为什么Alveoli give the lungs an enormous surface area, are one cell thick and are wrapped in capillaries, so oxygen has the shortest possible distance to travel.

It contracts and flattens

提示The dome of muscle below your lungs pulls downwards to make more room.

为什么As the diaphragm flattens and the ribs swing up and out, the volume of the chest rises and the pressure inside falls, so air flows in from outside.

carbon dioxide

提示The waste gas you breathe out, the one that turns limewater cloudy.

为什么Respiration happens in every living cell, all the time, releasing the energy stored in glucose. It is not the same as breathing, which only moves air in and out.

Twice

提示One loop serves the lungs, a separate loop serves everywhere else.

为什么Humans have a double circulation: heart to lungs and back, then heart to body and back. Returning to the pump between the two keeps the pressure high enough to reach every organ.

Arteries

提示They need thick, muscular walls because the pump sends blood out at high pressure.

为什么Arteries have thick, elastic walls and a narrow lumen to withstand the surge each heartbeat sends. Veins carry blood back at low pressure, so they have thin walls and valves.

Lactic acid

提示It is what makes your legs ache and burn near the end of a sprint.

为什么Without enough oxygen, glucose is only partly broken down, releasing far less energy and leaving a waste product behind. The oxygen you gulp afterwards clears it away.