Human body systems

The bone marrow

HintThe soft tissue inside your femur, not the hard outer shell.

The whyProtection, support and movement are the skeleton's other jobs. Red blood cells and many white blood cells are made in the marrow of the long bones, the ribs and the pelvis.

Relaxing

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

The whyMuscles 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.

Its distance from the pivot is much shorter, so it needs a bigger force

HintTurning effect equals the pull multiplied by its lever arm, and both effects about the elbow must balance.

The whyMoments balance about the elbow: force × distance is the same on each side. With the load acting 30 cm out and the muscle only 4 cm out, the biceps pulls with roughly seven or eight times the weight of the load. The body trades force for speed — a small shortening close to the joint swings the hand a long way.

Digestion

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

The whyStarch 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

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

The whyAmylase 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

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

The whyThose 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.

The alveoli

HintMillions of them bulge like bunches of balloons at the end of the smallest airways.

The whyTogether they give the lungs an enormous surface area; each is one cell thick and wrapped in capillaries, so oxygen has the shortest possible distance to travel.

The diaphragm contracts; the volume rises and the pressure falls.

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

The whyThe ribs swing up and out at the same time, pulled by the intercostal muscles — two changes working together to enlarge the chest. Breathing out reverses every step.

carbon dioxide

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

The whyRespiration 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

HintOne loop serves the lungs, a separate loop serves everywhere else.

The whyHumans 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

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

The whyArteries 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

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

The whyWithout 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.