Health and body systems, Year 9

To take in more oxygen and remove more carbon dioxide

提示Think what the working cells are using up, and what they are making.

为什么Working muscles need more energy, so they respire faster, using more oxygen and producing more carbon dioxide. Breathing faster and more deeply moves more air through the lungs, keeping up the supply of one gas and clearing the other.

The diaphragm and rib muscles have become stronger

提示Breathing is powered by something that responds to training just as a bicep does.

为什么Breathing is driven by the diaphragm and the muscles between the ribs. Like any muscles, they strengthen with regular use, so each breath can be deeper and gas exchange becomes more efficient.

Oxygen reached their muscles more efficiently, leaving less to catch up on

提示Think about how well each runner kept up with demand while the race was still on.

为什么How quickly breathing settles after exercise is called the recovery time, and it is a simple measure of fitness. Stronger breathing muscles and better-used muscles mean the trained runner falls less far behind on oxygen during the race, so there is less to put right when it ends.

They are smaller and weaker

提示A muscle responds to how much work it is asked to do — in both directions.

为什么Muscles grow larger and can exert more force when they are worked regularly, and they shrink when they are not used. This is why training has to be kept up, and why physiotherapy follows a long spell of rest.

Anaerobic respiration

提示Its first two letters mean "without".

为什么Cells normally release energy from glucose using oxygen. When oxygen is short they can still release some energy without it. In human muscle the glucose is only partly broken down, to lactic acid, so much less energy is released.

When oxygen cannot be supplied fast enough, as in a sprint

提示Picture the final seconds of a 100 m race, with your lungs falling behind your legs.

为什么During very hard exercise the muscles need energy faster than the blood can deliver oxygen. Anaerobic respiration tops up the supply for a short time. Aerobic respiration carries on as well; the anaerobic kind is an extra, not a replacement.

aerobic

提示One of them breaks the glucose down completely; the other stops partway.

为什么With oxygen, glucose is broken down fully to carbon dioxide and water and all the available energy is released. Without it, the breakdown stops at lactic acid, which still holds most of the energy. That is why anaerobic respiration can only support a short burst.

Extra oxygen is needed to break down the lactic acid

提示Something built up in the muscles during the race and now has to be cleared.

为什么Lactic acid made during anaerobic respiration cannot simply be left in the muscles. Once the effort stops, the body uses oxygen to break it down, so breathing stays fast and deep until the job is done.

ethanol

提示It is the alcohol in beer and wine.

为什么Microorganisms such as yeast respire without oxygen too, but they make different products from human muscle. The carbon dioxide makes bread dough rise, and the ethanol is what brewers and winemakers are after.

A drug

提示Caffeine, nicotine and paracetamol all count.

为什么Drugs include medicines, which are taken to treat illness, and recreational drugs, which are taken for their effect on mood or behaviour. Alcohol, nicotine and caffeine are legal recreational drugs; many others are illegal because of the harm they cause.

stimulant

提示The name comes from a verb meaning to excite or rouse into action.

为什么Stimulants speed up the messages travelling between the brain and the rest of the body, so reactions are quicker and the user feels more awake. Nicotine is another. Stronger, illegal stimulants put a dangerous strain on the body.

depressant

提示It is the opposite of a stimulant, and its name suggests pressing something down.

为什么Depressants slow the messages travelling between the brain and the body. Thinking, balance and reaction times all get worse, which is why judgement is poor after drinking alcohol. Large doses can slow breathing enough to kill.

Alcohol slows reactions, so the driver takes longer to brake

提示Think about the gap between seeing a hazard and the foot reaching the pedal.

为什么A car travelling at 30 mph covers about 13 metres every second, so even a small delay in reacting adds several metres before the brakes are applied. Alcohol also makes people overconfident, so they take more risks while being less able to cope.

Dependence (addiction)

提示The same word describes a baby's complete reliance on its parents.

为什么With repeated use the body and mind adjust to a drug until the person needs it to feel normal. Stopping brings unpleasant withdrawal symptoms, which makes giving up hard. This is one of the main risks of recreational drugs, including legal ones such as nicotine and alcohol.