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Blood is under pressure in the arteries so that it can reach all parts of the body. Respiration releases energy for cells from glucose. This can be aerobic respiration, which needs oxygen, or anaerobic respiration, which does not. During exercise, the breathing rate and heart rate increase. During hard exercise an oxygen debt may build up.

Blood pressure

Arteries carry blood away from the heart.

The blood in the arteries is under pressure because of the contractions of the heart muscles. This allows the blood to reach all parts of the body.

Blood pressure

Blood pressure is measured in millimetres of mercury, mmHg. There are two measurements:

  • systolic pressure - the higher measurement when the heart beats, pushing blood through the arteries, and
  • diastolic pressure - the lower measurement when the heart rests between beats

A young, fit person may have a blood pressure of about 120 over 70, which means their systolic pressure is 120 mmHg, and their diastolic pressure 70 mmHg.

Blood pressure varies with age. It also varies with lifestyle factors such as:

  • diet
  • stress
  • exercise
  • body mass
  • alcohol consumption


Respiration is the process by which energy is released from glucose.

Aerobic respiration

Aerobic respiration requires oxygen. It happens in cells when glucose reacts with oxygen. Here are the word and symbol - higher only - equations:

glucose + oxygen → carbon dioxide + water (+ energy)

C6H12O6 + 6O2 → 6CO2 + 6H2O (+ energy)

Energy is shown in brackets in each equation because it is not a chemical substance.

Anaerobic respiration

Anaerobic respiration does not need oxygen. It happens when there is not enough oxygen for aerobic respiration. Here is the word equation:

glucose → lactic acid (+ energy)

Much less energy is released by anaerobic respiration than by aerobic respiration.


During exercise, the muscle cells respire more than they do at rest. This means:

  • oxygen and glucose must be delivered to them more quickly
  • waste carbon dioxide must be removed more quickly

This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected by measuring the pulse rate.

During hard exercise, the oxygen supply may not be enough for the needs of the muscle cells. When this happens, anaerobic respiration takes place, as well as aerobic respiration. The lactic acid that forms causes muscle fatigue and pain.

Fitness versus health

Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate is likely to return to normal more quickly after exercise.

But being fit is not the same as being healthy. Healthy people are free from disease and infection: they may or may not be fit as well. It is possible to be fit but unhealthy, or healthy but unfit.

Crude oil is a mixture of hydrocarbons. These are separated into useful products, such as fuels, using a process called fractional distillation.

The demand for short hydrocarbon molecules is greater than their supply in crude oil, so a reaction called cracking is used. Cracking converts long alkane molecules into shorter alkanes and alkenes, which are more useful. The exploitation of oil can damage the environment - for example, through oil spills.

Crude oil

Fossil fuels

Crude oil, coal and gas are fossil fuels. They were formed over millions of years, from the remains of dead organisms:

  • crude oil and gas were formed from dead sea creatures
  • coal was formed from dead plants

The fossil fuels are finite resources - there is only a limited supply of them in the Earth’s crust. They took so long to form that they cannot be replaced once they have been used up. They are non-renewable fuels.

How crude oil was formed

Crude oil is found trapped in some of the sedimentary rocks of the Earth's crust.

Millions of years ago, huge numbers of microscopic animals and plants - plankton - died and fell to the bottom of the sea. Their remains were covered by mud.

As the mud sediment was buried by more sediment, it started to change into rock, as the temperature and pressure increased. The plant and animal remains were ‘cooked’ by this process, and slowly changed into crude oil.

Oil is less dense than the water in the rocks and will rise as a result of pressure from below, often escaping altogether if the rocks are permeable.

If some of the rocks above the oil are impermeable, the oil cannot rise through them, so it gets trapped underneath.

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