Fitness and respiration

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. You can see how the heart pumps the blood to the lungs and rest of the body by studying this animation:

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.

Fit for life - higher

Blood pressure

Extremes of blood pressure can create problems.

High blood pressure can cause:

  • kidney damage
  • burst blood vessels
  • damage to the brain, including strokes

Low blood pressure can cause:

  • fainting
  • dizziness
  • poor blood circulation

Muscle fatigue

During hard exercise when anaerobic respiration occurs with aerobic respiration, an oxygen debt builds up. This is because glucose is not broken down completely to form carbon dioxide and water. Some of it is broken down to form lactic acid. Panting after exercise provides oxygen to break down lactic acid. The increased heart rate also allows lactic acid to be carried away by the blood to the liver, where it is broken down.

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