The Respiratory System

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The Breathing System

The function of the breathing system is to take in oxygen from the air and get rid of carbon dioxide.  This is called gas exchange.

The diagram below shows the structure of the breathing system.

The lungs are found in the upper part of the body called the thorax.

They are protected by the ribcage and separated from the abdomen below by a sheet of muscle called the diaphragm.

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The Alveoli

Gas exchange occurs across the walls of the alveoli.   They are adapted for this function in several ways:

  • there are many of them, so they provide a very large surface area
  • they have a moist surface, so oxygen can dissolve before diffusing into the blood
  • they are surrounded by capillaries, so have a good blood supply to carry the gases

Oxygen diffuses into the blood from the air in the alveoli and attaches to red blood cells to be transported around the body.

Carbon dioxide diffuses out of the blood plasma into the air in the lungs to be breathed out.

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Breathing In and Out

Breathing is the movement of the diaphragm and muscles between the ribs which results in air moving into or out of the lungs.  This movement of air is called ventilation.

Breathing in is called inhalation.

  • The muscles between the ribs contract pulling the ribcage up and outwards.
  • At the same time the diaphragm contracts and flattens.
  • These movements increase the volume of the thorax.
  • The pressure inside the thorax decreases so air is forced into the lungs.

These changes are reversed during exhalation (breathing out).

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Aerobic Respiration

Aerobic respiration is a chemical reaction that occurs inside structures called mitochondria in cells.

The reaction is summarised by the equation:

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

Energy released is used:

  • to build up larger molecules using smaller ones
  • to enable muscles to contract
  • to produce heat
  • in the active transport of materials across boundaries

During vigorous exercise the circulatory system may not be able to supply enough oxygen to the muscle cells and they stop contracting efficiently.  The cells then use anaerobic respiration to release more energy.

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Anaerobic Respiration

Anaerobic respiration is without oxygen.  It does not break down the glucose completely, so much less energy is released than in aerobic respiration.  Lactic acid is produced, which is poisonous and causes cramp.

The equation for anaerobic respiration is:

glucose → lactic acid (+ a little energy)

After exercise you continue to pant in order to supply more oxygen to oxidise the lactic acid.  The amount of oxygen required is called the oxygen debt.

lactic acid + oxygen → carbon dioxide + water

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Maida Mahmud



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