Cambridge IGCSEtm Biology 4th Edition – D G Mackean

It renews the oxygen supply in the lungs and removes the remaining carbon dioxide. Horseshoe-shaped hoops of cartilage are present in the trachea and bronchi to stop them collapsing when we breathe in. The lungs contain no muscle fibres and are made to expand and contract by movements of the ribs and diaphragm. The diaphragm is a sheet of muscle tissue that separates the thorax from the abdomen (see Figure 11.2). When relaxed, it is domed slightly upwards. The ribs are moved by the intercostal muscles. The external intercostals (Figure 11.11) contract to pull the ribs upwards and outwards.

The internal intercostals contract to pull them downwards and inwards. Figure 11.12 shows the contraction of the external intercostales making the ribs move upwards. Inspiration The diaphragm muscles contract and pull the diaphragm down (Figure 11.13(a)). The internal intercostal muscles relax while the external intercostal muscles contract and pull the ribcage upwards and outwards (Figure 11.14(a)). ▲ Figure 11.11 Ribcage seen from left side, showing external intercostal muscles These two movements make the volume in the thorax bigger, so forcing the lungs to expand.

The reduction in air pressure in the lungs results in air being pulled in through the nose and trachea. This movement of air into the lungs is known as ventilation. ▲ Figure 11.12 Model to show action of intercostal muscles Expiration The diaphragm muscles relax, allowing the diaphragm to return to its domed shape (Figure 11.13(b)). The external intercostal muscles relax while the internal intercostal muscles contract, pulling the ribs downwards to cause a forced expiration (Figure 11.14(b)). The lungs are elastic and shrink back to their relaxed volume, increasing the air pressure inside them.

This results in air being forced out again. ▲ Figure 11.13 Diagrams of thorax to show mechanism of breathing ▲ Figure 11.14 Movement of ribcage during breathing A piece of apparatus called the bell-jar model (Figure 11.15) can be used to show the way in which movement of the diaphragm results in inspiration and expiration. The balloons start off deflated. When the handle attached to the rubber sheet is pulled down, the balloons inflate.

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