The Need for Energy

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In order to maintain a constant body mass, energy input must equal energy output. If we eat too much, without doing any exercise we will gain weight and may become over weigh. If we eat too little we will lose weight.

Oxygen for aerobic respiration is obtained from the atmosphere when we take air into our lungs to breathe. Carbon dioxide produced as a waste product of aerobic respiration, passes out of our bodies as we breathe out.

The lungs are a mammal's organs of gaseous exchange. Air enters through the nose or mouth and passes down the larynx and then the trachea. The trachea divides into two bronchi (singular, bronchus) one to each lung. Each bronchus divides further into smaller tubes called bronchioles ending in tiny air sac's called alveoli.

To breathe in - Intercostal muscles contract and raise the ribcage. (raises up and out). The diaphragm contracts and flattens the floor of the chest cavity. Both these contractions increase the volume and decrease the pressure of the chest cavity and the lungs inflate.

to breathe out - Intercostal muscles relax and the rib cage is lowered. The diaphragm relaxes and it returns to its original dome-shape. Both of these decrease the volume and increase the pressure of the chest cavity and the lungs deflate.

The trachea and bronchi are held permanently open by incomplete rings of cartilage. Without these they would collapse and close, leading to suffocation.

The trachea and bronchi are lined with tiny hair-like cilia and glandular cells which secrete a sticky mucus.

Rhythmic beating of the cilia sweeps mucus containing trapped dust and germs upwards to the larynx from where it passes into


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