Gas exchange in the lungs

When we breathe in, air passes through the mouth or nose. At the back of the nose then is a large space called the nasal cavity. This is divided up by bony partition giving a large surface area. The surfaces are lined with fine hairs called cilia and the secrete slimy mucus. This mucus plays an important part in trapping dust and germ before they can get into the lungs. Movements of the cilia sweep the mucus towards the throat where it is swallowed, or coughed up as catarrh or phlegm (pronounced "flem"). Mucus in the nasal passages can also be expelled by blowing your nose.

From the mouth or nose, the air passes through the larynx ("voice box") to the trachea (windpipe). The trachea branches into two bronchi (singular: bronchus), one to each lung. Rings of cartilage strengthen the trachea and bronchi to prevent them collapsing.

In the lungs, the bronchi divide into hundreds of thin tubes called bronchioles. The structure of the bronchioles is like the branches and twigs on a tree. The whole network is sometimes called the bronchial tree.

Each bronchiole ends in a bunch of tiny air sacks called alveoli. Each alveolus is only about 0.2 mm in diameter, but there are about 300 million alveoli in your lungs. This gives a very large surface area across which gases (oxygen and carbon dioxide) can diffuse.

The alveoli are covered with a network of capillaries like a string bag. The walls of the alveoli and the capillaries are extremely thin. Their surfaces are also covered in a thin layer of liquid. The thin walls and the layer of moisture make the diffusion of gases easier. Oxygen inside the alveoli dissolves in the layer of moisture and passes through the walls of the alveoli into the capillaries. It can then be carried away by red blood cells to different parts of the body.

For carbon dioxide, the process is reversed. Carbon dioxide is carried by the blood into the capillaries around the alveoli. Here it passes out of the blood and into the alveoli. It is then expelled from the lungs when we breathe out.

Cellular respiration

Respiration is the overall process of breathing, gas exchange in the lungs and the breaking down of chemicals to provide energy in living things.

Cellular respiration refers to the chemical reactions which occur in cells and which result in the release of energy from foods.

During respiration we use up foods and oxygen and produce carbon dioxide, water and energy. The same thing happens during combustion when fuels burn.

The food (or fuel) reacts with oxygen, so these reactions are called oxidation reactions. We say that the food (or fuel) is oxidised:

food (or fuel) + oxygen  carbon dioxide + water + energy.

Because of these similarities between foods and fuels, foods are sometimes called "body fuels" or "biological fuels".

Although cellular respiration and burning are similar processes, there are important differences. When energy food such as glucose is oxidised in our body cells, there are no flames and the reaction is very slow compared to burning. In fact, the oxidation of simple foods takes place through a large number of separate chemical reactions.

The cellular respiration of glucose can be summarised by the equation:

C6H12O6	+	6O2		6CO2	+	6H2O	+	2900 kilojoules (kJ).
glucose		oxygen		carbon dioxide		water		energy

When this process occurs in our cells, about twenty separate chemical reactions are needed to convert the reactants (glucose and oxygen) into the products (carbon dioxide and water). Notice, in the equation above, the large amount of energy produced. This energy from respiration enables other life processes to occur. It can be used as:

• heat to keep us – warm,

• mechanical energy in our muscles to help us move around and to keep our heart and breathing muscles working.

About half of the energy from cellular respiration is released as heat. This explains why you get hot whilst running or working hard. Your body uses up more food, therefore more heat is produced.