Gaseous Exchange in Animals

Larynx ? this is the Adam?s apple, which is where the tube in your throat splits up into 2: the trachea that takes air to and from the lungs and the esophagus that takes food to the stomach.
Trachea ? this tube has rings of cartilage that stop it from closing over (you can feel them in your throat. This ensures that air can always pass through.
Epiglottis ? this is a flap that is found in the larynx. Usually it closes over the oesophagus, but when you swallow it closes over the trachea to stop food going into your lungs (sometimes food or drink does go down the wrong tube and makes you cough).
Bronchi ? there are 2 lungs, so the trachea splits up into 2 bronchi: one for each lung.
Bronchioles ? the bronchi divide up into a network of tiny tubes called bronchioles.
Alveoli ? the bronchioles end on groups of small air sacs called alveoli (one is called an alveolus). This is where gas exchange takes place.
Pleural membranes ? it is important that the lungs do not rub on the inside of the chest. The pleural membranes prevent this.
Diaphragm ? this thin layer of muscle separates your thorax (chest cavity) from your abdomen (the area with most of your guts). When it contracts it increases the volume of the thorax and causes air to enter the lungs.
Intercostal muscles ? these are the muscles between the ribs. When they contract, they pull the ribs upwards and outwards to increase the volume of the thorax.

Gas exchange
The exchange of gases takes place in the alveoli. Oxygen crosses from the air in the alveoli to the haemoglobin in the blood. Carbon dioxide crosses in the other direction, from the blood to the alveoli.
The alveoli are especially designed to allow this to happen (again, it would help here to check a diagram). There are large numbers of blood vessels surrounding the alveoli; the alveoli and blood vessels both have thin walls (only one cell thick), so it is easy for gasses to diffuse through; the surfaces are moist, which allows the diffusion to take place more easily; the alveoli provide a large surface area for diffusion to occur. If the lungs were just bags, there would only be a very small amount of surface to exchange gasses, but the alveoli provide a surface area equal to about the size of a tennis court.
The gasses move across the surface by diffusion. Remember that this is the movement of particles from an area where it is highly concentrated to an area where it is less concentrated. Remember, also, that the membrane does not carry the particles across, they just move across randomly.

Inhaling and exhaling (again, check your handouts)
When you inhale (breath in), one set of intercostals muscles contracts pulling the ribs upwards and outwards, which increases the volume of the thorax (chest). At the same time the diaphragm contracts, which further increases the volume. This means that the air in the lungs is now taking up a bigger space, so it will be at a much lower pressure. The air outside the body is now at a higher pressure, so when you open your mouth the outside air pressure forces air to enter your lungs.
When you exhale, the opposite happens. The intercostals muscles pull the ribs downwards and inwards and the diaphragm relaxes, causing the volume of the thorax to decrease, which increases the pressure, forcing air out of the mouth.

Keeping the lungs clean
When you breathe in there are often small particles in the air and bacteria that enter your lungs. These can be harmful and need to be cleaned out. The surface of the lung has cells called goblet cells that secrete a sticky substance called mucous. The particles in the air stick to the mucous which can then be removed. This is done by the cilia, which are tiny moving hairs lining the surface of the lungs. The cilia waft the mucous and particles up the bronchi and trachea where it is usually swallowed. Coughing is a way of removing mucous from the lungs.

Smoke and air pollution
Although the lungs can clean some substances from the lungs, if there is too much it can cause serious damage. Many air pollutants can damage the delicate cells inside the lungs. The effect depends a lot on the type of pollutant that is present, but commonly the person will cough a lot and the efficiency of gas exchange will be less.

Smoking
Cigarettes contain a number of chemicals. Nicotine is the main drug that is found in cigarettes and it acts as a depressant, which means that it calms the body. Nicotine is also highly addictive, so many people smoke because they have a need caused by the addiction rather than getting any useful affect. A number of serious problems can be caused by smoking. The biggest danger is from lung cancer, but many other diseases such as emphysema, bronchitis, heart disease and a whole load of other problems, are associated with smoking. One problem is that the cilia lining the lungs stop being formed, so the lungs are not able to clean themselves. This means that the dangerous chemicals build up and have a greater effect. When someone gives up smoking the cilia grow back and start cleaning the lungs again. Smoking also affects the efficiency of the lungs, so smokers often get short of breath very quickly when they do exercise.