Respiratory system

The respiratory system is divided into two parts – the upper respiratory tract and the lower respiratory tract. The upper respiratory tract includes the mouth and nose, the pharynx (which includes the adenoids and tonsils) and the larynx, or voice box, which is located at the top of the trachea (also known as the windpipe).

The lower respiratory tract starts where the neck meets the chest. It includes the airways that lead to the lungs and the lungs themselves. The body has two lungs – one on the left and one on the right. The lungs are cone-shaped and sponge-like.

Air gets into the lungs through the trachea, which divides just below the top of the breastbone into two bronchi, one leading to the left lung and one to the right lung. These two bronchi subdivide into about 24 further airways, which are called bronchioles. At each division, the width of the airway gets smaller, and they spread out into the lung like the branches of a tree. The airways are made of rings of cartilage which help keep them open during respiration, and rings of muscle that can expand or contract to adjust the width of the airway.

At the end of each branch, there is a tiny sack known as an alveoli, or alveolar sac. This is where the air taken into the lungs enters the blood.

The lungs
The lungs have an extremely good supply of blood. The alveoli are surrounded by a lot of tiny blood vessels. The oxygen in the alveoli dissolves into the blood in these vessels. This blood is then taken to the heart and is pumped around the body to supply the rest of the body with oxygen. The other process that happens in the alveoli is that the carbon dioxide in the blood dissolves into the air in the lungs so it can be breathed out or expired.

The lungs are therefore where oxygen enters the body and carbon dioxide leaves the body. If this exchange is not performed efficiently, the body will have too much carbon dioxide and not enough oxygen. Both of these situations – when the body is starved of oxygen, or when there is too much carbon dioxide – are potentially fatal.

This respiratory tract also has a very active defence mechanism. The airways are all lined by a layer of cells that produce thick mucus. This mucus is intended to warm and moisten the air as it passes into the lungs, and also trap any foreign objects, such as dust, that find their way into the airways. The airways are also lined with thousands of tiny hairs, called cilia. These are in constant movement, constantly sweeping mucus and dust particles upwards where they are can be coughed or sneezed out, or swallowed. When this defence mechanism is in working properly, it provides an efficient defensive system against respiratory diseases.