Bronchi and Bronchioles
As we have already said, this section of the respiratory tract begins when the trachea divides into the two bronchi on their path towards the lungs. Following this split these tubes then continue to divide, much like the branches of a tree, into the smaller and more numerous bronchioles.
The lung structure is extremely complex and intricate. The huge number of bronchioles means that it has a very high overall surface area. However, at the end of each of these branches there are several clusters of minute sacks known as alveoli. In all there are millions of these tiny air spaces in our lungs. It is in the alveoli that respiration truly takes place. Alveoli are extremely thin and delicate– being composed of only a few cells in total. They have a very rich blood supply, in fact they are completely encapsulate by networks of capillaries that look like a net.
The blood that flows through these capillaries comes directly from the pulmonary arteries that originate on the right side of the heart. This blood has previously travelled to the heart from the rest of the body. The oxygen it once contained has been used up, and it is now laden with carbon dioxide.
When we breathe in the air that reaches the alveoli is rich in oxygen and contains little carbon dioxide. Because of this relative difference between the blood and the air, the oxygen from the air diffuses into the blood and the carbon dioxide in the blood is drawn into the air.
The now, richly oxygenated blood from these capillaries is then pumped back to the left side of the heart via the pulmonary veins (These are the only veins to carry oxygenated blood).