08 Sep The respiratory system
Air enters our body via our nose and to a lesser extent, via our mouth. The air that enters via the nostrils passes into two nasal cavities appropriately equipped with capillary vessels and covered with hairs and mucus: upon contact with the capillaries, the air warms and is provided with humidity, while the mucus, which is dense and viscous, and the respiratory cilia filter it, keeping out the germs and dust particles. Air which enters the mouth is not warmed or filtered, hence breathing through the nose is recommended.
The air then passes into the pharynx, which is connected at the bottom, on one side, to the esophagus, which leads to the stomach and on the other side, the larynx, which leads to the trachea. The larynx features a flap, the epiglottis which rises as air passes but closes in the presence of food, stopping it from entering the trachea. This explains how it is impossible to swallow and breathe at the same time. The trachea is a long pipe, approximately 12 cm long, which divides into two branches near the lungs, called bronchial tubes.
The bronchial tubes enter the lungs and form 23 sets of branches (more than 8 million in total) which subdivide into ever smaller tubes, smaller in diameter than one hundredth of a millimeter, and called bronchioles. This reduction in diameter is useful to stop particles which have escaped the nasal filter and prevent them from reaching the alveolus, which are the last part of the bronchioles. They are small spherical elements, like grape bunches, surrounded by a close network of capillaries. They represent the true functional area of the lung: from here the oxygen enters the blood to be transported to all cells within our body. To ensure a large surface area of exchange, there are approximately 300 million alveoli and they cover a total surface area of 100-140 sq. m., the same as that of a large apartment.