The function of the respiratory system is to transport air into and out of the lungs and to facilitate the diffusion of oxygen into the blood stream, in order to carry it around the body and help remove carbon dioxide. The respiratory system is made up of tissues and organs in the body that enables us to breathe. It is divided into the upper and lower respiratory tracts. The upper respiratory tract consists of firstly the mouth, nose and nasal cavity, its function is to warm, filter and moisten the incoming air. The pharynx is another part of the upper respiratory tract and here the throat divides into the trachea also known as the windpipe and oesophagus. There is also a small flap of cartilage called the epiglottises which prevents food from entering the trachea. The last part of the upper respiratory tract is the larynx. This is what we call the voice box as it is where sound is generated. It also helps protect the trachea by producing a strong cough reflex if any solid objects pass the epiglottises. The lower parts of the respiratory tract consist of the trachea; this is the tube which carries air from the throat into the lungs. The inner membrane of the trachea is covered in tiny hairs called cilia, which catch particles of dust which we can then remove through coughing. The trachea is surrounded by C-shaped rings of cartilage at the front and side which help protect it and keep it open. They are not complete circles due to the position of the oesophagus directly behind the trachea and the need for the trachea to partially collapse to allow the expansion of the oesophagus when swallowing. .
The bronchi is another part of the lower respiratory tract and this is when the trachea divides into two tubes, one entering the left and one entering the right lung. The left bronchi are narrower, longer and more horizontal than the right. Irregular rings of cartilage surround the bronchi whose walls also consist of smooth muscle.