Gaseous exchange system

• The lungs are a pair of structures with a large surface area located in the chest cavity with the ability to inflate. The lungs are surrounded by the rib cage which serves to protect them
• A lubricating substance is secreted to prevent the friction between rib cage and lungs during inflation and deflation
• External and internal intercostal muscles between the ribs which contract to raise and lower the ribcage respectively. A structure called the diaphragm separates the lungs from abdomen area
• The air enters through the nose, along the trachea, bronchi and bronchioles which are structures well adapted to their role in enabling passage of air into the lungs
• The gaseous exchange takes plance in the walls of alveoli, which are tiny sacs filled with air
• The trachea, bronchi and bronchioles enable the flow of air into and out of the lungs. The airways are held open with the help of rings of cartilage, incomplete in the trachea to allow passage of food down the oesophagus behind the trachea.
• Trachea and bronchi are similar in structure, with the exception of size bronchi are narrower. They are composed of several layers which together make up a thick wall
• The wall of the trachea and bronchi is composed on incomplete C rings of cartilage
• The cartilage in the trachea and bronchi is a layer of glandular and connective tissue, elastic fibres, blood vessels and smooth muscles which is referred to as 'loose tissue'
• The inner lining is an epithelial layer composed of ciliated epithelium and goblet cells
• The bronchioles are narrower than the bronchi. Only the larger bronchioles contain cartilage. Their wall is made out of smooth muscle and elastic fibres
• The smallest of bronchioles have alveoli clusters at the ends