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The bronchi stem from the trachea acting as openings for air into the lungs. The left bronchus is about 5cm long and narrow, whereas the right is 2.5cm long and wider. The bronchi (more cartilaginous in structure) further expand and branch off into bronchioles (which are less cartilaginous and more smooth muscle and epithelial). The bronchioles further lead on to the end of the lower respiratory tract where the gaseous exchange can take place. Further down the bronchioles, ciliated epithelium are replaced with non-ciliated epithelium and mucus secreting goblet cells disappear. The respiratory functions of the bronchioles are to regulate the control of air entry (through contraction or relaxation of smooth muscle in the walls), and provide warming and humidifying to the air.
The bronchioles divide further into smaller respiratory bronchioles and end with the alveoli. These are thin membrane pockets (or sacks) which are surrounded by capillaries and supported by elastic fibres. Gaseous exchange happens between the membrane wall of the alveoli and capillary, which are fused firmly together and known as the respiratory membrane. Septal cells lying between the squamous cells of the alveoli secrete surfactant which prevents the alveoli from drying out. There are about 150 million alveoli in the adult lung which provide gaseous exchange. The respiratory functions of the alveoli are to provide the centre of gaseous exchange between O2 and CO2.