The oxygen present in the air reaches the lungs through the upper and lower respiratory tract. Air enters the body through the nasal and oral cavity, then passes through the pharynx, larynx and trachea and then reaches the lungs.
The trachea divides to the left and right bronchi at the fifth thoracic vertebrae and enters the lungs. Where it is further divided into secondary and tertiary bronchi and bronchioles. Terminal bronchioles have a number of vascularised bag-like structures known as alveoli. The gaseous exchange takes place between the alveoli and capillaries.
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