The major structures of the lower respiratory tract include the trachea (windpipe), and within the lungs, the bronchi, bronchioles, and the alveoli.
After its journey through the larynx, inhaled air reaches the trachea. The trachea is made up of firm, C-shaped cartilage rings that give the trachea its rigidity and allows it to stay open continuously. The trachea is about about 4 inches long and 1 inch in diameter and is very flexible in nature. Like the nasal cavity, the trachea helps to filter, warm and humidify the air that passes through it.
