The lower respiratory system is the bottom half of the respiratory tract. It includes the trachea, or windpipe; the bronchi, which are the two branches of the trachea that penetrate the lungs; the lungs themselves; and the bronchioles and alveoli within the lungs, where gas exchange takes place. In the lower respiratory system, air that has been filtered of pathogens and warmed in the upper respiratory tract is transported to the lungs. There, oxygen will be pulled from the inhaled air and deposited into the bloodstream, while carbon dioxide pulled from the bloodstream is released back into the lungs to be expelled from the body upon exhaling. This exchange of gases is the primary function of the respiratory system.
When air is inhaled, it first passes through the upper respiratory system. It moves from the nasal passages or mouth into the throat, also known as the pharynx, and finally into the voicebox, also known as the larynx. At the top of the larynx is an important tissue structure called the epiglottis, which opens during inhalation to allow inhaled air to pass into the trachea and closes during swallowing, covering the laryngeal opening so that food and drink cannot enter the airways. Beyond the larynx the air enters the trachea, the start of the lower respiratory system.
The trachea is a tube about an inch across, several inches in length, and is constructed of stacked semicircles of cartilage, much like the arrangement of the vertebrae in the spine. It performs two functions: it moves air downward from the larynx into the paired bronchi, contributing to the filtering and warming of the air as it does so, and it moves mucus and other materials up out of the lungs, as in coughing. As such, it is partially covered by a muscle called the trachealis, which lends elasticity to the interior of the trachea and permits objects to be pushed upward during coughing.
At the lower end of the trachea is a division into two airways of the lower respiratory system, the bronchi. Forking downward and outward from the base of the trachea, each bronchus penetrates one of the lungs, carrying air that has not only been filtered and brought up to body temperature but humidified to either organ. To distribute this air throughout the lungs, each bronchus subsequently divides into multiple smaller airways; the first divisions are known as the secondary or lobar bronchi, and their subdivisions are the tertiary or segmental bronchi.
Much like the heart, the lungs are made up of several chambers known as lobes: two in the left lung, and three in the larger right lung. Thus the lobar bronchi are those that carry air to each lobe, inside of which the smaller segmental bronchi are found. The segmental bronchi give rise to another branch of the lower respiratory system, the bronchioles. These tiny airways bring the inhaled air to the alveolar sacs, tiny pouches all over the respiratory or final bronchioles that are the sites of gas exchange.
Inside of the alveolar sacs are the alveoli, the final units of the lower respiratory system. These fill the walls of the alveolar sacs and are where oxygen and carbon dioxide are exchanged during respiration. This is because the alveolar walls are also dense on their exterior side with capillaries, which receive oxygen from the air into the bloodstream and in turn release carbon dioxide from the bloodstream into the lungs.