The Respiratory System
The primary function of the respiratory system is to supply oxygen to all of the cells of the body. However, the because the respiratory organs themselves do not reach to every cell of the body, the respiratory system interfaces with the circulatory system. The respiratory system also works to expel carbon dioxide from the body. If carbon dioxide accumulates in the body, it can react with water to form carbonic acid, which is harmful to body tissues.
Nasal cavity, pharynx, and trachea
Air is forced into the body by the contractions of the diaphragm, a dome-shaped muscle directly beneath the lungs. Air enters the body through either the mouth or nose, travels through the muscular pharynx, or throat, down the trachea, and into the bronchial tree of the lungs. Air is potentially very harmful to the delicate lungs when it enters the body. It is much colder than the warm body temperature of the lungs. It also contains millions of miniscule dust particles, bacteria, and the like. However, the body has a remarkable way of coping with these problems. Because the air must travel through such an extensive series of passageways to reach the lungs, it has time to gradually become warm before it reaches the delicate lungs. Also, the entire respiratory tract is covered with tiny hairlike extensions of epithelial cells called cilia. These cilia constantly beat in order to sweep dirt away from the lungs. The cells of the respiratory tract also secrete mucous in order to trap bacteria and contaminants to small to be caught by the cilia themselves. The moist membranes also serve to moisten the air. This allows diffusion of oxygen to occur more efficiently in the alveolae, or air sacs.
Lungs and alveolae
Each lung is formed from a bronchial tree which divides into smaller and smaller segments until it reaches the smallest unit, a clump of alveolae. An alveola is a tiny round sac, and they occur in clumps resembling a bunch of grapes. The wall of each alveola is only one cell thick, and each alveola is surrounded by capillaries. It is here that oxygen and carbon dioxide are exchanged in the bloodstream. Alveolae also contain cells called macrophages to destroy and bacteria or contaminants that happen to get past the other defenses previously described. Since these air sacs are so small and delicate and still so vital to survival, the body takes every possible measure to protect them.
