ation is dark colored, low on oxygen, and high in carbon dioxide. The blood is pumped by the right side of the heart into the pulmonary arteries, which leads to the lungs. The pulmonary arteries divide into smaller and smaller blood vessels, which ending with pulmonary capillaries in the walls of the alveoli(World Book L 528). These alveoli are the smallest of a series of cavities in the lungs.The lungs are designed to receive air, which enters the body through the mouth or the nose. The air passes through the pharynx and the larynx and enters the airways, beginning with the trachea and then into the two primary bronchi(World Book L 527). After the primary bronchi enter the lungs they subdivide into smaller and smaller branches, called secondary and tertiary bronchi. Finally ending in the smallest of the conducting passageways, the bronchioles. This branching and re-branching is often referred to as the bronchial tree. The bronchioles lead into the respiratory zone structures, even smaller conduits, which eventually terminate in alveoli (Marieb 380). Unlike the heart, which is made entirely of muscle, the lungs have no muscles in them. They are forced to expand and allowed to contract through movements of the ribs and the diaphragm. Like your heartbeat, the rhythmic motion of your lungs expanding and relaxing is a constant process. You can however control your breathing to a certain extent. For instance, if you feel like taking a deep breath you can do so instantly. But you cannot make your heart beat faster or slower on impulse. You have the power to hold your breath, but only for a short time. If you hold your breath long enough you will fall into unconsciousness and then resume breathing automatically. The reason behind this is that the muscles that control your breathing are controlled by your bodys nervous system (Dunbar 5). You see if your brain senses that it is not getting enough oxygen it will automatically f...
