The Eight Major Systems
The digestive system The excretory system The muscular systemThe respiratory system
The skeletal system The nervous system The circulatory system The immune system
The Respiratory System
The respiratory system is one of the most interesting. We could enter it through the mouth, but since we've already toured there for the digestive system, we'll go through the nose and nasal cavities. This area is surrounded by mucus membranes that are designed to trap dust before it reaches the lungs. Be careful not to get to close to the walls, or you might get caught. After the nasal cavities, we pass through the pharynx, and then down into the larynx, which is where the vocal chords are. Now we slip down the trachea, the walls of which are coated with small hair-like cilia which push foreign material and mucus up out of the lungs. You should also be careful to avoid these, as you could easily get seperated from the rest of the tour.
Now we must choose one path to follow as the trachea branches off into many bronchi. As we follow one bronchus, it also divides into smaller and smaller passageways called bronchioles. Off of the bronchioles, we enter the chambers where the true oxygen exchange takes place. These are called the alveoli. They are small flexible sacs about .1 to .2 millimeters in diameter. The walls of the alveoli are one cell layer thick, as are the walls of the capilaries that carry blood to absorb oxygen. Respiration is partly voluntary control, but also has involuntary mechanisms. For instance, you can not commit suicide by holding your breath, because as soon as you pass out, the involuntary breathing kicks in, and your oxygen content returns to normal. This automatic reaction to to much carbon dioxide in the bloodstream is controlled by the carotid artery, which is located in the walls of the arteries leading to the brain.