10. If you were a molecule of oxygen, how would you get from the air we breathe in via the lungs, to the lower leg? Be sure to discuss the pathway of arteries and veins that might be used to get this molecule to the lower body.
Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (where speech sounds are produced) and the trachea which is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to tiny sacs called alveoli. The average adult's lungs contain about 600 million of these spongy, air-filled sacs that are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste-rich blood from the veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when you exhale.
The main way oxygen is carried through the body is within red blood cells, bound to hemoglobin. It gets there via a process of diffusion across the walls of alveoli into capillaries in the lungs. That is how O2 gets to where it needs to be and is carried throughout the body, and how CO2 is returned to the lungs for disposal (it binds to the hemoglobin in the periphery after the O2 is dropped off--oversimplification, but basically that's what goes on).