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The human circulatory is one of, if not the, most important system in the body. �It consists of such organs as the heart, and lungs� (Dunbar 4). �However every organ and organ system in the body is nourished and kept alive through the use of the circulatory system� (2-4). The main organ in the circulatory system is the heart. �Basically, the heart is a pump that keeps fresh blood coursing through your body, bringing oxygen and nutrients to all your organs and cells� (2). �A mathematical description of what the heart accomplishes is astonishing. Your heart keeps approximately ten pints (4.73 liters) of blood constantly circulating through seventy �five �thousand miles (one �hundred �thousand kilometers) of blood vessels� (2). Those thousands of miles of blood vessels reach every organ in your body. �Once they reach the organ they deliver much needed oxygen and take away carbon dioxide and other waste products that your body cannot use. This must be done without interruption if you are to remain healthy and alive� (2). �Every living cell in your body depends on oxygen for life. Any cell deprived of oxygen for more than thirty minutes will die. The cells in your brain are even more dependent on fresh oxygen. If your brain cells are deprived of oxygen for only five seconds it will cause you to become unconscious. If your brain is deprived of oxygen for more than four minutes your brain will die� (2). But where does all of this oxygen come from? , You may ask yourself. The lungs, I believe, are the second most important organs in the circulatory system. For one reason they are the only way for oxygen to be received into the body. It is in the lungs that cells absorb fresh oxygen, and where carbon dioxide is released to be expelled from the body. The transfer of these two gasses takes place in the alveoli in the lungs. �Alveoli are �small...