Answer: The mouth is considered part of the digestive system, because it is the entry point for food going into your body. Additionally, digestion begins in the mouth. Saliva contains a enzyme called amylase that begins digesting carbohydrates before they enter your stomach. If you hold a piece of bread in your mouth for around a minute, it will begin to taste different -- this is the amylase at work.
Answer: The heart is the only real organ, but your cardiovascular system includes the heart, major arteries and veins (totally different from your arteries), your capillaries, and your blood. Also note that these are the basic main elements of your cardiovascular system, in a more detailed description you would find the specific arteries and veins (jugular vein, pulmonary artery, etc.) and your right and left ventricles.
Answer: there is no such system in the body. the 12 body systems are integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, immune, respiratory, disgestive, urinary, and reproductive.
Answer: You need long strides, therefore hamstrings, butt, quads, and calves. A strong core is also VERY important, it stops your body from wavering and allows you to stay tall while driving up your knee.