In addition to the stimulant nicotine, cigarettes contain many harmful chemicals, like tar and the poisonous gas carbon monoxide. These chemicals present health risks that range from bronchitis to cancer. Doctors believe that cigarette smoking is the cause of 90 percent of all cases of lung cancer. Heart disease, heart attack, and stroke are far more common in smokers. One of the effects of nicotine is constricting the blood vessels, which causes high blood pressure. Another effect is that smoking raises your heart rate, which adds extra stress on your heart. Smoking also affects every part of the body’s circulatory system. Your blood becomes thicker and stickier, making it harder for your heart to work well. The lining of the blood vessels is damaged, allowing fat deposits to stick, which most likely causes arteriosclerosis, or hardening of the arteries. Smoking also stains teeth, fingernails, and lung tissue and causes bad breath.