A cardiologist is a heart doctor. He or she is the one to visit if you feel a tightness in your chest and shortness of breath.
We know that the suffix -ologist refers to someone who studies some area. To that, we add cardio-, which comes from the Greek kardia, meaning "heart," and we get "someone who studies the heart." The actual study of the heart is called cardiology, with cardiologist coming along a few years later. Besides treating the heart, a cardiologist is also concerned with the circulatory system, including arteries and veins.