The normal heart is a continuously powerful pump made up of muscles, about the size of my fist. The heart has four chambers, the upper two chambers are the atria and the lower two chambers are the ventricles. (Figure A) Blood flows through the chambers of the heart guided by four valves that open and close to allow blood to flow in a single direction. These four valves are: (1) tricuspid valve: located between the right atrium and the right ventricle (2) pulmonary valve: located between the right ventricle and the pulmonary artery (3) mitral valve: located between the left atrium and the left ventricle (4) aortic valve: located between the left ventricle and the aorta Each valve consists of a set of leaflets. Except for the mitral valve, which has two leaflets, the other three valves are composed of three leaflets. After the body circulation, dark red blood (low in oxygen) returns through the veins to the right atrium of the heart, which sends blood through the tricuspid valve to the right ventricle (Figure B) The right ventricle sends blood at a lower pressure through the pulmonary valve to the pulmonary artery, where this blood enters the lungs, where it receives fresh oxygen (Figure C), turns bright red, returns through the pulmonary veins to the left atrium, and then passes through the mitral valve to the left ventricle (Figure D ). The left ventricle pumps the bright red blood through the aortic valve into the aorta (Figure E), which sends this blood into the body circulation. The pressure in the left ventricle is essentially the same as the blood pressure measured in the arm.