Two pumps, two loops
The heart is two pumps side by side. The right side (blue) sends oxygen-poor blood to the lungs, and the left side (red) sends oxygen-rich blood to the body. Note: the heart is seen from the front, so its right side is on your left.
Return from the body
Oxygen-poor blood from the body enters the right atrium through the venae cavae. It passes the tricuspid valve into the right ventricle.
Lung loop
The right ventricle squeezes, the pulmonary valve opens, and blood travels up the pulmonary artery to the lungs. There it drops carbon dioxide and picks up oxygen.
Back to the left side
Now oxygen-rich, the blood returns through the pulmonary veins to the left atrium, then through the mitral valve into the left ventricle.
Body loop
The powerful left ventricle pushes blood through the aortic valve into the aorta, which carries it to every tissue. Cells take the oxygen, and the now oxygen-poor blood returns to the right atrium.
Valves and septum
Four valves open only one way, so blood can never flow backwards. The septum keeps the two kinds of blood from mixing.