The heart has four chambers. The small upper chambers are called atria and the lower chambers are called ventricles. So here, right atrium, left atrium, right ventricle, left ventricle.
The heart also has four valves. These are like little doors which only open one way. This ensures...
that the blood is flowing in the right direction and doesn't flow backwards. Tricuspid and pulmonary valve are on the right. The mitral and aortic valves are on the left. It's important to remember that veins carry blood to the heart and arteries carry blood away from the heart.
The superior and inferior vena cava are large veins that deliver deoxygenated blood into the heart. The superior vena cava delivers blood into the heart from the superior part of the body. This includes the head, neck, arms and chest. The inferior vena cava delivers blood into the heart from inferior parts of the body. This includes organs in the abdomen, legs, feet, etc.
The pulmonary vein carries oxygenated blood from the lungs to the heart. The aorta is the main artery that carries oxygenated blood away from the heart to the rest of the body. The pulmonary artery carries deoxygenated blood from the heart to the lungs.
Now let's go through the full process. Deoxygenated blood from the body flows into the right atrium via the superior and inferior vena cava. The right atrium contracts and the blood flows through the tricuspid valve into the right ventricle.
The ventricle contracts and the blood flows up through the pulmonary valve into the pulmonary artery. This is then taken out of the heart into the lungs. At the same time the oxygenated blood from the lungs enters the heart through the left atrium via the pulmonary veins.
The left atrium contracts and blood then flows through the mitral valve into the left ventricle. The left ventricle contracts and oxygenated blood flows up through the aortic valve into the aorta. This blood is then sent around the body via arteries. Remember in one heartbeat blood enters and exits the heart from both sides.