The normal heart is made up of four main chambers:
The two upper collecting chambers are the atria (right and left atrium)
The two lower pumping chambers are the ventricles (right and left ventricle)
The blue or used blood that has been around the body returns to the right atrium via two large veins- the superior and inferior vena cava. Here it goes through a one way valve called the tricuspid valve into the right ventricle. The blood is then pumped via another one way valve, the pulmonary valve, into the pulmonary or lung arteries into the lungs.
The pink or oxygenated blood returns to the heart via the pulmonary veins into the left atrium. Here it goes through a one way valve called the mitral valve into the left ventricle. The blood is then pumped via another one way valve, the aortic valve, into the aorta or body artery around the body.
Before babies are born there is a hole connecting the two atria called the foramen ovale and a tube connecting the aorta and the pulmonary artery call the ductus arteriosus. These have to be open before the baby is born and are programmed to close after birth.