The atria pump blood into the ventricles, although the force of the atrial pumping action is not as strong as that of the ventricles. These are the stronger pumping chambers, responsible for moving blood to the lungs and to the body. Of the two ventricles, the right ventricle is not as strong a pump as is the left ventricle. The right ventricle only pumps blood through the lungs or pulmonary circulation, which has a low resistance and therefore requires less pressure than the left ventricle, which pumps blood around the entire rest of the body. Thus although the amount of flow needs to be equal through the right and left sides of the heart, the pressures are quite different on the two sides.
Blood flows from the body into the right atrium.
That blood is 'blue' and is in need of oxygen, because oxygen has been removed from the blood by the body's organs and muscles in the course of blood flow. This blood is referred to as venous blood. It then passes from the right atrium into the right ventricle, which pumps the blood through the pulmonary arteries to the lungs where the blood picks up a fresh supply of oxygen. The oxygenated or 'red' blood then returns to the heart, flowing from the lungs through the pulmonary veins to the left atrium. The oxygenated blood next passes into the left ventricle, from where it is pumped through the aorta to the body.