Blood circulation in human beings is double and complete, It is double because it was two circuits (General or Systemic, and pulmonary) and complete because the blood in the veins never mixes with blood in the arteries. The arteries go from the heart to the tissues, and the veins go from the tissues to the heart.
Oxygen deprived blood from the superior and inferior vena cava, enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs. Gas exchange occurs in the lungs, whereby CO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the left atrium.