We can describe the human circulatory system as "Double circulation".
The three main blood vessels in our body help transport blood. Artery, which transports blood away from the heart to other organs; veins, which transport blood from other organs back to the heart; and capillaries, which allows substances exchange in organs.
The heart is composed of 4 chambers. Left and right atrium and ventricle. Let's trace the path of a drop of blood. First, the blood starts off with being deoxygenated and full of waste like carbon dioxide. It arrives the right atrium from other organs through the vena cava (vein). The heart then relaxes and the blood goes to the right ventricle. When the heart contracts again, the blood is forced out and is now being transported to the lungs, by the Pulmonary artery. In the lungs, gas exchange takes place. Carbon dioxide diffuses out from the blood and new oxygen diffuses in. Now the blood is oxygenated and full of nutrients. Then, the blood is traveled back to the heart, through the Pulmonary vein, to the left atrium. Again, then to left ventricle and getting forced out of the heart. Now, the blood is being transported to all the other organs (except lungs) to provide oxygen and nutrient to the organs to function. And of course, there it goes again, back to the right atrium through the vena cava.
That's why we call our human circulation a double circulation, as the same blood has to travel to the heart twice before starting the cycle again.
