Double circulation

In every full circuit of the body, blood passes through the heart twice. This is double circulation.

Pulmonary circulation

Right ventricle → lungs → left atrium

Blood picks up oxygen and gives up carbon dioxide.

Systemic circulation

Left ventricle → body → right atrium

Blood gives oxygen and food to the cells and collects their waste.

Figure to be added

Figure — double circulation: the heart in the middle, a loop to the lungs and a loop to the rest of the body, oxygen-rich blood in red and oxygen-poor blood in blue.

Because the two sides of the heart are separate, oxygen-rich and oxygen-poor blood never mix, and the body gets blood at high pressure.