Q.What do you mean by double circulation?

In double circulation, blood makes two separate loops through the heart per full circuit of the body — a pulmonary loop (heart↔lungs) and a systemic loop (heart↔rest of body) — keeping oxygenated and deoxygenated blood fully separated.

Humans (and other birds/mammals) have a four-chambered heart with a complete septal division between the right (deoxygenated) and left (oxygenated) sides. This anatomy forces blood through two distinct circuits, both of which pass through the heart:

1. Pulmonary circulation: deoxygenated blood collected in the right atrium (from the venae cavae) passes to the right ventricle, which pumps it via the pulmonary artery to the lungs. There it picks up oxygen and releases CO2, becoming oxygenated, and returns via the pulmonary veins to the LEFT atrium.
2. Systemic circulation: this oxygenated blood passes from the left atrium to the left ventricle, which pumps it (via the aorta, at high pressure) to every organ/tissue of the body. There it delivers oxygen and picks up CO2, becoming deoxygenated, and returns via the venae cavae to the RIGHT atrium — completing the loop. …