Briefly mention the steps in double-circulation through human heart.

As per NCERT: 'Oxygen-rich blood from the lungs comes to the thin-walled upper chamber of the heart on the left, the left atrium... It then contracts, while the next chamber, the left ventricle, relaxes, so that the blood is transferred to it. When the muscular left ventricle contracts in its turn, the blood is pumped out to the body. De-oxygenated blood comes from the body to the upper chamber on the right, the right atrium... This transfers blood to the right ventricle, which in turn pumps it to the lungs for oxygenation... it goes through the heart twice during each cycle in other vertebrates. This is known as double circulation.' This separation is essential because it prevents oxygenated and deoxygenated blood from mixing, allowing a highly efficient supply of oxygen to the body — vital for animals like birds and mammals that constantly use energy to maintain their body temperature.