Describe double circulation in the heart

During a single cycle blood goes twice in the heart,  is known as double circulation. It is necessary in human being to separate oxygenated and de-oxygenated blood since it makes the circulatory system more efficient and helps in maintain the body temperature.

It helps maintain body temperature and it also beats twice.

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an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme