Question

What is double circulation actually?

Answer 1

In the human heart, blood passes through the heart twice in one cardiac cycle. This type of circulation is called double circulation. One complete heart beat in which all the chambers of the heart contract and relax once is called cardiac cycle.
It is called a double circulatory system because it has two loops, one from the heart to the lungs and one from the heart to the rest of the body. In a first order circulatory circuit, blood is pumped to the lungs, thus acquiring oxygen while simultaneously releasing carbon dioxide.

Systemic Vein $\Rightarrow$ Sinus Venosus $\Rightarrow$ Right Auricle $\Rightarrow$ Right Ventricle $\Rightarrow$ Pulmonary Artery $\Rightarrow$ Lungs$\Rightarrow$ Pulmonary Vein $\Rightarrow$ Left Auricle$\Rightarrow$ Left Ventricle\Rightarrow Aorta $\Rightarrow$ Systemic Circulation