In mammals, there is a double circulation whereby the blood flows through the heart twice in one complete circuit.
Double blood circulation in mammals. Click here
The two blood circulations are (1) pulmonary circulation and (2) systemic circulation. Pulmonary circulation involves the flow of blood between the heart and the lungs. Systemic circulation involves the flow of blood between the heart and the other parts of the body.
The heart is an important muscular organ that pumps blood through both the pulmonary and systemic circulations. The heart lies in the thorax behind the chest bone and between the two lungs. It is surrounded by a double layer membranous bag called the pericardium. Between the double membrane layers lies the pericardial fluid that helps to reduce friction when the heart is beating.