 Now we are going to talk about another thing, that is the double circulation. Human blood circulatory system has a double circulation. Let's have a look on another diagram. Human body have two types of circulations. Look at this circuit diagram. This is not like a normal body but this is just to show that how the blood flows. You can see a heart, body and lungs. Blood, we start from the body. Deoxygenated blood is returned from the body to the heart towards the right atrium. From right atrium it goes to the right ventricle. From right ventricle it goes towards the lungs to be oxygenated. This part of circulation and then lungs oxygenate that blood and then oxygenated blood from lungs is returned to the heart towards the left atrium. Through left atrium heart goes back to the left ventricle. Left ventricle when contracts push the blood towards the body and it is returned towards the body. We call it a double circulation because on the top you can see there is a pulmonary circuit. Blood is going towards lungs and from lungs blood is going to come back towards heart. So, circuit is from heart to lungs and from lungs to heart. This is called the pulmonary circuit. The other one if you see starts from aorta. From aorta towards the body and from body towards the right atrium and then to the ventricle and so on. This circuit is called systemic circuit or systemic circulation. So, human body have a double circulation which is a pulmonary circuit and a systemic circuit. Now, the importance of the valves of the heart in preventing the backflow. Actually, heart valves are just like that of the valves present in the vein are to prevent the backflow. We have seen previously during the cardiac cycle that when the atria are filled the heart, the tricuspid and the bicuspid valve, the atrioventricular valves are open and blood goes down towards the ventricles. When ventricle contracts AV valves are closed. So, blood cannot go back to the atria and then the semi-lunar valves they open. And when the atria contract at that time semi-lunar valves are closed so that there is no backflow towards the ventricles. When the ventricles contract semi-lunar valves are open. When the ventricles relax then there is a chance that blood from aorta and pulmonary vein could return back. But what happened that semi-lunar valves close during ventricular diastole. So, this prevents the blood flow back towards the ventricle. So, the heart valves are very important in preventing the backflow both the tricuspid and bicuspid valves and the semi-lunar valves. This was about the heart and the vessels of blood circulatory system of the human beings. I hope that this is everything is clear and discussed.