The picture shows the blood circulation pattern of human body, please analyze and answer the relevant questions according to the picture. (1) The power of blood circulation comes from _ _ _ _ _. (2) Blood circulation is divided into (1) The blood circulation system is composed of the heart and blood vessels, which are the conduits for blood circulation. The wall of the heart is composed of myocardium, which can continuously contract and relax, promote blood circulation in the blood vessels, and is the power organ of blood circulation, just like a pump, which can pump blood to the whole body.

(2) The blood circulation of human body can be divided into systemic circulation and pulmonary circulation, in which the route of systemic circulation is: left ventricle → aorta → arteries at all levels → capillary network of all parts of the body → veins at all levels → superior and inferior vena cava → right atrium; The route of pulmonary circulation is: right ventricle → pulmonary artery → pulmonary capillary → pulmonary vein → left atrium. The starting point of pulmonary circulation is right ventricle and the end point is left atrium.

(3) Four left ventricles are the most developed cardiac muscles. The contraction of ventricles transports blood to the capillaries of the whole body or lungs, while atrial contraction only needs to transport blood to the ventricles. According to their respective functions, the walls of ventricles are thicker than those of atria. Similarly, the contraction of left ventricles transports blood to the whole body, while the contraction of right ventricles transports blood to the lungs. Compared with the two, the distance of left ventricles transporting blood is longer, and correspondingly, the walls of left ventricles are thicker than those of right ventricles.

(4) There are atrioventricular valves between atria and ventricles, which only open to ventricles, so that blood can only flow from atria to ventricles, but not back. There are arterial valves between arteries and ventricles, which only open to arteries, so that blood can only flow from ventricles to arteries; Therefore, the blood in the heart can only flow from atrium to ventricle to artery. In the heart structure in the figure, there are atrioventricular valves between 1 and 2, and between 3 and 4 to prevent blood from flowing backwards.

(5) Arteries send blood rich in nutrients and oxygen to the capillary network of various organs of the body, and the blood exchanges with tissues and cells, so that the transported nutrients and oxygen can be used by cells, and at the same time, the carbon dioxide and other wastes generated by cells can be transported away. Thus, the blood changes from arterial blood rich in oxygen to venous blood with little oxygen. Therefore, when the blood flows through tissues and cells, the blood changes from bright red arterial blood to dark red venous blood.

So the answer is: (1) heart.

(2) left atrium

（3）4

(4) Valve

(5) The bright red arterial blood becomes dark red venous blood.