The picture shows the heart structure. According to the picture, the answer is: (1)9 The heart is mainly composed of _ _ _ _ _ _ _ _ _ parts.

(1) The heart is mainly composed of myocardium, which has the characteristics of automatic rhythmic contraction, and the contraction of myocardium provides power for blood circulation. Therefore, the heart is the power organ of blood circulation. The thicker the heart wall, the more developed the muscles, and the stronger the contraction and relaxation. Among the four chambers of the heart, the wall of the left ventricle is the thickest, which is consistent with the distance that each chamber of the heart contracts to transport blood: the contraction of the ventricle transports blood to the capillaries of the whole body or the capillaries of the lungs, while the contraction of the atrium only needs to transport blood to the ventricle. According to their respective functions, the walls of ventricles are thicker than those of atria. Similarly, the contraction of the left ventricle transports blood to the whole body, and the contraction of the right ventricle transports blood to the lungs. Compared with the two, the blood supply distance of the left ventricle is far, and the wall thickness of the left ventricle is thicker than that of the right ventricle, which is suitable for functional differences.

(2) The circulation of blood in the pipeline system composed of the heart and all blood vessels is called blood circulation. According to different circulation routes, blood circulation can be divided into systemic circulation and pulmonary circulation.

In the figure, 2 is the right ventricle, 5 is the pulmonary vein, and 6 is the pulmonary vein, so the process of blood coming out of 2 is the right ventricle, passing through 5 is the pulmonary artery, being transported to the lung, exchanging gas with the lung, increasing oxygen in the blood and reducing carbon dioxide, then passing through 6 is the pulmonary vein, and reaching the left atrium is the pulmonary circulation.

(3) Blood enters aorta 7 from left ventricle 4, and then flows through arteries, capillary networks and veins at all levels of the body. Oxygen in blood is consumed by the physiological activities of tissues and cells, producing a large amount of carbon dioxide. The circulation in which blood changes from arterial blood to venous blood, and finally collects in the inferior vena cava and returns to 1 right atrium is called systemic circulation.

So, the answer is: (1) muscle; Left ventricle; ?

(2) lungs; ?

(3) arteries; vein