A. Blood circulation, systemic circulation and pulmonary circulation in the human body all start from the ventricle. The heart is composed of a large number of muscle tissues, which can contract and relax and end in the atrium of the heart through some blood vessels. Therefore, blood vessels are the conduit of blood circulation, and the heart is the power organ of blood circulation, which does not meet the problem.

B, the route of pulmonary circulation is: right ventricle → pulmonary artery → pulmonary vein → left atrium. When the blood flows through the capillary network outside the alveoli, it exchanges gas with the alveoli. The carbon dioxide in the blood enters the alveoli, and the oxygen in the alveoli enters the blood, changing from venous blood to arterial blood.

C. There are atrioventricular valves, middle venous valves and arterial valves between the ventricles and atria of the heart, which can prevent blood from flowing backwards and ensure blood to flow in a certain direction.

D. In systemic circulation, when blood flows through the capillaries at the tissue, it exchanges substances with the tissue cells: the transported nutrients and oxygen are utilized by the tissue cells; Remove carbon dioxide and other wastes produced by cells. In this way, the blood changes from arterial blood to venous blood. So this option meets the problem.

So choose: C.