(2) Between atria and ventricles, and between ventricles and arteries, there are valves that can be opened and closed: these valves can only be opened in one direction: atrioventricular valves can only be opened to ventricles, and arterial valves can only be opened to arteries. This ensures that blood can only flow in one direction: blood can only flow from atrium to ventricle, and from ventricle to artery, but not vice versa.
(3) The gas always diffuses from the place with high concentration to the place with low concentration until it reaches equilibrium; The content of oxygen in alveoli is more than that in blood, while the content of carbon dioxide is less than that in blood. So oxygen diffuses from alveoli to blood, and carbon dioxide diffuses from blood to alveoli; In this way, after the blood flows through the pulmonary capillaries, it changes from venous blood to arterial blood.
The blood cells that transport oxygen in the blood are red cells. Oxygen is carried by these cells, diffuses between tissues through blood circulation and transportation, and enters tissue cells through breathing process, so that tissue cells can breathe, decompose organic matter and release energy.
(4) The heart has four chambers: left atrium, right atrium, left ventricle and right ventricle. The left atrium is connected with pulmonary vein, the right atrium is connected with superior and inferior vena cava, the left ventricle is connected with aorta, and the right ventricle is connected with pulmonary artery. The way of blood circulation and the changes of blood in the process of circulation are shown as follows: the blood flowing through pulmonary artery, pulmonary vein, right ventricle and left atrium is venous blood, arterial blood, venous blood and arterial blood in turn.
So the answer is: (1) body?
(2) prevent blood from flowing backwards (or prevent blood from flowing from the ventricle to the atrium)
(3) gas diffusion; Breathe?
④ Arteries