心室收缩时，三尖瓣关闭，肺动脉瓣开放，血液进入肺动脉；心室舒张时，肺动脉瓣关闭，三尖

         There is a zoo mour city. My parents often take me there on holidays. I like animals. I have a lot of
toy animals in my room. In the zoo, I can see tigers, elephants, monkeys, pandas , bears , snakes(蛇) and many other animals. Some animals are friendly, but some are not. Tigers, bears and some snakes are dangerous(危险). That is why they have to stay in cages. But I don't think it's good for animals to stay in cages. They should be free(自由). The animals in cages can't be    happy.
        Tigers usually live in forests and mountains. They can run very fast. They catch and eat small animals
 like rabbits (兔子)and deer(鹿),but now they live in small rooms. They have nothing to do every day. So
they walk round and round in the cages, and they want to get out. When they are tired(累了),they sleep.
 I feel(感到)sorry for them.
        I think the most interesting animal in the zoo is the dolphin. I like watching them swim and jump. They swim fast and they jump very high. They can play with a ball. They can stand up and walk on water! They are very  friendly  to  people.  If you fall into(掉进)the water and can't swim,  they may (可能)come up to
help you.