托福听力文本素材：Conversation1

　　Narrator Listen to a conversation between a student and his biology professor.

　　Student Well, you know, I am writing that paper about whales and the path they travel as they swim through the ocean, their migration patterns.

　　Professor Yes. I remember.

　　Student And well, I was thinking about it and I realized I don't understand how they hold their breath underwater. lt's a little crazy for me to be writing about migration patterns without actually knowing how they stay underwater for so long.

　　Professor Did you do any research to find out how they do it?

　　Student Yeah, I did. I searched on the internet and there was a lot of information about whales, their habitats, the way they communicate, you know, their songs. But if there was anything about whales and how they hold their breath, I missed it. I've got a bunch of books. Actually, I have got so much information, it's a little overwhelming.

　　Professor I am surprised there is nothing about it in any of those books.

　　Student Well, to be honest, l've only skimmed them so far. I am still working on finding sources.

　　Professor OK. I know I encourage everyone in class to look at a substantial number of sources, but I don't want you to get overwhelmed. Looking at a number of sources gives you a good knowledge base, but students only have a limited amount of time to work on each paper. I don't expect you to read a dozen books on whales for this assignment. Focus on just a few.

　　Student Ok. Thanks.

　　Professor You know, since you are already here. I can give you a quick summary of how whales hold their breath underwater. lt's just a matter of certain adaptations in their anatomies, specifically in their circulatory system.

　　Student So the blood flow is what makes the difference?

　　Professor Yes, and in a couple of ways. First, blood makes up a larger share of a whale's weight than in other mammals.

　　Student So they can store more oxygen because they have more blood?

　　Professor Yes, but that's only part of it. They also have a greater capacity than land animals to store oxygen in their blood.

　　Student So how does having more oxygen in their blood help them stay underwater longer?

　　Professor It's the way the whale's blood carries oxygen to the rest of its body. Whales carefully conserve their oxygen when underwater in a couple of ways. When a whale dives, its metabolic rate drops, causing its heartbeat to slow down. And the blood flow to its muscles and some of its non-vital organs, like its kidneys, is also cut off. A whale's muscles and non-vital organs are able to function without oxygen for an extended period of time.

　　Student I see. Well, now I can concentrate on my topic.