NARRATOR：

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

MALE STUDENT：

Well, you know, I’m writing my paper about whales and the paths they travel as they swim through the ocean, their migration patterns…

FEMALE PROFESSOR：

Yes, I remember.

MALE STUDENT：

And, well, I was thinking about it and I realized I don’t understand how they hold their breath underwater.

It’s a little crazy for me to be writing a paper about migration patterns without actually knowing how they stay underwater for so long.

FEMALE PROFESSOR：

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

MALE 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’ve got so much information it’s a little overwhelming.

FEMALE PROFESSOR：

Well, I’m surprised there was nothing about it in any of those books.

MALE STUDENT：

Well, to be honest, I’ve only skimmed them so far. I’m still working on finding sources.

FEMALE PROFESSOR：

OK. I know I encouraged 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.

MALE STUDENT：

OK, thanks.

FEMALE PROFESSOR：

You know, since you’re already here, I can give you a quick summary of how whales hold their breath underwater; it’s just a matter of certain adaptations in their anatomy — specifically in their circulatory system.

MALE STUDENT：

So the blood flow is what makes the difference?

FEMALE PROFESSOR：

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

MALE STUDENT：

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

FEMALE PROFESSOR：

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

MALE STUDENT：

[Quizzically]So, how does having more oxygen in their blood help them stay underwater longer?

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 nonvital organs, like its kidneys, is also cut off.

A whale’s muscles and nonvital organs are able to function without oxygen for an extended period of time.

MALE STUDENT：

I see… well, now I can concentrate on my topic!