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

Well, you know, I'm reading the papers about whales, and the path they travel as they swim through the ocean, their migration patterns.

Yes, I remember.

Well, I was thinking about it, and I realized I don't understand how they hold their breaths under water. 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.

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

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 breaths, I missed it. I've got a bunch of books.Actually, I've got so much information. It's a little overwhelming.

I'm surprised that there is nothing about it in any of those books.Well, to be honest, I've only skimmed them so far. 

I'm still working on finding sources.

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 of books on whales for this assignment.Focus on just a few.

Ok, thanks.

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

So, the blood flow was what makes the difference?

Yes, and in a couple of ways. 

First, blood makes up a larger share of whale's weight than any other mammals.

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

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

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 weight drops, causing its heart beat to slow down. And the blood flows to its muscles and some of its none-vital organs, like its kidneys, is also cut off. Whales muscles and none-vital organs are able to function without oxygen for an extended period of time.

I see, well, now I can concentrate on my topic.