Diving Deep To Appreciate The Spleen

17:34 minutes

Bajau diver
A Bajau diver hunts fish underwater using a traditional spear. Credit: Melissa Ilardo

Consider the spleen. Many may not appreciate or even think about them very much at all, unless they’ve had them removed, but the Bajau people of Southeast Asia rely on them every day without even knowing it. The Bajau are “sea nomads,” meaning they get everything they need to live by diving up to 65 feet under water, multiple times, for up to eight hours a day. But it’s not their large lung capacity that give them an advantage during a dive—it’s their extra large spleens. The spleen, which stores oxygenated blood, contracts as part of a human “dive response,” sending an extra boost of O2 to the system, like a built-in oxygen tank. Dr. Melissa Ilardo, post-doctoral researcher in the Department of Molecular Medicine at the University of Utah joins Ira to discuss the finding.

Plus, the Bajau have evolved to have larger spleens to account for their diving lifestyle, but they’re not the only group of humans to have a genetic adaptation to a low-oxygen environment. Dr. Cynthia Beall, professor of anthropology at Case Western Reserve University joins Ira to discuss how the people of Tibet carry a genetic trait that keeps them breathing comfortably at high altitude.

Segment Guests

Melissa Ilardo

Melissa Ilardo is a post-doctoral researcher in the Department of Molecular Medicine at the University of Utah in Salt Lake City, Utah.

Cynthia Beall

Cynthia Beall is a professor of Anthropology at Case Western Reserve University in Pittsburgh, Pennsylvania.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. How often do you think about your spleen? Maybe when you hear of someone who’s had it removed. Doesn’t feel like a particularly useful organ. But the Bajau people of Southeast Asia rely on their spleens every day without even knowing it. The Bajau are sea nomads. They get everything they need to live by by diving, holding their breath multiple times for up to eight hours a day. Sometimes they can go down as much as 200 feet. You’d think they must have very large lungs to do that.

But in fact, it’s their extra large spleens that give them an advantage during a dive. Joining me to talk more about this new research is Melissa Ilardo, former researcher at the University of Copenhagen, now in the Department of Molecular Medicine at University of Utah. Welcome to Science Friday.

MELISSA ILARDO: Hi. Thanks for having me.

IRA FLATOW: I want to invite our listeners if they have questions about super spleens, or other adaptations for living in extreme environments, to send us a tweet @scifri.

You know, we don’t think about our spleens much, do we, the average person?

MELISSA ILARDO: No. Yeah, we really don’t. It’s kind of a bizarre organ. Because as you said, sometimes it gets taken out, so it seems like we can live without it. So how important could it really even be?

IRA FLATOW: Yeah. So tell us why it’s so important to the Bajau people.

MELISSA ILARDO: So it turns out that one of the things that the spleen does is that it stores oxygenated red blood cells. And so this is useful when you’re diving, because there’s something called the human dive response, and that’s actually triggered by holding your breath and being in the water. And what happens is your heart rate slows down, your blood vessels constrict, and then your spleen contracts. And so this gives you an oxygen boost. So in a way, it’s kind of like a biological scuba tank.

IRA FLATOW: That’s interesting, because diving is really important to their culture, isn’t it?

MELISSA ILARDO: It is. Yeah, it’s a huge part of their life. Traditionally they’re traveling around in these houseboats and getting everything they need from the sea.

IRA FLATOW: So you needed to measure the size of their spleens, right? To know that they’re bigger. How did you go about doing that?

MELISSA ILARDO: Yes. So I didn’t want to just show up and say, hey, can I measure your spleen? So I started with an initial visit. Yeah, exactly. It was already kind of weird that I was coming out of nowhere. But I went on an initial visit just to introduce myself, and introduce the science. And make sure that they really understood it and were interested to participate. And so on my second trip, once they had kind of opened up to the idea, I brought a portable ultrasound machine with me. And I took ultrasound images of their spleens.

IRA FLATOW: And how did you know it was a genetic adaptation that occurred, and not just a mechanical change? You know, if you use your muscles, your muscles get bigger. Maybe if you use your spleen more, it just gets bigger?

MELISSA ILARDO: Yeah, exactly. That was something that we really wanted to clear up early on. And so we took measurements both from Bajau divers, as well as Bajau non-divers. And so we were able to see that their spleens were approximately the same size. So this indicates that if there’s something going on, it is happening at a genetic level, rather than a plastic response to the activity of diving.

IRA FLATOW: Could you have a genetic adaptation that happened so quickly, over a period of hundreds of years? Or I don’t know, maybe thousands?

MELISSA ILARDO: Yeah. We’re not actually sure how long they’ve been diving, so it’s hard to say. But, yeah, we really know very little about how long it takes for these kinds of adaptations to occur. But it seems like it’s been long enough for the Bajau.

IRA FLATOW: I want to bring on another guest, because this is not the only example of modern humans undergoing a genetic change to adapt to life with less oxygen. I want to bring in my next guest, Cynthia Beall. She is professor of anthropology at Case Western Reserve University, also in Cleveland. Welcome to Science Friday.


IRA FLATOW: Now, I understand you study Tibetan people who are able to live at higher altitudes without suffering from the effects of hypoxia. How are they able to do that?

CYNTHIA BEALL: Well, they have adapted both physiologically and genetically. Like Melissa, we were interested in linking the physiology and the genetics. And one of the surprising things about Tibetans is that they live at high altitudes without the high levels of hemoglobin that you or I would get if we were living at high altitude. Or that the Andean Highlanders have at the high altitudes.

So they have an unexpected biological response in the sense of not having much of a hemoglobin response. And we’ve been able to link that to a genetic locus called EPAS1 that associates with the low, or the lack of response, of hemoglobin. So we have a genetic locus, and a biological response.

IRA FLATOW: And so does the genetic locus do for them? How does it make them do this?

CYNTHIA BEALL: Yeah. What the genetic locus does is it basically turns down the hemoglobin response that you or I would have, or that an Andean Highlander has. So it turns down the whole pathway leading from an oxygen sensor to erythropoietin, the hormone that initiates the production of red blood cells full of hemoglobin.

So in Tibetans, that whole pathway is turned down, dampened.

IRA FLATOW: Interesting. So what happens to a person who doesn’t have this genetic adaptation, like you or I, if we tried to live with the Tibetans for a year?

CYNTHIA BEALL: Well, I’ve done that. Many others have done that.

IRA FLATOW: Tell us, tell us.

CYNTHIA BEALL: Yes, tell us. Tell us. Well, we maintained the high hemoglobin concentration. And you know, our blood is a little bit thicker. It flows a little slower. You and I would also, then, develop some pulmonary hypertension, because our heart is working hard against our lungs to push that sluggish blood around. And the Tibetans do not have that pulmonary hypertension and the extra work on their heart.

IRA FLATOW: Melissa, let’s talk about the– back to the Bajau now. Do we know, with being deprived of oxygen on these dives, maybe they have any other effects, any long term effects?

MELISSA ILARDO: In terms of long term physiological effects, we haven’t really had a chance to look into that yet. But seeing them, they seem like very healthy fit people. But they do also have a number of other adaptations it seems to this diving activity.

IRA FLATOW: Such as?

MELISSA ILARDO: So one gene that we identified that’s been under selection is this gene BDKRB2, and that has to do with the vasoconstriction response that I mentioned that happens while you’re diving. So it seems like they have an increased ability to constrict their blood vessels in order to preserve oxygenated blood for their heart and their lungs and their brain.

IRA FLATOW: Is this something that other diving mammals or animals have also? That basically we’re mimicking what maybe a seal does?

MELISSA ILARDO: Yeah. Definitely in the case of the spleen, that was one of the first clues that that was something that we should target in our search. Because species of diving seals that can hold their breath for longer than other species of seals have been shown to have these disproportionately large spleens. And it’s been hypothesized that that gives them an advantage in diving.

IRA FLATOW: Now Cynthia, if we know that the spleen is involved in breathing here for the Bajau, perhaps the Tibetans may have something. Do they have a larger spleen, perhaps, that would help them breathe at a higher altitude?

CYNTHIA BEALL: I have no idea. I have never seen a measure of spleen size for high altitude populations.

IRA FLATOW: Go back with your machine and go measure it.


MELISSA ILARDO: Please come with us.

CYNTHIA BEALL: I’d be happy to join you.


CYNTHIA BEALL: OK. It’s a plan. I did hear from someone this morning who is, along with me, attending a conference on hypoxia. He had looked at spleen size in deer mice who live at a range of altitudes here in the US. And he said that at least the hypoxic deer mice do not have enlarged spleens. So we have one species there for you to compare with.

MELISSA ILARDO: Yeah. One reason I would say that might be the case, too, is that the Tibetans are undergoing chronic hypoxia, so they’re constantly living at these lower levels of oxygen. Whereas in the Bajau, it’s this very acute hypoxia. So we might expect there to be different adaptations to these different conditions.

IRA FLATOW: Well, let me just turn that around. Oh, go ahead. I’m sorry.

CYNTHIA BEALL: I was going to say, I agree completely that the nature of the stress is very different at high altitude and in diving. And so I agree that we would expect the adaptations to likely be very different.

IRA FLATOW: So we would think in general, then, some sort of stress in any way might lead to some sort of genetic difference over the years. Maybe not hypoxia, maybe not having the diving– how about weak folks who are living with stress all that time in our lives. Might we be developing some sort of genetic mutation for those who handle it better?

MELISSA ILARDO: Well, one thing that we need for natural selection to work is something that’s either killing you or that’s making you have more children. So I’m not sure.

IRA FLATOW: Ding ding.

MELISSA ILARDO: Yeah. Maybe stress is killing us, but maybe not as quickly as diving. Because diving can actually be extremely dangerous.

IRA FLATOW: Right. Right. Let’s now go in the opposite direction now, Cynthia. If the Tibetan people were to come down from their mountain and try to live at sea level for a year, would they have any trouble doing that? Would they be getting too much oxygen now, if they’re not used to all this oxygen?

CYNTHIA BEALL: No. It seems from what little we know that the Tibetans do just fine at low altitude. Their responses to altitude, when they are at altitude, are dampened, and the response to the relief from hypoxia don’t seem to bother them at all.

IRA FLATOW: Melissa, let’s talk more about the spleen, because I promised my listeners that we would learn everything we could.


IRA FLATOW: It’s an organ that we said before, it’s an organ that does not get a lot of recognition. So tell us what is its purpose?

MELISSA ILARDO: Yeah. It’s completely under-appreciated, I think. But it seems to be this contraction is not only happening in diving, but it also happens during physical activity to a lesser extent. So that could be generally it’s acting as this reservoir for oxygenated red blood cells. But it’s also involved in the immune response to certain bacteria.

IRA FLATOW: Hmmm. And tell me more about that. I hadn’t heard about that.

MELISSA ILARDO: That’s not something I’ve looked into so much, because I’ve been mostly focused on the dive component. But yeah.

IRA FLATOW: Yeah. So I’m going to just think out loud– maybe our spleen has a microbiome in it also?

MELISSA ILARDO: It absolutely could.

IRA FLATOW: That keeps it healthy? We know that the larger spleens make the Bajau better divers, but could it give them other super abilities? Like could they be better athletes, because they have a bigger spleen? You know, run long distance races better?

MELISSA ILARDO: Yeah, that’s certainly something we’re interested in. Especially because another animal that’s often talked about in terms that the physiology of the spleen is the horse. And it’s thought that the spleen in horses is so large because it has something to do with their running and their running capabilities. So it could be that there is something to do with athletic performance, generally, and spleen size.

IRA FLATOW: You know, Cynthia, we’ve heard of athletes going to higher levels– you know, I mean altitudes like Mexico City, whatever, to train because it affects their blood. They used to call it blood doping, things like that. Could there be spleen doping, Melissa? Where you practice for years underwater?

MELISSA ILARDO: Well, yeah, I’ve already been asked–

IRA FLATOW: If they get bigger.

MELISSA ILARDO: Yeah. So we actually found that the spleen size, the difference, we believe is at least in part attributable to these higher levels of the thyroid hormone T4. And so I’ve already been asked if people could inject themselves with T4 to get this bigger spleen. And I say, no, please don’t do that. Because there are a lot of health consequences associated with hyperthyroidism. So we’re not exactly sure what’s happening in terms of the physical consequences of this relationship between the thyroid hormone and spleen size.

IRA FLATOW: You don’t want a mean spleen.

MELISSA ILARDO: No. Certainly not.

IRA FLATOW: This is Science Friday from WNYC Studios. I’m Ira Flatow, talking about our spleen, of all things, with Melissa Ilardo of the University of Utah, and Cynthia Beall of Case Western Reserve.

How did you get into studying the spleen? Why go to medical school or whatever and say, I want to study the spleen? How does that happen?

MELISSA ILARDO: Yeah, it was kind of an accidental discovery. So first I heard about the sea nomads, and that was what I was interested in. Because I thought, this seems like a really interesting possibility for studying natural selection. And then I started looking into the physiology of diving. And that was when I came across the dive response, a component of which, as I said before, is this contraction of the spleen. And so the studies that I had seen to do with the spleen contraction mostly talked about seals, and how the seals that can dive for longer have these larger spleens.

So, yeah, it wasn’t really something I spent a lot of time thinking about before I started this research. I wasn’t even really sure where it was or what it did before. But now I’m a big fan.

IRA FLATOW: And you, Cynthia, how did you get involved in this?

CYNTHIA BEALL: I got involved– you mean in studying evolution or studying high altitude?

IRA FLATOW: High altitudes.

CYNTHIA BEALL: High altitudes– I got involved when I was in graduate school studying how people adapt to the environment. And various students chose various environments, and I liked high altitude. It’s a fun environment, partly because it’s a chronic stress. It’s chronic and unavoidable, and everyone at a given altitude has the same stress. Which makes it very different from the dive response, which is an interesting contrast.

IRA FLATOW: Yeah. Let me see if I can get a quick call in from Tempe, Arizona. Alicia, welcome to Science Friday.

CALLER: Hi. So I went to Peru a few years ago. I was working there for five months. And I started feeling really terrible all the time. Eventually I go to Lake Titicaca, and I felt awful. I mean, I’m only 23. I should have been able to do the activities that were there. And then I came back to the US, it turns out that I had severe anemia. I wound up in the hospital with a hemoglobin of 5, and I had to get two blood transfusions and iron transfusions.

So my question is, why wasn’t I able to– the worst of it was when I was at Lake Titicaca. Why wasn’t I able to do more? Because if I have less hemoglobin, shouldn’t I have been able to be more active?

IRA FLATOW: Well, Alicia, you know we don’t try to give out personal medical advice, but I’ll ask the experts. We’ll see if they have an idea here. Any idea what happened to her?

MELISSA ILARDO: I defer to Cynthia.

CYNTHIA BEALL: Well, I was going to defer to you. The first question is, why were you anemic? Because a hemoglobin of 5 is well below the average of 13.5 or 14 that would be expected for a young adult woman here at low altitude. So I don’t think it had anything to do with your ability to respond to altitude. It had something to do with what was causing you to become anemic. And I’m glad you got over it.

IRA FLATOW: So where do you go with your research from here? What else do you want to know?

MELISSA ILARDO: Who are you talking to you? Me? OK. So there are a lot of different– so we identified a number of, as I mentioned, genes that have been under selection in the Bajau that weren’t associated with spleen size. And so we’d definitely like to look into some of the physiological effects of those adaptations. There was the one I mentioned that had to do with vasoconstriction, but then we also found something that seems to have to do with the regulation of carbon dioxide in the blood. And this is interesting for diving, because when you hold your breath, what triggers you to want to breathe isn’t actually dangerously low oxygen, but rather the buildup of carbon dioxide. So it could be that there’s something going on there in the Bajau.

But also, the Bajau aren’t the only sea nomads. So there are a number of other sea nomad populations in Southeast Asia.

IRA FLATOW: I’ve got to say goodbye. Because I’ve asked too many good questions.


IRA FLATOW: Melissa Ilardo, Department of Medicine University of Utah. Cynthia Beall, professor of anthropology at Case Western Reserve University. I love when I run out of time, because we’re getting into good stuff. Thank you both for taking time to be with us.

We’ll be right back after this break with the–

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