This time of year, it’s hard to escape sharks – on TV at least. But perhaps that heartbeat-like theme from “Jaws” – ba-dump, ba-dump – has you wondering, “What might I learn about my own heart from a shark?”
Strap on your scuba gear, because we are diving into that very topic.
You might assume that sharks and humans do not have much in common at heart. After all, our last shared link on the evolutionary tree was perhaps 400 to 450 million years ago.
But shark hearts and human hearts are similar in at least one basic way, said Dr. Georgina Cox, an assistant professor at Washington State University Vancouver. “Basically, the heart is a pump, no matter what species you look at.”
And that pump isn’t just delivering oxygen-rich blood, said Cox, a comparative cardiovascular physiologist. “It unites all of our bodily systems,” helping to remove waste and deliver hormones that enable organs to communicate with one another.
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Cox teaches human anatomy and biochemistry while studying fish cardiovascular systems and how fish respond to stressors such as increased temperatures and pollution. In both sharks and humans, she said, the heart never stops beating. “And like us, if a fish’s heart stops working, that’s it,” she said.
But while they are alike in function, shark and human hearts differ in form, said Dr. Jules Devaux, a research fellow in the School of Biological Sciences at the University of Auckland in New Zealand.
Humans and other mammals have a four-chambered heart – two upper chambers, called atria, and two lower chambers, or ventricles – that push blood from the heart to our lungs to get oxygen, then back to the heart, then out to the body.
Sharks, like other fish, have what is usually described as a two-chambered heart – one atrium and one ventricle. Inside a rigid, boxlike cavity, sharks also have a collection sac, called the sinus venosus, and a series of valves, the conus arteriosus, that keep things flowing in the right direction.
It all pumps blood to the gills, where it is oxygenated and then flows along to the shark’s body.
But while the basic heart structures are quite different, “there is one thing that is weirdly common to both of us, and that’s our coronary circulation,” said Cox, who wrote her doctoral dissertation on blood flow in shark hearts.
Sharks and humans have “fully vascularized” hearts, she said. That’s in contrast to most bony fish species, which have no coronary arteries and rely on their hearts to pick up whatever oxygen is left in the blood that has already circulated through their bodies. Having coronary arteries means a shark’s heart receives oxygenated blood directly from the gills.
In humans, the blockage of coronary arteries can cause heart attacks, which starve the heart of blood and oxygen, killing the heart muscle.
Some sharks, however, have adapted to survive in places with little oxygen. Devaux’s research has included work with both epaulette sharks and grey carpet sharks, which live on coral reefs along the coasts of Australia and the Indo-Pacific region.
“These sharks prefer to inhabit the reef platform during low tide to avoid” larger predator sharks, he said. At night, when photosynthetic corals and algae stop producing oxygen, the water can become oxygen-poor, or anoxic.
But the grey carpet shark can endure such waters for several hours by storing excess red blood cells, Devaux said, something “akin to having a spare dive tank.”
The epaulette shark, also known as the “walking shark,” does not have this additional oxygen reserve, Devaux said. Instead, it goes into a state that resembles a coma. “Remarkably, it does so during nearly all nocturnal low tides, equivalent to surviving approximately five strokes per week without sustaining any brain or heart damage” from oxygen deprivation.
Devaux’s work has focused on mitochondria, the parts of cells that use oxygen to produce energy, and a substance called succinate, which builds up during oxygen deprivation and is converted into damaging “free radical” molecules when oxygen is reintroduced. The epaulette shark does not process as much succinate when oxygen returns, which means it suffers less damage.
By unraveling the mechanisms that carpet sharks have evolved, “we may be able to offer innovative therapeutic strategies to help stroke and heart attack victims,” Devaux said. “Understanding how these resilient creatures navigate oxygen deprivation could offer invaluable lessons for human health and medicine.”
Understanding the reoxygenation process, also called reperfusion, is important, said Cox. “It’s both the lack of oxygen and the reperfusion that causes a lot of cardiac damage in humans,” she said.
Her work has included studying hagfish, which are not sharks but can live in oxygen-poor waters, and leopard sharks, which don’t live in oxygen-poor waters. She has done open-heart surgery on them to measure their blood flow in an effort to understand the role of the coronary circulation in the shark heart’s response to elevated ocean temperatures.
She jokes about having never been in a shark cage. “They’re for wimps,” Cox said, quickly adding, “I’m just kidding.”
But her interest in what we can learn from shark hearts comes from a serious place. Her father is a survivor of both heart failure and a heart attack, and her career has been a way to merge her interests in fish and the human heart.
Although she’s unaware of any sharks who have had heart attacks, Cox couldn’t rule out the possibility.
“I mean, it’s hard,” she said. “They don’t come in for annual checkups. And usually when something goes wrong, they die, and then get eaten.” But while cutting off arteries that feed the heart muscle would be fatal to a human, a fish can still absorb oxygen from blood that has circulated through its body. So it is very likely that a shark would be able to survive a heart attack, Cox said.
But she noted that sharks get plenty of exercise and eat a balanced diet – which are key parts of preventing heart attacks and strokes in humans.
You might have heard that sharks are champions at physical activity, but not all sharks have to keep moving constantly, Cox said. They do need to make sure water is flowing over their gills. And muscle contractions in their tails help move blood back to the heart, much like walking helps return blood to the heart in humans.
And the definition of “balanced diet” varies by species.
Most sharks are carnivores, Cox said, meaning “they eat things that eat vegetables, and that’s how they break things down.” Humans, as omnivores, can get nutrients from both plants and other animals, and research has emphasized the heart-health value of a plant-rich diet.
In a way, some sharks share an affection for junk food. In sharks, that’s literal. “Some shark species will eat a suit of armor,” she said. “Just because they’re curious.”
But when it comes to prolonging your life, paying attention to your heart is much more likely to be of value than worrying about being eaten by a shark.
According to the Florida Museum of Natural History, which tracks shark attacks internationally, the lifetime odds of someone dying because of a shark is 1 in 4,332,817. That’s compared to a 1 in 5 risk of dying from heart disease, and a 1 in 24 risk of dying from a stroke.
Humans are, in fact, a much greater risk to shark health than the other way around. One way is through climate change, which highlights another basic anatomical difference.
While humans can maintain a steady body temperature, most sharks can’t. As seawater temperature increases, Devaux said, so does a shark’s need for oxygen, and that can overload their hearts. “In fact, it is the first organ to fail in fish exposed to acute marine heat waves,” he said.
Humans also threaten sharks through pollution and through fishing, either as accidental bycatch with other species or through intentional hunting, sometimes in pursuit of “weird remedies” that have no medical basis, Cox said. “We are way more dangerous to them than they are to us.”
But they remain captivating and awe-inspiring, with millions of people tuning in to watch programs about them each year. So this week, if you do happen to catch a show that notes how a mako shark can hit speeds of more than 45 miles per hour, or that a silky shark can cover 17,000 miles in a year and a half, keep in mind that you have at least a little bit in common with them.
“The power that you see in sharks, and their grace and their beauty, is the result of a solidly working cardiovascular system,” Cox said.
covers heart and brain health. Not all views expressed in this story reflect the official position of the American Heart Association. Copyright is owned or held by the American Heart Association, Inc., and all rights are reserved.
