The biggest heart on Earth belongs to the blue whale. It is a muscle about as heavy as a grand piano and roughly the size of a small car. Its size is only half the story, though. What surprises scientists is how slowly it can beat while the whale dives, and that combination of enormous size and extreme slowness is what lets the largest animal ever feed deep underwater.
How Big and Heavy a Blue Whale’s Heart Really Is
A blue whale’s heart is about 5 feet long, 4 feet wide and 5 feet tall.1National Geographic. How Big is a Blue Whale’s Heart? It weighs about 400 pounds.2National Geographic. How Big is a Blue Whale’s Heart? A heart weighed from a stranded North Atlantic blue whale came in at 180 kg (400 lb), the largest heart known from any animal.3Wikipedia. Blue whale Another blue whale heart, preserved for a museum, weighed 175 kg, or 386 pounds.4The Journal of Plastination. The Challenges of Plastinating a Blue Whale (Balaenoptera musculus) Heart
Size depends on the whale. Standard formulas for mammals predict that an average 70,000 kg blue whale, about 23 m long, would have a heart of about 319 kg.5PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal But blue whales range much larger, with the biggest reported near 200,000 kg.6PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal The measured weight comes in below the formula’s prediction for a few reasons. The formula assumes typical proportions, while real hearts vary for their size. Stranded whales are usually found some time after death, and the heart may lose fluid or begin to break down before anyone weighs it.
For everyday comparison, a human heart weighs under a pound, roughly 300 grams, so a blue whale’s is several hundred times heavier. Its 5-foot height is close to that of an average adult woman.
The heart is a four-chambered organ like ours, with two upper chambers and two lower pumping chambers, just scaled up enormously. The thick muscular wall of the main pumping chamber accounts for much of the weight, so this is mostly dense muscle, not a hollow bag. Museum and documentary hearts are usually shown preserved and propped open, so they look rounder and fuller than a heart inside a whale, where it sits compressed among other organs.
How Much Blood It Pumps With Each Beat
A blue whale’s heart pumps about 60 gallons of blood with each beat.7NOAA. The Big-Hearted Blue Whale Mammal scaling formulas put that output at about 80 liters per beat for an average-sized blue whale.8PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal That is roughly a bathtub’s worth of blood every beat, and a heart this large is needed to supply a body that can weigh 70,000 kg or more.9PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal No one can strap a blood-flow meter inside a living blue whale, so the 60-gallon and 80-liter figures are estimates from scaling rules, not direct measurements.
A human heart moves about 70 milliliters per beat, a few tablespoons, and beats about 60 to 100 times a minute at rest. The whale moves around a thousand times more blood per beat while beating several times slower. The body is tens of meters of vessels long, so a big volume per beat is what keeps blood moving over that distance with few beats.
The same formulas predict a resting rate of about 15 beats a minute for an average blue whale.10PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal For the very largest blue whales the predicted resting rate falls to about 11 beats a minute.11PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal Bigger animals have slower hearts, because their bodies use energy more slowly per pound.
Why the Heart Slows to a Few Beats a Minute During Dives
At the bottom of feeding dives a blue whale’s heart beats only about 4 to 8 times a minute.12PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal That is roughly a third to a half of the whale’s expected resting rate.13PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal These rates fall even below the predicted resting rate of 11 beats a minute for the largest blue whale on record.14PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal The slowdown happens whatever the dive’s depth or length.15PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal
The longer the dive, though, the lower both the average and the slowest heart rate.16PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal This is part of the dive response, which saves oxygen while the whale holds its breath.17PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal Even during lunges after prey, a blue whale’s rate rises only to about 22 beats a minute, then falls to about 8 within a minute.18PubMed Central. Sprint-like cardiac dynamics support repeated acrobatic lunges in foraging rorqual whales
The dive response is a set of automatic changes that begin when a mammal holds its breath underwater. In whales it is far stronger than in humans. A whale does not breathe underwater, so every bit of oxygen for the dive must be carried in its blood and muscles when it leaves the surface. Slowing the heart means the heart muscle itself uses less oxygen, and the saved oxygen can go to the brain and other organs.
Blood flow is cut back to tissues that can tolerate a short oxygen shortage, such as muscles and the gut, and kept up for the brain and heart. Muscle also holds a store of oxygen bound to a protein, so working muscles can keep going on reduced blood supply. The payoff is time enough to find and gulp dense patches of krill, which is the purpose of the whole dive.
At 4 to 6 beats a minute, there is a 10 to 15 second gap between beats. A human with that rate would faint, but the whale’s stretchy main artery keeps blood flowing between beats.
How the Heart Races Back Up at the Surface
At the surface, whales hit near-maximum heart rates to swap gases and refill tissues with blood.19Scripps Institution of Oceanography. Scientists Report First Recording of a Blue Whale’s Heart Rate The longer the dive, the higher the heart rate climbs after it.20PubMed Central. Extreme bradycardia and tachycardia in the world’s largest animal In rorqual whales feeding at depth, surface rates averaged about 36 beats a minute, easing from about 40 to 33 over 3 to 4 minutes and 6 to 10 breaths.21PubMed Central. Sprint-like cardiac dynamics support repeated acrobatic lunges in foraging rorqual whales During shallow feeding with very short breaks, rates stayed near 39 to 40 beats a minute.22PubMed Central. Sprint-like cardiac dynamics support repeated acrobatic lunges in foraging rorqual whales
As the whale breaks the surface, it takes a series of breaths, and over those breaths the heart speeds up to bring oxygen into the blood quickly. The fast heartbeat then carries the freshly oxygenated blood out to muscles and organs that were running low during the dive. The heart eases down gradually across several minutes of breathing, which shows in the drop from about 40 to 33.
The swing is dramatic: from about 4 beats a minute to about 40, a tenfold change, every dive cycle. The cycle repeats many times an hour while feeding.
After a long deep dive, more oxygen has been used up, so the recovery is bigger and longer. After shallow feeding with very short breaks, there is little time to recover, so the heart stays elevated.
Scientists learned all this by attaching a suction-cup tag with heart sensors to a wild blue whale. The tag recorded every beat during real feeding dives, and it was the first heart rate record from a blue whale.
How Other Whale Hearts Compare
A resting wild humpback’s heart beats about 18 to 19 times a minute, matching a land mammal of similar size.23PubMed Central. Sprint-like cardiac dynamics support repeated acrobatic lunges in foraging rorqual whales Humpback rates jump about 60 to 70 percent during feeding.24PubMed Central. Sprint-like cardiac dynamics support repeated acrobatic lunges in foraging rorqual whales But a humpback 17 seconds after a lunge is still near 19 beats, while a blue whale has already dropped to about 8 a minute later.25PubMed Central. Sprint-like cardiac dynamics support repeated acrobatic lunges in foraging rorqual whales
Heart shape in whales seems linked to how deep and long they dive.26PubMed. The cardiac ventricles of a baleen whale (Balaenoptera acutorostrata: Minke whale) and a toothed whale (Hyperoodon ampullatus: Bottlenose whale) The ridged muscle pattern inside the pumping chambers may relate to body size.27PubMed. The cardiac ventricles of a baleen whale (Balaenoptera acutorostrata: Minke whale) and a toothed whale (Hyperoodon ampullatus: Bottlenose whale) Killer whale hearts have an especially dense mesh of these muscular ridges inside the chambers.28PubMed Central. The Heart of the Killer Whale: Description of a Plastinated Specimen and Review of the Available Literature Whales also share a type of heart wiring, the fibers that spread the beat signal, with sheep and goats.29PubMed. Morphological varieties of the Purkinje fiber network in mammalian hearts, as revealed by light and electron microscopy
Hunting styles help explain the variety. Baleen whales such as blue and humpback whales filter krill or small fish in big gulps, so their feeding is a repeating pattern of lunges and dives. Toothed whales such as killer whales and beaked whales chase individual prey, and some beaked whales dive extremely deep, so their hearts face different demands.
How Scientists Preserve and Display a Whale Heart
After a blue whale washed up in Newfoundland, the team plugged the heart’s openings with buckets, bottles and even toilet plungers. Then they pumped in 2,800 liters of formaldehyde solution to stop decay.30Smithsonian Magazine. How Big Are Whale Hearts? Size, Weight, and Function They wanted it fixed fully swollen, the moment just before a beat when the heart is largest.31Smithsonian Magazine. How Big Are Whale Hearts? Size, Weight, and Function
The heart was then plastinated, a process replacing body fluids with plastic, in Guben, Germany.32Royal Ontario Museum. Blue Whale Heart Arrives at the ROM Body water and fat are swapped for a plastic that hardens, leaving a dry specimen. Colored silicone was injected into the heart’s own blood vessels to show them, but because the vessels had broken down, much of it leaked.
Handling it was a struggle. Its 386 pound weight and slippery surface meant a lift and netting were needed for every step.33The Journal of Plastination. The Challenges of Plastinating a Blue Whale (Balaenoptera musculus) Heart It was so hard to turn, even floating, that workers could only inspect the top.34The Journal of Plastination. The Challenges of Plastinating a Blue Whale (Balaenoptera musculus) Heart A heavy internal frame was built to hold the nearly 400 pound heart open for display.35The Journal of Plastination. The Challenges of Plastinating a Blue Whale (Balaenoptera musculus) Heart It arrived in Toronto in May 2017 for the Royal Ontario Museum.36Royal Ontario Museum. Blue Whale Heart Arrives at the ROM