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How well do you really understand your heart? Make a fist. That’s roughly its size. The adult human heart weighs approximately 300 grams and is essential for delivering oxygen and nutrients to the body’s tissues and organs, while also eliminating carbon dioxide and other waste materials.
To accomplish this efficiently, your heart is equipped with four chambers—an incredible feat of evolutionary design. The two chambers on the right side handle deoxygenated blood. The upper chamber (right atrium) receives it, and the lower chamber (right ventricle) sends it to the lungs for oxygen. The left side of the heart takes in the oxygen-rich blood from the lungs (in the left atrium) and pumps it out (through the left ventricle) to the rest of the body.
Human blood appears red because of hemoglobin, a protein that contains iron. This iron attracts oxygen molecules, and the bond between iron and oxygen reflects red light. Even though veins seem blue beneath your skin, the blood inside them is red. Deoxygenated blood is still red, just darker. If your blood appears blue when you cut yourself, it’s time for a DNA test—you might be an octopus.
At rest, the average human heart beats 60 to 100 times per minute. Top athletes usually have a lower resting heart rate than the general population. Your maximum heart rate is about 220 minus your age.
Not all hearts are the same. Some creatures in the animal kingdom have circulation systems that defy belief.
10. Octopus
An octopus, like other cephalopods, has three hearts. Two of these hearts pump blood to the gills, while the third heart circulates blood to the rest of the body. Unlike humans, octopuses have blue blood. They lack hemoglobin, the protein that gives human blood its red color, and instead use hemocyanin, a copper-rich protein that is more effective in low-oxygen and cold environments for oxygen transport.
Ever wonder why octopuses prefer crawling over swimming? The reason is fatigue. When the octopus swims, the heart that pumps blood throughout the body stops beating, causing it to tire much faster.
By the way, the plural form of “octopus” is “octopuses,” not “octopi.” If the word “octopus” had been derived from Latin, “octopi” would be correct. However, since the word comes from Greek, the proper plural form is simply “octopuses.”
9. Frogs
Fairy tale princesses, take note: Frogs possess three-chambered hearts. If you transform a frog into a prince, he might need constant medical care to adjust to life in human form.
Like humans, frogs have two atria, but only one ventricle. While humans have a four-chamber heart to keep oxygenated and deoxygenated blood separate, frogs’ blood mixes as both the left and right atria empty into the single ventricle. This system may seem inefficient, but frogs, being small and having low metabolisms, don’t require the same efficiency as larger animals. And here’s where the frog-turned-prince might face difficulty—unless he retains the ability to breathe through his skin. Frogs can absorb oxygen through both their skin and lungs. This skin-based oxygen supply helps them cope with the inefficiency of their single-ventricle heart.
A final piece of advice for princesses: Keep the bath towels hidden. If your prince can breathe through his skin, it must stay moist at all times.
8. Wood Frog
When we think of Arctic creatures, frogs don’t usually come to mind. After all, the Arctic is extremely cold. Wouldn’t they freeze? The answer, unsurprisingly, is yes. But for one species of wood frog, Rana sylvatica, this is not an issue.
During the winter, when temperatures drop drastically, the wood frog freezes, and its heart stops beating—sometimes for days, even weeks. However, once the temperature rises, the frogs thaw, and their hearts resume beating. The amphibians emerge completely unharmed. This extraordinary survival mechanism could also have potential benefits for humans.
Scientists working in the field of organ transplantation are eager to uncover the secret behind the wood frog’s cryoprotectants (the processes that allow them to endure freezing). The ability to freeze and then thaw human hearts could completely transform the transplantation process. No longer would patients need to be in the right place at the right time. Donor hearts could be frozen and transported to recipients anywhere, potentially saving countless lives. Be still, my beating heart...
7. Giraffe
Pumping blood to a giraffe’s brain requires immense power, something a human heart could never handle. Thankfully, the giraffe’s heart is built to push blood all over its massive body. Weighing in at 11 kilograms (24 lb) and measuring 60 centimeters (24 in) long, the giraffe heart is a heavyweight. Not only does the blood need to defy gravity for 1.8 meters (6 ft) to reach the animal’s head, but it must travel a similar distance to reach its feet. Gravity helps on the downward journey, but once the blood reaches the bottom, it requires enough pressure to make the return trip. Without it, giraffes would have extraordinarily swollen ankles.
Ever felt light-headed after standing up too quickly? That’s the result of a sudden drop in blood pressure. Giraffes, however, move their heads around constantly, which should make them perpetually dizzy. But thanks to a series of clever adaptations, they don’t. One of these is their unique jugular vein. While most animals’ jugular veins lack muscle, the giraffe’s contains muscle. This muscled jugular, combined with its thick skin and powerful heart, boosts the giraffe’s blood pressure to about double that of humans.
6. Blue Whale
It’s no surprise that the largest animal on Earth also has the largest heart. But is the blue whale’s heart truly the size of a small car? Without actual blue whale specimens to examine, scientists could only speculate about the size of this giant mammal’s heart. In 2014, however, scientists from the Royal Ontario Museum embarked on a bold mission to preserve the remains of a blue whale that had died in ice off Newfoundland. Typically, most blue whale carcasses sink to the ocean floor, so this was a rare opportunity.
Recovering the heart was a top priority. Scientists first opened the chest cavity before carefully entering the carcass to remove the heart. Surrounded by blood and lungs, the scientists worked to free the heart from the tissue. It then took four of them to push the enormous organ through the opening they had made.
The heart weighed a staggering 180 kilograms (400 lb) and was capable of pumping 150 liters (40 gal) of blood with every beat. With a diameter of 1.2 meters (4 ft), it wasn’t quite the size of a car, but it was comparable to the size of a golf cart. Retrieving a blue whale heart, however, is not a task for the faint of heart.
5. Cockroaches
Cockroaches have hearts with 12 or 13 chambers, but instead of blood, their heart circulates a yellowish-white substance called hemolymph. This nutrient-rich fluid feeds the organs but doesn’t carry oxygen, as insects obtain oxygen through openings in their body.
Even with the most sensitive stethoscope, you won’t be able to hear a cockroach’s heartbeat. Their hearts don’t actually beat. Instead, muscles in the body cavity contract and relax, helping to move the gooey hemolymph through the heart.
4. Earthworms
Do earthworms even have hearts? Well, kind of. They possess five pseudohearts that wrap around their esophagus. Instead of pumping, these pseudohearts compress the blood vessels to circulate the blood. So, similar to cockroaches, earthworms don't have a traditional heartbeat.
Unlike cockroaches, however, earthworm blood is red due to the presence of hemoglobin. Interestingly, they don't actually need hemoglobin to deliver oxygen throughout their bodies. Instead, earthworms absorb oxygen through their skin.
3. Pygmy Shrew
The title for the mammal with the fastest heart rate goes to the pygmy shrew. Generally, the smaller the animal, the faster its heart beats—and the pygmy shrew is minuscule. Weighing only around 3 grams, it’s so tiny that you might lose it in your cereal bowl.
The pygmy Shrew’s heart rate exceeds 1,000 beats per minute—equivalent to 16 beats per second! Imagine the challenge of tracking its target heart rate if it were on a fitness plan. That definitely wouldn’t follow the simple formula of 220 minus age...
2. Ocellated Icefish
The ocellated icefish is a creature that continues to puzzle scientists. It is the only vertebrate known to lack hemoglobin in its blood, making it clear rather than red. Remarkably, the heart of this fish is five times larger and more powerful than that of a typical fish. Researchers believe this adaptation may be connected to the frigid environment it inhabits.
The waters around Antarctica, although harsh, contain more oxygen than warmer waters. It’s possible that these fish absorb oxygen directly through their skin, instead of relying on hemoglobin. In such cold waters, oxygen might dissolve into the blood plasma. Imagine if human blood were clear—horror movie scenes would never be the same.
1. Zebrafish
Most fish hearts have just two chambers, and the zebrafish, native to Southeast Asia, follows this norm. Although it has half the number of chambers of a human heart, the zebrafish heart does the same job—pumping oxygenated blood to various organs. The oxygen is extracted through the gills, not lungs, which works beautifully for these aquatic creatures.
What makes this striped tropical fish particularly fascinating is its ability to regenerate heart tissue. A study from 2002 revealed that zebrafish can regrow heart tissue within just two months after damaging up to 20 percent of their heart muscle. Perhaps that’s why we don’t often encounter brokenhearted zebrafish.
You might think there’s little in common between humans and zebrafish, but scientists disagree. Zebrafish are becoming increasingly popular in medical research because they share many physiological traits with humans. They are vertebrates, with a backbone, brain, spinal cord, and organs like the heart, liver, pancreas, kidneys, bones, and cartilage. In fact, 70 percent of human genes can be found in zebrafish. So, if you have these remarkable creatures in your aquarium, take a closer look and appreciate them for the important role their counterparts play in advancing global medical research.
