Go ahead — squeeze it. I have extras.
I keep a heart on my desk in a clear plastic bag. It’s reddish-purple with yellow pockets of fat at the edges. It’s a little bigger than half a sandwich, but it weighs a lot more. A lot of people say they think it’s gross, but then they pick it up and marvel at it anyway. I ordered it from a science supply company that also sells turkey testicles and preserved pigeons. It cost three dollars and fifty-five cents, which is a pretty good deal for a no-fail conversation starter.
I’ve had other hearts in the past, but the one on my desk right now is from a sheep. I like it because it’s the same size and shape as a human heart and it reminds me that even when my day sucks, my heart keeps beating to get me through to the next day.
I’m a high school biology teacher, so a big part of my job is getting kids interested in what’s going on inside their bodies. And for the most part, I find that it’s easier to grab their attention by handing them a body part than by assigning them pages to read in a textbook.
A lot of the organs in our bodies are hopelessly complicated. The brain is an invisible mess of tangled electricity and the kidneys are full of strange twists and turns with names like the “Loop of Henle.” But the heart is simple. By the age of three or four, most kids understand that they have a heartbeat that keeps them alive.
You probably understand that your heart’s job is to pump blood through your body. You probably know that your blood delivers oxygen to and picks up waste carbon dioxide from every cell in your body.
But you might not realize what an amazing feat of evolution, plumbing, and wiring you’re carrying around right there in your chest with you from before you’re born to the moment you die.
A short history of blood
For the first 2 billion years, life on planet earth was tiny. Microscopic single-celled organisms do not need a circulatory system. As more complex organisms evolved, however, they needed a way to transport stuff around their bodies.
Early circulatory systems, found in simple worms and other organisms without a backbone, consisted of just one muscle. The muscle would contract, causing the liquids around it to swish around. Insects still have this kind of circulatory system, and due to their small size, it works.
But bathing our organs in an open cavern of swishing liquids around wouldn’t work for us. If our blood wasn’t contained in our arteries and veins, it would just pool at our ankles, leaving our brains starving for oxygen.
This is why we — and birds, turtles, reptiles, and all other mammals — evolved to have a closed circulatory system. The blood in our system is contained in a pressurized system. You can see proof of this in the bulging veins of bodybuilders or the tiny drops of blood that squeeze out of your finger when you get a papercut.
At its core, our circulatory system is made up of two separate pumps connected by miles and miles of tubes. The left side of our heart is the pump that squeezes blood out to all of the cells in our body, delivering fresh oxygen and nutrients. The right side of our heart is the other pump, and it pushes oxygen-depleted blood to our lungs to resupply.
Just like the plumbing in your house, your circulatory system has to operate at a certain pressure. When the pressure gets too high or too low, problems ensue.
If you have low water pressure at your house, your showers are lame and your dishes don’t get clean in the dishwasher. In your body, low blood pressure means your tissues aren’t getting an adequate supply of oxygen. Your brain needs a steady supply of oxygen to function. Fortunately, when your brain isn’t getting enough oxygen, it overrides your whole system and causes you to pass out.
This might seem like a bad thing, as it can be pretty embarrassing to drop like a leaf while you’re waiting for a table at your favorite restaurant, but it’s actually a good thing. When you pass out, you fall to the ground, allowing blood to flow back to your head.
Too much pressure in your circulatory system also causes trouble. In your house, your pipes can burst from too much pressure, which creates a hell of a mess. In your body, too much pressure can cause burst blood vessels and heart problems. Exercise, medications, and healthy eating can all help reduce high blood pressure.
Have you ever wondered why your heart even beats in the first place?
Like our triceps and biceps, the heart is a muscle that contracts and relaxes. But unlike our skeletal muscles — the ones we workout at the gym — cardiac muscle is driven by its own little patches of cells called nodes.
The atrioventricular node and the sinoatrial node (AV and SA nodes) are little patches of tissue on the outside of your heart that generate electricity. Each little pulse of electricity causes your heart muscle to squeeze rhythmically. If you’ve ever listened to your heart through a stethoscope, what you’re hearing is valves of your heart slamming shut as your heart muscle contracts and relaxes.
Generally, a slower heartbeat is better. When your heart muscle is powerful and strong, and your blood vessels are healthy and clear, one pump can push a lot of blood. If you could listen to the heart of an endurance athlete, you would hear it beat once every two seconds.
Most healthy people have a resting heart rate between sixty and one hundred beats per minute.
But there are a variety of factors that can cause your heart to beat faster. In the short term exertion, fear, anxiety, excitement, or arousal can all cause your heart rate to quicken, giving you the energy you need to outrun a bear, finish your TED talk, or — well — bang.
But while it’s good for your heart to beat faster for brief periods, it’s not for your heart to beat fast all the time. The harder your heart has to work, the quicker it will wear out. Things like an unhealthy diet, a sedentary lifestyle, genetic factors, smoking, and obesity all make your heart work harder.
What’s not to love?
If you came into my classroom, I’d let you squeeze my heart. You would probably be surprised at how firm it is. Sometimes people are amazed at how heavy it is. If you were really interested, I might even take it out of the bag and show you how I can squirt water several feet in the air when I put it under the water of my fish tank and squeeze it.
The heart inside your chest is awesome too. Not because it helps you fall in love or follow your passions — that’s mostly your brain — but because it does such a damn fine job just keeping you alive.
You don’t have to be a cardiovascular surgeon to appreciate your heart. You just have to be alive. And chances are if you’ve read this far, you are. So take a second to put your hand on your chest. Sit quietly and close your eyes.
Can you feel it?
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