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Your heart is a pump, and it’s a lot like the pumps that people manufacture for industrial and mechanical purposes.
Any pump needs a source of energy to function. If you work a pump with your hands, that pump is powered by mechanical energy. Or you can trigger it with electricity, which is how your heart works.
The tiny electric sparks your heart makes cause cardiac (heart) muscles to “tense up,” or contract. The contractions, or heartbeats, are steady and regular, thanks to a built-in timer. If your heart’s timer stops ticking, you can get an artificial one. It’s a small box called a pacemaker.
Your heart is triggered by electricity, but it also has moving parts: valves open and shut, muscles squeeze and push, and blood flows through expandable tubes.
The heart pump’s one job is to control the flow of blood. It pumps blood uphill against gravity and keeps it flowing to all parts of the body.
’Round and ’Round You Go
Your heart is part of your body’s circulatory system. (The word circulatory means “in circles.”) Your blood actually does flow in two circles, kind of like a figure eight: one circle goes from right heart to lungs to left heart. The other goes from left heart to somewhere in the body to right heart. Let’s take a little trip and see how it all works. Pretend you’re a bright red, oxygen-rich blob of blood, just sitting in the left atrium of the heart, about to start your journey at Step 1. ▼
The brain hogs one-fourth of the body’s pumped blood. (Oxygen is the energy the brain uses to think and carry on its functions.)
A bundle of nerve cells creates a spark, which causes muscles around you, the blob, to contract. The atrium walls close in.
The closing walls push you into a huge ventricle with walls made of powerful muscles. It, too, contracts, which shoots you into a thick-walled artery, traveling at 16 inches per second.
The arteries get smaller and smaller, so you’re going slower and slower. Soon the arteries are capillaries, which are tiny blood vessels just one cell thick. You’re creeping along at .02 inches per second. That’s like a jet slowing in a few seconds from 800 miles per hour to 1 mile per hour.
Your “cargo” of oxygen, white blood cells, and so on squeezes through the cells of the capillary walls into the brain. Carbon dioxide and other waste flow in. Your color has changed to dark purple.
The capillary turns into a thin-walled vein. The force of your flow makes the walls expand, but they snap back together behind you, causing a one-way valve to shut—you can’t go back. The veins you’re traveling keep getting bigger, until you’re just above the heart.
You plunge into the right atrium. With another squeeze, or contraction, you’re in the right ventricle, which means you’re halfway through your figure-eight journey.
One heartbeat later, you shoot into an artery and head for the lungs. Capillaries in the lungs have thin walls that allow your carbon dioxide to pass through, and then the lungs expel that gas. Meanwhile, your red blood cells have attracted another gas—oxygen—and you’ve become bright red again.
Veins carry you back to the heart, and you drop into the left atrium, where you started. That whole trip took just 60 seconds!
If you weigh 90 pounds, your blood weighs about 7 pounds. The dimpled disks are red blood cells. They carry oxygen. The small white dots are platelets, or pieces of blood cells, which are important in clotting, or helping seal wounds. Blood also has hormones (chemical messengers), white blood cells (disease fighters), and clotting agents to help platelets seal wounds. All of this—and more—floats in a river of liquid called plasma.
◀ Like a tiny car wash, the kidneys clean blood of useless or harmful parts such as poisons. The bone marrow makes new red blood cells, which live for about 120 days.
Try This!
When giraffes bend over to drink, blood rushes toward the brain. Valves in their blood vessels slow the flow of blood so the giraffe doesn’t faint when it bends down. How does spreading the legs, which lowers the heart, slow the flow? Let’s experiment by filling a turkey baster with water. The bulb represents the giraffe’s heart, and the tip represents the head. Standing over a sink, hold the “head” up and squeeze the “heart.” How high and fast does the water travel? Now reload the baster and tip the “head” down, as if the giraffe is drinking. What happens when you squeeze? How does water flow when the “heart” and “head” are even, when the baster is sideways? ▼
