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Your heart is a pump, and it’s a lot like pumps that humans make.
For a pump to work, it needs a source of energy. You can make a pump work with your hands. That’s mechanical energy. Or you can trigger it with electricity—and that’s how your heart works.
Your heart makes tiny electric sparks. These sparks cause cardiac (heart) muscles to “tense up,” or contract. The contractions (or heartbeats) are steady and regular, thanks to a built-in timer. (What happens if your heart’s timer stops ticking? You can get an artificial one. It’s a box called a pacemaker.)
Although your heart is triggered by electricity, it also has mechanical (moving) parts. Valves open and shut. Muscles squeeze and push. Blood flows through expandable tubes.
The heart pump has one job: to control the flow of liquid. It pumps the liquid uphill against gravity. And it keeps the liquid circulating to all parts of the body.
’Round and ’Round You Go
The heart is part of the body’s circulatory system. Circulatory means “in circles.” Your blood flows in two circles, 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 say you’re a bright red, oxygen-rich blob of blood. You’re in the left atrium of the heart. Start your journey now at Step 1. ▼
The brain uses 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. The spark causes muscles around you 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 (squeezes). You shoot into a thick-walled artery, traveling at 16 inches per second.
The arteries get smaller and smaller. You slow down. Soon the arteries are capillaries—blood vessels just one cell thick. You’re down to .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”—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. You’re now a dark purple.
The capillary turns into a thin-walled vein. The force of your flow makes the walls expand. But they snap back in place behind you, causing a one-way valve to shut. There’s no going back. The veins get bigger and bigger until you’re just above the heart.
You plunge into the right atrium. Another squeeze, and you’re in the right ventricle. You’re halfway through your figure-eight journey.
A 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. The lungs expel this gas. Meanwhile, your red blood cells attract another gas—oxygen. You’re bright red again.
Veins carry you back to the heart. You drop into the left atrium, where you started. The whole trip took just 60 seconds!
If you weigh 90 pounds, you have about 7 pounds of blood in you. The dimpled disks are red blood cells. They carry oxygen. The small white dots are platelets. They’re important in clotting, or helping seal wounds. Blood also has hormones (chemical messengers) and white blood cells (disease fighters). It has clotting agents that help platelets seal wounds. All this stuff and more floats in a liquid called plasma.
◀ The kidneys remove useless or harmful things like poisons from the blood. The bone marrow makes new red blood cells. They live for about 120 days.
Try This!
When giraffes bend over to drink, blood rushes toward the brain. Valves in the blood vessels slow the flow so that the giraffe doesn’t faint. How does spreading the legs (and thus lowering the heart) also slow the flow? Fill a turkey baster with water. The bulb is the giraffe’s heart; the tip is the head. Over a sink, hold the “head” up and squeeze the “heart.” How high and fast does the water travel? Reload the baster. Then tip the “head” down, as if the giraffe is drinking. Squeeze. What happens? How does water flow when the “heart” and “head” are even (the baster is sideways)? ▼
