Special Circuits in the Circulatory System

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll discuss why it's important for you (but not for an insect) to have a closed circulatory system. We'll discuss your systemic circuit and your pulmonary circuit and how each works together to get oxygen where it needs to go.

Types of Circulatory Systems

If you were a single-celled organism, it would be pretty easy for you to get oxygen and other chemicals where you need them to go; you would just let them diffuse in. However, you're not a single-celled organism. You know you need oxygen, but how does that oxygen get from your lungs to where you may need it, say, in your big toe? From your blood, of course!

So how does your blood get where it needs to go? From that amazing pump in the middle of your chest, your heart.

If you were, say, an insect, your heart and circulatory system would be a pretty simple operation. We call the type of circulatory system that an insect has an open circulatory system. An insect doesn't have veins and arteries like you. Instead, the heart just fills with blood and squirts it all over the insect's insides. Imagine just squirting a balloon full of water all over your organs. That's about how an insect's heart works. This system is elegant but a little inefficient, because an insect can't control where its blood is going. An insect's blood cells get all mixed up together, so some of the blood cells that an insect's heart pumps to its organs will be full of oxygen, but some of it will already be oxygen-depleted.

You're bigger than an insect, so you can't afford to waste time and energy sending oxygen-depleted blood to your organs. Your closed circulatory system, so named because the blood is enclosed in arteries and veins, is a bit more complex. However, it allows your body to ensure that only oxygen-rich blood goes to your organs.

The Systemic Circuit

Let's assume that your big toe is in need of a little oxygen. (It always is, because all the living cells in your body need oxygen.) The systemic circuit is the pathway that blood takes from your heart to your big toe, and back to your heart. Your blood will also pick up waste carbon dioxide in your big toe so that you can get rid of it.

Your systemic circuit starts in the left side of your heart. Blood flows into your left atrium, the thin-walled chamber on the top left side of your heart that collects incoming blood. It then flows into the left ventricle, the thick-walled chamber on the bottom left side of your heart. Your ventricle is thicker because it has a harder job to do, so it needs a sturdy layer of muscle. Your atrium only has to pump blood as far as its next-door neighbor (the ventricle), but your ventricle has to pump blood all the way to your big toe. When your ventricle contracts, it squirts blood all over your body.

The Pulmonary Circuit

The whole reason blood needs to get to your big toe is to provide it with oxygen. How does that blood get the oxygen it needs?

You know you get oxygen from breathing in with your lungs. The pulmonary circuit is the pathway that your blood takes from your heart to your lungs in order to pick up oxygen. Your blood also unloads all that carbon dioxide it picked up during its path on the systemic circuit.

Your pulmonary circuit starts in the right side of your heart. Your right atrium, on the top right of your heart, collects incoming blood. Your right ventricle contracts, squirting blood from your heart to your lungs so the blood can pick up precious oxygen and get rid of carbon dioxide.

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