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Our circulatory system provides a network through which our blood flows constantly. However, there are two types of circulatory systems in the animal kingdom: open and closed. Which do humans have? Read on to find out.

A System For Internal Transport

The human body is a complex machine with many different systems automatically functioning day and night to keep life going.

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We breathe, our heart beats and our food is digested without conscious effort on our part. And although you cannot see it from the outside, within your body is an incredible superhighway system of transport. This system moves our blood through the body, carrying within it life-giving molecules such as oxygen, sugars and salts. It also picks up waste products for removal out of the body. It is the circulatory system, and we wouldn’t be alive without it.

Blood, our system of transport
Blood cells

In the animal kingdom, there are two main types of circulatory systems: the open circulatory system and the closed circulatory system. Humans have a closed circulatory system, which is exactly how it sounds: our blood stays enclosed within blood vessels and the heart. In this lesson, we will take a closer look at this system and gain a better understanding of how it works.

Open vs.

Closed Circulatory System

To better understand a closed circulatory system, let’s take a quick look at one that is open. Insects have open circulatory systems, and you guessed it: this means they don’t have blood circulating exclusively within vessels. Instead, a fluid called hemolymph travels through vessels for a short time before emptying into the body cavity.

Hemolymph is quite different from our blood in that it has no red blood cells and isn’t even red. This is why squashing an insect doesn’t yield a red bloody mess, unless it is a well-fed mosquito.After the hemolymph empties into the body cavity, essential molecules diffuse into tissues before the fluid returns to vessels. This system works great for insects, in part because their hemolymph is not responsible for carrying oxygen and carbon dioxide, as our blood is. The needs of a human body are more complicated, and so our circulatory system is a bit more complex.

Parts Of Our Closed Circulatory System

Now we will take a closer look at the closed circulatory system of the human body, also known as a cardiovascular system (cardio=heart, vascular=vessels).

You already know that our blood stays confined within the vessels and heart. You can compare this concept to a network of pipes within a house. Clean water is brought into the house via pipes, and waste water is neatly removed through different pipes.

Imagine the mess that would ensue if the water and waste were not enclosed in pipes and instead flowed freely through the house.

Pipes designated to carry water to a house, as blood vessels carry blood

Likewise, in the human body, blood moves through a complex system of vessels instead of mixing in an internal soup of liquids. We depend on the bloodstream to carry oxygen throughout the body and remove carbon dioxide. For this reason, blood flow must be efficient and organized.Let’s take a closer look, starting with the organ responsible for generating the movement of the blood: the heart.

The heart pumps the blood out into large vessels known as arteries. Arteries always carry blood away from the heart. (Think a=artery, away from the heart.


Diagram showing basic vessels of the human circulatory system
Circulatory system diagram

Arteries branch into smaller vessels called arterioles, which reach to every part of the body, ultimately branching again into the tiniest vessels called capillaries. It is here that transfer of essential molecules like oxygen happens through the ultra-thin walls of these miniscule vessels. Once molecules have been delivered and waste such as carbon dioxide has been picked up, it’s time for the blood to make a return trip back to the heart.Veins are the vessels that always bring the blood back to the heart.

Capillaries merge with tiny vessels known as venules, which eventually connect with larger veins. Blood is returned to the heart to begin the process all over again.All throughout its journey around the body, the blood stays confined in this closed system. Like a current flowing through a closed electrical circuit, this system only works properly if pathways remain closed.

If blood leaves the vessels somewhere along the way, there could be serious trouble.

Lesson Summary

There are two main types of circulatory systems in the animal world, open and closed. Humans have closed circulatory systems. This means that blood is always enclosed in vessels and the heart while circulating throughout the body. Traveling through arteries and veins, blood carries important molecules throughout the body and is always confined.

In comparison, animals such as insects have open circulatory systems. This means that the hemolymph travels through part of their bodies within vessels and then empties into the body cavity.

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