What is Double Circulation?

G.W. Poulos

Double circulation is the process by which the cardiovascular systems of many vertebrates such as mammals and birds circulate blood throughout their bodies. In this system, the heart pumps the blood twice to perform its function. The first pumping sends the blood to be circulated though the lungs, and the second pumping circulates the blood throughout the body.

Illustration of circulating blood.
Illustration of circulating blood.

Humans, like most other mammals, have a highly developed circulation system. In humans, the heart is divided into the right and left halves, which are actually two separate pumping and flow systems. The right side of the heart accepts blood that has been circulated throughout the body and pumps it at a relatively low pressure to the lungs. In the lungs, the blood is replenished with oxygen.

Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again.
Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again.

Once the blood has been completely circulated through the lungs, it returns to the heart. Arriving at the much larger left side of the heart, the blood is then pumped out to the entire body. As this side must push the blood though a greater amount of tissue, it squeezes much harder than the right, creating a greater amount of pressure. After the blood has been fully circulated throughout the body, it returns to the right side of the heart, where the process is repeated.

Blood circulates through the lungs to be refilled with oxygen before returning to the heart.
Blood circulates through the lungs to be refilled with oxygen before returning to the heart.

Though the blood is considered to be fully circulated upon its return to the heart, each blood cell does not actually visit each part of the body. In double circulation systems, there are many different paths called arteries that the circulating blood may take to reach different parts of the body. There are also many different paths called veins that the blood uses to return to the heart. The blood is delivered in the quantities needed by each portion of the body and then returned to the heart to be sent for reoxygenating in the lungs. Generally, the more blood needed by a specific part of the body, the more direct the path the blood takes to and from the heart.

Fish have a single circulation system.
Fish have a single circulation system.

Though most highly developed animals have double circulation systems, not all do. Fish, for example, have a single circulation system that lacks the double-sided hearts of mammals. In fish, the blood circulates throughout the body until it reaches the heart. Once it returns there, the blood is pumped again, passes through the fish’s gill system, and then circulates on through the body again.

Like most other mammals, humans have a highly developed circulation system.
Like most other mammals, humans have a highly developed circulation system.

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Discussion Comments

NathanG

@janismiller - This is amazing. I had never heard of a double circulatory system for the heart. That’s an intelligent system, if you ask me, and I’m coming from a computer science background.

Another amazing fact is that the heart performs these functions without any obvious guidance. It’s an involuntary function.

I think that’s what scientists call it, they make a distinction between involuntary and voluntary functions. If you lift your hand, that’s voluntary; when your heart beats, that’s involuntary.

What tells the heart to keep beating? Is it the mind? Then what do we make of people who are in a coma and their heart still beats?

I don’t know, I don’t claim to have answers to these questions, but I’m glad that my heart keeps doing its job without any guidance from me…I tend to be a little absent minded.

Janismiller

@MalakAslan – Those are amazing facts about our cardiovascular system. Our hearts just keep on pumping and we don’t really pay much attention until something goes wrong.

According to the CDC the number one and three leading causes of death, heart disease and stroke (cerebrovascular diseases), are circulatory diseases.

If we all just took better care of our bodies there would be a lot less circulatory system disease. Most of it can be prevented by a good diet and exercise program.

MalakAslan

The human circulatory system is amazing. The human heart only weighs around 10 ounces and yet it can pump about 2,000 gallons of blood per day.

I learned some other interesting cardiovascular system facts from a paper my daughter brought home from school.

Did you know that a healthy heart beats around 100,000 beats per day and that if added together the length of an average adult’s blood vessels could circle the earth twice?

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