How is Blood Circulation in Reptiles?

Abraham McLaughlin

The blood circulation in reptiles it is double, closed and incomplete. It consists of a heart with two atria (communicated by a hole called the Foramen of Panizza) and a ventricle, as well as blood vessels.

Reptiles are the animals that belong to the order of the saurians, chelonians, snakes and crocodilians. In all orders, except that of crocodilians, the blood circulation system works in the same way.

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It has a cavity between the two atria of the heart where oxygen-laden blood (from the left atrium) mixes with oxygen-poor blood (from the right atrium). For this reason it is said that the circulation is closed, since the blood never travels outside the blood vessels.

It is also said that it is double, because for the blood to complete a journey it must pass through the heart twice. Finally, it is said to be incomplete as oxygen-rich blood mixes with oxygen-poor blood..


In the case of crocodilian reptiles, the circulation is closed, double and complete. In other words, oxygenated blood never comes into contact with blood that lacks oxygen..

Regardless of the type of reptile, the circulation process will always be carried out in two circuits, one minor (pulmonary) and one major (systemic)..

Circulation in non-crocodilian reptiles

In non-crocodilian reptiles, the circulation process is divided into a minor and a major circuit.

Minor circuit

The minor circuit begins in the heart, where the right atrium contracts and causes oxygen-poor blood to travel to the ventricle, which is partially divided.

Subsequently, the ventricle contracts and causes oxygen-poor blood to pass to the lungs, through the pulmonary arteries..

There the blood is oxygenated and released from carbon dioxide. The oxygen-rich blood then passes from the lungs through the pulmonary veins into the left atrium..

Once the left atrium contracts, it causes the blood to travel to the ventricle, where it partially combines with the oxygen-poor blood, left over from the previous pumping. In this way, the process of the minor circuit is concluded.

Major circuit

In the case of the major circuit, the circulation process begins when the ventricle contracts and causes oxygen-rich blood to pass through the aorta artery to each of the body's cells..

During the process of the larger circuit, the blood collects the carbon dioxide present in all the cells of the body, as well as oxygenates them.

Once the blood passes through the whole body, and the carbon dioxide is collected, it passes through a network of capillaries (each with a different diameter), which converge in a type of veins known as vena cavae..

The vena cavae are responsible for carrying oxygen-poor blood to the right atrium, which contracts and allows blood to travel to the ventricle to start the minor circuit process again..

Circulation in crocodilian reptiles


The circulatory system of crocodilian reptiles has a heart divided into two atria and two ventricles (similar to that of mammals and birds).

Between the atria and ventricles there are valves, known as the tricuspid on the right side and the mitral valve on the left side..

The tricuspid and mitral valves prevent blood from backing up as it circulates within the heart. In this sense, the circulatory system of crocodilian reptiles is closed, double and complete..

It is said that the circulatory system of crocodilian reptiles is closed because the blood contained within it never travels to the outside of the blood vessels.

On the other hand, it is said to be double, since the blood must pass through the heart twice to carry out a single journey. Finally, the system is considered complete, since at no time is oxygenated blood mixed with blood that lacks oxygen.

On the other hand, it can be seen that in the heart of crocodilian reptiles the left side of the heart is much more developed than the right side.

This is because the left ventricle must pump the blood with enough force so that it can travel throughout the body when it leaves the heart..

Minor circuit

As in other reptiles, the crocodilian circulation process also takes place in two circuits.

The minor circuit begins when the right ventricle contracts, once oxygen-poor blood has been received and the tricuspid valve closed. In this way, blood that lacks oxygen is sent to the lungs through the pulmonary arteries..

In the pulmonary arteries the blood is oxygenated and carbon dioxide is released. Once this process takes place, the oxygen-rich blood leaves the lungs and travels through the pulmonary veins until it reaches the left atrium..

There it contracts and the mitral valve opens so that blood passes to the left ventricle..

Major circuit

The major circuit begins with the contraction of the left ventricle and the closure of the mitral valve. At this time, the oxygenated blood travels through the aorta artery to supply all the cells of the body..

During this process, the carbon dioxide contained in all the cells of the body is also collected. This distribution of blood throughout the body is possible thanks to a network of capillaries present in all reptile tissues..

These capillaries have different diameters and flow into the vena cavae, which flow into the right atrium. In this place, the blood is again propelled to the right ventricle and the whole process begins again..

Crocodilian reptiles are considered the most evolved of their kind, since they have a heart with four chambers. However, there are some species within this order that can have a heart with only three chambers..


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