How Is Blood Circulation in Reptiles?

The Blood circulation in reptiles Is double, closed and incomplete. It consists of a heart with two atria (connected by an orifice called Foramen de Panizza) and a ventricle, as well as blood vessels.

Reptiles are animals that belong to the order of the saurians, chelonians, opossums and crocodilians.

Turtle blood circulation, characteristic reptile Image via

In all orders, except in the case of crocodilians, the blood circulation system works in the same way.

It has a cavity between the two atria of the heart where oxygen-loaded bleeding (from the left atrium) is mixed with oxygen-poor blood (from the right atrium).

For this reason it is said that the circulation is closed, since the blood never moves outside the blood vessels.

It is also said to be double, because for the blood to complete a course must pass twice by the heart. Finally, it is said to be incomplete because oxygen rich blood is mixed with oxygen-poor blood.

In the case of crocodilian reptiles, the circulation is closed, double and complete. That is, oxygenated blood never comes into contact with oxygen-depleted blood.

Regardless of the type of reptile, the circulation process will always be carried out in two circuits one minor (pulmonary) and one major (systemic) (Explanation and schema circulating reptiles, birds and mammals, 2013).

Circulation in non-crocodilian reptiles

In non-crocodilian reptiles, the circulation process is divided into a smaller and one larger 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.

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

There the blood is oxygenated and released from carbon dioxide. Then, oxygen-rich blood passes from the lungs through the pulmonary veins towards the left atrium.

Once the left atrium is contracted, it causes the blood to travel to the ventricle, where it is partially combined with the oxygen-poor blood, leftover from the previous pump. In this way, the process of the smaller circuit is concluded.

Main 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 aortic artery to each of the body's cells.

During the process of the greater circuit the blood collects the carbon dioxide present in all the cells of the body, at the same time as it oxygenates them.

Once the blood passes through the body, and carbon dioxide is collected, it passes through a network of capillaries (each of different diameter), which converge in a type of veins known as cavas veins (Khanna, 2004).

Veins cavas are responsible for bringing poor blood oxygen into the right atrium, which contracts and allows the blood to travel to the ventricle to start the process of minor circuit.

Circulation in crocodilian reptiles

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

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

The tricuspid and mitral valves prevent blood from receding as it circulates inside the heart.

In that sense, the circulatory system of the crocodilian reptiles is closed, double and complete (Natural, 2013).

It is said that the circulatory system of the crocodilian reptiles is closed because the blood contained in its interior never moves to the outside of the blood vessels.

On the other hand, it is said to be double, since blood must pass twice through the heart to carry out a single stroke.

Finally, it is considered that the system is complete, since at no time does the oxygenated blood mix with that which lacks oxygen.

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

This is due to the fact that the left ventricle must pump the blood with sufficient force so that it leaves the heart and reaches the whole body.

Minor circuit

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

The minor circuit begins when the right ventricle contracts, once the blood is poor in oxygen and the tricuspid valve is closed.

In this way, blood that lacks oxygen is sent to the lungs through the pulmonary arteries (Kubesh, McNeilM, & Bellotto, 2009).

In the pulmonary arteries blood is oxygenated and carbon dioxide is given. Once this process takes place, 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 to allow blood to pass into the left ventricle.

Main circuit

The major circuit begins with contraction of the left ventricle and closure of the mitral valve.

At this point, the oxygenated blood moves through the aorta to irrigate all the cells of the body.

During this process the carbon dioxide contained in all 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 tissues of reptiles.

These capillaries have different diameters and converge in the cavas veins, which flow into the right atrium.

In this place, the blood is again pushed to the right ventricle and the whole process begins again (101, 2014).

Crocodylian reptiles are considered the most evolved within their type, since they have a heart with four cavities.

However, there are some species within this order that may have a heart with only three cavities.


  1. 101, C. (2014). Herpetology, An Introductory Biology of Amphibians and Reptiles: Biology of Amphibians and Reptiles. CTI Reviews.
  2. (3 of 2013). Retrieved from Explanation and outline reptiles, birds and mammals circulation:
  3. Khanna, D. (2004). Biology of Reptiles. New Delhi: Discovery Publishing House.
  4. Kubesh, K., McNeilM, N., & Bellotto, K. (2009). Coloma: Lapbook.
  5. Naturales, C. (February 2013). Obtained from Circulatory system in reptiles: natural

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