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In this lesson, we’ll delve into the reptile circulatory system. The lesson will cover the structure and function of reptile blood vessels and heart structure, as well as special adaptations of these systems.

The Reptile Circulatory System


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Lub-DUB. Lub-DUB. The sound of your very mammalian heart has a familiar and comforting rhythm. But what about reptiles? If you could press your ear against the chest of a crocodile (which I wouldn’t recommend), or a lizard, or a turtle, would you hear the same sound?Our mammalian circulatory system is superbly adapted for a speedy metabolism, just like bird circulatory systems.

Reptiles, in contrast, have a circulatory system that falls somewhere between rapid bursts of energy and long, lethargic days basking in the sun.All reptilian circulatory systems have a heart, blood vessels, including veins and arteries, and blood, just like mammals and birds. Not all reptiles have the same exact circulatory structure, though, especially with regard to the heart. We can trace the path of blood through a reptile’s body to get a complete picture of how this remarkable group of animals exchanges oxygen and carbon dioxide and gathers energy from environmental heat.Along the way, we’ll take a look at the overall structure and function of reptilian hearts, blood vessels, as well as more specialized adaptations of reptilian circulation.

Reptilian Hearts

Let’s imagine you’re a blood cell in a reptile’s body, and you’re in the animal’s heart. The maze you make your way through will vary in structure, depending on the reptile.

During this discussion, remember that ‘arteries move away’ from the heart, and ‘veins move toward’ the heart. This will help you remember the flow of blood through veins and arteries.Three-chambered hearts are present in most reptiles.

Like our four-chambered human hearts, three-chambered hearts have two upper atria. Unlike our own hearts, a three-chambered reptile heart has one large ventricle, or lower and more muscular portion of the heart.

B. Lizard, turtle, and snake heart anatomy, with three chambers.
lizard heart

In this single ventricle, both oxygenated – or oxygen rich – and deoxygenated – or oxygen poor – blood can mix together.

If you’re a deoxygenated blood cell, you want to pick up oxygen in the lungs. So, you travel into the heart via a large vein, the sinus venosus, and enter the right atrium. The right atrium then pushes the blood cell into the single lower ventricle, which pushes blood into the lungs through the pulmonary arteries. There, the blood cell will pick up oxygen.Oxygenated blood moves from the lungs through the pulmonary veins and into the left atrium and the main ventricle, where the path of the oxygenated and deoxygenated blood crosses. The vast majority of the oxygenated blood is pushed out of the heart through the aorta, a major artery.

Mixing of the blood is kept to a minimum through differential pressure during the beating of the aorta versus the ventricles. That gives the typical reptile heart a lub-DUB sound, though it is not quite as forceful as the sounds that our heart valves make when they are forced shut.

C. Crocodilian heart, with four chambers.
crocodilian heart

Crocodiles have a four-chambered heart with a fully divided set of atria and ventricles.

The septum tissue that divides the heart in the middle is not perforated, like it is in other reptiles’ three-chambered heart – where oxygenated and deoxygenated blood cross the same path. Very little mixing of oxygenated and deoxygenated blood (purple) occurs in crocodilians.

Reptile Veins

Reptile’s veins transport blood towards the heart. That includes taking our blood cell into the heart from the body through the sinus venosus, an analogue to our own mammalian inferior and superior vena cava. Pulmonary veins transport oxygenated blood from the lungs to the heart. Veins also transport blood back to the heart from its long circulation path throughout the body.In the transition between veins and arteries, tiny structures known as venules transition blood from the capillaries to the veins themselves.

Reptile Arteries

Arteries transport reptile blood away from the heart. Blood is transported away from the heart through the pulmonary arteries, which lead from the heart to the lungs. Arteries also transport blood away from the heart through the aorta, the major artery of both our own body and reptiles.

In some reptilian species, such as lizards, turtles, and crocodilians, two separate aortae are present.In the transition between arteries and veins, tiny structures known as arterioles transition blood from the arteries to the capillaries, and on to the venules and veins.

Special Adaptations

To combat the energetic inefficiency of crossing the path between oxygenated and deoxygenated blood, reptiles have evolved heart structures and special adaptations.Many non-crocodilian reptiles have evolved special chambers within the single ventricle that cordon off oxygenated and deoxygenated blood. This is an important factor in preventing the mixing of blood and supporting a higher metabolism.Another example of an circulatory adaptation involves snakes.

When snakes consume prey, think of how much energy it takes – all with its mouth full! Snakes can shunt oxygenated blood away from the lungs to other tissues while they feed. This keeps the snake from losing oxygen while powering the muscles that complete the feeding process.In order for turtles to make room for their hearts as they tuck their heads into their shells, turtle hearts are often situated in the center or the extreme lateral – side – of the body rather than near the chest or head. Crocodile and monitor lizard hearts are similarly located further towards the middle of the body than we might expect.Snake hearts are also quite movable, since they are pushed around by the consumption of larger prey.

Lesson Summary

Bringing things full circle, we have traced the path of blood through two different types of reptile hearts, learning about the structure and function of three-chambered hearts and four-chambered hearts along the way.

We’ve taken a look at the types of blood vessels and special adaptations in the reptile circulatory system. The system of blood flow in reptiles is as diverse and interesting as the group of creatures itself.

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