Lizards are fascinating reptiles that have captured people’s imaginations for ages. A common question many have is whether these cold-blooded creatures have 4 chambered hearts like mammals and birds.

If you’re short on time, here’s a quick answer: No, lizards do not have 4 chambered hearts. They have 3 chambered hearts, with 2 atria and 1 ventricle.

In this comprehensive article, we’ll explore the anatomy of lizard hearts, how they function, how they compare to the hearts of other animals, and the purpose they serve for these unique reptiles.

The Anatomy and Function of Lizard Hearts

Three chambers instead of four

Unlike mammals and birds, lizards have a three-chambered heart rather than four. This means they have two atria to receive blood and only one ventricle to pump blood, rather than separate ventricles like humans have.

The single ventricle mixes oxygenated and deoxygenated blood before pumping it out through the body. This is a less efficient system, but it works well enough for lizards’ relatively small bodies and lower energy needs compared to warm-blooded animals.

Single ventricle pumps blood

The single muscular ventricle makes up the bottom portion of the lizard heart. When it contracts, it pumps blood out to the lungs and body through a single large vessel called the truncus. Valves within the ventricle and truncus direct some blood towards the lungs to be oxygenated, while the rest circulates around the body.

The single ventricle allows a continuous flow of blood, rather than separate pulmonary and systemic circuits.

Two atria receive blood

Lizards have two upper atria which receive blood, just like mammals and birds. The right atrium receives deoxygenated blue blood from the body through large veins. The left atrium receives freshly oxygenated red blood from the lungs through the pulmonary veins.

Each atrium passes blood down into the shared ventricle through one-way valves. The mixing of the blood allows lizards to shunt blood away from the lungs when oxygen needs are low, conserving energy.

Comparison to Other Animal Hearts

The hearts of reptiles like lizards are often compared to the hearts of other animal groups like mammals, birds, fish and amphibians. Understanding how they differ provides great insight into the unique circulatory systems of lizards.

Mammals and birds have 4 chambers

The hearts of mammals and birds contain 4 chambers – two atria and two ventricles. This allows for complete separation of oxygenated and deoxygenated blood, enabling highly efficient circulation. The four chambers work in synchrony to pump blood to the lungs and body.

Fish have 2 chambers

Fish hearts contain just two chambers – one atrium and one ventricle. This means oxygenated and deoxygenated blood mix, reducing efficacy. However, the simple two chamber design meets the needs of fish living in water.

Amphibians have 3 chambers like lizards

Amphibians have 3-chambered hearts, just like lizards and some other reptiles. They contain two atria but only one ventricle. This means some mixing of oxygenated and deoxygenated blood occurs. While less efficient than the 4-chamber hearts of mammals and birds, a 3-chamber heart adequately meets the circulation needs of amphibians and lizards.

The Evolutionary Purpose of 3 Chambered Hearts in Lizards

Suited to Reptilian Physiology and Oxygen Needs

Lizards and other reptiles have a three-chambered heart, consisting of two atria and one ventricle, that has evolved to suit their specific physiological needs (source). This cardiac design provides sufficient oxygenation for their relatively low energy lifestyles compared to mammals and birds.

The single ventricle allows mixing of oxygen-rich blood from the lungs and oxygen-poor blood from the body before pumping it back out. This adequate oxygenation aligns with lizards’ lower aerobic activity.

Allows Blood Mixing to Regulate Body Temperature

The three-chambered heart supports lizards’ ectothermic regulation of body temperature (source). The partial mixing of blood allows heat to transfer from warmer blood to cooler blood. This facilitates warming up when lizards are cold or cooling down when they are hot.

Thus their cardiac design with some shunting aids thermoregulation critical to their survival.

Adequate for Lizards’ Activity Levels

The lower-pressure three-chambered heart sufficiently meets the blood circulation needs for lizards’ general activity levels (source). As reptiles with relatively modest exercise requirements compared to mammals, the partially mixed blood provides enough oxygen and nutrients to their tissues.

Their heart design did not need to evolve into a higher-pressure four-chambered model required by warm-blooded creatures with greater exertion needs.


In summary, lizards have 3 chambered hearts consisting of 2 atria and 1 ventricle, unlike mammals and birds which have 4 chambered hearts. This unique structure suits the evolutionary needs of these ectothermic reptiles by allowing blood mixing to regulate body temperature.

While simple in design compared to 4 chambered hearts, the lizard heart adequately meets the cardiac demands of these fascinating creatures.

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