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How do three-chambered hearts in amphibians differ from four-chambered hearts in mammals and birds?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

The primary difference between the hearts of amphibians and those of mammals and birds lies in the degree of internal separation between oxygenated and deoxygenated blood.

Amphibian Hearts

Amphibians typically possess a three-chambered heart consisting of two atria and a single ventricle. While the atria are divided, the single ventricle allows for the mixing of oxygenated blood (returning from the lungs) and deoxygenated blood (returning from the body). Although muscular columns within the ventricle and a spiral valve in the conus arteriosus help direct blood flow to reduce mixing, the blood pumped to the body is only partially aerated. This system is an adaptation that supports flexible gas exchange, as amphibians can also utilize their skin for respiration.123

Mammalian and Avian Hearts

In contrast, mammals and birds possess a four-chambered heart with a complete ventricular septum, which entirely separates the left and right sides of the heart. This structure creates two distinct, independent circuits:2
* Pulmonary circulation: The right side pumps deoxygenated blood to the lungs.24
* Systemic circulation: The left side pumps oxygenated blood to the rest of the body.24

This complete separation prevents the mixing of blood, significantly increasing the efficiency of oxygen transport to support the higher metabolic demands of these animals.2