Evolution of the Heart in Vertebrates

Evolution of the Vertebrate Heart

The heart originates from a bilateral structure in the embryo, where mesenchymal cells form a pair of endocardial tubes below the pharynx. These tubes fuse into a single endocardial tube, which, along with the surrounding splanchnic mesoderm, forms the heart. This two-layered tube consists of an inner endocardium and an outer myocardium. The heart becomes S-shaped due to differential growth and the formation of chambers and valves.

Single-Chambered Heart

In Amphioxus, a primitive chordate, a true heart is absent. Instead, a contractile part of the ventral aorta beneath the pharynx acts as a heart. This primitive heart is enclosed within a pericardial cavity, which is separated from the body cavity by a transverse septum. The conus arteriosus connects to the ventral aorta and functions during circulation.

Two-Chambered Heart

In Cyclostomes: The heart has four chambers arranged sequentially: a thin-walled sinus venosus, a slightly muscular auricle, a muscular ventricle, and a muscular conus arteriosus and bulbus cordis. These chambers lie within the body cavity alongside other visceral organs. Evolution has led to significant changes in the heart's structure.

In Elasmobranchs: In elasmobranchs (cartilaginous fish like dogfish), the heart is a muscular, dorsoventrally bent S-shaped tube with four compartments in a linear series: the sinus venosus and auricle for receiving venous blood, and the ventricle and conus arteriosus for pumping blood. The heart is termed a branchial venous heart, where only deoxygenated blood is pumped to the gills, oxygenated, and then distributed to the body.

In Teleosts: In teleosts, the heart resembles that of elasmobranchs but with some differences. The conus arteriosus is reduced and contains a single pair of valves. The ventral aorta's proximal part, the bulbus arteriosus, is thick-walled and elastic, aiding in blood circulation during ventricular contraction.

Three-Chambered Heart

In lungfish, the atrium is divided by a septum into right and left chambers, reflecting a shift towards a double circulatory system where both oxygenated and deoxygenated blood enter the heart separately. The right auricle receives deoxygenated blood and pumps it to the gills or lungs, while the left auricle receives oxygenated blood returning from the lungs.

In Amphibians: Amphibians exhibit a further evolution of the heart, where the atrium is completely divided into right and left chambers. The conus arteriosus divides into systemic and pulmonary vessels by a spiral valve, facilitating better separation of oxygenated and deoxygenated blood. However, some mixing still occurs, especially in lungless salamanders where the interatrial septum is incomplete.

Reptiles

In reptiles, the heart has advanced further with a complete separation of the atrium into right and left chambers. The ventricle is partially divided, with complete separation in alligators and crocodiles. This ensures better separation of oxygenated and deoxygenated blood. The embryonic conus arteriosus splits into the pulmonary arch and systemic aorta, enhancing the efficiency of blood circulation.

Four-Chambered Heart in Birds and Mammals

In birds and mammals, the heart is fully divided into four chambers: two atria and two ventricles. This complete separation allows for efficient double circulation without any mixing of oxygenated and deoxygenated blood. The systemic aorta leaves the left ventricle, supplying oxygenated blood to the body, while the pulmonary artery leaves the right ventricle, carrying deoxygenated blood to the lungs for oxygenation. The sinus venosus is incorporated into the right atrium, enhancing the efficiency of venous return.

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