Evolution of Heart and Aortic Arches

Evolution of heart Separation of right and left atrium increasing Separation of right and left ventricle of the heart Deoxygenated blood on right side of the heart Oxygenated blood on left side of the heart In ancestral condition, separation is not complete but structures like interatrial septum help to maintain separation.   The aortic arches of vertebrate animals from the arterial supply route to the tissue In the embryo, aortic arches develop in association with each of the developing pharyngeal arches An illustration showing the basic embryological pattern of aortic circulation is shown, and the fates of these arches in several representatives of the major vertebrate groups are discussed. Aortic arches are paired blood vessels that emerge from the ventricle of the heart and carry blood from the dorsal blood vessels to the ventral blood vessels. They are built on the same fundamental plan. The number of aortic arches is variable in different vertebrates. Ostracoderms had 10 pairs of aortic arches. In all vertebrates, aortic arches are bilaterally symmetrical but in amniotes these are asymmetrical. Lampreys have 9 pairs and in hagfishes the number of aortic arches is 15 pairs. In gnathostomes, there are 6 pairs of aortic arches found in the embryo of all vertebrates. Aortic arches are paired blood vessels that emerge from the ventricle of the heart. They vary in number and disposition in different vertebrates during embryonic stages. Embryonic Aortic Arches: During the embryonic stages, 6 pairs of aortic arches develop in most gnathostomes and are named according to the visceral clefts The number of aortic arches is different in different adult vertebrates, but they are built on the same fundamental plan.   The differences in the number of aortic arches are related to the complexity of heart circulation and the mode of living (from aquatic to terrestrial). Branchiostoma (Amphioxus) has 10 pairs of aortic arches, but Petromyzon has only 7 pairs, and Myxine has only 6 pairs. Aortic arches in fishes: The hearts of fishes are 2-chambered. They have only one atrium and one ventricle. In that respect, the fish heart resembles the embryonic condition of all other vertebrate animals.   A thin-walled space called the sinus venosus gathers blood from the veins before entering the atrium. Elasmobranchs have only 7 pairs of aortic arches; Chondrichthyes have only 6 pairs. Sharks have 5 pairs of functional aortic arches; the first pair is reduced or disappeared by the non-functional gill. In teleosts, each arch has one afferent and one efferent artery in each gill. In most teleosts or bony fishes, the 1st and 2nd aortic arches tend to disappear, and thus only the 3rd, 4th, 5th, and 6th pairs of aortic arches remain functional. In polypterids and lungfishes (dipnoi): Due to the terrestrial lifestyle, the 1st, 2nd, and 5th aortic arches disappeared. Thus, there are only 3rd, 4th, 6th aortic arches functional. In amphibians: Amphibians have a 3-chambered heart. The heart has a divided atrium segment. The right atrium receives blood from systemic circulation, while the left atrium receives blood from the lungs.   Due to the presence of lungs as the main respiratory organ, the importance of gills is diminished. In urodels: Four pairs of aortic arches from 3rd to 6th are functional in general.   The 5th pair is absent in Siren, Amphiuma. The 3rd pair forms the carotid artery, the 4th pair forms systemic arches, and the 6th pair forms the pulmonary aorta which supplies blood to skin and lungs. In anurans: At metamorphosis, with the loss of gills, the 1st, 2nd, and 5th aortic arches disappear altogether. Thus, 3 pairs of aortic arches (3rd, 4th, and 6th) remain functional in general. In reptiles: Reptiles have a 3.5-chambered heart. The ventricle is partially divided. This separation decreases the mixing of left-side oxygenated blood with deoxygenated blood of the right ventricle.   Reptiles are fully terrestrial vertebrates. Gills disappear altogether and are replaced by lungs. Only three functional arches (3rd, 4th, and 6th) are present. In birds and mammals: Birds and mammals are warm-blooded because the ventricle is completely divided, preventing the mixing of oxygenated and deoxygenated blood. Six arches develop in embryos, but only 3 arches (3rd, 4th, and 6th) persist in the adult. Both birds and mammals have a 4-chambered heart. Both atrial and ventricular regions of the heart are divided.