The Unique Morphofunctional Structure of the Reptilian Heart

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The paper analyzes the evolution of the thermoenergetic statuses of vertebrates and the associated evolutionary development of their heart. The analysis shows that in most modern lepidosaurs and turtles, the heart is not completely, conditionally five-chambered: it has two atria and one ventricle, in which two incomplete septas divide it into three functional chambers. In some of them, these two septas were modified in evolution so that they turned into one with vertical and horizontal elements, as a result of which the heart became functionally four-chambered, with improved separation of arterial and venous blood flows. Crocodiles have a fully morphologically four-chambered heart. But the hearts of all reptiles, both recent and extinct, perform two opposite functions in parallel – the separation of arterial and venous blood flows and at the same time their regulated mixing. To do this, there are special morphological and physiological mechanisms in their hearts. Such a strange functional duality in the work of the reptilian heart aims to regulate the metabolism level by controlling the amount of carbon dioxide entering the blood flow: increasing the amount of CO2 in the blood flow reduces the metabolic rate, reducing its amount increase metabolism. Mixed blood in reptiles’ blood flow is not an immature, primitive state, but a physiological necessity. Moreover, this method of regulating of metabolic rate is most adequate to the initial, ancestral thermoenergetic state in reptiles, because basal terrestrial tetrapods and most ancient reptiles were meso- and even tachymetabolic, i.e. almost or completely warm-blooded, endothermic animals. It was just these endothermic animals that needed such type of metabolism regulation. As a result, all recent reptiles have a complex morphophysiological organization of the heart, which was functionally more suitable for their almost warm-blooded ancestors. Recent reptiles use part of their ancestral properties as an adaptation to new environmental conditions, new environmental requirements, and their new morphophysiological state. This unique organization of the heart is characteristic of all modern and extinct reptiles, and, importantly, it is characteristic exclusively for reptiles due to their original endothermic state.

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V. A. Cherlin

Dagestan State University

Author for correspondence.
Email: cherlin51@mail.ru
Russian Federation, Makhachkala

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2. Fig. 1. A simplified diagram of the evolution of thermoenergetic statuses in vertebrates, their main indicators, and specific groups of animals that have followed different paths of this evolutionary development. The letters "R“ in the diagram indicate groups of vertebrates, which, in accordance with L.P. Tatarinov's position, assuming a horizontal type of vertebrate taxonomy (Tatarinov, 2009), can be considered reptiles. The partial overlap of gray ellipses denoting phylogenetic groups of vertebrates with black ellipses denoting the thermoenergetic state indicates that these phylogenetic groups have this thermoenergetic status.

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3. Fig. 2. Modern ideas about the evolution of vertebrates and their thermoenergetic statuses.

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4. Fig. 3. Two directions of manifestation of heart functions in reptiles in relation to arterial and venous blood flows: a decrease in the concentration of CO2 in the blood increases the level of metabolism, an increase in the level of CO2 slows down metabolism.

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5. Fig. 4. Simplified diagram of the heart structure of some reptile species and the blood flow in it. (a) – most scaly, tuataras and turtles; (b) – pythons (according to: Jensen et al., 2014). In fact, most scaly and tortoises have trabeculae in the arterial (8) and pulmonary (12) chambers of the ventricle, but in the figure they are erased so as not to interfere with the understanding of the division of the ventricle into chambers. The black thin arrows represent deoxygenated (venous) blood, the gray wide arrows and parts of the arrows (in figures II and IV) represent oxygenated (arterial) blood. I – the first diastole, II – the first systole, III – the second diastole, IV – the second systole. 1 – right atrium, 2 – left atrium, 3 – right aortic arch, 4 – left aortic arch, 5 – pulmonary artery, 6 – bulbous septum, 7 – venous chamber, 8 – arterial chamber, 9 – atrioventricular valve, 10 – vertical septum, 11 – muscular crest, 12 – pulmonary chamber, 13 – deoxygenated, venous blood, 14 – oxygenated, and

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6. Fig. 5. Diagram of the device and operation of a fully morphologically four-chambered crocodile heart (according to Benton, 2020). The black arrows represent venous blood, the gray arrows represent arterial blood. (a) – anatomy of the heart, (b) – blood circulation on land, (c) – blood circulation during diving. 1 – pulmonary artery, 2 – left aortic arch, 3 – right aortic arch, 4 – right ventricle, 5 – left ventricle, 6 – Panizza opening.

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7. Fig. 6. Diagram of shunts and valves providing mixing of two blood streams and regulation of the degree of its mixing in crocodiles. (a) – by: Seymour et al., 2004; (b) – by: Boas, 1884; Polezhaev, Shimkevich, 1891; Schmalhausen, 1947; (c) – by: Grigg et al., 2022. 1 – right atrium, 2 – left atrium, 3 – right ventricle, 4 – left ventricle, 5 – pulmonary artery, 6 – left aortic arch, 7 – right aortic arch, 8 – carotid arteries, 9 – interventricular septum, 10 – membranous septum, 11 – dorsal aorta, 12 – abdominal artery, 13 – extra–cardiac shunt, 14 - Panizza opening (intracardiac shunt), a 15 –tooth valve.

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