Heart anatomy and development
In all representatives of the class Reptilia (Reptilia), which are cold-blooded (poikilothermic) animals, the heart is located on the ventral side. During embryogenesis, it undergoes a complex path: it develops from paired mesodermal primordia that fuse into a single tube. Then, a two-chambered, later three-chambered heart with a common ventricle is formed, and finally — a three-chambered heart with an incomplete interventricular septum.
In adult specimens, the heart consists of completely separated atria that open into the ventricle through independent orifices. Inside the ventricle, there is an incomplete septum. At the moment of systole (contraction), this septum is capable of completely dividing the ventricle into two parts, which effectively prevents the mixing of blood flows.
An exception is crocodiles: their heart is four-chambered, with a complete interventricular septum. However, there is a centrally located foramen within the septum itself, and a connection between the arches (crossing of the right and left aortic arches) is also preserved.
Hemodynamics and arterial system
The conus arteriosus in adult reptiles is reduced, so three independent efferent vessels depart from the ventricle. Blood flow in the ventricle is distributed strictly by sections: venous blood is in the right part (entering from the right atrium), arterial blood is in the left part (from the left atrium), and a small portion of mixed blood accumulates above the septum.
Topography of the departing vessels:
- Pulmonary trunk: departs from the right part of the ventricle, carries venous blood, and divides into two pulmonary arteries.
- Right aortic arch: departs from the left part of the ventricle, carries arterial blood. The carotid arteries to the brain and subclavian arteries to the forelimbs branch off from it.
- Left aortic arch: departs from the region above the septum and carries mixed blood.
Behind the heart, the right and left aortic arches unite, forming the unpaired dorsal aorta. Since the left arch contributes a portion of mixed blood, the blood in the dorsal aorta also becomes mixed. It provides blood supply to the trunk and hind limbs.
Transformation of arterial aortic arches
In embryogenesis, 6 pairs of arterial aortic arches depart from the ventral aorta. During the development of the circulatory system, they undergo radical changes:
- Pairs I, II, and V — regress completely.
- III pair — transforms into the carotid arteries, providing the brain with a blood supply of pure arterial blood.
- IV pair — gives rise to the right and left aortic arches.
- VI pair — transforms into the pulmonary arteries.
Some species retain rudimentary embryonic structures. For example, lizards retain the carotid duct — a commissure connecting the carotid arteries with the aortic arches (between the III and IV arches). In turtles, one can find the ductus arteriosus (Botallo's duct), connecting the pulmonary arteries to the aortic arches (a commissure between the IV and VI arches).
Venous system
The heart of reptiles receives 5 afferent (incoming) vessels: three venae cavae and two pulmonary veins. The sinus venosus, characteristic of more primitive vertebrates, is reduced in reptiles.
- Return from the upper body: venous blood collects in two superior venae cavae (right and left) and flows into the right atrium.
- Return from the lower body: blood flows via the inferior vena cava, having previously passed through filtration in the hepatic portal system, and also flows into the right atrium.
- Return from the lungs: oxygen-rich arterial blood moves through the right and left pulmonary veins, flowing into the left atrium.
Briefly: answers to topical questions
How does the heart of crocodiles differ from that of other reptiles?
The heart of crocodiles differs from that of other reptiles by having four chambers and a complete interventricular septum.
| Sign | Crocodiles | Other reptiles |
|---|---|---|
| Number of chambers | Four-chambered | Three-chambered |
| Interventricular septum | Complete | Incomplete |
Despite the presence of a complete septum, crocodiles retain a connection between the arches (crossing of the right and left aortic arches). There is also a centrally located foramen (foramen of Panizza) within the septum itself, through which partial mixing of blood occurs.
Which vessels form the hepatic portal system in reptiles?
In reptiles, venous blood from the abdominal organs first passes through the hepatic portal system. Sources confirm the presence of this pathway, but do not provide a complete list of vessels forming the hepatic portal system in reptiles.
Where is the sinus venosus located and where does it drain in reptiles?
In adult reptiles, the sinus venosus is reduced. Venous blood from the upper body enters via two superior venae cavae, and from the lower body via the inferior vena cava; these three vessels drain into the right atrium.
To remember which blood goes where from the ventricle, use the side-matching rule: The right part of the ventricle (venous) gives rise to the pulmonary trunk to the lungs, and the left part of the ventricle (arterial) gives rise to the right aortic arch to the brain and forelimbs. A "crossing" of the names of the arches and the parts of the ventricle occurs.
Class and exam questions
Why does the brain of reptiles receive the most oxygen-rich blood?
The carotid arteries supplying the brain branch off from the right aortic arch. This arch originates in the extreme left part of the ventricle, where pure arterial blood coming from the left atrium collects.
What is the uniqueness of the crocodile heart?
Unlike other reptiles, the heart of crocodiles is four-chambered with a complete interventricular septum. However, there is a central foramen in the septum itself, and the crossing of the aortic arches is preserved, which leaves the possibility for partial blood mixing.
What happens in the reptile heart during systole?
During contraction (systole), the incomplete interventricular septum completely closes with the opposite wall, temporarily dividing the ventricle into two isolated chambers. This prevents the mixing of arterial and venous blood at the moment of ejection into the vessels.
What else to study on this topic
- Stages of amniote heart embryogenesis from the mesoderm
- Diagram and significance of the hepatic portal system in reptiles
- Features of species anatomy: carotid and Botallo's ducts
- Comparative analysis of phylogeny: reduction of the conus arteriosus and sinus venosus
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