From acrania to fish: origin of the heart
The evolution of the circulatory system begins with primitive forms that still lack a specialized muscular pump.
In acrania (exemplified by amphioxus), the circulatory system is closed, which aligns them with invertebrate animals, specifically annelids. They lack a true heart, and blood is pumped by the pulsation of the ventral aorta. An important distinction is the distribution of blood: the ventral aorta carries venous blood, and the dorsal aorta carries arterial blood. Circulation occurs in a single loop.
The first major progressive step is observed in fish. They develop a true two-chambered heart consisting of a single atrium and a single ventricle. Despite the structural complexity, the circulatory loop remains single. An important physiological feature of fish is that exclusively venous blood passes through their heart, which is then directed to the gills for gas exchange.
Transition to land: amphibians and reptiles
The transition to a terrestrial lifestyle required a radical restructuring of the vascular bed.
In amphibians, pulmonary respiration appears, leading to the formation of a second loop — the pulmonary circulation. The heart becomes three-chambered (two atria and one ventricle). Arterial and venous blood mix within the single ventricle, so mixed blood is delivered to internal organs. However, the mixing process is partially mitigated by special muscular ridges called trabeculae.
In reptiles, further complexity is observed. An incomplete septum forms in the ventricle (becoming complete in crocodilians). This makes the separation of blood flows more efficient, although partial mixing still persists, for instance, through septal defects or the specific foramen of Panizza. Three independent major vessels emerge from the reptilian heart: the pulmonary artery, as well as the left and right aortic arches.
Achieving homeothermy: birds and mammals
The pinnacle of cardiovascular system evolution is the complete separation of the arterial and venous beds, which served as the morphological basis for homeothermy.
In both birds and mammals, a fully functional four-chambered heart forms with a complete interventricular septum. The circulatory loops are completely isolated from each other.
Despite similarities, there are important class-level differences:
- In birds, only the right aortic arch is retained during embryogenesis.
- In mammals (including humans), only the left aortic arch is retained. An additional feature of mammals is the presence of anucleated erythrocytes, which increases their oxygen-carrying capacity.
Clinical significance: ontophylogenetic malformations
Understanding phylogeny is critically important for medicine, as many congenital cardiovascular malformations in humans are explained by the phenomenon of recapitulation — the arrest of embryonic development at a stage corresponding to the structure of ancestral forms.
Cardiac malformations:
- Two-chambered heart: complete arrest at the "fish stage". The pathology is entirely incompatible with life.
- Atrial septal defect (ASD): arrest at the "amphibian stage".
- Ventricular septal defect (VSD): arrest at the "reptilian stage" (retention of an incomplete septum).
Great vessel malformations:
- Persistent ductus arteriosus (Botallo's duct): retention of the embryonic connection between the aorta and the pulmonary artery. This is normal for a human embryo, as well as for adult amphibians and reptiles, but a severe pathology in the human postnatal period.
- Transposition of the great arteries.
- Aortic arch anomalies: formation of a right-sided arch (as in birds) or persistence of a double aortic arch (as in amphibians and reptiles). Such anomalies can lead to the formation of a "vascular ring" causing dangerous compression of the trachea and esophagus.
Briefly: answers to topical questions
What is the foramen of Panizza and between which vessels is it located?
The foramen of Panizza is a structure through which partial blood mixing occurs in reptiles; sources do not specify precisely between which vessels it is located.
To remember the aorta: Birds are always "RIGHT" (the right arch is retained), while mammals went "LEFT" (left arch).
Class and exam questions
Why is mixed blood delivered to amphibian organs?
Amphibians developed a pulmonary circulation and two atria, but the ventricle remained single. Arterial and venous blood mix within it, although trabeculae somewhat reduce this process.
What are ontophylogenetic developmental malformations?
These are congenital human anomalies arising from recapitulation — the arrest of fetal organ development at a stage that anatomically corresponds to the structure of evolutionary ancestors (e.g., a ventricular septal defect corresponds to the reptilian heart).
What else to study on this topic
- Homology of arterial branchial arches
- Genetic regulation of interventricular septum development
- Mechanisms of partial blood mixing (foramen of Panizza)
- Transposition of great vessels: embryogenesis of the malformation
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