Anatomical structure of the heart
Adult amphibians have a three-chambered heart located on the ventral side of the body. It consists of two atria and a single common ventricle.
Both atria open into the ventricular cavity through a single common opening. The right atrium contains deoxygenated blood collected from internal organs, while the left atrium contains pure oxygenated blood coming from the lungs. Five incoming vessels (three venae cavae and two pulmonary veins) approach the heart, and only one trunk emerges from it — the truncus arteriosus.
The inner surface of the ventricular floor has a complex relief of muscular outgrowths called trabeculae. Their main function is to prevent complete mixing of different blood flows. Due to the trabeculae, the ventricular space is functionally divided:
- Deoxygenated blood is retained in the right part.
- A mixed fraction forms in the central zone.
- Oxygenated blood concentrates in the left part.
Arterial system and blood distribution
Blood ejection from the heart occurs in a strictly sequential manner. The truncus arteriosus extends from the right part of the ventricle. Inside this vessel lies a long spiral valve that regulates hemodynamics by sequentially opening the arterial ostia and directing portions of blood into the respective pathways.
Order of vessel emergence from the truncus and blood distribution:
- Pulmocutaneous arteries (first pair): branch off first, carrying deoxygenated blood for subsequent gas exchange.
- Aortic arches (second pair): receive mixed blood. These large vessels wrap around the heart and fuse together to form the dorsal aorta.
- Common carotid arteries (third pair): branch off last, carrying the most oxygen-rich oxygenated blood to the brain.
Venous bed
The amphibian venous system ensures the return of blood to the heart:
- Blood from the posterior half of the body is collected into the unpaired posterior vena cava. Notably, before returning to the heart, this flow undergoes mandatory filtration in the hepatic portal system.
- Venous blood drains from the anterior part of the body via the right and left anterior venae cavae.
All three venae cavae empty into the sinus venosus, from which blood enters the right atrium. Simultaneously, pure oxygenated blood is discharged directly from the lungs into the left atrium via two pulmonary veins.
Ontogeny and evolution of branchial arches
The development of the amphibian circulatory system is accompanied by profound metamorphoses. At the larval stage (in tadpoles), the heart pumps exclusively deoxygenated blood and is two-chambered (one atrium and one ventricle), making it anatomically identical to the fish heart.
In embryos, 6 pairs of arterial branchial arches develop from the ventral aorta. During ontogenesis, they undergo the following changes:
| Arch pair | Fate in adult amphibian |
|---|---|
| Pairs I, II, V | Regress |
| Pair III | Transforms into carotid arteries |
| Pair IV | Becomes the right and left aortic arches |
| Pair VI | Forms the pulmocutaneous arteries |
In tailed amphibians, the commissure between the IV and VI branchial arches does not disappear, persisting in adults as the ductus arteriosus (botall duct).
Briefly: answers to topical questions
What role does the hepatic portal system play in the venous bed of amphibians?
The hepatic portal system provides preliminary purification of deoxygenated blood draining from the posterior part of the amphibian's body. This blood collects into the unpaired posterior vena cava, which empties into the sinus venosus.
To easily remember the order in which vessels emerge from the amphibian heart, use the mnemonic "Skin Breathes Somnolently": Skin/pulmocutaneous arteries (first, deoxygenated blood), Aortic arches (second, mixed), Carotid arteries (third, oxygenated).
Class and exam questions
Why doesn't blood mix completely in the ventricle of amphibians?
Special muscular projections—trabeculae—are located on the floor of the ventricle. They create a spongy structure that prevents free mixing, keeping arterial, venous, and mixed blood in different zones of the chamber.
How is the tadpole circulatory system structured?
At the larval stage, amphibians have a two-chambered heart (one atrium and one ventricle). Only deoxygenated blood passes through it, fully mirroring the structural principles of the fish circulatory system.
What type of blood supplies the brain of amphibians?
Pure oxygenated blood goes to the brain. It is ejected from the ventricle last and directed through the spiral valve of the truncus arteriosus into the common carotid arteries.
What else to study on this topic
- Mechanism of action of the spiral valve in the truncus arteriosus
- Physiology and significance of the hepatic portal system in amphibians
- Function of the ductus arteriosus (botall duct) in tailed amphibians
- Comparative hemodynamics of the larval stage and the adult
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