Heart Structure and Embryogenesis
During the embryonic period, the heart rudiment develops in the pharyngeal region. Its formation originates from mesodermal cells. The developmental process occurs in stages: first, a primary plate is formed, which then transforms into the heart tube.
In adult specimens, the heart is located on the ventral side of the body. It is two-chambered—anatomically consisting of a single atrium (atrium) and a single ventricle (ventriculus). The most important physiological feature of the fish circulatory system is that exclusively deoxygenated blood circulates inside the heart.
Blood Inflow and Outflow Pathways
Blood enters the heart via three afferent (incoming) vessels: the right and left Cuvierian ducts, and the hepatic vein. All of them carry deoxygenated blood and open into the sinus venosus (sinus venosus), from which the fluid flows into the atrium and then into the ventricle.
A single efferent vessel extends from the ventricle—the ventral aorta (aorta ventralis). The comparative anatomy of the cardiac outflow tract varies among different fish groups:
- In cartilaginous fish (selachians) and ganoids, an arterial conus (conus arteriosus) extends from the ventricle, equipped with valves to prevent blood regurgitation.
- In teleost fish, the arterial conus is reduced. Instead, the ventral aorta forms a characteristic expansion—the bulbus arteriosus (bulbus arteriosus), in which only a single pair of valves is retained.
Arterial System and Gill Apparatus
In fish embryos, the ventral aorta branches into 6–7 pairs of arterial gill arches. During development, the first and second pairs are completely reduced. The remaining arches (III, IV, V, and VI pairs) transform into 4–5 pairs of afferent branchial arteries.
Blood flow and gas exchange occur according to the following scheme:
- Deoxygenated blood is directed from the heart to the gills via afferent arteries.
- A dense capillary network is developed in the gill lamellae, where blood oxygenation (gas exchange) takes place.
- Oxygenated (arterial) blood is collected into efferent branchial arteries.
The carotid arteries, which supply blood to the head, branch off from the first pair of efferent arteries. The remaining efferent vessels merge into the roots of the dorsal aorta, which fuse into a single dorsal aorta carrying blood to all organs.
Venous System and Portal Blood Flows
The venous system of fish is characterized by the appearance of the renal portal system.
Venous blood drainage from body tissues occurs via the anterior and posterior cardinal veins (right and left). By fusing, they form the right and left Cuvierian ducts, which empty into the sinus venosus.
A special role is played by the hepatic portal system. Blood from the gastrointestinal organs is collected into the hepatic portal vein, passes through the hepatic tissue, and is collected into the hepatic vein. The hepatic vein, bypassing other large vessels, empties directly into the sinus venosus and further into the atrium.
Phylogeny of Vertebrate Aortic Arches
The basic structural pattern of six pairs of gill arches, laid down in fish embryogenesis, serves as the basis for the formation of the vascular bed in all vertebrates.
Analysis of vascular homology reveals evolutionary transformation: in fish, these arches function as the afferent and efferent vessels of the gill apparatus. In terrestrial classes (amphibians, reptiles) and mammals, some of the arches are reduced, while the remaining ones transform into the carotid arteries, systemic aortic arches, pulmonary arteries, and ductus arteriosus (ductus arteriosus).
Remembering the three vessels bringing blood to the fish heart is helped by the acronym "R-L-H": Right Cuvierian duct, Left Cuvierian duct, Hepatic vein.
Class and exam questions
What type of blood is found in the fish heart?
In the fish heart (both in the atrium and the ventricle), there is exclusively deoxygenated blood.
How many blood vessels bring blood to the fish heart?
Three afferent vessels approach the heart: the right Cuvierian duct, the left Cuvierian duct, and the hepatic vein. All of them open into the sinus venosus.
What happens to the arterial gill arches of fish during development?
Out of the initial 6–7 pairs, the I and II pairs are reduced. Arches III through VI transform into 4–5 pairs of afferent branchial arteries participating in gill circulation.
How does the structure of the ventricular outflow tract differ between cartilaginous and teleost fish?
Cartilaginous fish have an arterial conus with multiple valves. In teleost fish, it is reduced, and the ventral aorta forms a bulbus arteriosus, retaining only a single pair of valves.
What else to study on this topic
- Anatomical Differences Between the Arterial Conus and Bulbus Arteriosus
- Formation of the Second Circulatory Loop in Lungfish
- Microcirculation and Gas Exchange in Gill Lamellae Capillaries
- Comparative Homology of Aortic Arches from Cartilaginous Fish to Mammals
- Physiology of the Renal and Hepatic Portal Systems in Fish
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