For the movement of blood through the tissues of the body, a kind of pump is needed, the role of which is assigned to the heart muscle. In the simplest living creatures, such as worms or chordates, this organ is absent, and the structure of the circulatory system is a closed ring. Fish have a two-chambered heart, which pushes blood through the vessels to all parts of the body, giving them access to oxygen, nutrients, and freeing them from metabolic products, delivering them to the places of excretion.
How the circulation developed
The circulatory system is the basis of life for many living organisms. To be able to perform its tasks, blood must constantly circulate throughout the body. The stages of development of the circulatory systems are clearly traced when considering the cardiovascular structure of fish, amphibians, reptiles and birds.
- Fish are cold-blooded animals with a closed circulatory system. They have a two-chambered heart and one circulation.
- Amphibians and reptiles have two circlescirculation, their heart is divided into three chambers. The exception is crocodiles.
- In birds, humans and many animals, the organ that pumps blood is represented by four chambers, and the circulatory system is represented by two circles.
The heart muscle contracts and accelerates the blood through the arteries, which are divided into smaller vessels and suitable for all parts of the body. After, having given up oxygen and useful elements, the blood already through the vessels, called veins, returns back and is enriched.
How the heart works in fish
An animal with a two-chambered heart is usually referred to as cold-blooded. These are representatives of fish and larvae of amphibians. According to the studies of biologists who studied the development of the circulatory system, it is clear that the first full-fledged pumping organ was found in fish. These cold-blooded animals have a two-chambered heart, represented by an atrium with a valvular system and a ventricle. The circulatory system is formed by one whole circle, chasing venous blood.
The blood from the pump moves through the capillaries of the gills, where it is saturated with oxygen and fills the vessels. Next comes its distribution into the capillaries located in the tissues of the body, and their saturation with oxygen. After that, it goes to the veins without oxygen and returns through them to the heart sac.
Building
Primitive fish have a two-chambered heart, which is conventionally divided into four segments:
- the first segment is a section called the venous sinus, whichresponsible for receiving blood that has given oxygen to the body;
- second segment represented by atrium with valves;
- the third segment is called the ventricle;
- The fourth segment is an aortic cone with several valves that pumps blood into the peritoneal aorta.
After the blood has left the heart, it moves through the gills, where it is saturated with oxygen and flows into the spinal aorta, from where it is distributed to all tissues of the body.
In fish of a higher order, all segments are not located on the same line, but in the shape of the letter S, where the last two segments are above the first two. Such a structure is inherent in cartilaginous and lobe-finned fish. The bony representatives are distinguished by a slightly pronounced arterial cone, which is usually characterized as part of the aorta, and not the heart muscle.
Description of fish heart
Compared to land mammals, the heart of a fish is small and weak. Its weight varies from 0.3 to 2.5% of body weight. Due to the weak contraction, the pressure in the vessels is also weakened. Thanks to these features, fish are able to survive icing during harsh winters. At this time, the heart of the fish stops beating, and when defrosted, the contractions resume, and the blood begins to circulate throughout the body, bringing the fish out of hibernation.
This work of the circulatory system is due to the fact that fish lead a horizontal lifestyle and live in the aquatic environment, so there is no need to push the blood flow up and fight the earthattraction.
Features of hematopoiesis in fish
In the body of fish, several organs are capable of producing blood cells:
- gills;
- intestinal mucosa;
- epithelium and cardiac vessels;
- kidneys and spleen;
- blood from vessels;
- lymphoid organs formed by blood-forming tissues and located under the lid of the skull.
Fish blood contains red blood cells with a nucleus in the center. To date, a system is known, represented by 14 blood groups.
Who else has a two-chambered heart
With the transition of animals to a terrestrial form of life and with the formation of their lungs, the muscular heart vessels also changed. The organization of animals became more complicated and the heart from a two-chamber was transformed into a three- and four-chamber. The second circle of blood circulation was formed, and the heart muscle began to pump not only venous, but also arterial blood.
As evidence that animals began life from water, scientists cite the stages of reproduction of amphibians, whose larvae have a two-chambered heart, and their circulatory system is the same as that of fish.
Adult individuals develop a three-chambered heart, which is represented by two atria and a ventricle. Amphibians are the first animals to have a second circulation.
Oxygenated blood from the lungs and skin accumulates in the left atrium and is separated by a septum from mixing with the venous, which enters the rightatrium.
Answering the question of which animals have a two-chambered heart, we can safely say that in adults such an organ is preserved only in fish, and in amphibians - at the larval stage.
