For the movement of blood through the tissues of the body requires a kind of pump, the role of which is assigned to the heart muscle. In simple 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-chamber heart, which pushes blood through the vessels to all parts of the body, giving them access to oxygen, nutrients, as well as freeing them from metabolic products, delivering them to places of excretion.
How did blood circulation develop?
The circulatory system is the basis of the life of many living organisms. To be able to perform their tasks, blood must constantly circulate throughout the body. The stages of the development of circulatory systems are clearly visible when examining the cardiovascular structure of fish, amphibians, reptiles and birds.
- Fish belong to cold-blooded animals that have a closed circulatory system. They have a two-chamber heart and one circle of blood circulation.
- Amphibians and reptiles have two circles of blood circulation, 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 in two circles.
The heart muscle contracts and accelerates blood through the arteries, which are divided into smaller vessels and fit all parts of the body. After giving oxygen and useful elements, the blood already through the vessels called veins returns back and enriches.
How the heart works in fish
An animal with a two-chamber heart is usually referred to as cold-blooded. These are representatives of fish and amphibian larvae. According to the studies of biologists who studied the development of the circulatory system, it is clear that the first full swinging organ was detected in fish. These cold-blooded have a bicameral heart, represented by an atrium with a valvular system and a ventricle. The circulatory system is formed by one single circle, which drives venous blood.
Blood from the pump moves through the capillaries of the gills, where it is saturated with oxygen and fills the vessels. Next is its distribution in the capillaries located in the tissues of the body, and their saturation with oxygen. After that, she goes to the veins without oxygen and returns to the heart bag through them.
Structure
Primitive fish have a two-chamber heart, which is conditionally divided into four segments:
- the first segment is a section called the venous sinus, which is responsible for taking blood that has given oxygen to the body;
- the second segment is represented by an atrium with valves;
- the third segment is called the ventricle;
- the fourth segment is an aortic cone with several valves, which pumps blood into the peritoneal aorta.
After the blood leaves the heart, it moves through the gills, where it is saturated with oxygen and flows into the spinal aorta, from where it is distributed throughout all tissues of the body.
In higher order fish, 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 tasseter fishes. The bony representatives are distinguished by a mild arterial cone, which is usually characterized as part of the aorta, and not the heart muscle.
Fish heart description
Compared to land mammals, the heart of the 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 can survive icing during harsh winters. At this time, the heart of the fish stops beating, and when thawing, contractions resume, and blood begins to circulate throughout the body, removing the fish from hibernation.
This work of the circulatory system is associated with the fact that fish lead a horizontal lifestyle and live in an aquatic environment, so there is no need to push the blood stream up and fight gravity.
Features of blood formation 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 blood vessels;
- lymphoid organs formed by blood-forming tissues and located under the lid of the cranium.
Fish blood contains red blood cells with a core in the center. To date, the known system represented by 14 blood groups.
Who else has a two-chamber heart?
With the transition of animals to a terrestrial life form 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. A 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, the larvae of which have a two-chambered heart, and their circulatory system is the same as that of fish.
In adults, a three-chambered heart develops , which is represented by two atria and a ventricle. Amphibians are the first animals to have a second round of blood circulation.
Oxygen-rich blood from the lungs and skin is accumulated in the left atrium and is separated by a septum from mixing with venous, which enters the right atrium.
Answering the question about which animals have a two-chambered heart, we can safely say that in adults only such fish survived this organ, and in amphibians - at the larval stage.