Our planet is densely populated by animals of various classes, orders and species. Scientists study their structure and functional significance of individual organs. Read about the heart of amphibians and reptiles in the article.
How did a three-chambered heart turn into a four-chambered one?
Vertebrates came to land due to the fact that their pulmonary respiration began to develop intensively. The circulatory system began to rebuild. Gill-breathing fish have one circulation of blood, their heart consists of only two chambers. They cannot live on land.
Terrestrial vertebrates have a three- or four-chambered heart. They are distinguished by the presence of two circles of blood circulation. Their permanent habitat is dry land. Amphibians and reptiles have an organ with three chambers. Although some species of reptiles have an incomplete division into four parts. The development of a true four-chambered heart during evolution occurred in parallel in mammals, birds and crocodiles.
Reptiles and amphibians
These two classes of animals have two circulations of blood andheart with three chambers. Only one reptile has a defective but four-chambered heart. This is a crocodile. A complete heart organ first appeared in primitive mammals. In the future, a heart with such a structure was inherited by the descendants of dinosaurs - birds. It has been inherited by modern mammals as well.
Birds
Four-chambered hearts are feathered. Birds are distinguished by a complete separation of the circles of blood circulation: large and small, like in humans, when there is no mixing of blood - arterial and venous. The right and left halves of the organ are completely separated.
Birds have a four-chambered heart, its structure is represented by two atria and the same number of ventricles. Venous blood enters the ventricle through the right atrium. From it comes the pulmonary artery, which is divided into left and right branches. As a result, venous blood is in the corresponding lung. At this time, the blood in the lungs is oxidized and enters the left atrium. This circulation is called its pulmonary circulation.
The large circle of blood circulation originates from the left ventricle. A single vessel departs from it, which is called the right aortic arch, which immediately at the exit from the heart separates two nameless arteries: left and right. The aorta itself unfolds in the region of the right bronchus and runs parallel to the spinal column already as the dorsal aorta. Each innominate artery divides into carotid and subclavian. The first goes to the head, and the secondagain divided into chest and shoulder. Large arteries depart from the dorsal aorta. Unpaired ones are designed to supply blood to the stomach and intestines, and paired ones - to the hind limbs, organs of the pelvic cavity and muscles of the walls of the peritoneum.
Birds have a four-chambered heart, it differs in that in birds the movement of blood is carried out mainly through large vessels, and only a small part of it enters the renal capillaries. Birds are distinguished by the presence of a large heart with frequent contractions and only pure arterial blood entering the organs. This made it possible to consider birds as warm-blooded animals.
Mammalian circulatory system
Mammals have a four-chambered heart, like humans or birds. Its formation with a complete separation of the circles of blood circulation is caused by the need to develop such a quality as warm-bloodedness. This is explained as follows: warm-blooded animals have a constant need for oxygen, which can only be satisfied by pure blood of the arteries with a large amount of oxygen. Only a four-chambered heart can provide it to the body. And the mixed blood of vertebrates, in which the heart has three chambers, is not able to give the desired body temperature. Therefore, such animals are called cold-blooded.
Due to the presence of complete partitions, the blood does not mix. In a large circle of circulation, only arterial blood flows, which is supplied to the required extent to all organs of a mammal, which contributes to the acceleration of metabolism. This process helps maintaintemperature at a constant level. Mammals, birds and other classes of animals have a four-chambered heart, which is vital for a constant and stable body temperature. Now the environment does not affect them.
Lizards
In fact, the heart of these reptiles has three chambers with two atria and one ventricle. But the principle of its work makes it possible to assert that lizards have a four-chambered heart. The explanation for this phenomenon is as follows. The venous cavity is filled with oxygen-poor blood, the source of which is the right atrium. Arterial blood enriched with oxygen comes from the opposite atrium.
The pulmonary artery and both aortic arches communicate. It would seem that the blood should be completely mixed. But this does not happen, since the presence of a muscle flap in combination with a biphasic contraction of the ventricle and further work of the heart prevent the mixing of blood. It is available, but in very small quantities. Therefore, in terms of functional significance, the three-chambered heart of lizards is similar to the four-chambered one.
Reptiles
The crocodile has a four-chambered heart, although the circles of blood circulation are not completely separated by a septum. In a reptile, the organ (heart), which is responsible for supplying the whole organism with nutrition through the blood, has a special structure. In addition to the pulmonary artery, which departs from the ventricle on the right side, there is an additional, left one. Through it, the bulk of the blood enters the digestive system.
Between the two arteries, right and left, the crocodile's heart has a hole. Through it, blood from the veins has the ability to enter a large circle of circulation, and vice versa. Scientists have long believed that the reptilian heart is of a transitional type on its way to developing a full-fledged four-chambered heart, as in warm-blooded mammals. But it's not.
Turtles
The vascular and heart system of these reptiles is the same as that of other reptiles: a heart with three chambers, interconnected veins and arteries. The content of insufficiently oxidized blood increases when external pressure increases. This can happen when the animal is diving or moving fast. The heart rate decreases, although the concentration of carbon dioxide increases significantly.
Turtles have a four-chambered heart, although the physiological structure of the organ has only three chambers. The fact is that the turtle's heart is distinguished by an incomplete ventricular septum, around which the blood functions, having a different amount of oxygen.
