The circulatory and respiratory systems are interconnected structurally and functionally. Together they provide the vital activity of the body, allow you to supply tissues and organs with oxygen and nutrients. And starting from the first animals that partially conquered the land, the unity of these systems is observed. It provides a higher level of structural organization and optimization of physiology to living conditions on land.
The respiratory and cardiovascular system of mammals, amphibians, birds and reptiles consists of lungs, heart and blood vessels. In this case, the scheme of the pulmonary circulation is entirely represented by the lungs, that is, the pulmonary capillaries, to which blood enters through the arteries, and is discharged through the veins. It is noteworthy that there are no structural barriers between the circulation circles, which is why the respiratory tract and the cardiovascular system are considered a single functional unit.
Sequential scheme of the pulmonary circulation
A small circle is a closed chain of vessels through which blood is sent from the heart to the lungs and returns back. At the same time, despite the differences in the physiology of hemocirculation, the scheme of the pulmonary circulation of mammals does not differ from that of amphibians, reptiles, and even birds. Mammals have more in common with the latter than with the rest. In particular, we are talking about a 4-chambered heart.
Since there are no boundaries between the vessels of the body, the conditional beginning of the pulmonary circulation is considered the right ventricle of the heart of a mammal. From it, oxygen-deprived blood flows through the pulmonary trunk to the pulmonary capillaries. The processes of diffusion of gases occurring in the alveolar epithelial cells end with the release of carbon dioxide into the lumen of the alveoli and the capture of oxygen. The latter combines with hemoglobin and is sent to the left side of the heart through the pulmonary veins. As the diagram of the pulmonary circulation shows, it ends in the left atrium, and the systemic circulation begins from the left ventricle.
Avian pulmonary circulation
In terms of the physiology of the respiratory and cardiovascular systems, birds are most similar to mammals, since they also have a 4-chambered heart. Amphibians and reptiles have a 3-chambered heart. As a result, the scheme of the pulmonary circulation of birds is the same as that of mammals. Here, venous blood flows from the right ventricle to the pulmonary capillaries. Oxygenation enriches the blood with oxygen, which is transported by erythrocytes with arterial blood to the left atrium, and from there to the ventricle and systemic circulation.
Pulmonary circulation in birds and mammals
Probably, you should figure out what kind of blood flows in the veins of the pulmonary circulation in birds, mammals, reptiles and amphibians. So, in mammals, venous blood flows through the pulmonary artery to the capillaries, depleted in oxygen and containing carbon dioxide in large quantities. After oxygenation, arterial blood is sent through the veins to the heart. It is noteworthy that in the systemic circulation, arterial blood from the heart always flows only through the arteries, and venous blood returns to the heart through the veins.
Pulmonary circulation in reptiles and amphibians
The scheme of the pulmonary circulation of the frog does not differ from that of mammals. However, they are different in physiology: due to the presence of a 3-chambered heart, venous and arterial blood mix. Therefore, a mixed biological fluid flows through the arteries of the body, including the lungs. And the venous through the veins of the body returns to the heart, and then mixes again in the three-chambered heart. Therefore, the partial pressure of oxygen in the arteries of the pulmonary and systemic circulation is practically the same. Because amphibians are cold-blooded.
Reptiles also have a three-chambered heart, but in the upper and lower sections of the common ventricle there is a rudiment of a septum. Crocodiles even have a partition betweenthe right and left ventricles are practically formed. It has only a few holes. As a result, crocodiles are tougher and larger than other reptiles. At the same time, it is not yet known what kind of heart dinosaurs, also belonging to the class of reptiles, possessed. They probably also had a practically complete septum in the ventricles. Although evidence is unlikely to be obtained.
Analysis of the scheme of the pulmonary circulation of a person
In humans, gas exchange takes place in the lungs. Here the blood gives off carbon dioxide and is saturated with oxygen. This is the main significance of the pulmonary circulation of blood. Any academic diagram of the pulmonary circulation, created on the basis of research into the physiology of the respiratory system, begins with the right ventricle. Directly from the valve of the pulmonary artery departs the pulmonary trunk. Due to its division into two parts, a branch of the pulmonary artery departs to the right and left lungs.
The pulmonary artery itself divides many times and splits up to capillaries, densely penetrating the tissue of the organ. Gas exchange proceeds directly in them through the air-blood barrier, consisting of alveolar epithelial cells. After oxygenation of the blood, it is collected in venules and veins. Two depart from each lung, and already 4 pulmonary veins flow into the left atrium. They carry arterial blood. This is where the pulmonary circulation scheme ends and the systemic circulation begins.
Biological significance of the pulmonary circulation
A small circle in phylogeny appears in organisms that begin to populate the land. In animals that live in water and receive dissolved oxygen, it is absent. Evolution created another respiratory organ: first, simple tracheal lungs, and then complex alveolar ones. And just with the advent of the lungs, the pulmonary circulation also develops.
From now on, the evolution of the development of organisms living on land is aimed at optimizing the capture of oxygen and its transportation to consumer tissues. The lack of mixing of blood in the cavity of the ventricles is also an important evolutionary mechanism. Thanks to it, the warm-bloodedness of mammals and birds is ensured. Also, more importantly, the 4-chambered heart ensured the development of the brain, because it consumes a quarter of all oxygenated blood.
