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, there is a unity of these systems. It provides a higher level of structural organization and optimization of physiology to living conditions on land.
The respiratory and cardiovascular systems of mammals, amphibians, birds, and reptiles are composed of lungs, heart, and blood vessels. In this case, the circulatory system is represented entirely by the lungs, that is, pulmonary capillaries, to which blood flows through the arteries, and is discharged through the veins. It is noteworthy that there are no structural barriers between the circles of blood circulation, which is why the respiratory tract and the cardiovascular system are considered as a single functional unit.
Sequential circulatory system
A small circle is a closed chain of vessels through which blood from the heart goes to the lungs and returns. Moreover, despite the differences in the physiology of hemocirculation, the circulatory system of mammals does not differ from that of amphibians, reptiles, and even birds. With the latter, mammals have more in common than with the rest. In particular, we are talking about a 4-chamber heart.
Since there are no boundaries between the vessels of the body , the right ventricle of the heart of a mammal is considered the conditional beginning of the pulmonary circulation. From it through the pulmonary trunk, blood, deprived of oxygen, is sent to the pulmonary capillaries. Gas diffusion processes occurring in alveolar epithelial cells are completed by 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 heart through the pulmonary veins. As the scheme of the pulmonary circulation shows, it ends in the left atrium, and systemic blood flow begins from the left ventricle.
The pulmonary circulation
In physiology of the respiratory and cardiovascular systems, birds are most similar to mammals, since they also have a 4-chamber heart. Amphibians and reptiles have a 3-chamber heart. As a result, the pattern 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 red blood cells with arterial blood to the left atrium, and from there to the ventricle and systemic circulation.
Pulmonary circulation of birds and mammals
Probably, you should understand what kind of blood flows in the veins of the pulmonary circulation in birds, mammals, reptiles and amphibians. So, in mammals, venous blood, depleted in oxygen and containing large amounts of carbon dioxide, flows through the pulmonary artery to the capillaries. After oxygenation, arterial blood flows through the veins to the heart. It is noteworthy that in a large circle of blood circulation, arterial blood from the heart always flows only through the arteries, and venous returns to the heart through the veins.
Pulmonary circulation of reptiles and amphibians
The scheme of the frog’s pulmonary circulation does not differ from that of mammals. However, in physiology they are different: due to the presence of a 3-chamber heart, venous and arterial blood mix. Therefore, mixed body fluid flows through the arteries of the body, including the pulmonary. And venous through the veins of the body returns to the heart, and then mixes again in a three-chambered heart. Therefore, the partial pressure of oxygen in the arteries of the small and large circle of blood circulation is practically no different. Because amphibians are cold-blooded.
Reptiles also have a three-chambered heart, however, in the upper and lower parts of the common ventricle there is a primordium of the septum. In crocodiles, the septum between the right and left ventricle is completely formed. It has only a few holes. As a result, crocodiles are more resilient and larger compared to other reptiles. At the same time, it is still unknown what kind of heart dinosaurs possessed, also belonging to the class of reptiles. They probably also had an almost complete septum in the ventricles. Although evidence is unlikely to be obtained.
Analysis of the circulatory system of a person
In humans, gas exchange occurs in the lungs. Here the blood gives off carbon dioxide and is saturated with oxygen. This is the main significance of pulmonary blood circulation. Any academic circuit of the pulmonary circulation created on the basis of studies of the physiology of the respiratory system begins with the right ventricle. The pulmonary trunk departs directly from the pulmonary artery valve. Due to its division into two parts, the branch of the pulmonary artery leaves to the right and left lung.
The pulmonary artery itself is repeatedly divided and crushed to capillaries, densely penetrating the tissue of the organ. Gas exchange flows directly through the airborne barrier consisting of alveolar epithelial cells. After blood oxygenation, it is collected in venules and veins. Two departs from each lung, and 4 pulmonary veins flow into the left atrium . They carry arterial blood. On this, the pulmonary circulation scheme ends, and the systemic circulation begins.
The biological significance of the pulmonary circulation
A small circle in phylogenesis appears in organisms that begin to inhabit land. In animals that live in water and receive dissolved oxygen, it is absent. Evolution also created another respiratory organ: first, simple tracheal lungs, and then complex alveolar. And just with the appearance of the lungs, a small circle of blood circulation also develops.
From this moment, the evolution of the development of organisms living on land is aimed at optimizing the uptake of oxygen and its transport to consumer tissues. The lack of mixing of blood in the ventricular cavity is also an important evolutionary mechanism. Thanks to him, warm-blooded mammals and birds are provided. Also, more importantly, the 4-chamber heart has provided brain development, because it consumes a quarter of all oxygenated blood.