Respiratory organs and cardiovascular system are interconnected and perform extremely vital functions of the organism, the main goals being performing gas exchange with the environment and emitting carbon dioxide produced in the organism into the environment. Therefore, we carried out comparative histo- and cytomorphometric evaluation of morphological structures of the heart and lungs of cattle, sheep and horses. Using complex methods of research, we determined specifics of microscopic structure and histometric parameters of parenchyma of the lungs and myocardium of the ventricles of the hearts of clinically healthy animals in species aspect – cattle, sheep and horses. The studies revealed that the histoarchitectonics of the studied animals were similar in structure, characteristic of other species of agricultural mammals, and distinct morphometric peculiarities of their histostructures. The study indicated that the respiratory zone of the lungs is most developed in horses. This parameter was lower in ruminants – cattle and sheep. Connective tissue septum of parenchyma of lungs was better expressed in cattle and sheep, and less in horses. At the same time, mean volume of lung alveoli in clinically healthy animals varied: being highest in horses – 699 ± 106 thou µm3, then cattle – 337 ± 43 thou µm3 and sheep – 158 ± 37 thou µm3. Such variation in histometric parameters of parenchyma of the lungs in experimental animals indicates adaptive specifics of the organism of animals in terms of living conditions. Therefore, the respiratory zone was most developed in the lungs of horses, animals that experience significant physical and physiological load on corresponding organs and systems. As a result of histometric studies of myocardium, we determined patterns of sizes of thickness of its cardiomyocytes and volume of their nuclei. We determined that in myocardium of the heart ventricles, in the same microscope field of view, cardiomyocytes of varying thicknesses occur. At the same time, thickness of cardiomyocytes, their mean volume of nuclei in the ventricle myocardium were expressed the most in cattle, then in horses and sheep, and their histometric parameters in myocardium of the left ventricle of the heart in the experimental animals were higher than in the right one. Therefore, thickness of cardiomyocytes of the left ventricle in cattle equaled 14.06 ± 0.41 µm, and volume of nuclei of cardiomyocytes respectively 124.55 ± 7.99 µm3. Similar changes in such parameters of histometry were found in sheep and horses. We attribute such varying histometric parameters of the thickness of cardiomyocytes and volume of their nuclei in myocardium of the left ventricle of the heart in experimental animals, compared with such parameters in the right ventricle, to the activities of the ventricles (the left one generally functions as a pump, right one – as a volumetric) and functional specifics of this myocardium tissue, which is capable of spontaneous rhythmic contractions, resulting in blood flow in the vessels: cardiomyocytes of the left ventricle carry greater load, promoting blood flow in vessels of greater (somatic) blood circulation, respectively cardiomyocytes of the right ventricle – less load, promoting blood flow in vessels of lesser (pulmonary) blood circulation.
