Dr.Biol., Professor  Yu.S. Vanyushin¹
PhD, Associate Professor R.R. Khairullin¹
PhD, Associate Professor D.E. Elistratov¹
PhD, Associate Professor N.A. Fedorov¹
N.F. Ishmukhametova^2
1Kazan State Agrarian University, Kazan
²Kazan State University of Architecture and Civil Engineering, Kazan

Keywords: cardiorespiratory system, load, respiration, blood circulation, gas exchange, athletes.

Background. Numerous studies made it possible to establish the cardiorespiratory system value typical for humans and animals. It is shown that this system develops and improves in the process of post-natal ontogenesis and motor activity [3-5]. The cardiorespiratory system is a private functional system [9], consisting of the circulatory and respiratory organs. It is studied in two ways:
- compensatory-adaptive reactions of the cardiorespiratory system to different functional loads;
- the state and functionality of the cardiorespiratory system aimed to provide oxygen to the body.
Objective of the study was to analyze the peculiarities of adaptation of the cardiorespiratory system of athletes engaged in endurance sports using a set of non-invasive research methods.
Methods and structure of the study. During the study, we simultaneously recorded the indicators of the cardiovascular system: heart rate (HR), stroke volume (SV), minute blood volume (MBV), blood circulation index (BCI), cardiac index (CI); external respiration rates: respiratory rate (RR), breathing capacity (BC), respiratory minute volume (RMV); gas exchange rates: oxygen utilization rate (O2U). The central hemodynamic indices (HR, SV, MBV) were determined using the method of a differential rheography by W. Kubicek et al. (1974), modified by Y.T. Pushkar et al. (1977) and Y.S. Vanushin (2003). The external respiration rates (RR, BC, RMV) were determined using the method of pneumotachography, and the gas exchange function (O2U) was evaluated by means of a calculation. For this purpose, the gas analysis of the exhaled air was carried out using a paramagnetic oxygen analyzer AK-5. The functional load was to change the spatial position of the body of athletes with the high total body sizes, perform muscular work on a cycle ergometer with a stepwise increasing load (from 50 to 200 W) with the power of 3 W/kg. The duration of each step was 4 minutes. The pedaling speed was 60 rpm.
Sampled for the study were 72 male athletes aged from 15 to 60 years, who were divided into 4 groups: Group 1 - 11 adolescent athletes aged 15-16 years; Group 2 - 22 young athletes aged 17-21 years; Group 3 - 20 athletes aged 22-35 years; Group 4 - 19 male athletes aged 36-60 years. All the athletes involved were engaged in such endurance sports as cross-country skiing and middle- and long-distance running in the track and field athletics.
Results and discussion. The first direction - "Compensatory-adaptive reactions of the cardiorespiratory system to different functional loads" - was analyzed while the male athletes changed the spatial position of the body and performed the cycle ergometer exercise. This helped determine the leading compensatory-adaptive reactions of the cardiorespiratory system and the degree of their involvement under different functional loads (see Fig. 1).

Figure 1. Schematic representation compensatory-adaptive reactions of the cardiorespiratory system to different functional loads

Thus, when the body position changes, a whole array of cardiorespiratory functional elements take part in the compensatory-adaptive reactions, with no driving factor to be distinguished. Changes occurring in the cardiorespiratory system during active orthostasis can be conditionally deemed as minimum loads [7], and they are manifested as the compensatory-adaptive reactions aimed to eliminate the primary effects. At the same time, cycle ergometer exercises are performed with the involvement of such bodily systems as blood circulation, external respiration, and gas exchange function. The 200 W physical load can be regarded as a threshold load, based on which we identified the main systems and functions of the body of athletes engaged in endurance sports and their involvement in the compensatory-adaptive reactions depending on age. In some cases, we observed an increase in several indicators of the cardiorespiratory system. This was particularly evident in the group of athletes with the high total body sizes, who performed individually selected loads on the cycle ergometer, sometimes reaching 300 W, which was considered as maximum loading. Such physical loads lead to tension of the bodily organs, in particular the locomotor muscles and myocardium. The signals from these organs appear to cause adaptive adjustments. Determination of the reserve capabilities under intense physical loads enabled to suggest that two, and in some cases three indicators of the cardiorespiratory system are needed to meet the increased oxygen demand in the working skeletal muscles: HR, SV, and RMV. The correlation between the strength and HR values, along with the Frank-Starling low, is a fundamental mechanism of self-regulation of the heart, which ensures effective cardiac performance under increasing physical loads leading to increased HR and heart muscle strength.
As regards the second direction, the oxygen supply to the body is determined by the degree of development of the oxygen regulation system and the optimal interaction of various links of the cardiorespiratory system, which includes external respiration, blood circulation, and gas exchange. Therefore, among the ways to improve the athletic performance, in endurance sports, in particular, is to enhance the cardiorespiratory system functionality.
Increased cardiac output is known to be the most effective mechanism of oxygen supply to the body. However, the results obtained [2] show a decrease in the MBV rates when transiting from one load mode to another, which, due to the shortening of diastole and insufficient cardiac muscle contraction, is achieved uneconomically - through the growth of HR with a limited increase in the cardiac output. Improvement of the heart functions, in this case, is limited by the intensity of the main processes that determine the contraction capacity of the cardiac muscle: excitation processes, interaction between excitation and contraction and relaxation, power supply of cardiac myocyte and power of the structures supporting these processes [8]. This suggests that there may be other mechanisms aimed to meet the oxygen demand of the body during muscle activity. One of the mechanisms is external respiration, reckoned by a number of researchers [6] among the factors that limit the possibility to achieve high sports results.
At the level of the respiratory system, adaptation is characterized by the maximum mobilization of external respiration, which is manifested in the increased lung ventilation due to the increased respiratory rhythm and respiration depth. It can be assumed that there is in coordination between the regional blood flow in the lungs and ventilation of the relevant areas of pulmonary tissue, as well as incoordination between respiration and movements [1]. The limiting factors here are the anatomical and functional possibilities of the organs of external respiration and functional possibilities of control of breathing [1].
The highest rates of lung ventilation were recorded in the groups of young athletes aged 15-16 years and adult athletes aged 36-60 years. Apparently, the mechanism associated with increased external respiration under physical load of increasing power performed on the cycle ergometer was prevalent in these groups, and physical working capacity is ensured by the considerable tension of the cardiorespiratory system. At the same time, there are various ways to achieve the maximum lung ventilation rates: in the group of youngsters - at the cost of increased respiratory rate; in the group of adults - at the cost of increased respiration depth. This fact can be explained from the standpoint of the age-specific characteristics since, by the age of 16, the morphofunctional formation of the respiratory system and the educational and training process should be oriented towards the development of the respiratory system potential, which will enable to increase the aerobic working capacity of the body. It should be noted that the data on lung ventilation are not a criterion of a sufficiently high level of training, as the oxygen and energy costs of breathing increase. Under these conditions, adaptation to functional loads is best achieved by activating and improving the efficiency of the oxygen transportation and utilization system. This is confirmed by the high values of blood circulation and HR in the 17-21 year-old males. Due to the development of hypertrophy and increased speed and amplitude of contraction of the respiratory muscles [10], the VC and O2U rates increased in the group of 22-35 year-old athletes characterized by the same oxygen consumption when performing physical loads of increasing power. It appears that the increase in the mitochondria mass of the skeletal muscles, there is a significant increase in the aerobic power of the body and improvement of the ability of the respiratory center to maintain excitation at the threshold level for a long time.
It is recommended to use a coefficient of comprehensive assessment  of oxygen supply, consisting of the cardiorespiratory system indicators, to evaluate the athletes' compensatory-adaptive reactions to functional loads. The coefficient showed that there are large functional reserves in the groups of young and adult athletes aged 22-35 years, as well as the substitution of functions in the body of young and adult athletes aged 36-60 years under physical loads of 100-200 W. The significant decrease of the value of the coefficient of comprehensive assessment of oxygen supply to the body in the groups of young and adult athletes aged 36-60 years when performing the 200 W load on the cycle ergometer reflects a large "physiological cost" of oxygen supply to the working skeletal muscles. The identified age-specific peculiarities of oxygen supply to the body make it possible to target the functional loads during the training process and solve the problems of formation and development of motor qualities in different periods of ontogenesis more rationally.
Conclusion. The study of the cardiorespiratory system using a set of non-invasive research methods revealed that the functional system structure is characterized by constant changes in the degree of involvement of its functional elements and peculiarities of their combination. Among the physiological determinants of the cardiorespiratory system are: inotropic, chronotropic, vascular and respiratory reactions, the activation of which depends on the functional loads. During active orthostasis, a whole array of cardiorespiratory functional elements take part in the compensatory-adaptive reactions, with no driving factor to be distinguished. Under physical loads from 50 to 200 W, there was a decline in the growth in cardiac output, which, depending on the athletes’ age, was compensated by one of the cardiorespiratory system reactions: external respiration, blood circulation or gas exchange function. Under the maximum load (3 W/kg), a mixed type of reaction was observed, which was characterized by an increase to the individual limit of indicators of the chronotropic function of the heart, external respiration and, in some cases, stroke output. Consequently, under the functional loads of increasing power, there arise complex adaptive relationships of cardiorespiratory parameters manifested in a variety of reactions.

References

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Corresponding author: kaf.fv.kgau@mail.ru

Abstract
Objective of the study was to analyze the peculiarities of adaptation of the cardiorespiratory system of athletes engaged in endurance sports.
Methods and structure of the study. The study involved a set of non-invasive research methods. Besides, we tested the athletes’ cardiovascular, external respiration and gas exchange systems.
Results of the study. Among the physiological determinants of the cardiorespiratory system are: inotropic, chronotropic, vascular and respiratory reactions, the activation of which depends on the functional loads. During active orthostasis, a whole array of cardiorespiratory functional elements take part in the compensatory-adaptive reactions, with no driving factor to be distinguished. Under physical loads from 50 to 200 W, there was a decline in the growth in cardiac output, which, depending on the athletes’ age, was compensated by one of the cardiorespiratory system reactions: external respiration, blood circulation or gas exchange function. By the prevailing nature of individual reactions of the circulatory and respiratory organs, the types of adaptation of the cardiorespiratory system were identified. Under the maximum load (3 W/kg), a mixed type of reaction was observed, which was characterized by an increase to the individual limit of indicators of the chronotropic function of the heart, external respiration and, in some cases, stroke output.
Conclusion. The functional system structure is characterized by constant changes in the degree of involvement of its functional elements and peculiarities of their combination.