Radiobiology - section of radiology, studying the action of ionizing radiation on living organisms. Radiobiology sets dependence reactions from a number of physical and biological factors - the type of radiation the dose of radiation (see Doses of ionizing radiation), the time factor exposure (see Irradiation), the radiosensitivity (see) of the organism, etc. Radiobiology explores how is the transfer of energy of ionizing radiation living matter what the primary physical-chemical processes and biochemical phenomena appear and what morphological and functional changes they bring (see Radiation sickness, Radiation damage). Radiobiology is the theoretical basis of application of ionizing radiation in medicine (see ionizing Radiation, medical Radiology).

Radiobiology (from lat. radius - ray, gr. bios - life, logos - doctrine - the study of the action of ionizing radiation on living organisms.
The discovery of x-rays at the end of the last century of x-rays (see X-rays, radioactivity phenomenon (see), the allocation of radium (see) contributed to the emergence of radiobiology as a separate discipline. Further discoveries in nuclear physics demanded study the effects of different types of radiation on biological objects, determination of the quantitative dependences of radiation effect, studying of influence of spatial and temporal distribution of the dose. There is a need in the knowledge of the primary processes underlying biological effects of radiation in the study of the initial changes in the physiological systems of the body.
Because radiobiology includes studying the effects of radiation on different levels (subcellular, cellular and whole organism, its systems, organs and tissues), there were three main sections: General radiobiology (the dependence of the effect on the dose, the dose distribution in time, the type and spatial distribution of the radiation from a number of biological factors - the type of animal, sex, age, the initial state of the organism, environmental factors, and so on); theoretical bases of primary biological effects of radiation; functional and structural characteristics of radiation reactions (radiation pathophysiology, pathomorphology, biochemistry, Microbiology and so on). Radiobiological laws form the basis of conventional radiation therapy (see), radiotherapy (see), while ensuring the safety of space flights, the development of preventive and therapeutic tools from radiation lesions (see Antiradiation protection). Radiobiology solve practical questions of radiation hygiene (see), problems of remote consequences of actions on the people of ionizing radiation, which is of great importance in connection with the use of radiation in medicine and national economy. One of the Central places in the modern radiobiology is the problem of recovery of irradiated organism, as a clarification of the conditions that enhance regenerative processes, promotes removal of the threat of remote consequences. Radiobiological patterns are used for the purposes of regioselective (see radiation Genetics and radiation sterilization of food and medical supplies (see Radiation Microbiology). Knowledge about the effects of radiation on living organisms are used in the solution of other issues of medical science, such as the suppression of the irradiation immunospecificity properties of the body is used to research the possibility of transplantation of tissues.
On the basis of experimental and clinical studies identified radiobiological patterns, showing the dependence of radiation effect from a number of physical and biological factors. Found various biological effectiveness of different types of radiation and radiation of various energies associated with the peculiarities of their interaction with matter and various spatial distribution in tissues (see ionizing Radiation, the relative biological effectiveness). This knowledge is used in medicine when selecting the source and method of irradiation for the treatment of various diseases, as well as in the practice of hygiene regulation.
The dependence of the damaging action of radiation from the amount (dosage) of the absorbed energy, which in a General form can be expressed graphically in the form of S-shaped curve of mortality and curve characterizing life expectancy (see Doses of ionizing radiation, Fig. 1 and 2). On the impact of radiation on mortality within a certain period (usually 30 days) adopted: 1) the minimum lethal dose (MRD) is the amount of radiation that causes mortality in 10% of cases; 2) dose half-survival (SD); 3) the minimum absolutely lethal dose (IADI) - the minimum amount of radiation that causes mortality 100% irradiated. For humans and many species of warm-blooded animals in General a single exposure FFM is 200 with SD - 400 p, IADI - 600 rub.
With increasing radiation dose to reduce the period of life of irradiated animals. At doses exceeding several thousand CT scans IADI, it is 3.5 days. The dose of more than 15 000 R leads to death within days. Further increase in dose shortens the lifetime of several hours and minutes, resulting in the deaths of under ray". Depending on the dose of radiation in a clinical manifestations of the disease is dominated by a variety of symptoms and found various mechanisms death. It is also established dose dependence of the damaging effect of radiation in relation to various skin reactions (see Skin, radiation damage).
Along with the quality and the dose of radiation on the biological effect is influenced by the factor of time of exposure (dose of extended or fractional irradiation by the dose of a single short-term exposure that causes the same biological effect). Fractional radiation exposure less efficiently than once. Significantly affect the effect of the size of the individual factions, the interval between exposures, and the rhythm of radiation exposure. Change the length of exposure by reducing the dose rate (below 20 R/min) also reduces the biological effect. The change in dose in the range of 20-150 R/min for most biological reactions no significant effect. In recent years, a marked reduction of the damaging effect when you use a dose of the order of several thousand CT scans done in a minute.
There is evidence of stimulating action of ionizing radiation on living organisms, however, this issue is controversial and requires further study.
The study of the response of biological reactions to radiation exposure detects different sensitivity of the organisms, physiological systems and separate structural units to ionizing radiation. Comparative sensitivity to radiation of various biological objects allows to speak about their relative sensitivity or radioresistance. It is necessary to take into account the assessment criteria and the period of reaction. There is a concept according to which the radiosensitivity of an object is determined by its ability to irradiation recovery.
Sensitivity to radiation great influence species, sex, age individuals exposed to radiation. Growing organisms have a higher radiosensitivity in comparison with adults. In the experimental study, and in the radiology clinic clear individual radiosensitivity. The radiosensitivity of an organism to a great extent determined by the typological peculiarities of the higher nervous activity. Better tolerate exposure animals and people with a strong balanced type of higher nervous activity. The main weakness of cortical processes involves an unfavorable course of the reaction to radiation exposure. Another important factor determining the reaction of the organism to radiation exposure, is the initial functional state of the Central nervous system.
Increased excitability of Central nervous system immediately prior exposure or exposure adversely affects the course of radiation injury; the decrease in excitability and level of cortical activity decreased biological effect. The change of the initial functional state of an organism (in particular, using drugs) can significantly affect their radiosensitivity. This is used if necessary in the radiology clinic.

Great influence on the course of radiation damage has also the initial functional state of the endocrine glands - pituitary-adrenal system, thyroid and sex glands. Certain dependency links the level of estrogen in the blood with natural radiosensitivity of animals. Known to increase the biological effect in terms of increased thyroid function, in case of insufficiency of the adrenal cortex. It is now recognized that the preliminary stimulation of the pituitary-adrenal system has a positive effect on the course and outcome of radiation damage. The preliminary impact of stress factors physical and chemical nature creates a specific time to the increased resistance of the organism to action of radiation.
On radiosensitivity of living organisms affected and some environmental factors. Marked diurnal and seasonal variations of radiosensitivity; have a value of air temperature, physical activity and other factors. Essentially changes the radiosensitivity of the body, the oxygen concentration in the environment and tissues at the time of exposure. The lower oxygen concentration in the time of exposure below atmospheric reduces biological action of radiation. The increase radiobiological effect of the irradiation in the presence of oxygen is called the oxygen effect. Oxygen effect is more versatile and occurs when the radiation effect on almost all types of living
matter. In addition, oxygen effect is used as a method of studying primary parts of the mechanism of biological action of ionizing radiation. Look at the mechanism of biological action of ionizing radiation based on perceptions about direct effects of radiation on the biological substrate (theory of target) or indirect (indirect) effect through intermediate active substances (theory activated water). Theory of target, developed on the basis of the analysis of the curves of dependence of the effect on the dose, based on mathematical analysis of the probability of "hits" in the most vulnerable volume in the cell ("target"). The validity of the theory target is limited to a rather narrow limits because it does not explain the action of radiation on complex biological objects.
According to the theory of indirect actions radiation radiation affects the substance indirectly, originally by changing the solvent molecules and causing the emergence of the so-called active radicals. Generated free unstable and chemically very active radicals atomic hydrogen (H·), hydroxyl group (OH-), hydroperoxide (2 ) - interact with molecules of the dissolved substance, causing primary radiation-chemical reactions. These reactions can have the character of oxidation or reduction processes.
Experimental data show that the primary action of ionizing radiation is both a direct and indirect effect, coexisting in any radiation-chemical and radiological process. Is becoming increasingly important problem of the primary mechanisms of biological action of radiation on the molecular and cellular levels. Further development of theoretical problems of radiobiology necessary to protect the person from the harmful effects of radiation, finding more effective ways of radiotherapy of malignant tumors, the use of radiation for the purpose of regioselective and other practical problems.