The body is self-existing unit of the organic world, representing a self-regulating system that responds as a whole on the various changes in the external environment. The organism can exist only in constant interaction with its surrounding environment and samoupravlenia the result of such interaction. As emphasized by I. M. Sechenov, "the body without the external environment that supports his existence is impossible."
Characteristic of every organism is a certain organization of its structures. In the simplest living organisms - viruses - there are only organization of its constituent molecules of protein and nucleic acids. Here you can talk about the molecular level of the organization of the body. A more complex single-celled organisms such as Paramecia, characterized by a more complex structure: inside cells differentiate the nucleus, mitochondria, surface and vnutripolostnoe membrane, vacuoles. Here already there molecular, cellular level of organization in which there is some division, differentiation of functions of different intracellular structures. For example, motor function is intracellular contractile fibrils, flagella and cilia; functions of digestion and excretion in some cells are vacuoles, etc.
In multicellular organisms in the course of their evolutionary development is differentiated cells, i.e. there are any differences in their size, shape, structure and functions. From the same differentiated cells are formed tissue, a characteristic property which are the structural consolidation, morphological and functional community and interaction of cells. Various fabrics are specialized in their functions, i.e. adapted to run different processes of life. So, muscle tissue is specialized on the implementation motor function, and its characteristic feature is the airway, the glandular tissue is specialized on the education and selected cells certain chemicals (hormones, enzymes and other). Being adapted to perform a certain type of activity, well-differentiated cells, tissues, however, carry out common to all cells function: metabolism, nutrition, breathing, selection. The presence of interaction between the cells that make up the fabric, the complexity of structure and specialization of functions of tissues determine their morphological and functional characteristics, which is the basis for allocation of tissue levels of organization of living organisms.
At a certain stage of the specific and individual development organisms are formed bodies, constructed from various tissues. Organs are anatomical education, characterized by a kind of structural and functional combining different fabrics. They are working apparatus of the body, specialized in the implementation of complex activities necessary for the existence of an integral organism. So, the heart acts as a pump, pumping blood from the veins in the arteries, kidney function excretion from the body of the final products of metabolism and function of maintaining the stability of the concentration of electrolytes in the blood, bone marrow function, blood, etc.
The presence of structurally and functionally different organs in the body allows to speak about organ levels of its organization.
Set of bodies involved in the execution of any complex act of activity, form of anatomical or functional Association - system bodies. These include the nervous and endocrine systems, regulating the activity of all organs and systems of organs of locomotion (movement), respiration, circulation, digestion, excretion, and reproduction. Among all these systems are of particular importance in the whole organism has a nervous system that combines and regulating the status and activities of all other systems of the body and determines its behavior in the environment. The presence of organ systems, each of which is specialized in the performance of any kinds of activity of the organism as a whole, defines the system level of the organization.
Each of these levels of organization of living organisms characterized by its special, inherent physiological regularities, which cannot be understood by studying at other levels. To clarify the processes occurring at different levels of organization of living organism that require different methods and different instrumental technique. It should be emphasized that for the knowledge of the function of higher organisms must study all - molecular, cellular, tissue, organ, and system - level organization of the body and synthesis of all the information that you get the researchers. This is because, having a complex organization, a living organism is a single whole, in which the activities of all its structures, cells, tissues, organs and their systems - agreed and Spadchyna that whole.