In the body of the higher animals elaborated to adapt counteracting the many influences of the external environment, relatively constant conditions of existence of cells. This is essential for the livelihoods of the entire organism. Illustrate this with examples. The cells of the organism of warm-blooded animals, i.e., animals that have a constant body temperature, normally operate only within a narrow temperature limits (the person within 36-38 degrees). The temperature shift beyond these borders leads to disruption of cell activity. However, the organism of warm-blooded animals can normally take place on a much larger fluctuations of temperature of the external environment. For example, polar bears can live at a temperature of 70 degrees and +20-30 degrees. This is because in the whole organism is governed by its heat exchange with the environment, i.e. the generation of heat (intensity, chemical processes with the release of heat) and heat dissipation. Thus, at low ambient temperature increases thermogenesis and heat transfer decreases. So, at fluctuations of the external temperature (within certain limits) remains a constant body temperature.
Cell function of the organism's normal only at relatively constant osmotic pressure caused by the persistence of detention in the cells of water and electrolytes. Change osmotic pressure reduction or increase, lead to abrupt breakdown of the functions and structure of the cells. The organism as a whole can some time to exist and excessive flow and deprivation of water, and in large and small quantities of salt in food. This is explained by the presence in the body of devices that help maintain
constant quantity of water and electrolytes in the body. In case of excess of water but a significant number are quickly excreted from the body excretion organs (kidneys, sweat glands, skin), and with the shortage of water it is retained in the body. Equally excretory organs regulate the content of electrolytes in the body: they quickly take excessive their number or keep them in the body fluids of an insufficient supply of salts.
The concentration of certain electrolytes in the blood and tissue fluids, on the one hand, and in the cytoplasm of cells on the other, different. In the blood and tissue fluids contain more sodium ions, and in the cytoplasm of cells more potassium ions. The difference ion concentration inside the cell and outside is achieved by a special mechanism that keeps potassium ions inside the cell and not allowing to accumulate in the cell ions of sodium. This mechanism, whose nature is not yet clear, called the sodium-potassium pump and is associated with the metabolism of cells.
The cells are very sensitive to changes in the concentration of hydrogen ions. Changes in the concentration of these ions in one direction or another sharply disturbs the activity of the cells. For the internal environment of an organism is characterized by the constant concentration of hydrogen ions, depending on the presence in the blood and tissue fluid so-called buffer systems (page 48) and from the activities of organs of excretion. With the increasing content of acids or alkalis in blood they are rapidly eliminated from the body and thus supported the constancy of the concentration of hydrogen ions of the internal environment.
Cells, particularly nervous, very sensitive to changes in blood sugar that serves as an important nutrient. Therefore, of great importance for the process of life is a constant blood sugar. It is achieved by the fact that with the increase in blood glucose level in the liver and muscles is synthesized from it postponed in cells polysaccharide is glycogen, and lowering of blood sugar levels glycogen is broken down in the liver and muscles and is released grape sugar entering the blood.
The constancy of the chemical composition and physical-chemical properties of the internal environment is an important feature of organisms higher animals. To indicate this constancy U. cannon coined the term, widely spread, is homeostasis. Expression homeostasis is the presence of a number of biological constants, i.e. stable quantitative indicators characterizing the normal condition of the body. Such constant in magnitude indicators are: the body temperature, the osmotic pressure of the blood and tissue fluids, the concentration of sodium ions, potassium, calcium, chlorine and phosphorus, protein and sugar, the concentration of hydrogen ions and several others.
Noting the constancy of the composition, physico-chemical and biological properties of the internal environment, it should be emphasized that it is not absolute, but relative and dynamic. This consistency is achieved continuously performed the work of a number of organs and tissues, which are aligned taking place under the influence of external environment changes and the functioning of the body shifts in the composition and physico-chemical properties of the internal environment.
The role of different bodies and their systems in maintaining homeostasis different. So, the system of the digestive system ensures the supply of blood nutrients in the form in which they can be used by the cells of the body. System of organs of blood circulation carries out continuous movement of blood and transport of different substances in the body, resulting in nutrients, oxygen and various chemical compounds that are formed in the organism, proceed to the cells, and the decay products, including carbon dioxide, released by the cells are transferred to the organs which is removed from the body. Respiratory provide oxygen to the blood and remove carbon dioxide from the body. The liver and other organs have implemented a large number of chemical transformations - the synthesis and breakdown of many chemical compounds of importance in the life of the cells. Excretory organs - kidneys, lungs, sweat glands, the skin is removed from the body of the final products of decomposition of organic substances and maintain the consistency of the content of water and electrolytes in the blood, and therefore in the tissue fluid in the cells of the body.
In homeostasis most important role belongs to the nervous system. Responds to various changes in the external or internal environment, it regulates the activities of organs and systems that are notified and cleared developments and violations that occur or could occur in the body.
Thanks to development of devices, ensuring a relative constancy of the internal environment of the organism, the cells are less susceptible to volatile environmental influences. According To CL. Bernard, "constancy of the internal environment is a prerequisite of a free and independent life."
Homeostasis is defined borders. During the stay, particularly long, the body in conditions that differ significantly from those to which it is adapted, homeostasis is broken and there might be changes that are incompatible with normal life. So, when a significant change of the ambient temperature to the side as her increase and decrease body temperature can rise or fall and may come overheating or cooling of the body, leading to death. Similarly, when a considerable limitation of exposure of water and salts or complete withdrawal of these substances relative constancy of the composition and physico-chemical properties of the internal environment after a while violated and life ceases.

The high level of homeostasis occurs only at certain stages of species and individual development. The lower animals do not have sufficiently developed devices to mitigate or eliminate the effects of changes in the external environment. For example, relatively constant body temperature (homoiothermy) is supported only in warm-blooded animals. The so-called cold-blooded animals body temperature close to the temperature of the external environment and is variable (poichiloterme). The newborn animal no such constant body temperature, composition and properties of the internal environment, as an adult organism.

Even small homeostasis cause pathology, and because relatively constant physiological parameters, such as body temperature, blood pressure, composition, physico-chemical and biological properties of the blood and so on, is of great diagnostic value.