The body and the external environment

Every organism - celled or multicellular - requires certain conditions for the existence, provided that the environment to which the body of a certain kind adapted in the way of its evolutionary development. The functions of the body can normally be carried out only under the condition that the external environment provides the possibility of obtaining a food that is necessary to replenish material and energy costs, and if the body is in need him water or air at a certain temperature, barometric pressure, intensity and spectrum of light, etc.
Forms of adaptation of different organisms to all the listed factors in the external environment and their relationship with the environment is extremely diverse. So, animals need organic nutrients and are not able to synthesize complex organic compounds from inorganic substances. On the contrary, green plants during the critical biological processes - photosynthesis - carry out the synthesis of organic substances from inorganic. This difference in relation to the external environment of animals and plants ensures the circulation of substances in nature.
Vegetable organisms use of inorganic nitrogen compounds (ammonia, nitrate), we obtain the root system from the soil, and synthesize nitrogen-containing protein substances. Animals same organisms are not able to synthesize proteins by using inorganic nitrogen compounds. In animal cells proteins are created only from dietary amino acids. In the process of life, deamidization and decarboxylation, constantly decay of amino acids with more simple nitrogen compounds (ammonia, urea and other), which again are used by plants and some microorganisms for the synthesis of protein compounds.
The cells of most animals are aerobic, i.e. they need a molecular oxygen, which enter the body from external environment and necessary for the processes of oxidation of nutrients to carbon dioxide and water. Some organisms are anaerobic; they do not need the free oxygen that they are not used and even harmful for them. They are necessary for their metabolic energy by anaerobic - anoxic - splitting nutrients.
Quite different is the attitude of different organisms to temperature, barometric pressure, air humidity and content of electrolytes in the aquatic environment. Thus, some types of organisms can exist only in fresh water lakes and rivers, salt content of which does not exceed 0,1%; other organisms exist only h sea water contains about 3% of electrolytes.
Various changes in the external environment, such as: changes in the number and composition of nutrients, changes in the composition of the air - oxygen and carbon dioxide, composition and concentration of electrolytes in water, temperature and barometric pressure, spectrum and intensity of the light, and the presence of toxic substances in the environment, radioactive radiation, and others - have a powerful effect on physiological functions. Some factors influence them positively, others oppress them and cause the body to death.
Living organisms can normally only be made in certain environmental conditions. Range of changes to these terms are relatively small, i.e., should their relative constancy. It should be borne in mind that the limits of fluctuations of the external environment, tolerated by the body of the higher animals, much broader than those needed for normal functioning of the majority of its cells. The reason for this is that the environment for the cells of the body is the internal environment of the body, which varies considerably less than the external. The internal environment is the blood, lymph and tissue fluid that bathes the cells of the body.