Chapter 4. Internal environment of the organism

4.1. Bodily fluids. Homeostasis

Water in the human organism accounts for about 60% of body weight and is distributed in two main water spaces: intracellular, which accounts for about 40%, and extracellular (20%). The intracellular fluid is represented by the liquid phase of the cytoplasm and nucleus. The main components of extracellular fluid are plasma, interstitial (tissue, intercellular) fluid, and lymph fluid.

Plasma, lymph, and interstitial fluid form the internal environment of the organism, which is characterized by the relative constancy of its composition and physico-chemical features and creates optimal conditions for the existence of the organism’s cells. The position of the constancy of the body’s internal environment as a condition of its independent existence was formulated by the physiologist C. Bernard. Further, W. Cannon in order to designate the constancy of the internal environment proposed the term ”homeostasis.” Speaking about the constancy of the composition, biological and physico-chemical properties of the internal environment, it should be understood that it is relative and dynamic. Currently, homeostasis is referred to as dynamic constancy of the body’s internal environment in association with the mechanisms that ensure the maintenance of this constancy; the term “homeokinesis” is also accepted.

In the blood, lymph, and interstitial fluid, the “pre-final,” in relation to the metabolic processes in the tissues, results of the activity of numerous functional systems of the homeostatic level, such as CBV and the number of formed elements, blood and osmotic pressure, temperature, medium reaction (pH), level of nutrients, gases, excretion products, etc., are concentrated. Each of these indicators is maintained at the level optimal for vital activity by a specific functional system, and due to this, metabolism, particularly in the tissues of the organism, is ensured.

As a result of the associative activity of these functional systems, general homeostasis is secured (Fig. 4.1).

Fig. 4.1. Associative auto-regulatory activity of various functional systems that secure homeostasis