HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.3. Biology of Individual Development

Systemic Mechanisms of Human Homeostasis at the Organismal Level

Homeostasis is the ability of living organisms to maintain the relative dynamic constancy of their internal environment despite changing environmental conditions. The foundation of homeostasis consists of mechanisms that emerged during evolution and are therefore genetically fixed. The effectiveness of homeostasis largely depends on the genotypes of individuals, whose diversity within the species Gene pool explains individual differences in the norm of reaction to the same environmental change.

Living organisms are open, self-regulating systems. They automatically activate and deactivate processes that maintain homeostasis. The interconnection between individual Links of the system is carried out through signals. A signal is a change that has occurred in a certain link of the system. In response to the signal, a specific process is triggered that restores the disrupted property of the system. The principles underlying the existence of self-regulating systems are studied by cybernetics. Cybernetic ideas have deeply penetrated biology (biocybernetics). The regulatory mechanisms of homeostasis in living systems are based on negative feedback loops (which lead to changes that reduce the dependence of the response on the stimulus). A negative feedback system implies the presence of a detector that records changes in a stable system, an integrating center that receives signals from the detector and regulates activity, and an effector that directly participates in restoring the system's homeostasis. Every organ system takes part in ensuring homeostasis. Let us examine the endocrine, nervous, and immune mechanisms of homeostasis.

Endocrine mechanisms of homeostasis. The Endocrine System consists of Glands of Internal Secretion (Endocrine glands). These glands produce chemical signals known as Hormones. Hormones enter the Blood AND Lymph directly and are transported throughout the Organism, yet only competent Cells possessing receptors for these signals respond to them. The central endocrine gland in humans and higher animals is the Pituitary Gland, which controls the endocrine activity of most subordinate (peripheral) glands. For instance, when the level of thyroid hormone rises in the blood, the thyrotropic function of the pituitary gland (which stimulates The Thyroid Gland) is suppressed, leading to a decrease in thyroid activity. There are cases when The activity of the thyroid gland increases (hyperfunction), basal METABOLISM rises, and oxidation processes intensify, but no negative feedback occurs; the pituitary gland fails to respond to the excess thyroid hormone and does not inhibit its activity. As a result, homeostasis is not restored, and a deviation from the norm known as thyrotoxicosis develops. In turn, the functional activity of the pituitary gland is regulated by neuroendocrine cells located in specialized nuclei of the Hypothalamus.

NERVOUS MECHANISMS OF homeostasis. The Nervous system regulates the Functions of the organism, coordinates the activity of cells, Tissues, Organs, and their systems, and ensures the interaction between the organism and the external environment. The working principle of the nervous system is reflex-based. Receptor Neurons perceive signals, encode them in the form of electrical impulses, and transmit them to other cells and organs capable of responding to them. The nervous and endocrine systems function in a coordinated manner. The centers of coordination are the hypothalamus and the pituitary gland.

Immune mechanisms of homeostasis. In the course of evolution, the organism has developed non-specific and specific defense mechanisms against Bacteria, Viruses, Protozoa, and helminths—pathogens of human diseases. Non-specific defense mechanisms include: the barrier properties of intact Skin and mucous membranes, the antimicrobial properties of salivary Lysozyme, and phagocytosis, which was discovered by I.I. Mechnikov. Specific defense reactions are provided by The Immune System. The immune system has been most thoroughly studied in mammals and humans. It comprises the Thymus gland, Bone Marrow, Spleen, Lymph Nodes, and lymphoid aggregates. The primary active element of this system is the lymphocyte (a type of leukocyte). The function of the immune system is to protect the organism from genetically foreign information and to maintain immunological homeostasis. The immune system distinguishes "self" from "non-self" and destroys the "non-self". The mechanism by which the organism defends itself against genetically foreign information is called Immunity. Examples of immunity include Resistance to Infectious diseases and graft rejection during Organ and tissue transplantation (transplantation immunity). Carriers of foreign Genetic information can be both external agents (bacteria and their toxins, viruses, helminths and their toxins, other organisms and their cells, certain synthetic polymers) and the organism's own mutant somatic cells (for instance, malignant tumor cells). All of them carry Antigens. Antigens are high-molecular-weight compounds that are genetically foreign to a given organism and capable of triggering an Immune Response. Antigens selectively interact with lymphocyte receptors, induce the synthesis of Antibodies, and react specifically with them.

The formation of the immune defense system begins with the generation of T- and B-lymphocytes. A T-lymphocyte is formed in the thymus from a precursor Cell originating in the bone marrow. A B-lymphocyte is also formed from cellular elements of the bone marrow. The fundamental property of T- and B-lymphocytes is their ability to specifically recognize antigens. The specific immune response comprises two types: cellular and humoral (cellular and humoral immunity). The cellular type consists in the immune response being mediated primarily by the activity of T-cells. Several types (subpopulations) of T-cells are distinguished: 1) T-helper cells; 2) T-suppressor cells (which suppress the immune response); 3) T-effectors, or T-killers (killer lymphocytes that produce cytotoxic substances); 4) T-counter-suppressors (whose action is opposite to that of T-suppressors). The main elements of the HUMORAL IMMUNE RESPONSE are antibodies. Antibodies are IMMUNOGLOBULINS (Ig) that specifically bind to antigens. Antibodies are synthesized by plasmacytes, which derive from B-cells. This form of reaction occurs mainly in antibacterial immunity. Immunoglobulins (antibodies) bind bacterial antigens with their active sites, preventing their multiplication or neutralizing Bacterial toxins. Immunoglobulins belong to serum soluble Proteins. They are Glycoproteins whose protein moiety consists of four polypeptide chains. Each immunoglobulin molecule contains two heavy H-chains and two light L-chains that are pairwise identical. Five classes of human immunoglobulins are known: IgM, IgG, IgA, IgE, and IgD. The science that studies the Patterns of inheritance of antigenic Specificity and the genetic determination of immune responses is called immunogenetics.