This section on specific causes of direct tissue injury is an extension of Section I, in which general mechanisms of tissue injury were described in some detail with only passing reference to specific agents.
Mechanisms of Tissue Injury
Causes of tissue injury act via the general mechanisms described in Section I. The reader may wish to review that section briefly before going on. In this section, tissue injury will be considered from the perspective of the specific agents causing injury, and the pathogenic mechanisms will be explored in greater detail. The chapters in this section will not emphasize the details of the diseases caused by these agents—that discussion is reserved for the organ system chapters (beginning with Chapter 20: The Blood Vessels) because the clinical consequences of injury are best studied in the context of the impact of the injury on the structure and function of the organ involved.
Tissues may be damaged by deprivation of nutrients or exposure to toxic agents.
Lack of Nutrients Essential for Cellular Metabolism
Nutrient deficiency may be generalized, involving the body as a whole (eg, protein-calorie and vitamin deficiency, hypoxia, hypoglycemia), or may involve a single organ as a result of compromise of the blood supply to that organ.
Noxious agents include physical agents such as radiation and heat as well as chemical agents (heavy metals, drugs, etc), infectious agents (bacteria, viruses, etc), and endogenous substances that accumulate in the body in inborn errors of metabolism (eg, phenylketonuria) or in failure of organs such as the liver, kidney, and lungs. Immunologic injury occurs when the individual's immune system is inappropriately directed at antigens or host tissues. The production of cytotoxic chemicals (eg, antibodies, complement activation) or cytotoxic cells (eg, effector T lymphocytes) is responsible for immunologic injury.