Etiology of Cell Injury Cell injury is a sequence of events that occur if the limits of adaptive capability are exceeded or no adaptive response is possible. This can be due to physical, chemical, infectious, biological, immunological factors and nutritional cellular abnormalities.
A) Acquired cause:
Acquired causes of cell injury are further categorized as:
(a) Oxygen deprivation (Hypoxia)
(b) Physical agents
(c) Chemical agents and drugs
(d) Microbial agents
(e) Immunologic agents
(f) Nutritional derangement
(g) Psychological factors
(h) Idiopathic agents
(a) Oxygen deprivation: Hypoxia is a deficiency of oxygen, which causes cell injury by reducing aerobic oxidative respiration. Hypoxia is an extremely important and common cause of cell injury and cell death. Causes of hypoxia include reduced blood flow (ischemia), inadequate oxygenation of the blood due to cardiorespiratory failure, and decreased oxygen-carrying capacity of the blood, as in anemia or carbon monoxide poisoning (producing a stable carbon monoxyhemoglobin that blocks oxygen carriage) or after severe blood loss. Depending on the severity of the hypoxic state, cells may adapt, undergo injury, or die. For example, if an artery is narrowed, the tissue supplied by that vessel may initially shrink in size (atrophy), whereas more severe or sudden hypoxia induces injury and cell death.
(b) Physical agents for cell injury: Mechanical trauma (e.g., Road accident), Thermal trauma (e.g., Heat and cold), Electricity, Radiation (e.g., U.V. radiation), Rapid changes in atmosphere pressure.
(c) Chemicals and Drugs: The list of chemicals that may produce cell injury defines compilation. Simple chemicals such as glucose or salt in hypertonic concentrations may cause cell injury directly or by deranging electrolyte balance in cells. Even oxygen at high concentrations is toxic. Trace amounts of poisons, such as arsenic, cyanide or mercuric salts, may damage sufficient number of cells within minutes or hours to cause death. Other potentially injurious substances are our daily companions: environmental and air pollutants, insecticides, and herbicides; industrial and occupational hazards, such as carbon monoxide and asbestos; recreational drugs such as alcohol; and the ever-increasing variety of therapeutic drugs.
(d) Microbial agents: Injuries by microbes include infection caused by bacteria, rickettsiae, viruses, fungi, protozoa and other parasites.
(e) Immunological Agents: The immune system serves an essential function in defence against infectious pathogens, but immune reactions may also cause cell injury. Injurious reactions to endogenous self-antigens are responsible for several autoimmune diseases. Immune reactions to many external agents, such as viruses and environmental substances, are also important causes of cell and tissue injury. Example: Hypersensitivity reactions, anaphylactic reactions, autoimmune diseases.
(f) Nutritional derangement: A deficiency or an excess of nutrients may results in nutritional imbalances. Nutritional deficiency diseases may be due to overall deficiency of nutrients (starvation), protein calorie (Marasmus, Kwashiorkor), and minerals (Anaemia) or of trace elements. Nutritional excess is a problem of society which results from obesity, in atherosclerosis, heart diseases and hypertension.
(g) Psychological factors: There are number of specific biochemical or morphological changes in common acquired mental diseases due to mental stress, strain, anxiety, overwork and frustration. Problems of drug addiction, alcoholism and smoking results in various diseases such as liver damage, chronic bronchitis, lung cancer, peptic ulcer, hypertension, ischemic heart diseases etc.
(h) Idiopathic factor: The causative factor of cell injury is unknown.
B) Genetic cause:
In western countries, genetic defects constitute about 50% total mortality in infancy and childhood, while in developing and under developing countries 95% of infant mortality occurs. Genetic cases are such as, Developmental defect (Errors in morphogenesis), cytogenic defects (chromosomal abnormalities), Single-gene defects (Mendelian syndrome), Storage diseases (Inborn Errors of metabolism), Disorders with multifactorial inheritation.
