Cell injury is defined as the effect of a variety of stresses due to etiologic agents a cell encounters resulting in changes in its internal and external environment. The response to a cell injury may vary between individuals. The cellular response to stress may vary and depends upon the following two variables.
- Host i.e. the type of cell and tissue involved.
- Factors pertaining to injurious agent i.e. extent and type of cell injury.
Etiology of cell injury
Basically, it is classified into – genetic causes and acquired causes (Acquired causes are discussed here). Based on the underlying agent, the acquired causes of cell injury can be further categorized.
- Hypoxia and ischemia
- Physical agents
- Chemical agents and drugs
- Microbial agents
- Immunologic agents
- Nutritional derangements
- Aging
- Psychogenic diseases
- Iatrogenic factors
- Idiopathic diseases
HYPOXIA and ISCHEMIA
All cells require oxygen to generate energy and perform metabolic functions. Deficiency of oxygen or hypoxia results in failure to carry out these activities by the cells. Hypoxia is the most common cause of cell injury. Hypoxia may result from the following 2 ways:
- Reduced blood supply to the cells due to interruption i.e. ISCHEMIA
- Impaired blood supply from causes other than interruption e.g. disorders of oxygen-carrying RBCs (e.g. anemia, carbon monoxide poisoning), heart diseases, lung diseases, and increased demand of tissues.
PHYSICAL AGENTS
Physical agents in the causation of disease are as under
i) mechanical trauma (e.g. road accidents)
ii) thermal trauma (e.g. by heat and cold)
iii) electricity
iv) radiation (e.g. ultraviolet and ionizing)
v) rapid changes in atmospheric pressure.
CHEMICAL AGENTS
An ever-increasing list of chemical agents and drugs may cause cell injury. Important examples include the following: i) chemical poisons such as cyanide, arsenic, mercury; ii) strong acids and alkalis; iii) environmental pollutants; iv) insecticides and pesticides; v) oxygen at high concentrations; vi) hypertonic glucose and salt; vii) social agents such as alcohol and narcotic drugs; and viii) therapeutic administration of drugs.
MICROBIAL AGENTS
Injuries by microbes include infections caused by bacteria, rickettsiae, viruses, fungi, protozoa, metazoa, and other parasites.
IMMUNOLOGIC AGENTS
Immunity is a ‘double-edged sword’—it protects the host against various injurious agents but it may also turn lethal and cause cell injury e.g.
ii) anaphylactic reactions
iii) autoimmune diseases.
NUTRITIONAL DERANGEMENTS
An imbalance in nutritional value may also cause cell injury. Nutritional deficiency diseases may be due to overall deficiency of nutrients (e.g. starvation), protein-calorie (e.g. marasmus, kwashiorkor), of minerals (e.g. anemia), or trace elements. Nutritional excess is a problem of affluent societies resulting in obesity, atherosclerosis, heart disease, and hypertension.
AGEING
Cellular Aging or senescence leads to impaired ability of the cells to undergo replication and repair and ultimately leads to cell death culminating in the death of the individual.
PSYCHOGENIC DISEASES
There are no specific biochemical or morphologic changes in common acquired mental diseases due to mental stress, strain, anxiety, overwork, and frustration e.g. depression, schizophrenia. However, problems of drug addiction, alcoholism, and smoking result in various organic diseases such as liver damage, chronic bronchitis, lung cancer, peptic ulcer, hypertension, ischaemic heart disease etc.
IATROGENIC CAUSES
Although as per the Hippocratic oath, every physician is bound not to do or administer anything that causes harm to the patient, there are some diseases as well as deaths attributed to iatrogenic causes (owing to physician). Examples include the occurrence of disease or death due to error in judgment by the physician and untoward effects of administered therapy (drugs, radiation).
IDIOPATHIC DISEASES
Idiopathic means “of unknown cause”. There still remain many diseases for which the exact cause is undetermined. For example, the most common form of hypertension (90%) is idiopathic (or essential) hypertension. Similarly, the exact etiology of many cancers is still incompletely known.
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