The cells may be broadly injured by two major ways:
A. Genetic causes
B. Acquired causes
The acquired causes of disease comprise vast majority of common diseases afflicting mankind. Based on underlying agent, the acquired causes of cell injury can be further categorised as under:
1. Hypoxia and ischaemia
2. Physical agents
3. Chemical agents and drugs
4. Microbial agents
5. Immunologic agents
6. Nutritional derangements
8. Psychogenic diseases
9. Iatrogenic factors
10. Idiopathic diseases.
HYPOXIA AND ISCHAEMIA Cells of different tissues essentially 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:
-> The most common mechanism of hypoxic cell injury is by reduced supply of blood to cells due to interruption i.e. ischaemia.
-> Hypoxia may also result from impaired blood supply from causes other than interruption e.g. disorders of oxygen-carrying RBCs (e.g. anaemia, carbon monoxide poisoning), heart diseases, lung diseases and increased demand of tissues.
PHYSICAL AGENTS (i) mechanical trauma (e.g. road accidents); (ii) thermal trauma (e.g. by heat and cold); (iii) electricity; (iv) radiation (e.g. ultraviolet and ionising); and (v) rapid changes in atmospheric pressure.
CHEMICALS AND DRUGS (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. i) immunodeficiency diseases; ii) hypersensitivity reactions; iii) anaphylactic reactions; and iv) autoimmune diseases.
NUTRITIONAL DERANGEMENTS A deficiency or an excess of nutrients may result in nutritional imbalances. Nutritional deficiency diseases may be due to overall deficiency of nutrients (e.g. starvation), of protein calorie (e.g. marasmus, kwashiorkor), of minerals (e.g. anaemia), or of trace elements. Nutritional excess is a problem of affluent societies resulting in obesity, atherosclerosis, heart disease and hypertension. AGEING Cellular ageing or senescence leads to impaired ability of the cells to undergo replication and repair, and ultimately lead to cell death culminating in 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 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).
IDIOPATHIC DISEASES Idiopathic means “of unknown cause”. Finally, although so much is known about the etiology of diseases, there still remain many diseases for which exact cause is undetermined. For example, most common form of hypertension (90%) is idiopathic (or essential) hypertension.