CAUSES OF CELL INJURY

Cell damage can occur in many ways. For purposes of dis­cussion, the ways by which cells are injured have been grouped into five categories: (1) injury from physical agents, (2) radiation injury, (3) chemical injury, (4) injury from biologic agents, and (5) injury from nutritional imbalances.

Injury From Physical Agents

Physical agents responsible for cell and tissue injury include mechanical forces, extremes of temperature, and electrical forces. They are common causes of injuries due to environ­mental exposure, occupational and transportation acci­dents, and physical violence and assault.

Mechanical Forces. Injury or trauma due to mechanical forces occurs as the result of body impact with another ob­ject. The body or the mass can be in motion, or as some­times happens, both can be in motion at the time of impact. These types of injuries split and tear tissue, frac­ture bones, injure blood vessels, and disrupt blood flow.

Extremes of Temperature. Extremes of heat and cold cause damage to the cell, its organelles, and its enzyme systems. Exposure to low-intensity heat (43° to 46°C), such as oc­curs with partial-thickness burns and severe heat stroke, causes cell injury by inducing vascular injury, accelerating cell metabolism, inactivating temperature-sensitive en­zymes, and disrupting the cell membrane. With more in-

CELL INJURY

Cells can be damaged in a number of ways, including phys­ical trauma, extremes of temperature, electrical injury, ex­posure to damaging chemicals, radiation damage, injury from biologic agents, and nutritional factors.

Most injurious agents exert their damaging effects through uncontrolled free radical production, impaired oxygen delivery or utilization, or the destructive effects of uncontrolled intracellular calcium release.

Cell injury can be reversible, allowing the cell to recover, or it can be irreversible, causing cell death and necrosis.

In contrast to necrosis, which results from tissue injury, apoptosis is a normal physiologic process designed to remove injured or worn-out cells.

tense heat, coagulation of blood vessels and tissue proteins occurs. Exposure to cold increases blood viscosity and in­duces vasoconstriction by direct action on blood vessels and through reflex activity of the sympathetic nervous system. The resultant decrease in blood flow may lead to hypoxic tissue injury, depending on the degree and dura­tion of cold exposure. Injury from freezing probably results from a combination of ice crystal formation and vasocon­striction. The decreased blood flow leads to capillary stasis and arteriolar and capillary thrombosis. Edema results from increased capillary permeability.

Electrical Injuries. Electrical injuries can affect the body through extensive tissue injury and disruption of neural and cardiac impulses. The effect of electricity on the body is mainly determined by its voltage, the type of current (i.e., direct or alternating), its amperage, the resistance of the intervening tissue, the pathway of the current, and the duration of exposure.⁴

Lightning and high-voltage wires that carry several thousand volts produce the most severe damage.² Alter­nating current (AC) is usually more dangerous than direct current (DC) because it causes violent muscle contractions, preventing the person from releasing the electrical source and sometimes resulting in fractures and dislocations. In electrical injuries, the body acts as a conductor of the elec­trical current. The current enters the body from an electri­cal source, such as an exposed wire, and passes through the body and exits to another conductor, such as the moisture on the ground or a piece of metal the person is holding. The pathway that a current takes is critical because the elec­trical energy disrupts impulses in excitable tissues. Current flow through the brain may interrupt impulses from respi­ratory centers in the brain stem, and current flow through the chest may cause fatal cardiac arrhythmias.

The resistance to the flow of current in electrical cir­cuits transforms electrical energy into heat. This is why the elements in electrical heating devices are made of highly resistive metals. Much of the tissue damage produced by electrical injuries is caused by heat production in tissues that have the highest electrical resistance. Resistance to electrical current varies from the greatest to the least in bone, fat, tendons, skin, muscles, blood, and nerves. The most severe tissue injury usually occurs at the skin sites where the current enters and leaves the body. After elec­tricity has penetrated the skin, it passes rapidly through the body along the lines of least resistance—through body fluids and nerves. Degeneration of vessel walls may occur, and thrombi may form as current flows along the blood vessels. This can cause extensive muscle and deep tissue injury. Thick, dry skin is more resistant to the flow of elec­tricity than thin, wet skin. It is generally believed that the greater the skin resistance, the greater is the amount of local skin burn, and the less the resistance, the greater are the deep and systemic effects.