Injurious agents and cell damage

Question

What types of injurious agents are there?

Which of these agents cause irreversable or reversable injury to certain cells in the body?

Thanks for your help.

Answer 1

There are many adverse "forces" that can damage cells. Because the number is so large, it is customary to devise a manageable list.

Deficient blood flow (ischemia) can damage cells.

Ischemia tops the list: it is responsible for almost 1,000,000 deaths in the United States each year. It is the underlying problem in heart attacks, strokes, and a number of other diseases. "Ischemia" refers to blocked or decreased blood flow to a tissue or an organ. Ischemia causes its damaging effects by depriving cells of oxygen and nutrients -- effects that are compounded by the accumulation of waste products.

Inadequate supplies of oxygen can damage cells.

If cells do not receive adequate supplies of oxygen, they will be injured. The terms "hypoxia" and "anoxia" refer to this situation. In "hypoxia" cells receive decreased amounts of oxygen (hypo- = under). The term "anoxia" is used when cells receive no oxygen at all (an- = without). Hypoxia and anoxia are the obvious results of ischemia; it is inadequate oxygen supply that is the major problem with decreased blood flow (ischemia). However, hypoxia and anoxia may occur without ischemia -- drowning is an example. Decreased oxygen transport can also occur when there is damage to red blood cells. Respiratory diseases, such as emphysema, may hinder proper oxygen transfer from air spaces to the blood stream.

Radiation, heat, and other physical agents can damage cells.

There are a host of physical agents that can damage cells. Radiant energy like that found in X-rays and sunlight can cause cell damage. Heat can damage cells by direct damage or by increasing the rate of cellular activity so that supply of oxygen becomes inadequate (as in a fever). Electrical energy can damage cells directly or by creating great amounts of heat. Trauma is an all too common cause of cell damage and cell death. Sudden, violent trauma (acute trauma) may injure cells directly while prolonged, less-intense trauma (chronic trauma) may provoke, at least for a while, cellular adaptations to it. Cold is another physical agent that may damage cells either by forming crystals that puncture cells or by slowing metabolic activities to the point that they simply stop.

External and internal chemicals can damage cells.

There are a large number of chemical compounds that may damage cells. There are too many of these to list here. Such chemical compounds may originate from outside or from inside the body. "Exogenous chemical agents" are those that are introduced from the outside world by ingestion, inhalation, or injection. The term "poison" is used if small amounts cause cell damage. The "endogenous chemical agents" are those that arise from cellular metabolism (i.e., waste products).

Viruses, bacteria, and other biologic agents can damage cells.

It is obvious that viruses and bacteria can cause cell damage and cell death. While a number of these circumstances will be described in a later chapter, a complete discussion will be left to microbiologists except to state that while viral and bacterial infections concern us the most, other biologic agents -- fungi, Rickettsia, protozoa, and even worms (helminths) -- can produce cell damage too.

Immune reactions can damage cells.

Immunity is usually portrayed as being beneficial. Most of the time that portrayal is accurate; however, there are times when immune reactions make things worse. In recent years, greater understanding of these potentially harmful reactions is included under the term "hypersensitivity." Some hypersensitivity reactions can cause cell injury and cell death. As but one example, a hypersensitivity reaction, the type IV variety, is responsible for the tissue death associated with pulmonary tuberculosis. As another, immune reactions directed at a patient's own cells produce a group of conditions known as "autoimmune diseases."

Damaged cells accompany diseases caused by abnormal genes.

Defective genes can cause disease. One is a disease in which glycogen accumulates abnormally in body cells. This uncommon disease is caused by the absence of an enzyme that releases glucose from glycogen. Because glucose cannot be released from its storage form, glycogen accumulates in increasing amounts. Patients with this disease are missing the gene that carries the code for glucose-6-phosphatase, the missing enzyme.

Lack of essential nutrients can damage cells.

Absences of essential nutrients may cause cell injury and cell death. Lack of vitamin C, for example, interferes with the formation of collagen, a prominent component of connective tissue. Since cells that producing collagen, are dependent on vitamin C, they are not able to produce adequate amounts of collagen and the disease scurvy results.

Increased work- load upon cells can transform or damage them.

Sometimes cells are called upon to perform at higher than normal activity levels. Elevation in body temperature is one cause of increased activity. It is a common observation that increased physical exercise brings changes in muscle cells. It is, perhaps, less commonly known that increased secretion of hormones can cause target cells to undergo changes.

Cells are less adaptable and more easily damaged as they grow older.

Some scientists have noted that cells grown outside the body undergo a finite number of cell divisions before the population dies out. That often repeated experiment has led some to believe that human cells have a finite life expectancy. They go on to extrapolate that human life expectancy may never be able to exceed, say, 150 years. These observations aside, it is true that cells in older individuals are less adaptable and more likely to undergo degeneration and death.

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