MECHANISMS OF DISEASE AND INJURY
The term disease means a specific process in which pathologic(abnormal) changes result in malfunction of a particular part or system of the body. A variety of biologic or other mechanisms can cause disease. However, because none of the body's part or systems exists in isolation, pathologic changes in one part of the body frequently affect the operation of another part. Thus, the process that is the etiology(cause) of one disease may itself be a disease process or the result of another disease. The term injury is often associated with disease because the processes that produce, or are, disease may damage and destroy cells, the microscopic units that compose a tissue, organ or system.
The mechanisms of disease and injury can be divided into 10 general types:
1. Infectious
2. Inflammatory
3. Allergic
4. Ischemic
5. Metabolic
6. Congenital
7. Developmental
8. Degenerative
9. Neoplastic
10. Traumatic
1. Infectious Process
Humans share this planet with many kinds of animals and plants, including unnumbered varieties of very small organisms invisible to the unassisted eye. These microorganisms are present in the air we breathe, in the food we eat, and on the objects we touch. They are even present inside our bodies. In many cases, we live in harmony with these bacteria, fungi, and viruses and even benefit from some of them. For eg, bacteria presetn in the intestine feed on waste matter and produce a vitamin the promotes blood clotting. On the other hand, penetration of some microorganisms through the body's natural defenses and into the tissues can have very damaging results. Invasion and multiplication of these harmful microorganisms, called infection, can injure cells by competing for nutrients and producing toxic substances or simply by interfering with the cells' normal activities and reproduction.
Bacterial and fungal infections frequently begin in the tissues immediately surrounding the microorganism's point of entry. Such infections are described as local. If they are unchecked, the infections may spread to surrounding tissues and become diffuse. In some cases, infections can get into the bloodstream and cause trouble at sites quite remote from the point of entry.
2. Inflammatory Process
The body generally reacts to infection first by local protective tissue response called inflammation. Specialized cells move to the affected area and act to destroy the injurious agent, while other cells release fluids to dilute any toxic substances produced by the infectious agent. Still other cells then proceed to wall off both the offender and the damaged tissue. Inflammation generally produces pain, heat, redness and swelling in the region affected. Although inflammation develops in response to infection, it may also occur following any injury or damage to tissue. Inflammation is often the body's response to foreign substances, such as bacteria or chemicals. The body releases its army of scavenger cells to try to neutralize the invading organisms or substance. The medical term for inflammation of a tissue or organ is obtained by adding the suffix -itis to the name of the tissue or organ. Thus, inflammation of the iris is known as iritis, inflammation of the retina retinitis, and so forth.
Inflammation that flares up quickly and remains for only a short period is called acute inflammation. If the condition persists for a long period, it is called chronic inflammation. The terms acute and chronic are also applied to infection and other disease processes, depending on whether they are brief or persistent. Although the purpose of inflammation starts out as protective, the changes it produces can result in a loss of function of the tissue or organ involved. Inflammation thus can itself act as a disease process.
3. Allergic Process
The body's initial inflammatory response to infection is generally followed by a wider and more complex immune reaction. Part of this response is the development of the antibodies to proteins present in the specific infecting microorganism. If a person is later re-exposed to the same invader, the antibodies will serve to neutralize the microorganism and may prevent recurrence of the infection. Even if reinfection is prevented, however, inflammation may still occur.
Many people have an overactive immune system that produces antibodies not only to infecting microorganisms, but to foods they eat or to plant pollens in the air they breathe. Re-exposure to these foods or pollens causes these people to have allergic reactions. Generally, the reactions are no more serious than a runny nose and watery eyes, but in some people they produce difficulties in breathing, such as asthma, or even death. Allergy, therefore, is an important disease mechanism. As with microbial infection, allergic reactions may be accompanied by inflammation.
4. Ischemic Process
Ischemia is the term given to a severe reduction in the blood supply to any part of the body. The cells of most body structures depend on the blood carried by nearby vessels for nutrients and oxygen and as a means to remove waste products. Interruption of the blood flow to a particular body part can occur if the vessels become occluded(blocked) - for eg, by a blood clot- or if the vessels break as the result of injury or high blood pressure. Even a relatively short period of ischemia and the resulting hypoxia (loss of oxygen) can lead to damage or death of the cells the vessels serve.
5. Metabolic Process
Metabolism refers to the combination of all the physical and chemical processes by which the body converts food into the building blocks of the body's tissues and into the energy the body uses. An extraordinary number of individual processes are involved to both build and repair the machine that is the body and to keep it running. In addition to food and water, the metabolic processes require various chemicals and materials to assist the chemical reactions. Examples include minerals, vitamins, substances called enzymes, which speed the processes, and hormones, which regulate them.
Most of the enzymes and hormones needed are produced by special tissues or organs in the normal individual. In some people, a defect may cause enzyme or hormone production to increase, decrease, or stop altogether, causing a whole series of problems in other body functions and leading to damage in other organs, sometimes including the eye. Examples of overproduction of a hormone include increased thyroid in the disease hyperthyroidism and increased cortisol levels in Cushing's disease. A familiar example of decreased hormone production is the disease diabetes mellitus. People with this condition are unable to produce enough of the hormone insulin required for the metabolism of sugar. Sugar then accumulates in the blood and urine, upsetting other body systems and damaging tissues, including the eye.
6. Congenital Process
Various disease processes develop in the child or adult because of an outside influence, such as infection or injury, or for no known reason. In some individuals, these processes or their effects may be present from the time of birth; that is, they are congenital. Congenital disease, as well as congenital malformations or malfunctions of the eye and other body structures, may be genetic (inherited), or they may be acquired during development of the fetus or during delivery.
7. Developmental Process
Development of the body tissues and structures begins at conception and continues throughout gestation. Growth and development continue after birth and through puberty until a person reaches adulthood. In some fetuses, babies, and young people, one or another organ, body structure, or body system may not develop properly or at all, due to genetic factors, infection, trauma, or unknown causes. Faulty development of these structures or systems results in their inability to function properly.
8. Degenerative Process
Automobile parts tend to wear out with age. This is also the case with parts of the eye and other organs of the body. Gradual deterioration in the structure or function of body tissues is called a degenerative disease process, and such diseases often occur in old people. Age is not the only cause, however; genetic factors may be responsible for degenerative pathology in young people and even children, as well as in the elderly. Injury, infection, and inflammation may also lead to degenerative disease.
9. Neoplastic Process
A neoplasm is a new growth of different or abnormal tissue, such as a tumor or a wart. The growth may be benign__that is, not dangerous to the well-being of the individual__ or it may be malignant(cancerous). The cells in malignant tissue are deformed and multiply at an extraordinary rate. These cells may also metastasize__ move to other parts of the body __ and begin to produce new tumours, eventually overwhelming the normal function of the body structures and draining them of their food and oxygen. While generally more inconvenient than dangerous, benign neoplasms can cause problems by physically interfering with the operation of other structures.
10. Traumatic Process
Trauma is the sudden wound or injury to the eye or other part of the body, often from outside the person. The injury may be a cut or a blow or a fragment of wood or metal penetrating the eye. These are examples of physical or mechanical trauma, but the body is vulnerable to other kinds of assault. Toxins (poisons) may be a serious cause of trauma, whether they are received by mouth, contact with the skin, inhalation, or from a snake or insect bite. Thermal trauma is the term for burns or freezing of tissues. Chemical trauma is a major concern of the ophthalmologist because of the serious and rapid damage caused by such chemicals as acid or alkali entering the eye. By its nature, the traumatic disease process often requires emergency care and treatment.
Signs, Symptoms and Syndromes
All of the disease processes described can affect the vision of individuals and prompt them to seek the help of an ophthalmologist. The changes in vision they experience and the pain or other effects they feel are called symptoms. Abnormal changes observed by the physician on examination of the patient are called signs. Some signs and symptoms may be the same in a particular condition. However, distinction between the two terms is useful because symptoms tend to be more subjective or personal, while signs are usually objective. Obviously, both signs and symptoms are important in the diagnosis of disease. Syndrome is the term given to a set of signs or symptoms that is characteristic of a specific condition or disease.
The term disease means a specific process in which pathologic(abnormal) changes result in malfunction of a particular part or system of the body. A variety of biologic or other mechanisms can cause disease. However, because none of the body's part or systems exists in isolation, pathologic changes in one part of the body frequently affect the operation of another part. Thus, the process that is the etiology(cause) of one disease may itself be a disease process or the result of another disease. The term injury is often associated with disease because the processes that produce, or are, disease may damage and destroy cells, the microscopic units that compose a tissue, organ or system.
The mechanisms of disease and injury can be divided into 10 general types:
1. Infectious
2. Inflammatory
3. Allergic
4. Ischemic
5. Metabolic
6. Congenital
7. Developmental
8. Degenerative
9. Neoplastic
10. Traumatic
1. Infectious Process
Humans share this planet with many kinds of animals and plants, including unnumbered varieties of very small organisms invisible to the unassisted eye. These microorganisms are present in the air we breathe, in the food we eat, and on the objects we touch. They are even present inside our bodies. In many cases, we live in harmony with these bacteria, fungi, and viruses and even benefit from some of them. For eg, bacteria presetn in the intestine feed on waste matter and produce a vitamin the promotes blood clotting. On the other hand, penetration of some microorganisms through the body's natural defenses and into the tissues can have very damaging results. Invasion and multiplication of these harmful microorganisms, called infection, can injure cells by competing for nutrients and producing toxic substances or simply by interfering with the cells' normal activities and reproduction.
Bacterial and fungal infections frequently begin in the tissues immediately surrounding the microorganism's point of entry. Such infections are described as local. If they are unchecked, the infections may spread to surrounding tissues and become diffuse. In some cases, infections can get into the bloodstream and cause trouble at sites quite remote from the point of entry.
2. Inflammatory Process
The body generally reacts to infection first by local protective tissue response called inflammation. Specialized cells move to the affected area and act to destroy the injurious agent, while other cells release fluids to dilute any toxic substances produced by the infectious agent. Still other cells then proceed to wall off both the offender and the damaged tissue. Inflammation generally produces pain, heat, redness and swelling in the region affected. Although inflammation develops in response to infection, it may also occur following any injury or damage to tissue. Inflammation is often the body's response to foreign substances, such as bacteria or chemicals. The body releases its army of scavenger cells to try to neutralize the invading organisms or substance. The medical term for inflammation of a tissue or organ is obtained by adding the suffix -itis to the name of the tissue or organ. Thus, inflammation of the iris is known as iritis, inflammation of the retina retinitis, and so forth.
Inflammation that flares up quickly and remains for only a short period is called acute inflammation. If the condition persists for a long period, it is called chronic inflammation. The terms acute and chronic are also applied to infection and other disease processes, depending on whether they are brief or persistent. Although the purpose of inflammation starts out as protective, the changes it produces can result in a loss of function of the tissue or organ involved. Inflammation thus can itself act as a disease process.
3. Allergic Process
The body's initial inflammatory response to infection is generally followed by a wider and more complex immune reaction. Part of this response is the development of the antibodies to proteins present in the specific infecting microorganism. If a person is later re-exposed to the same invader, the antibodies will serve to neutralize the microorganism and may prevent recurrence of the infection. Even if reinfection is prevented, however, inflammation may still occur.
Many people have an overactive immune system that produces antibodies not only to infecting microorganisms, but to foods they eat or to plant pollens in the air they breathe. Re-exposure to these foods or pollens causes these people to have allergic reactions. Generally, the reactions are no more serious than a runny nose and watery eyes, but in some people they produce difficulties in breathing, such as asthma, or even death. Allergy, therefore, is an important disease mechanism. As with microbial infection, allergic reactions may be accompanied by inflammation.
4. Ischemic Process
Ischemia is the term given to a severe reduction in the blood supply to any part of the body. The cells of most body structures depend on the blood carried by nearby vessels for nutrients and oxygen and as a means to remove waste products. Interruption of the blood flow to a particular body part can occur if the vessels become occluded(blocked) - for eg, by a blood clot- or if the vessels break as the result of injury or high blood pressure. Even a relatively short period of ischemia and the resulting hypoxia (loss of oxygen) can lead to damage or death of the cells the vessels serve.
5. Metabolic Process
Metabolism refers to the combination of all the physical and chemical processes by which the body converts food into the building blocks of the body's tissues and into the energy the body uses. An extraordinary number of individual processes are involved to both build and repair the machine that is the body and to keep it running. In addition to food and water, the metabolic processes require various chemicals and materials to assist the chemical reactions. Examples include minerals, vitamins, substances called enzymes, which speed the processes, and hormones, which regulate them.
Most of the enzymes and hormones needed are produced by special tissues or organs in the normal individual. In some people, a defect may cause enzyme or hormone production to increase, decrease, or stop altogether, causing a whole series of problems in other body functions and leading to damage in other organs, sometimes including the eye. Examples of overproduction of a hormone include increased thyroid in the disease hyperthyroidism and increased cortisol levels in Cushing's disease. A familiar example of decreased hormone production is the disease diabetes mellitus. People with this condition are unable to produce enough of the hormone insulin required for the metabolism of sugar. Sugar then accumulates in the blood and urine, upsetting other body systems and damaging tissues, including the eye.
6. Congenital Process
Various disease processes develop in the child or adult because of an outside influence, such as infection or injury, or for no known reason. In some individuals, these processes or their effects may be present from the time of birth; that is, they are congenital. Congenital disease, as well as congenital malformations or malfunctions of the eye and other body structures, may be genetic (inherited), or they may be acquired during development of the fetus or during delivery.
7. Developmental Process
Development of the body tissues and structures begins at conception and continues throughout gestation. Growth and development continue after birth and through puberty until a person reaches adulthood. In some fetuses, babies, and young people, one or another organ, body structure, or body system may not develop properly or at all, due to genetic factors, infection, trauma, or unknown causes. Faulty development of these structures or systems results in their inability to function properly.
8. Degenerative Process
Automobile parts tend to wear out with age. This is also the case with parts of the eye and other organs of the body. Gradual deterioration in the structure or function of body tissues is called a degenerative disease process, and such diseases often occur in old people. Age is not the only cause, however; genetic factors may be responsible for degenerative pathology in young people and even children, as well as in the elderly. Injury, infection, and inflammation may also lead to degenerative disease.
9. Neoplastic Process
A neoplasm is a new growth of different or abnormal tissue, such as a tumor or a wart. The growth may be benign__that is, not dangerous to the well-being of the individual__ or it may be malignant(cancerous). The cells in malignant tissue are deformed and multiply at an extraordinary rate. These cells may also metastasize__ move to other parts of the body __ and begin to produce new tumours, eventually overwhelming the normal function of the body structures and draining them of their food and oxygen. While generally more inconvenient than dangerous, benign neoplasms can cause problems by physically interfering with the operation of other structures.
10. Traumatic Process
Trauma is the sudden wound or injury to the eye or other part of the body, often from outside the person. The injury may be a cut or a blow or a fragment of wood or metal penetrating the eye. These are examples of physical or mechanical trauma, but the body is vulnerable to other kinds of assault. Toxins (poisons) may be a serious cause of trauma, whether they are received by mouth, contact with the skin, inhalation, or from a snake or insect bite. Thermal trauma is the term for burns or freezing of tissues. Chemical trauma is a major concern of the ophthalmologist because of the serious and rapid damage caused by such chemicals as acid or alkali entering the eye. By its nature, the traumatic disease process often requires emergency care and treatment.
Signs, Symptoms and Syndromes
All of the disease processes described can affect the vision of individuals and prompt them to seek the help of an ophthalmologist. The changes in vision they experience and the pain or other effects they feel are called symptoms. Abnormal changes observed by the physician on examination of the patient are called signs. Some signs and symptoms may be the same in a particular condition. However, distinction between the two terms is useful because symptoms tend to be more subjective or personal, while signs are usually objective. Obviously, both signs and symptoms are important in the diagnosis of disease. Syndrome is the term given to a set of signs or symptoms that is characteristic of a specific condition or disease.
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