Introduction to Pathology

What is disease?

Disease is the failure to maintain homeostatic conditions and pathophysiology is the study of functional changes caused by disease. In pathophysiology, we understand that there are four aspects that form its core:

1. Etiology
2. Mechanisms of development
3. Structural alterations induced in the cells and organs of the body (morphological changes)
4. Functional consequences of the morphological changes

Etiology is the identification of casual factors that provoke the particular disease, e.g. a bacterium.


A disease of unknown origin is known as an idiopathic disease.


Prevalence is the number of people affected with a particular disease at a given time.


Pathogenesis is the development of evolution of a disease, i.e. invasion of the body, proliferation, spread, evolving inflammatory response, immunologic defences of the host and destruction of cells and tissues.


A symptom is a patient's perception of a change in their normal body function. They are difficult to measure and rely on asking patients appropriate questions.


A sign is a physical manifestation of the disease and can be measured.


Stress is any change or stimulus causing the disruption of cellular homeostasis. Causes of stress include:

• Physical agents, e.g. weight lifting
• Chemical agents, e.g. alcohol
• Micro-organisms
• Hypoxia
• Free radical injury (damage to cell membrane, mutations in DNA or RNA)

Adaptation is cells making adjustments to changes to keep the organism functioning in a normal state. The types of cellular adaptations include:

• Atrophy, where the cell becomes smaller
• Hypertrophy, where the cell becomes bigger
• Hyperplasia, where the number of cells increases
• Metaplasia, where the cell changes their type because it is stressed or assaulted
• Dysplasia, where the cell changes type abnormally (cancer precursor)

There are several types of reversible injuries:

• Cellular swelling, due to increased water as a result of malfunctioning Na+/K+ pump
• Cellular accumulation, due to build up of lipids, carbohydrates, glycogen, bilirubin, coal and calcium salts

Apoptosis and necrosis, two forms of cell death, are not reversible. Apoptosis is programmed cell death where the cell breaks junction with the cells around it and blebs into small, self-contained products that are digested by other cells (phagocytosis). Necrosis is a pathologic form of cell death, most commonly due to anoxia (absence of oxygen) and hypoxia (low oxygen). Both anoxia and hypoxia can be caused by conditions such as thrombosis, embolism and atherosclerosis.


Necrosis can be further divided into 4 categories:

1. Coagulative necrosis, in which calcium accumulation occurs in dead cells and the cells are then replaced by scar cells.
2. Liquefactive necrosis is most common in the brain, where degradative enzymes are rich and there is little supportive connective tissue. Here, the cells are transformed in to a liquid viscous mass.
3. Fat necrosis is when enzymes attack cell membranes of fat cells, releasing triglyceride stores.
4. Caseous necrosis is a characteristic of lung tissue damaged by tuberculosis, making cells look like clumpy cheese.

Neoplasms are defined as the development of an abnormal growth, which also happens to be unresponsive to control mechanisms. Often in neoplasms, the proto-oncogene, which is normal, can be mutated to become the oncogene, which can ultimately cause cancer.


Neoplasms can be both benign or malignant, meaning they are not always a form of cancer. Benign tumors have the suffix 'oma', e.g. fibroma and adenoma. Malignant tumors have the suffix 'carcinoma' when they are of epithelial origin, or 'sarcoma' when they are of mesenchymal origin.


Tumor growth fraction is the percentage of tumor cells that are dividing. The higher the fraction, the more rapidly growing the tumor.