Cells are the fundamental units of life, and their ability to adapt and respond to stress determines the health of tissues and organs. When cells are exposed to harmful stimuli, they may undergo injury, adaptation, or death depending on the severity and duration of the insult. At the same time, the body has built-in mechanisms like inflammation and repair to deal with such damage and restore balance.
This unit explores the principles of cell injury and adaptation as well as the mechanisms of inflammation and repair — two core foundations of pathology and disease progression.
Introduction to Homeostasis and Feedback
Homeostasis is the process by which living organisms maintain a stable, relatively constant internal environment despite changes in the external or internal conditions. Think of it as the body’s way of staying balanced, ensuring that critical variables like body temperature, blood glucose levels, and pH remain within a narrow, healthy range. This “dynamic equilibrium” is crucial for survival.
The Role of Feedback Loops
Homeostasis is primarily maintained through feedback loops, which are regulatory mechanisms that monitor a variable and trigger a response to keep it within a set range. There are two main types of feedback loops: negative and positive.
1. Negative Feedback Loops
Negative feedback is the most common type of regulatory mechanism for maintaining homeostasis. It works to counteract a change, bringing a variable back to its set point. When a stimulus causes a change in the body, the negative feedback loop initiates a response that opposes or negates that change.
Components of a Negative Feedback Loop:
- Stimulus: A change in the internal environment (e.g., a rise in body temperature).
- Sensor/Receptor: Detects the change (e.g., nerve cells in the skin and brain).
- Control Center: Receives information from the sensor and determines the appropriate response (e.g., the hypothalamus in the brain).
- Effector: Carries out the response to reverse the change (e.g., sweat glands and blood vessels).
Example: Body Temperature Regulation If your body temperature rises above its set point (e.g., during exercise), the hypothalamus signals sweat glands to release sweat and blood vessels in the skin to dilate. Evaporation of sweat cools the skin, and vasodilation allows more heat to radiate from the body, bringing the temperature back down.
2. Positive Feedback Loops
Positive feedback loops are much less common in homeostasis because they amplify or intensify the initial stimulus, pushing the variable further away from its set point. These loops are typically used for processes that need to be driven to completion, not for maintaining a stable state.
Example: Childbirth During labor, the baby’s head pressing on the cervix sends nerve signals to the brain. In response, the brain releases the hormone oxytocin, which causes stronger uterine contractions. These contractions push the baby further down, increasing pressure on the cervix and causing the release of even more oxytocin. This amplifying cycle continues until the baby is born. The process ends once the stimulus (pressure on the cervix) is removed.
A video about homeostasis and feedback loops can further clarify these concepts.
Causes of Cellular Injury
Cellular injury occurs when a cell is unable to maintain its normal state of homeostasis due to a variety of stresses. These stresses, or injurious stimuli, can be a wide range of factors that can either cause immediate, irreversible damage leading to cell death or initiate a chain of events that the cell can initially adapt to but may eventually succumb to.
Here is a breakdown of the major causes of cellular injury:
- Hypoxia/Ischemia – oxygen deprivation due to reduced blood flow.
- Physical agents – trauma, heat, radiation, cold.
- Chemical agents and drugs – toxins, alcohol, heavy metals.
- Biological agents – bacteria, viruses, parasites.
- Genetic factors – mutations, chromosomal abnormalities.
- Nutritional imbalances – deficiencies (e.g., vitamin D deficiency) or excess (e.g., obesity).
Inflammation and Repair
Inflammation and repair are two sequential and interconnected processes that are vital for the body’s response to injury or infection. Inflammation is the initial, protective response that eliminates the cause of cell injury, while repair is the subsequent process of restoring tissue structure and function.
Inflammation
Inflammation is a rapid and complex immune response to harmful stimuli, such as pathogens, damaged cells, or irritants. Its primary purpose is to localize and eliminate the injurious agent and to initiate the healing process.
The five cardinal signs of acute inflammation, which are visible hallmarks of the process, were first described by a Roman physician centuries ago:
- Redness (Rubor): Caused by increased blood flow to the injured area.
- Swelling (Tumor): Due to fluid leaking from blood vessels into the tissue.
- Heat (Calor): Also caused by increased blood flow.
- Pain (Dolor): From the release of chemicals that stimulate nerve endings.
- Loss of Function (FunctioLaesa): A result of the swelling and pain.
There are two main types of inflammation:
- Acute Inflammation: This is an immediate, short-term response that lasts for minutes to a few days. It is characterized by the rapid accumulation of fluid and immune cells, primarily neutrophils, at the site of injury. Once the threat is neutralized, the acute inflammatory response resolves and the healing process begins.
- Chronic Inflammation: This occurs when the inflammatory response persists for weeks, months, or even years. It is often caused by a persistent irritant, an autoimmune condition, or an unresolved acute infection. Chronic inflammation is characterized by a different set of immune cells, mainly macrophages and lymphocytes, and is often accompanied by simultaneous tissue destruction and repair, which can lead to progressive organ damage.
Tissue Repair
Following the inflammatory phase, the body begins the process of tissue repair to restore the damaged area. The goal is to return the tissue to its normal state, though this isn’t always possible.
Tissue repair occurs through two main mechanisms:
- Regeneration: This is the ideal outcome of repair, where the damaged tissue is completely replaced by new, healthy tissue of the same type. It involves the proliferation of uninjured cells and tissue stem cells. Regeneration is only possible in tissues with a high capacity for cell division, such as the skin, liver, and some components of the blood.
- Scarring (Fibrosis): This occurs when the tissue is too severely damaged to regenerate or if the cells lack the capacity to divide (e.g., in the heart or brain). The injured tissue is replaced by fibrous connective tissue (a type of scar tissue), which fills the gap but does not have the same functional properties as the original tissue. While scarring provides structural integrity, it can lead to a loss of function in the affected organ.
In essence, inflammation clears the battlefield, and repair rebuilds the tissue. An effective inflammatory response is essential for setting the stage for successful tissue repair, while a dysregulated or chronic inflammatory state can impair healing and lead to pathological scarring.