Homeostasis in Physiology

Homeostasis 1

• Homeostasis is a core concept in physiology, crucial for understanding regulatory mechanisms.
• Disruptions in homeostatic mechanisms lead to disease, and conversely, diseases can disrupt homeostasis.
• Defining and understanding homeostasis is important for understanding human health and disease.

Learning Objectives

• Define homeostasis and its significance in human health and disease.
• Detail the components of a homeostatic control system, avoiding common misinterpretations.
• Illustrate physiological variables that are homeostatically regulated.
• Describe thermoregulation as an example of homeostasis.

Historical Perspective

• Historical view of health as a balanced state of "elements" within the human body, and illness as an imbalance thereof.
• Claude Bernard introduced the concept of "fixed internal milieu," proposing a stable internal environment despite external changes.
• Walter Cannon coined the term "homeostasis," signifying a state of balance and stability.

Defining Homeostasis

• Homeostasis, "staying similar", maintains a stable, relatively constant internal environment.
• Homeostasis is a self-regulating process maintaining internal stability while adapting to environmental changes.
• Active regulatory mechanisms are employed to minimize disturbances to internal conditions, making it a dynamic, not static, process.

Internal Conditions

• Physiological variables are regulated to maintain relatively constant levels, preventing potentially lethal changes.
• Examples include body temperature, body weight, blood glucose levels, blood gases, and levels of ions (e.g., calcium, iron, sodium, and potassium), blood pressure, and blood pH.

Disruptions to Internal Conditions

• Disruptions in homeostasis of these variables can result in diseases or physiological issues.
• Some examples are hypothermia, obesity, anorexia, diabetes, hypoxia, anemia, hyponatremia, hypertension, and acidemia.

Environmental Changes

• Anything causing alterations to internal conditions, whether external or internal disturbances, can disrupt homeostasis.
• Examples include extreme temperatures, low oxygen levels, infectious agents, dietary iron deficiency, and blood loss.
• Eating, for example, serves as an environmental change in blood glucose homeostasis.

Active Regulatory Mechanisms

• Interrelated physiological processes maintain a variable within normal limits, responding to environmental changes.
• These are part of a control system.

Homeostatic Regulatory System

• A system with multiple key components:
• A sensor that measures a variable.
• A set point, defining normal values or ranges for the variable.
• A control center that compares the measured value with the set point.
• Effectors that can modify the variable.
• Responses of effectors to adjust the variable.

Homeostatic Control Systems: A Model

• The system uses negative feedback, where a change in a variable triggers a response that moves the variable back to its normal range.
• The model comprises sensors, set points, control centers, and effectors.

Thermoregulation

• Thermoregulation maintains a body temperature of 36.1-37.4°C via:
• Skin receptors and the hypothalamus that detect temperature changes.
• The hypothalamus, acting as a control center that coordinates responses.
• Blood vessels, muscles, and shivering for regulating body temperature.
• Mechanisms include adjusting blood vessel constriction, shivering, and sweat production to either reduce or increase heat loss.

Common Misconceptions

• Set point is fixed:  Set points are ranges, not fixed values. Body temperature, for example, has natural fluctuations throughout the day, and a circadian rhythm.
• Set point is fixed: Homeostatic set points are adjustably changeable and can be affected by disease or physiological processes.
• Homeostasis is an on/off switch: Homeostatic mechanisms are always active, and their strength/amplitude adjusts to changes in their environment.
• Effector response is the physiological change:  The physiological change results from effector responses

Lecture Summary

• Homeostasis is essential for human health and maintaining a stable internal environment.
• Disruption of homeostasis causes disease.
• Homeostatic processes involve continuous feedback loops.