Overview
This lecture covers the concept of homeostasis, the mechanisms that maintain it (negative and positive feedback loops), key physiological variables, and examples of feedback in the human body.
Homeostasis: Definition and Importance
- Homeostasis is the maintenance of a relatively constant internal environment despite external changes.
- It involves dynamic constancy, meaning variables stay within a livable range, not a fixed value.
- Physiology is the study of homeostasis and what happens when it is disrupted.
- Disease results when the body cannot maintain homeostasis.
Components of Homeostatic Regulation
- Receptors detect changes in physiological variables and monitor the internal environment.
- The control center processes the information and determines the response.
- Effectors carry out the response to bring variables back to the normal range.
- Receptors must be present for a variable to be regulated.
Key Homeostatic Variables and Ranges
- Blood glucose: generally 70–110 mg/dL (know this for Test 1).
- pH: 7.35–7.45 (easy to remember alongside sodium).
- Sodium: 135–145 mEq/L.
- Calcium: Normal range varies—know to check the units.
- Bicarbonate: Typically 22–26 or 24–28 mEq/L (ranges vary by lab).
- Partial pressure of oxygen: know this for Tests 3 and 4.
Negative Feedback Loops
- Negative feedback brings variables back toward the set point when they go out of range.
- Process: stimulus detected → receptor → control center → effector → response → return to range → stop response.
- Example: Increased body temperature detected by the hypothalamus triggers sweating and vasodilation to cool the body.
- Once the variable returns to range, the response is turned off.
Positive Feedback Loops
- Positive feedback amplifies the original stimulus instead of counteracting it.
- These loops require an endpoint to stop the process.
- Example: Blood clotting—platelet activation causes more platelets to stick, forming a clot until blood loss stops.
- Example: Birth—oxytocin increases uterine contractions, which leads to more oxytocin release, continuing until delivery.
Adaptation and Acclimation
- The body adapts physiologically (beyond behavioral changes) to extreme environments (heat, cold, altitude) to maintain homeostasis.
Key Terms & Definitions
- Homeostasis — Maintenance of a stable internal environment within a livable range.
- Dynamic Constancy — The concept that the internal environment is maintained within a normal range, not a fixed point.
- Receptor — Sensor that detects changes in a physiological variable.
- Control Center — Processes information from receptors and initiates a response.
- Effector — Organ or cell that carries out the response.
- Negative Feedback Loop — Mechanism returning a variable to its normal range.
- Positive Feedback Loop — Mechanism that amplifies a stimulus until an endpoint occurs.
Action Items / Next Steps
- Memorize the normal blood glucose range for Test 1.
- Review the normal ranges for pH and sodium.
- Practice writing your own examples of negative and positive feedback loops.
- Prepare for the next lecture on medical imaging.