Overview
This lecture introduces the core principles of general physiology, focusing on homeostasis, feedback mechanisms, and body fluid compartments essential for maintaining internal stability.
Homeostasis
- Homeostasis refers to maintaining a stable internal environment through coordinated organ system responses.
- The body uses narrow normal ranges for critical conditions like blood glucose and blood pressure.
- Negative feedback loops restore conditions to their set point by counteracting deviations.
- Positive feedback loops exist but are less common and typically drive processes further from the set point.
Negative Feedback Example: Blood Glucose Regulation
- Sensors detect changes in blood glucose from the normal set point (≈90 mg/100 mL).
- After eating, increased blood glucose prompts pancreatic beta cells to release insulin.
- Insulin enables cells to absorb glucose, lowering blood glucose back to normal.
- If fasting lowers glucose, pancreatic alpha cells release glucagon, raising glucose by breaking down liver glycogen.
Body Fluid Compartments
- Body fluids are divided into intracellular fluid (ICF) and extracellular fluid (ECF).
- ICF is fluid inside cells, comprising about 60% of total body water.
- ECF is fluid outside cells (40% of total body water), split into interstitial fluid and blood plasma.
- Interstitial fluid makes up 80% of ECF; blood plasma is 20% of ECF.
- Composition varies: ICF and ECF have different concentrations of sodium, potassium, and proteins.
Fluid Balance and Osmolarity
- Osmolarity (solute concentration) is similar in ECF and ICF (≈285 mOsm/L), enabling equilibrium and water movement.
- Interstitial fluid and plasma are similar, except interstitial fluid lacks proteins.
- Water loss increases solute concentration, shifting fluid between compartments to maintain balance.
- Significant fluid loss (5 L ECF) is needed before plasma volume drops by 1 L due to compartment ratios.
- Plasma loss can be treated with isotonic saline infusion to restore homeostasis.
Key Terms & Definitions
- Homeostasis — Maintenance of stable internal physiological conditions.
- Negative Feedback — A regulatory mechanism that opposes changes from a set point.
- Set Point — The desired or normal value for a physiological condition.
- Intracellular Fluid (ICF) — Fluid inside cells, about 60% of body water.
- Extracellular Fluid (ECF) — Fluid outside cells, includes interstitial fluid and plasma.
- Osmolarity — The concentration of solute particles in a solution.
- Isotonic Saline — A saline solution with solute concentration equal to body fluids, used to restore fluid balance.
Action Items / Next Steps
- Review normal ranges for physiological variables in the provided textbook table.
- Practice outlining negative feedback loops for blood pressure scenarios.
- Study the diagram of body fluid compartments in your textbook.