Homeostasis - MBBS 1st Year (Physiology)

1. Definition and Origin of the Concept

• Claude Bernard (1813-1878), the French physiologist, first described this idea with his concept of the milieu intérieur (the "internal environment"). He wrote: "The constancy of the internal environment is the condition for free, independent life." He recognized that each organ system - cardiovascular, respiratory, renal, gastrointestinal, endocrine - works to preserve the composition of the extracellular fluid that bathes every cell.
• Walter B. Cannon (1871-1945), the American physiologist, later coined the actual term "homeostasis" in the 1930s to describe the mechanisms that restore the normal state once it has been disturbed.

"The body is a social order of about 35-40 trillion cells... Each functional structure contributes its share to maintaining homeostasis in the extracellular fluid, which is called the internal environment." - Guyton & Hall

2. What Does the Body Keep Constant?

At the level of the milieu intérieur (whole body):

At the level of the single cell:

• Cell volume, intracellular Na+, Ca2+, H+, and energy (ATP) are all individually regulated by each cell.

3. The Concept of Steady State vs. Equilibrium

Example: Blood glucose is constant not because it just sits there, but because glucose absorption from gut + glycogen breakdown is exactly balanced by glucose uptake into cells + glycogen synthesis. Energy is always being spent to maintain this balance.

4. Negative Feedback - The Core Mechanism

SENSOR → detects the vital parameter (or something related to it)
  ↓
COMPARATOR → compares input signal to the SET POINT
  ↓
ERROR SIGNAL → difference between actual value and set point
  ↓
EFFECTOR → acts to OPPOSE the deviation and return to set point

Classic Example: Blood Osmolality Regulation

1. Dehydration → Blood osmolality rises (>299 mOsm)
2. Osmoreceptors in the hypothalamus detect this (sensor)
3. Vasopressin (ADH) is released from posterior pituitary
4. ADH acts on renal collecting duct → inserts aquaporins → water reabsorption from urine increases
5. Simultaneously, the thirst center is activated → water intake increases
6. Blood osmolality returns to normal
7. The drop in osmolality feeds back to suppress vasopressin release (negative feedback closes the loop)

5. Components of a Feedback Loop (Ganong's)

6. Negative vs. Positive Feedback

7. Hierarchy and Redundancy of Feedback Loops

• Example: Hypothalamus → Anterior Pituitary → Adrenal Cortex → Cortisol → Blood glucose
• Each level controls the one below it, and each level receives negative feedback from downstream hormone levels.

8. Role of the ANS and Endocrine System

• ANS regulates: airflow, blood flow, blood gas composition, blood glucose, blood pressure, body temperature, digestion, electrolyte balance, heart rate, and urination (Ganong's)
• Endocrine system: slower but longer-lasting corrections via hormones

• Loss of temperature regulation → Hyperthermia / Heat stroke / Hypothermia
• Loss of glucose regulation → Diabetes mellitus
• Loss of water/salt balance → Dehydration, Hypertension, Cardiac arrhythmias
• Loss of blood pressure control → Hypertension / Hypotension / Heart failure

9. Homeostasis and Medicine (Guyton & Hall)

"As long as normal conditions are maintained in the internal environment, the body's cells continue to live and function properly. Each cell benefits from homeostasis and, in turn, each cell contributes to the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function."

10. Quick Summary for Exams