Feedback Control Systems for Homeostasis

What is Homeostasis?

• Arterial blood pressure
• Body core temperature
• Plasma O₂, CO₂, glucose
• K⁺, Ca²⁺, H⁺ concentrations
• Blood osmolality (normal: 275-299 mOsm)
• Guyton and Hall Textbook of Medical Physiology
• Medical Physiology (Boron & Boulpaep)

Components of a Feedback Control System

Negative Feedback (the dominant mechanism)

• Pressure rises → baroreceptors in aortic arch detect stretch → signals sent to medullary vasomotor center → sympathetic activity reduced → heart pumping decreases + vasodilation → pressure falls back toward normal
• Pressure falls → baroreceptors less active → vasomotor center more active → vasoconstriction + increased heart rate → pressure rises back toward normal

Gain of a Control System

Gain = Correction / Error

• Without baroreceptors: blood transfusion raises pressure by 75 mmHg (100 → 175 mmHg)
• With baroreceptors: same transfusion raises pressure by only 25 mmHg
• Correction achieved = 175 - 125 = -50 mmHg
• Remaining error = +25 mmHg
• Gain = -50 / +25 = -2

• Guyton and Hall Textbook of Medical Physiology, p. 26

Feed-Forward and Adaptive Control

Positive Feedback - Vicious Cycles and Physiological Completion

• Parturition (childbirth): Uterine contractions → pressure on cervix → oxytocin release → more contractions, until delivery completes
• Blood clotting cascade: Each step amplifies the next until a clot forms
• LH surge at ovulation: Rising estrogen triggers a surge of LH that completes follicular rupture

• Guyton and Hall Textbook of Medical Physiology, p. 26

Endocrine Feedback Axes

Worked Example: Blood Osmolality Regulation

• Dehydration → osmolality rises → osmoreceptors in hypothalamus detect it → vasopressin released from posterior pituitary → collecting duct aquaporins inserted → water reabsorbed from urine into blood → osmolality falls back to normal → vasopressin release inhibited
• Simultaneously, the thirst center is activated → water intake increases → further dilution of blood

Hierarchy and Interaction of Feedback Loops

• Synergistic loops: Vasopressin and the thirst mechanism both lower osmolality, working together
• Antagonistic loops: Insulin lowers blood glucose; epinephrine and cortisol raise it - the net glucose level reflects competition between these loops
• Hierarchical loops: Hypothalamus → anterior pituitary → adrenal cortex → cortisol → blood glucose (a cascade where higher centers govern lower ones)
• Medical Physiology (Boron & Boulpaep), p. 15

Summary Table

Feedback Control Systems for Homeostasis - Simply Explained

Start Here: What Problem is the Body Solving?

The Thermostat Analogy (Most Important Concept)

The 4 Parts of Every Feedback System

  DISTURBANCE
      ↓
  [SENSOR] → detects the change
      ↓
  [CONTROL CENTER] → compares to normal (set-point), decides what to do
      ↓
  [EFFECTOR] → carries out the correction
      ↓
  [RESULT] → parameter returns to normal → sensor quiets down

Type 1: Negative Feedback ✅ (The Good One - Used 99% of the Time)

Simple Rule:

Too much of something → body reduces it Too little of something → body increases it

3 Easy Examples:

You eat a meal
    ↓
Blood glucose RISES (too high)
    ↓
Pancreas senses this
    ↓
Releases INSULIN
    ↓
Cells absorb glucose → blood sugar FALLS back to normal
    ↓
Insulin secretion STOPS (negative feedback complete)

You get scared / exercise
    ↓
Blood pressure RISES
    ↓
Baroreceptors in aorta sense the stretch
    ↓
Signal to brain → reduce heart rate + dilate vessels
    ↓
Blood pressure FALLS back to normal ✓

You go out in the cold
    ↓
Body temperature FALLS
    ↓
Hypothalamus detects it
    ↓
Commands: shiver, constrict blood vessels, feel cold (put on a jacket)
    ↓
Temperature RISES back to 37°C ✓

Type 2: Positive Feedback ⚠️ (Rare - Amplifies the Change)

Simple Rule:

More of something → even more of it → keeps going until the job is DONE

2 Easy Examples:

Baby pushes on cervix
    ↓
Cervix stretches → triggers oxytocin release
    ↓
Oxytocin causes stronger uterine contractions
    ↓
Baby pushes harder on cervix
    ↓
Even MORE oxytocin → even stronger contractions
    ↓
... keeps escalating ...
    ↓
BABY IS BORN → loop ends ✓

Blood vessel is cut
    ↓
Clotting factors activated
    ↓
Each factor activates more factors (amplification cascade)
    ↓
Clot forms quickly and completely ✓

Why Positive Feedback Can Be Dangerous (Vicious Cycle)

Massive blood loss
    ↓
Heart has less blood to pump → pressure drops
    ↓
Less blood reaches the heart muscle itself
    ↓
Heart gets WEAKER → pumps even less
    ↓
Even less coronary blood flow → heart weaker still
    ↓
↓↓↓ spiraling downward ↓↓↓
    ↓
DEATH (if not treated)

The "Gain" Concept - How Good is the System?

Gain = How much was corrected ÷ How much error remains

• A bad system brings you to within 25 points of target (corrected 50, missed 25) → Gain = 50/25 = 2
• A great system brings you to within 2 points → Gain = 33

Feed-Forward Control - The Body Predicting Problems

The Big Picture - One Diagram to Remember

        DISTURBANCE
        (↑ blood sugar)
              ↓
         [SENSOR]
      (β cells of pancreas)
              ↓
      [CONTROL CENTER]
    (detects high glucose)
              ↓
         [EFFECTOR]
      (releases insulin)
              ↓
        CORRECTION
      (glucose absorbed)
              ↓
    ← ← ← ← ← ← ← ← ← ←
    ↑   NEGATIVE FEEDBACK  ↑
    (correction turns OFF   )
    (insulin when glucose   )
    (is back to normal      )

Quick Comparison - Side by Side

One-Line Memory Tricks

• Negative feedback = "The body says STOP, go back to normal!"
• Positive feedback = "The body says KEEP GOING until it's DONE!"
• Homeostasis = The body's obsession with staying in the middle, always correcting, never settling for too much or too little.

Feedback Control & Homeostasis - Short & Simple

Homeostasis = Body keeping everything "just right"

How? Using Feedback Loops

• Room too hot → AC turns ON → cools down → AC turns OFF
• That's it. Your body does the same thing.

Every feedback system has 3 parts:

Sensor detects the problem → Brain decides what to do → Organ fixes it

2 Types of Feedback

🔵 Negative Feedback - "Undo the change"

Rule: Too much → reduce it. Too little → increase it.

🔴 Positive Feedback - "Keep going until done"

Rule: Used only to finish something quickly. Has a clear endpoint.

Key Difference in One Line:

Negative feedback = Stabilizes. Positive feedback = Completes.

Why it matters:

• When negative feedback fails → disease (e.g., diabetes = insulin feedback broken)
• When positive feedback has no endpoint → death (e.g., hemorrhagic shock)

• A professor or teacher?
• A researcher or doctor?
• A public figure?

Astigmatism

What is it? (The Simple Version)

The Core Problem

Normal eye (sphere):
All light → focuses at ONE point on retina → CLEAR vision

Astigmatic eye (oval/rugby ball):
Light in one meridian → focuses here  ──┐
                                         → TWO points = BLURRY vision
Light in other meridian → focuses here ──┘

Causes

• The cornea (most common) - not perfectly round
• The lens inside the eye - curved unevenly
• Goldman-Cecil Medicine

2 Types

Symptoms

• Blurry vision at all distances (near AND far)
• "Shadowing" - objects appear to have a ghost/double image
• Difficulty seeing fine detail
• Eye strain and headaches (from squinting to compensate)
• In children - can cause amblyopia (lazy eye) if uncorrected

Related Condition: Keratoconus

• Starts in adolescence, progresses over 5-10 years
• Usually affects both eyes (but not equally)
• Causes worsening vision that glasses can't fully fix
• Treatment: Rigid contact lenses for mild cases; corneal collagen cross-linking stops progression; corneal transplant for severe cases

Treatment Options

• Kanski's Clinical Ophthalmology, 10th ed.

Quick Summary

Astigmatism = unequal corneal curvature → light focuses in two places → blurry vision. Regular type is very common and easily corrected with glasses or laser. Irregular type (from scarring or keratoconus) needs rigid lenses or surgery.

Myopia vs Hypermetropia vs Astigmatism

The One Thing They All Share

The Core Difference - Where Does Light Focus?

NORMAL EYE:
Light → Lens → focuses EXACTLY on retina → Clear vision ✅

MYOPIA:
Light → Lens → focuses IN FRONT of retina → Blurry far vision ❌

HYPERMETROPIA:
Light → Lens → focuses BEHIND retina → Blurry near vision ❌

ASTIGMATISM:
Light → Lens → focuses at TWO DIFFERENT POINTS → Blurry/distorted ALL distances ❌

Side-by-Side Comparison

• Goldman-Cecil Medicine; Kanski's Clinical Ophthalmology, 10th ed.

The Simplest Visual Explanation

🔵 MYOPIA - Eye TOO LONG
   [Light] ──→ focuses ✦ ···· [Retina]
                  (in front)

🔴 HYPERMETROPIA - Eye TOO SHORT
   [Light] ──→ ···· [Retina] ···· ✦ focuses
                              (behind)

🟡 ASTIGMATISM - Cornea UNEVEN (oval not round)
   [Light] ──→ ✦ focuses here (one meridian)
            ──→ ···✦ focuses here (other meridian)
                (two different spots = blurry + distorted)

What Causes Each?

Can You Have More Than One?

• Myopia + Astigmatism (most common combo) - blurry far vision AND distortion
• Hypermetropia + Astigmatism - blurry near vision AND distortion
• All three together

Sphere (myopia/hypermetropia) + Cylinder (astigmatism) + Axis (direction of astigmatism)

Memory Trick

Myopia = My eye is too long → far things blurry Hypermetropia = Hyper (over) = eye overshoots → near things blurry Astigmatism = A-stigma = no single point of focus → everything slightly off