Endocrine Responses to Stress & Blood-Glucose Regulation

Blood-Glucose Homeostasis & Hypoglycemia

  • Blood-glucose (BG) levels must stay within \approx 70!\text{–}!110\,\text{mg·dL}^{-1} for normal cognitive and metabolic function.
  • Skipping meals or going the entire day without food → BG falls.
    • Clinical term: hypoglycemia (low blood glucose).
    • Symptoms mentioned: “fuzzy-brained,” dizziness, nausea, general malaise.
    • Mechanistic reason: insufficient glucose delivery to neurons → decreased ATP production in the CNS.

Endocrine System: Primary vs. Secondary Organs

  • Primary endocrine organs (devoted mainly to hormone secretion)
    • Hypothalamus
    • Pituitary gland
  • Secondary endocrine organs (have a primary non-endocrine role yet secrete hormones)
    • Kidneys (renin, erythropoietin, calcitriol, etc.)
    • Heart (atrial natriuretic peptide)
    • Intestines (multiple peptide hormones regulating digestion/absorption)
    • Reproductive organs (testes, ovaries—sex steroids & inhibins)
  • Key point: These secondary organs integrate with primaries to maintain homeostasis, especially during stress.

Stress Response = General Adaptation Syndrome (GAS)

Hans Selye’s 3-stage model, mapped here to lecture details.

1. Alarm Phase (Immediate / “Fight-or-Flight”)
  • Trigger: acute stressor (e.g., being startled, upcoming exam in 1–2 h).
  • Neural driver: Sympathetic division of autonomic nervous system (ANS).
  • Endocrine output: Adrenal medulla releases epinephrine + norepinephrine.
  • Physiological effects (all cited in transcript)
    • ↑ Mental alertness, arousal, “perk-up.”
    • Mobilization of glycogen & lipid reserves → Liver, skeletal muscle, adipose tissue dump glucose & fatty acids into circulation.
    • ↑ Heart rate (HR) & respiratory rate (RR) → ↑ systemic circulation.
    • Possible ↑ sweating (thermoregulation).
    • Temporary shutdown of: digestive tract, urinary tract, reproductive system (blood shunted away).
  • Ethical/practical note: Short-lived; generally adaptive.
2. Resistance Phase (Hours → Weeks)
  • Begins if stress ≳ a few hours.
  • Still sympathetic-driven but hormone spectrum broadens.
  • Hormonal milieu
    • Pituitary: ↑ Growth hormone (GH) → tissue maintenance, lipolysis.
    • Pancreas: releases glucagon (instructor verbally said “glycogen”) to raise BG.
    • Adrenal cortex: ↑ glucocorticoids (e.g., cortisol).
    • Kidney related: Aldosterone-mediated electrolyte regulation.
  • Metabolic goals
    1. Mobilize energy reserves
    • Fat stores (adipose) → free fatty acids.
    • Glycogen → glucose via glycogenolysis:
      (C<em>6H</em>10O<em>5)</em>n+n H<em>2O→n C</em>6H<em>12O</em>6(\text{C}<em>6\text{H}</em>{10}\text{O}<em>5)</em>n + n\,\text{H}<em>2\text{O} \rightarrow n\,\text{C}</em>6\text{H}<em>{12}\text{O}</em>6
    1. Conserve glucose (reduce renal excretion) → net ↑ BG.
    2. Conserve salts & water (Na$^+$, Cl$^−$, H2O\text{H}_2\text{O}) while losing K$^+$ & H$^+$.
  • Electrolyte specifics
    • Loss of K$^+$ compromises excitable-membrane function.
    • Recall Na$^+$/K$^+$ exchange:
      3 Na+<em>in+2 K+</em>out+ATP→3 Na+<em>out+2 K+</em>in+ADP+Pi3\,\text{Na}^+<em>{\text{in}} + 2\,\text{K}^+</em>{\text{out}} + \text{ATP} \rightarrow 3\,\text{Na}^+<em>{\text{out}} + 2\,\text{K}^+</em>{\text{in}} + \text{ADP}+\text{P}_i
    • Loss of H$^+$ → challenges blood-buffer systems, jeopardizing pH (7.35!–!7.45)\text{pH}\,(7.35!\text{–}!7.45) maintenance.
3. Exhaustion Phase (Weeks → Months) ❗ Life-Threatening
  • Occurs if stress persists and reserves are depleted.
  • Consequences detailed by instructor
    • Electrolyte balance fails → muscle & cardiac dysfunction (Ca$^{2+}$ handling also implicated).
    • No lipid reserves → no substrates for gluconeogenesis.
    • Progressive organ damage.
    • Inability to secrete glucocorticoids → impaired tissue repair.
    • High mortality risk.

Electrolyte & Energy Patterns Across Phases

  • Alarm: rapid glucose + O$_2$ delivery; K$^+$/H$^+$ normal.
  • Resistance: conserve Na$^+$, Cl$^−$, H$_2$O; lose K$^+$, H$^+$; sustained hyperglycemia.
  • Exhaustion: global electrolyte collapse; BG unable to be sustained; organ failure.

Autonomic–Endocrine Integration

  • Sympathetic ANS directly stimulates adrenal medulla (neuroendocrine interface).
  • Endocrine hormones then exert systemic, longer-lived effects, illustrating nervous–endocrine “double-check.”

Real-World & Philosophical Context

  • Stress isn’t always a personal “fault” (“not something we did in the closet”).
  • Acute stress is evolutionarily beneficial; chronic stress undermines health.
  • Ethical takeaway: lifestyle & societal structures should minimize prolonged stress exposure.

Minor Logistical Note from Class

  • Time of day in lecture: 11:00 AM.
  • Choice offered: wait until 12:20 or take a 10-min break and start immediately.
  • Student consensus: 10-minute break; reconvene downstairs.

Study Reminders

  • Link hypoglycemia symptoms to alarm-phase triggers.
  • Distinguish glucagon (pancreatic hormone) vs glycogen (stored carbohydrate).
  • Rehearse electrolyte shifts & the Na$^+$/K$^+$ pump for exam essays.
  • Recognize indicators of progression from resistance → exhaustion in clinical scenarios.