Homeostasis Study Guide

📖 Core Concepts Homeostasis – maintenance of stable internal physical & chemical conditions (temperature, pH, ion concentrations, glucose, etc.). Control System Triad – Receptor (sensor) → Control Center (integrator) → Effector (response organ). Negative Feedback – effector action reduces the original stimulus, turning the loop off. Set Point – the target value for a variable; can shift (circadian rhythm, fever, acclimatization). Allostasis – behavioral or anticipatory adjustments made before physiological mechanisms engage (e.g., seeking shade, predictive insulin release). 📌 Must Remember Key variables: temperature, extracellular pH, Na⁺, K⁺, Ca²⁺, glucose, iron, water. Temperature: vasoconstriction conserves heat; shivering & non‑shivering thermogenesis generate heat; sweating & panting dissipate heat. Glucose: ↑ blood glucose → β‑cells release insulin → ↑ GLUT4 uptake, glycogen synthesis, lipogenesis. ↓ glucose → α‑cells release glucagon → hepatic glycogenolysis & gluconeogenesis. Calcium: low Ca²⁺ → PTH → bone resorption, renal ↓ phosphate, ↑ 1,25‑OH₂ vitamin D → ↑ intestinal Ca²⁺ absorption. High Ca²⁺ → calcitonin → bone deposition. Sodium: low Na⁺/BP → juxtaglomerular cells release renin → Ang I → ACE → Ang II → vasoconstriction + aldosterone → Na⁺ reabsorption. Potassium: high K⁺ → zona glomerulosa depolarization → aldosterone release → renal K⁺ secretion. Water: ↑ osmolality → hypothalamic ADH release → collecting duct water reabsorption + thirst. ↓ osmolality → ADH suppression → dilute urine. pH (Henderson–Hasselbalch): $$\text{pH}=pKa+\log\frac{[\text{HCO}3^-]}{0.03\,P{CO2}}$$ Normal ratio = 1:20 → pH ≈ 7.4. Renal‑Respiratory Compensation – lungs adjust CO₂ (acute), kidneys adjust HCO₃⁻ (chronic). 🔄 Key Processes Negative‑Feedback Loop (generic) Stimulus → Receptor → Control Center → Effector → Response → ↓ Stimulus → Receptor activity stops. Thermoregulation (cold) ↓ core temp → thermoreceptors → hypothalamic center → sympathetic → vasoconstriction + shivering → heat production. Thermoregulation (heat) ↑ core temp → thermoreceptors → hypothalamus → sympathetic → vasodilation + sweat gland activation → evaporative cooling. Insulin‑Glucagon Glucose Control ↑ glucose → β‑cell → insulin → ↑ GLUT4, glycogen synthase, lipogenesis → ↓ glucose. ↓ glucose → α‑cell → glucagon → hepatic glycogenolysis + gluconeogenesis → ↑ glucose. Renin‑Angiotensin‑Aldosterone System (RAAS) ↓ Na⁺/BP → JGA → renin → Ang I → ACE → Ang II → vasoconstriction + aldosterone → Na⁺ reabsorption, K⁺ excretion. ADH‑Mediated Water Retention ↑ plasma osmolality → osmoreceptors → posterior pituitary → ADH → V2 receptors in collecting duct → aquaporin‑2 insertion → water reabsorption. 🔍 Key Comparisons Insulin vs. Glucagon – insulin ↓ blood glucose; glucagon ↑ blood glucose. PTH vs. Calcitonin – PTH raises plasma Ca²⁺; calcitonin lowers plasma Ca²⁺. Renin‑Angiotensin vs. ADH – RAAS primarily restores Na⁺/volume & BP; ADH primarily restores water balance/osmolality. Peripheral vs. Central Chemoreceptors – peripheral detect O₂ & CO₂ Pa; central detect CSF pH (driven by CO₂). Shivering vs. Non‑shivering Thermogenesis – shivering = rapid muscular contractions; non‑shivering = ↑ metabolic rate (brown fat, thyroid hormones). ⚠️ Common Misunderstandings “Set point is fixed.” – It shifts with circadian rhythms, menstrual cycle, fever, acclimatization. “Insulin always lowers glucose instantly.” – Predictive (anticipatory) insulin release occurs before glucose rise; negative feedback still needed. “ADH only acts on kidneys.” – It also triggers thirst via hypothalamic centers. “Calcitonin is a major regulator of calcium.” – Its effect is modest; PTH dominates calcium homeostasis. “Hyperventilation always improves oxygenation.” – At altitude it helps O₂ but lowers CO₂, causing respiratory alkalosis; renal compensation follows. 🧠 Mental Models / Intuition Thermostat Analogy – The hypothalamus = thermostat; receptors = temperature probes; effectors = heater (shivering) or AC (sweating). Bank Account – Homeostatic variables are “balances.” Negative feedback = deposits/withdrawals that keep the balance near a target; “interest” (allostatic/anticipatory) adds money before a known expense. Push‑Pull Levers – RAAS pushes (vasoconstriction, Na⁺ reabsorption) when volume drops; ADH pulls water back when osmolality rises. 🚩 Exceptions & Edge Cases Fever – Set point is deliberately raised by cytokines → body generates heat (chill) until new point reached. High Altitude – Chronic hypoxia → ↑ EPO → polycythemia; also renal bicarbonate loss → compensatory respiratory alkalosis. Hemochromatosis – Hepcidin deficiency → excess iron absorption despite adequate stores. Type 1 Diabetes – No insulin → unopposed glucagon → severe hyperglycemia & ketoacidosis. 📍 When to Use Which Diagnosing hypercalcemia → check PTH (high → primary hyperparathyroidism; low → malignancy, vitamin D excess). Hyponatremia with low ADH → consider diabetes insipidus (ADH deficiency) vs. excessive water intake. Acute metabolic acidosis → expect respiratory compensation: ↑ ventilation (hyperventilation) to lower PaCO₂. Persistent hypertension → assess RAAS activity (renin, aldosterone levels) before prescribing ACE inhibitors. 👀 Patterns to Recognize “Low → hormone ↑ → opposite effect” – Low Na⁺ → renin ↑ → Ang II ↑ → BP ↑. “High → opposite hormone ↓” – High Ca²⁺ → calcitonin ↑ → bone deposition. “Combined sensor‑effector response” – Any deviation in temperature, glucose, or osmolality triggers a three‑part cascade (sensor → center → effector). 🗂️ Exam Traps Mistaking peripheral chemoreceptor stimulus – they respond to low O₂ and high CO₂, not just O₂. Confusing aldosterone’s primary target – it acts on distal tubule & collecting duct (Na⁺ reabsorption, K⁺ secretion), not the proximal tubule. Assuming ADH acts on all nephron segments – only the collecting duct expresses V2 receptors. Over‑attributing calcium regulation to calcitonin – most long‑term calcium balance is via PTH & vitamin D. Equating hyperventilation with “more O₂” – primary effect is CO₂ washout → pH rise; O₂ increase is secondary and limited by hemoglobin saturation. --- Use this guide to scan quickly before the exam – focus on the triad, feedback loops, and the hormone‑target relationships that drive each variable back to its set point.