Homeostasis

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Homeostasis — Key Facts for MDCAT

Definition: Homeostasis (Greek: homoios = similar, stasis = standing) is the mechanism by which living organisms maintain a stable internal environment despite changes in the external environment. It is a fundamental characteristic of all living systems.

Why It Matters: Every organ system in the body operates within narrow ranges. Even small deviations (e.g., body temperature rising from 37°C to 42°C) can be fatal. Homeostasis keeps us alive.

Core Concept — Internal Environment:

• Claude Bernard (1857): The internal environment is the extracellular fluid (ECF) bathing all cells
• Cells depend on this internal fluid for nutrients, oxygen, and waste removal
• External environment surrounds the organism; internal environment surrounds cells

The Big Picture: $$\text{External Environment} \xrightarrow{\text{Organ systems}} \text{Internal Environment} \xrightarrow{\text{Cells}} \text{Life}$$

Negative Feedback Loop (most common):

1. Stimulus: A change away from the set point (e.g., blood glucose rises after eating)
2. Receptor: Detects the change (e.g., pancreatic beta cells detect high glucose)
3. Control Center: Processes the information (e.g., brain/spinal cord or hormonal glands)
4. Effector: Produces a response that opposes the change (e.g., insulin release → cells absorb glucose)
5. Response: Variable returns toward set point

⚡ MDCAT Tip: In negative feedback, the response OPPOSES the stimulus. This is the most important distinction — remember it for matching questions!

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Homeostasis — Detailed Study Guide

Types of Feedback Mechanisms

1. Negative Feedback (self-correcting)

Example	Stimulus	Receptor	Effector	Response
Thermoregulation	Body temp rises >37°C	Skin thermoreceptors	Sweat glands, skin arterioles	Cooling: vasodilation, sweating
Blood glucose	Glucose rises after meal	Pancreatic beta cells	Insulin-secreting cells	Glucose enters cells, glycogen formed
Blood pressure	BP rises above normal	Baroreceptors in carotid	Heart rate, arteriole diameter	Cardiac output decreases, vasodilation

2. Positive Feedback (amplifying)

Example	Stimulus	Cascade	Outcome
Childbirth	Oxytocin release	Uterine contractions stretch cervix	More oxytocin released → stronger contractions
Blood clotting	Platelet activation	Thromboplastin release	More platelets aggregate → clot forms
Ovulation	LH surge	LH peak	Surge triggers ovulation

⚡ MDCAT Trap Question: “Which of the following is an example of positive feedback?” Answer options may include childbirth and blood clotting. Both are valid. The key is: response AMPLIFIES the stimulus (not opposes it).

Key Organ Systems in Homeostasis

1. Thermoregulation (Temperature Regulation)

Mechanism	Hot Conditions	Cold Conditions
Skin arterioles	Vasodilation → more blood → heat loss	Vasoconstriction → less blood → heat conserved
Sweat glands	Active → evaporation cooling	Inactive
Hair (mammals)	Flat → no trapped air	Erect → trapped air insulates
shivering	Not activated	Shivering Thermogenesis — rapid muscle contractions generate heat
Brown fat (infants)	Not active	Non-shivering thermogenesis — BAT generates heat via uncoupling proteins

Set point: Normal human body temperature = 37°C (98.6°F)

• Hypothalamus in the brain acts as the thermostat
• Hypothalamus receives input from peripheral thermoreceptors
• Commands are sent via autonomic nervous system to effectors

2. Osmoregulation (Water-Salt Balance)

Fluid Compartment	Location	Approximate Volume
Intracellular fluid (ICF)	Inside cells	~25 L (40% body water)
Extracellular fluid (ECF)	Outside cells	~15 L (20% body water)
Plasma	Inside blood vessels	~3 L
Interstitial fluid	Between cells	~12 L

Osmolality: Normal blood osmolality = 285–295 mOsm/kg

• Osmoreceptors in hypothalamus detect [H₂O] changes
• ADH (Antidiuretic Hormone / Vasopressin) from posterior pituitary
• High [H₂O] → ADH release → kidney tubules reabsorb water → dilute urine
• Low [H₂O] → ADH withheld → lots of dilute urine (diuresis)

3. Blood Glucose Regulation

State	Insulin	Glucagon	Blood [Glucose]
Fed (high glucose)	Released (beta cells)	Inhibited	Falls toward normal
Fasted (low glucose)	Inhibited	Released (alpha cells)	Rises toward normal

Normal fasting blood glucose: 70–100 mg/dL Diabetes mellitus: Chronic hyperglycemia

• Type 1: Autoimmune destruction of beta cells → no insulin
• Type 2: Insulin resistance → insulin present but cells unresponsive
• Gestational: During pregnancy, insulin resistance

⚡ MDCAT PYQ (2019): “The thermostat of the human body is located in the: (a) Cerebrum (b) Hypothalamus (c) Cerebellum (d) Medulla oblongata” → Answer: (b) Hypothalamus

Kidney as Homeostatic Organ

The kidney regulates:

• Water balance (via ADH-controlled reabsorption)
• Electrolyte balance (Na⁺, K⁺, Cl⁻, Ca²⁺, phosphate)
• Acid-base balance (HCO₃⁻ buffer system, H⁺ excretion)
• Blood pressure (via renin-angiotensin-aldosterone system)
• RBC production (via erythropoietin)
• Vitamin D activation (calcium homeostasis)

Nephron Function Summary:

1. Glomerular filtration — non-selective, based on size
2. Tubular reabsorption — selective, reclaiming useful substances (glucose, amino acids, water, Na⁺)
3. Tubular secretion — active removal of wastes (H⁺, K⁺, drugs)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Homeostasis — Complete Notes for MDCAT

Detailed Feedback Loops

Thermoregulation — Hypothalamic Set Point

The preoptic area of the hypothalamus contains temperature-sensitive neurons:

• Warm-sensitive neurons: Fire more when temperature rises
• Cold-sensitive neurons: Fire more when temperature falls

Heat GAIN mechanisms (cold stress):

1. Vasoconstriction: Blood diverted from skin → less heat loss
2. Piloerection: Hair erected (in animals) — less effective in humans
3. Shivering: Involuntary muscle contractions (40% efficiency in generating heat)
4. Non-shivering thermogenesis: Brown adipose tissue, thyroid hormone (T3/T4)
5. Behavioral: Seeking warmth, curling up, clothing

Heat LOSS mechanisms (heat stress):

1. Vasodilation: Skin arterioles dilate → 7× more blood flow to skin → heat radiates
2. Sweating: Eccrine glands secrete sweat → evaporation (540 cal/g H₂O)
3. Behavioral: Seeking shade, removing clothing, swimming

Blood Pressure Regulation

Baroreceptor reflex (short-term):

• High BP → carotid sinus stretch → vagus nerve (X) → ↓heart rate → ↓BP
• Low BP → ↓carotid stretch →sympathetic activation → ↑heart rate + vasoconstriction → ↑BP

Renin-Angiotensin-Aldosterone System (long-term):

Low BP → Juxtaglomerular cells release renin
         ↓
Renin converts Angiotensinogen → Angiotensin I (lung capillary ACE)
         ↓
Angiotensin I → Angiotensin II (potent vasoconstrictor)
         ↓
Angiotensin II stimulates aldosterone (adrenal cortex)
         ↓
Aldosterone → Na⁺ reabsorption in kidney distal tubule
         ↓
Water follows Na⁺ → ↑blood volume → ↑BP

ACE Inhibitors (e.g., enalapril, lisinopril) block this pathway → used in hypertension treatment

Homeostasis Failure — Disease States

Disorder	Failure	Symptom	Compensation
Diabetes insipidus	No ADH production	Massive dilute urine (20L/day)	Replace ADH (desmopressin)
Addison’s disease	No aldosterone	Na⁺ loss, K⁺ retention, hypotension	Replace aldosterone (fludrocortisone)
Cushing’s syndrome	Excess cortisol	Hyperglycemia, hypertension, edema	Reduce cortisol production
Hypothyroidism	Low T3/T4	Low metabolic rate, cold intolerance	Replace thyroid hormone
Hyperthyroidism	High T3/T4	High metabolic rate, heat intolerance, weight loss	Antithyroid drugs, radioiodine

Counter-Current Multiplier System (Kidney)

The loop of Henle creates a concentration gradient in the medulla using counter-current multiplication:

• Descending limb: Permeable to water → water exits → filtrate becomes hypertonic
• Ascending limb: Impermeable to water, actively pumps Na⁺/K⁺/Cl⁻ OUT → filtrate becomes hypotonic
• ** Osmotic gradient**: 300 mOsm/L (cortex) → 1200 mOsm/L (inner medulla)
• ADH effect: Increases collecting duct permeability to water → water exits into hypertonic medulla → concentrated urine produced

Maximum urine concentration (with full ADH): up to 1200 mOsm/L

Comparative Homeostasis

Organism	Challenge	Mechanism
Desert Kangaroo Rat	Water conservation	Very concentrated urine (3000 mOsm/L), metabolic water from seeds
Marine fish	Water loss by osmosis	Drink seawater, excrete concentrated salts via gills
Freshwater fish	Water flooding in	Produce large volumes dilute urine, actively uptake Na⁺
Camel	Heat + dehydration	Allows body temp to rise 6°C during day, nocturnal feeding

⚡ MDCAT Strategy: Homeostasis questions often ask for the INTEGRATION of multiple systems. For example: “How does the kidney respond to dehydration?” — answer involves ADH, RAAS, and renal physiology.

⚡ MDCAT PYQ (2021): “Which part of the brain acts as the thermostat? (a) Thalamus (b) Hypothalamus (c) Medulla (d) Cerebrum” → Answer: (b) Hypothalamus

⚡ MDCAT PYQ (2020): “Positive feedback is seen in: (a) Body temperature regulation (b) Blood clotting (c) Heart rate regulation (d) Blood pressure regulation” → Answer: (b) Blood clotting

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