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🫀 PHYSIOLOGY MCQ MASTER GUIDE

Stuart Fox & Ganong | Cardiac, Vascular, Respiratory

Exam Strategy: Numbers, curves, and exceptions are the most tested. Every value below is MCQ-ready.

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SECTION 1: CARDIAC OUTPUT & VENOUS RETURN ⭐⭐⭐ (HIGHEST YIELD)

What is Cardiac Output (CO)?

• Normal CO at rest = 5 L/min
• Normal SV = 70 mL/beat
• Normal HR = 72 beats/min
• Cardiac Index = CO / Body Surface Area = 3.2 L/min/m² (corrects for body size)

Cardiac Reserve

• Maximum CO during heavy exercise = ~20–25 L/min
• Cardiac Reserve = Max CO − Resting CO = ~15–20 L/min above resting

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Frank-Starling Law (THE most tested concept)

"The heart pumps what it receives"

• Preload = end-diastolic volume (EDV) = the stretch on ventricular fibers BEFORE contraction
• More stretch → more cross-bridge overlap → greater force of contraction → greater SV → greater CO
• Works by increasing the sensitivity of troponin C to Ca²⁺ and optimizing actin-myosin overlap
• Normal EDV = ~120 mL; ESV = ~50 mL → SV = 70 mL
• Ejection Fraction (EF) = SV/EDV = 70/120 = ~58% (normal >55%)

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The Cardiac Function Curve vs. Vascular Function Curve

• Intersection = steady-state operating point where CO = Venous Return
• The two curves must intersect - this is where the body operates
• Normal operating point: RAP ≈ 0–2 mmHg, CO ≈ 5 L/min

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How to Shift the Curves (VERY HIGH YIELD)

• ↑ Sympathetic stimulation (inotropic effect)
• Digitalis / catecholamines
• ↓ Afterload

• Heart failure
• ↓ Sympathetic tone
• ↑ Afterload (e.g., hypertension, aortic stenosis)

• ↑ Blood volume (e.g., IV fluids, hypervolemia)
• ↑ Venous tone (venoconstriction - sympathetic activation)
• Exercise (muscle pump, respiratory pump)
• Lying down (increased venous return)

• ↓ Blood volume (hemorrhage, dehydration)
• Vasodilation (venous pooling)

Slope of vascular function curve = determined by arteriolar resistance. Vasodilation → steeper slope. Vasoconstriction → flatter slope.

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Mean Systemic Filling Pressure (MSFP)

• Pressure in systemic circulation when heart stops = ~7 mmHg
• X-intercept of the vascular function curve
• Increases with blood volume ↑ or venous tone ↑
• When RAP = MSFP, venous return = 0

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Factors Affecting Stroke Volume (3 pillars)

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Heart Rate Effects on CO

• ↑ HR → ↑ CO BUT only up to ~160–180 bpm
• Above 180 bpm: diastolic filling time ↓ → EDV ↓ → SV ↓ → CO may fall
• Diastasis (last phase of diastolic filling) is first eliminated at high heart rates

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SECTION 2: BLOOD VOLUME & VASCULAR DYNAMICS ⭐⭐

Normal Blood Volume

• Total blood volume = 5 L (70 mL/kg)
• Plasma = ~3 L (60%); Formed elements (RBCs etc.) = ~2 L (40%)
• Hematocrit (Hct) = % of blood that is RBCs = 42% females, 45% males

Distribution of Blood Volume

MCQ KEY: Most blood is in the VEINS, not the arteries. Veins = "capacitance vessels" = blood reservoirs.

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Starling Forces at the Capillary (Fluid Exchange)

• Net at arterial end: Filtration (fluid moves OUT)
• Net at venous end: Absorption (fluid moves IN)
• Small net excess is drained by lymphatics (~2–4 L/day)

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Edema Formation (tested frequently)

1. ↑ Capillary hydrostatic pressure (heart failure, venous obstruction)
2. ↓ Plasma oncotic pressure (hypoalbuminemia - nephrotic syndrome, liver failure, malnutrition)
3. ↑ Capillary permeability (inflammation, burns, anaphylaxis)
4. Lymphatic obstruction (filariasis - elephantiasis)

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Compliance & Capacitance

• Compliance (C) = ΔV / ΔP (change in volume per change in pressure)
• Veins are ~20× more compliant than arteries → function as volume reservoirs
• Arteries function as pressure reservoirs (Windkessel effect) - maintain pressure during diastole
• Pulse pressure = Systolic BP - Diastolic BP = ~40 mmHg normally

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SECTION 3: BLOOD PRESSURE & REGULATION ⭐⭐⭐

Mean Arterial Pressure (MAP)

MAP = Diastolic BP + 1/3 Pulse Pressure OR: MAP = CO × Total Peripheral Resistance (TPR)

• Normal MAP = 93 mmHg (≈ 80 + 1/3 × 40 = 93)
• Normal BP = 120/80 mmHg
• MAP is controlled by changing: CO and/or TPR

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Short-Term Regulation of BP (Seconds-to-Minutes)

1. Baroreceptor Reflex (MOST IMPORTANT)

• Location: Carotid sinus (CN IX) + Aortic arch (CN X)
• Most sensitive at normal BP range (60–180 mmHg)
• ↑ BP → Stretches baroreceptors → ↑ Afferent firing → Nucleus tractus solitarius (NTS) → ↑ Parasympathetic / ↓ Sympathetic → ↓ HR, ↓ contractility, ↓ vasoconstriction → ↓ BP
• Rapid response - within seconds
• Resets during sustained hypertension (adapts over days) - does NOT provide long-term BP control

2. Chemoreceptors

• Peripheral chemoreceptors: Carotid bodies (main) + aortic bodies → respond to ↓ PO₂, ↑ PCO₂, ↓ pH
• Central chemoreceptors: Medulla → respond mainly to ↑ PCO₂ / ↓ pH of CSF
• Hypoxia → peripheral chemoreceptor activation → vasoconstriction + ↑ HR

3. CNS Ischemic Response (Cushing Reflex)

• When cerebral perfusion ↓ severely → medullary ischemia → massive sympathetic discharge → extreme hypertension
• Seen in raised intracranial pressure (ICP)
• Cushing Triad: Hypertension + Bradycardia + Irregular breathing

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Medium-Term Regulation (Minutes to Hours)

• Renin-Angiotensin-Aldosterone System (RAAS)
• Vasopressin (ADH)
• Capillary fluid shift

Long-Term Regulation (Days to Weeks)

• Kidney - pressure natriuresis (most important long-term regulator)
• ↑ BP → ↑ urinary Na⁺ and water loss → ↓ blood volume → ↓ CO → ↓ BP
• Aldosterone: ↑ Na⁺ reabsorption → ↑ volume → ↑ BP
• ANP/BNP: ↑ Na⁺ excretion → ↓ volume → ↓ BP

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Vasomotor Center

• Located in medulla oblongata (reticular formation)
• Vasoconstrictor area (C1 area) → tonic sympathetic discharge → maintains resting vascular tone
• Vasodepressor area → inhibits vasoconstrictor area
• Influenced by higher centers: hypothalamus, cortex (blushing, fainting from emotion)

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SECTION 4: REGIONAL BLOOD FLOW ⭐⭐

Coronary Circulation

• Left coronary fills during DIASTOLE (systolic contraction compresses vessels)
• Right coronary fills in both systole and diastole
• Adenosine is the primary metabolic vasodilator for coronary vessels
• Other vasodilators: hypoxia, ↑ CO₂, ↑ K⁺, ↑ H⁺
• Autoregulation maintains coronary flow between MAP 60–140 mmHg

MCQ KEY: Coronary flow to the left ventricle occurs mainly in DIASTOLE. During systole, the contracting myocardium compresses intramural vessels.

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Cerebral Circulation

• ↑ CO₂ (hypercapnia) → most potent cerebral vasodilator
• ↓ O₂ (hypoxia) → also causes vasodilation
• Autoregulation: constant flow despite changes in MAP (myogenic mechanism)
• Blood-Brain Barrier (BBB) maintained by tight junctions of endothelium + astrocyte foot processes
• Disrupted BBB in: infection, trauma, tumors, uremia

MCQ KEY: CO₂ (not O₂) is the primary regulator of cerebral blood flow. Hyperventilation → ↓ PCO₂ → cerebral vasoconstriction → dizziness/syncope.

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Skeletal Muscle Blood Flow

• At rest: sympathetic adrenergic tone dominates (vasoconstriction via α₁ receptors)
• During exercise: local metabolic factors override sympathetics → massive vasodilation
• Metabolic vasodilators: ↓ O₂, ↑ CO₂, ↑ K⁺, ↑ H⁺, ↑ lactate, ↑ adenosine, ↑ osmolality
• Also: sympathetic cholinergic vasodilator fibers (anticipatory vasodilation before exercise begins - mainly in animals, debated in humans)
• Exercise → muscle pump → ↑ venous return → ↑ CO (Frank-Starling)

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Skin Blood Flow

• Primary purpose: thermoregulation (not metabolic needs)
• Regulated by hypothalamus via sympathetic nervous system
• ↑ Body temperature → sympathetic withdrawal → vasodilation → ↑ skin blood flow → heat loss
• Also has unique arteriovenous anastomoses (AVAs) in fingers, toes, nose, lips - shunt blood directly from arteries to veins to conserve heat
• ↑ Temperature → vasodilation + sweating
• ↓ Temperature → vasoconstriction + AV shunt closure

MCQ KEY: Skin circulation is neurogenic (controlled by sympathetics), NOT primarily by local metabolic factors. Unique because it serves thermoregulation, not tissue metabolism.

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SECTION 5: RESPIRATORY SYSTEM & LUNG VOLUMES ⭐⭐⭐

Lung Volumes (ALL NUMBERS ARE EXAM GOLD)

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Critical MCQ Points on Lung Volumes

RV, FRC, TLC - because RV cannot be exhaled. Any capacity containing RV cannot be spirometrically measured.

TV, IRV, ERV, VC, IC

1. Helium dilution method - closed circuit; helium is insoluble in blood, equilibrates with FRC
2. Body plethysmograph (Boyle's Law method) - measures ALL gas including trapped gas; most accurate, especially in obstructive disease

Because plethysmograph measures trapped gas in bullae that helium cannot reach.

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Obstructive vs. Restrictive Pattern

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Dead Space

• Anatomical dead space = conducting airways that don't participate in gas exchange = ~150 mL
• Alveolar dead space = alveoli ventilated but not perfused
• Physiological dead space = anatomical + alveolar dead space = measured by Bohr equation

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Mechanics of Breathing

• Inspiration = ACTIVE (diaphragm + external intercostals contract)
• Quiet expiration = PASSIVE (elastic recoil)
• Forced expiration = active (internal intercostals + abdominals)
• Surfactant (dipalmitoylphosphatidylcholine / DPPC): reduces alveolar surface tension, prevents atelectasis, produced by type II pneumocytes

• Smaller alveoli would collapse into larger ones WITHOUT surfactant
• Surfactant lowers T more in small alveoli → stabilizes them

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Ventilation-Perfusion (V/Q) Matching

• V/Q = 0 (perfusion without ventilation) = shunt → blood bypasses gas exchange → hypoxemia NOT correctable with O₂
• V/Q = ∞ (ventilation without perfusion) = dead space → wasted ventilation
• Hypoxic pulmonary vasoconstriction (HPV): ↓ local PO₂ → vasoconstriction → diverts blood away from poorly ventilated areas → optimizes V/Q

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SECTION 6: TYPES OF BLOOD VESSELS & LYMPHATIC SYSTEM ⭐⭐

Classification of Blood Vessels

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Capillary Types

MCQ KEY: The liver has sinusoidal capillaries with large gaps → allows proteins, lipoproteins to cross.

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Microcirculation & Precapillary Sphincters

• Precapillary sphincters: rings of smooth muscle at capillary entry; regulate capillary perfusion
• Vasomotion: intermittent opening/closing of sphincters based on local metabolic needs
• Controlled by local tissue O₂, CO₂, and metabolites (NOT neural control primarily)

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Lymphatic System

• Filariasis (Wuchereria bancrofti) → elephantiasis
• Surgical removal (mastectomy → arm lymphedema)
• Cancer metastasis
• Congenital (Milroy disease)

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⚡ RAPID-FIRE MCQ FACTS (Last-Minute Review)

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🎯 TOP 10 MCQ TRAPS TO AVOID

1. Baroreceptors do NOT provide long-term BP control - they reset with sustained hypertension
2. Veins hold most blood (~64%) - not arteries
3. Left coronary fills in DIASTOLE - systolic compression blocks it
4. CO₂ not O₂ is the main cerebral vasodilator - PCO₂ most potent
5. RV cannot be measured by spirometry - plethysmography or helium dilution required
6. FEV₁/FVC is NORMAL or HIGH in restriction - only low in obstruction
7. Skin blood flow is neurogenic - thermoregulation, not metabolic demand
8. V/Q shunt (=0) hypoxemia NOT fixed by O₂ - blood bypasses lungs entirely
9. Frank-Starling works only within limits - over-stretch → heart failure
10. Pulse pressure = SBP - DBP - widens in aortic regurgitation, narrows in cardiac tamponade

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