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Short Answer Physiology Questions - Detailed Answers

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1. State Starling's Law of Force of Contraction (Frank-Starling Law)

• Increased end-diastolic volume stretches the sarcomeres to a more optimal length (approximately 2.0-2.2 µm)
• At this length, actin-myosin cross-bridging is maximized
• This leads to greater myofilament Ca²⁺ sensitivity (myofilament length-dependent activation), producing a stronger contraction
• The heart therefore automatically adjusts its output to match venous return

• The Frank-Starling curve can shift left (enhanced inotropy - e.g., with exercise, adrenergic stimulation, digitalis) or right (decreased inotropy - e.g., heart failure)
• It keeps cardiac output of left and right ventricles balanced

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2. Functions of the Juxtaglomerular Apparatus (JGA)

• Macula densa (specialized DCT cells)
• Juxtaglomerular (granular) cells (in afferent arteriole wall)
• Extraglomerular mesangial (Lacis) cells

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3. Define Osmosis

• Osmotic pressure: The hydrostatic pressure required to stop net osmotic flow; it is proportional to the number of solute particles (osmolarity)
• Formula (van't Hoff): π = nCRT (where n = number of particles, C = concentration, R = gas constant, T = temperature)
• Osmolarity: Normal plasma osmolarity = 285-295 mOsm/L
• Isotonic solution: 0.9% NaCl (normal saline) or 5% dextrose - no net water movement across cell membrane
• Hypotonic solution: Water moves INTO cells → cell swelling/lysis
• Hypertonic solution: Water moves OUT of cells → cell shrinkage/crenation

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4. Name the Neuroglial Cells. Functions of Astrocytes

Neuroglial (Glial) Cells of the CNS:

1. Astrocytes (most abundant)
2. Oligodendrocytes (myelin in CNS)
3. Microglia (CNS immune cells/macrophages)
4. Ependymal cells (line ventricles, produce CSF)

Functions of Astrocytes:

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5. Functions of Angiotensin II

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6. Intrapulmonary Pressure and Its Normal Value

• Intrapleural (intrathoracic) pressure: Normally -5 mmHg at rest (more negative = -8 mmHg during inspiration)
• Transpulmonary pressure = Intrapulmonary pressure - Intrapleural pressure = distending pressure keeping lungs inflated
• At FRC, lung recoil inward is balanced by chest wall recoil outward; intrapulmonary pressure equals atmospheric

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7. Name the Contractile Proteins

Principal Contractile Proteins:

1. Myosin - the thick filament; contains globular heads (S1 fragments) that form cross-bridges with actin; has ATPase activity
2. Actin - the thin filament; two helical chains of G-actin monomers form F-actin; interacts with myosin heads during contraction

Regulatory Proteins (associated with thin filament):

1. Tropomyosin - double-stranded α-helical protein that covers actin-myosin binding sites at rest
2. Troponin complex (three subunits):
   • Troponin T (TnT): binds to tropomyosin
   • Troponin I (TnI): inhibitory subunit; inhibits actin-myosin interaction
   • Troponin C (TnC): binds Ca²⁺; conformational change removes tropomyosin inhibition → contraction begins

Other structural proteins:

1. Titin - noncontractile elastic protein connecting myosin to Z-disc; acts as molecular spring
2. α-Actinin - anchors actin at Z-discs

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8. Define Dead Space. Types of Dead Space

Types of Dead Space:

• The volume of air occupying the conducting airways (nose, pharynx, larynx, trachea, bronchi, bronchioles up to but not including respiratory bronchioles)
• These structures have no alveolar walls and cannot exchange gases
• Approximately 2 mL/kg body weight (or 1 mL per pound of body weight)

• Volume of air in alveoli that are ventilated but not perfused (VA/Q = ∞)
• Examples: pulmonary embolism (alveoli ventilated but capillary flow blocked)
• Normally minimal in healthy individuals

• The total wasted ventilation
• Measured using the Bohr equation: $$\frac{V_{D_{phys}}}{V_T} = \frac{PaCO_2 - P\bar{E}CO_2}{PaCO_2}$$
• Where: V_D = dead space volume, V_T = tidal volume, PaCO₂ = arterial CO₂, PĒCO₂ = mixed expired CO₂
• Normal: Physiological dead space ≈ Anatomical dead space (~150 mL; ~30% of tidal volume of 500 mL)
• In disease (COPD, pulmonary embolism), alveolar dead space increases → physiological dead space >> anatomical dead space

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9. Draw a Labeled Diagram of ECG Waves

         R
         |
    P    |        T
   /\    |       /\
  /  \   |      /  \
-/----\--+--Q--/----\--S----  (isoelectric line)
         |
    PR   QRS    ST   QTc

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10. Define Muscle Tone

• It is not a voluntary contraction - it is a reflex phenomenon
• Not all motor units are active simultaneously - they take turns firing (asynchronous firing), preventing fatigue
• The muscle remains in a state of readiness and maintains posture

• Maintained by the tonic stretch reflex via the gamma motor neuron system
• Gamma motor neurons maintain tension on muscle spindles (intrafusal fibers)
• Muscle spindle afferents (Ia fibers) continuously send signals to alpha motor neurons → maintaining baseline contraction
• The reticular formation, cerebellum, and basal ganglia modulate tone via descending pathways

1. Postural tone - maintains erect posture against gravity
2. Resting tone - baseline muscle stiffness at complete rest
3. Phasic tone - fluctuating tone during movement

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Summary Table