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SECTION A (40 Marks)

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Q.1 - Cardiac Cycle (1+2+7 = 10 Marks)

Definition (1 Mark)

Phases of the Cardiac Cycle (2 Marks)

Pressure and Volume Changes with Labelled Diagram (7 Marks)

• P wave on ECG triggers atrial depolarization and contraction
• Left atrial pressure rises; atrial blood is ejected into the left ventricle through the open mitral valve
• Ventricular volume increases slightly (final ventricular filling)
• S4 heart sound (not audible normally)
• Venous "a" wave appears

• QRS complex triggers ventricular depolarization
• Ventricular pressure rises sharply
• Both mitral and aortic valves are closed - volume remains constant (end-diastolic volume ~130 mL)
• S1 heart sound (mitral valve closure)

• Ventricular pressure exceeds aortic pressure (~80 mmHg); aortic valve opens
• Blood rapidly ejected into aorta
• Aortic and ventricular pressures rise to peak (~120 mmHg)
• Ventricular volume falls steeply
• ST segment on ECG

• Ventricular ejection continues but at a slower rate
• Aortic pressure begins to fall as blood runs off to periphery
• T wave on ECG (ventricular repolarization)
• Ventricular volume reaches minimum (end-systolic volume ~50 mL)
• Stroke volume = 130 - 50 = ~80 mL

• Ventricles relax; ventricular pressure falls rapidly
• Aortic valve closes (producing the dicrotic notch on aortic pressure trace)
• S2 heart sound (aortic valve closure)
• Both valves closed - ventricular volume remains constant

• Ventricular pressure falls below atrial pressure; mitral valve opens
• Blood flows passively from atrium to ventricle
• Ventricular volume increases rapidly
• S3 heart sound (in disease states)
• Venous "v" wave

• Filling slows down; ventricle nears end-diastolic volume
• No valves open or close; no heart sounds
• Cycle ends and repeats

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Q.2(a) - Facilitated Diffusion Including Factors Affecting It (6 Marks)

Definition

Mechanism

1. The substance to be transported binds to a specific receptor site on the carrier protein at one side of the membrane
2. The carrier protein undergoes a conformational change (changes shape)
3. The pore opens to the opposite side of the membrane
4. Due to weak binding, thermal motion of the substance causes it to be released on the other side
5. The carrier returns to its original conformation

Factors Affecting Facilitated Diffusion

1. Concentration gradient - Greater the difference in concentration across the membrane, the faster the rate (up to Vmax)
2. Number of carrier proteins - More carriers available = higher Vmax. Insulin increases GLUT4 transporter density in muscle, adipose, and heart (10-20 fold increase)
3. Temperature - Higher temperature increases molecular kinetic energy and rate of diffusion
4. Molecular size - Smaller molecules diffuse faster (up to a limit defined by carrier specificity)
5. Saturation of carriers - When all carriers are occupied, adding more substrate does not increase the rate (Vmax is reached)
6. Competitive inhibition - Structurally similar molecules compete for the same carrier (e.g., galactose and fructose compete with glucose for GLUT transporters)
7. Specificity of carrier - Each carrier is specific for one or a few structurally similar molecules

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Q.2(b) - Genesis of Resting Membrane Potential (6 Marks)

Definition

Ionic Basis

Mechanism - Step by Step

1. K+ dominates the RMP: The cell membrane at rest is far more permeable to K+ and Cl- than to Na+ and Ca2+
2. K+ tends to diffuse outward down its concentration gradient, carrying positive charge out - this makes the interior of the cell negative
3. Na+ wants to diffuse inward but has low permeability, so it contributes little to RMP
4. The RMP therefore lies close to the K+ equilibrium potential (approximately -94 mV, calculated by the Nernst equation)
5. The actual RMP is slightly less negative than E_K+ because of the small inward Na+ leak

The Chord Conductance Equation

Role of Na+-K+-ATPase Pump

1. Direct electrogenic effect: Pumps 3 Na+ out for every 2 K+ in - creates a small additional negativity
2. Indirect (more important) effect: Maintains the K+ concentration gradient across the membrane, which sustains the K+ diffusion potential that drives the RMP toward E_K+

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SECTION B (40 Marks)

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Q.1 - Gastric Juice: Composition, Functions, HCl Secretion Mechanism, Regulation of Gastric Secretion (2+2+2+4 = 10 Marks)

Composition and Functions of Gastric Juice (2+2 Marks)

Mechanism of HCl Secretion (2 Marks)

1. Inside the parietal cell, CO2 + H2O → H2CO3 (catalyzed by carbonic anhydrase) → H+ + HCO3-
2. H+ is actively pumped into the canaliculus (lumen) via H+/K+-ATPase (proton pump) in exchange for K+
3. Cl- is secreted into the canaliculus passively alongside H+ (via Cl- channels), forming HCl
4. HCO3- exits the basolateral side of the cell in exchange for Cl- (via Cl-/HCO3- exchanger) - this creates the "alkaline tide" in portal blood after eating
5. Na+/K+-ATPase on the basolateral membrane maintains intracellular K+ and Na+ gradients

Regulation of Gastric Secretion (4 Marks)

• Triggered by sight, smell, taste, thought of food
• Mediated via the vagus nerve (CN X)
• Vagal stimulation releases acetylcholine (ACh) acting on M3 muscarinic receptors on parietal cells
• Also stimulates G cells to release gastrin
• Result: Acid and pepsinogen secretion begins before food enters stomach

• Triggered when food enters the stomach
• Two mechanisms:
  • Distension of stomach wall stimulates stretch receptors → local (enteric) and vagal reflexes → ACh release → stimulates parietal cells
  • Chemical stimulation by peptides and amino acids → stimulates G cells to release gastrin → gastrin acts on parietal cells (via CCK-B receptors) to stimulate acid secretion
• Gastrin is the dominant hormone of this phase

• Chyme entering the duodenum initially provides a small stimulatory effect (duodenal gastrin)
• As the duodenum becomes acidic and fills with fat/hypertonic chyme, inhibitory signals dominate:
  • Secretin (released by S cells in response to acid in duodenum) - inhibits gastric acid and stimulates pancreatic HCO3-
  • Cholecystokinin (CCK) - released by I cells in response to fat/protein - inhibits gastrin
  • Gastric inhibitory peptide (GIP) - released by fat and glucose in duodenum
  • These form the enterogastric reflex (neural) and hormonal inhibition to slow acid secretion as digestion progresses

• ACh (vagal/local)
• Gastrin
• Histamine (from ECL cells, acts on H2 receptors) - these three act via their own receptors on parietal cells and potentiate each other

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Q.2(a) - Oxygen Dissociation Curve (6 Marks)

Definition

Key Features of the Curve

• The curve is sigmoid (S-shaped) - due to cooperative binding (binding of O2 to one heme group increases O2 affinity of remaining heme groups in the same Hb molecule)
• P50 = the PO2 at which 50% of Hb is saturated = 26.5 mmHg (normal)
• At PO2 = 100 mmHg (arterial blood): Hb ~97-98% saturated
• At PO2 = 40 mmHg (mixed venous blood): Hb ~75% saturated
• At rest, only ~25% of delivered O2 is extracted (large reserve)

Physiological Significance of the Sigmoid Shape

• The flat upper portion (PO2 > 60 mmHg): Hb remains nearly fully saturated even with significant drops in PO2 (protective at altitude or in mild lung disease)
• The steep middle portion (PO2 20-60 mmHg): Small drops in PO2 at tissue level cause large release of O2 - efficient delivery to metabolizing tissues

Shifts of the Curve

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Q.2(b) - Cheyne-Stokes Breathing (6 Marks)

Definition

• Gradual crescendo increase in depth and rate of breathing
• Followed by gradual decrescendo decrease
• Followed by a period of apnea (cessation of breathing for 10-60 seconds)
• The cycle then repeats

Mechanism

• Disruption of neural pathways that normally inhibit respiration leads to overresponse to CO2
• Elevated CO2 causes hyperventilation → CO2 falls below normal
• The resultant hypocapnia inhibits the respiratory centers → apnea
• During apnea, CO2 rises again → respiratory center is again overstimulated
• This oscillation continues cyclically

• Seen in heart failure where cardiac output is low and circulation time is prolonged
• When hyperventilation lowers lung PCO2, it takes longer than normal for this low-CO2 blood to reach the brainstem
• During this delay, the lungs continue to lower CO2 further
• When the low-CO2 blood finally reaches the brain, it inhibits breathing → apnea
• During apnea, PCO2 in pulmonary blood rises; this high-CO2 blood then reaches the brain → drives hyperventilation again
• The negative feedback loop between lungs and brain is abnormally long, causing oscillation

Clinical Associations

• Congestive heart failure (most common)
• Uremia (renal failure)
• Brain damage/stroke (midbrain lesions)
• High altitude in some normal individuals
• During sleep in some normal elderly individuals

Significance

• Indicates serious underlying cardiorespiratory or neurological disease
• Causes irregular blood oxygenation and CO2 levels
• Can produce hypoxemia during apneic phases

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Q.2(c) - Tubuloglomerular Feedback (TGF) Mechanism (6 Marks)

Definition

Anatomical Basis

Mechanism (Step by Step)

1. Detection: When GFR increases, more NaCl is delivered to the distal tubule. The macula densa cells detect elevated NaCl concentration via NKCC2 co-transporters (Na-K-2Cl) in their apical membrane
2. Signal transduction: NaCl uptake by NKCC2 leads to:
   • Depolarization and ionic changes in macula densa cells
   • ATP release from macula densa cells → stepwise hydrolysis by ecto-ATPases → adenosine
3. Effector response: Adenosine acts on A1 adenosine receptors (A1AR) on afferent arteriolar smooth muscle cells → causes afferent arteriolar vasoconstriction via:
   • Inhibitory G-protein (Gi) → phospholipase C activation
   • Release of Ca2+ from intracellular stores
   • Entry of Ca2+ through L-type Ca2+ channels
   • → Vasoconstriction of afferent arteriole
4. Outcome: Afferent arteriolar constriction → reduces glomerular capillary pressure → GFR falls back toward normal

Negative Feedback Summary

Importance

• Maintains GFR and distal NaCl delivery within narrow limits
• Prevents tubular overload
• Furosemide and bumetanide block NKCC2 and abolish TGF

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Q.2(d) - Cystometrogram (6 Marks)

Definition

Three Components/Phases of the Normal Cystometrogram

• As fluid is first introduced, there is a slight initial rise in pressure (elastic property of bladder wall)

• As bladder volume increases from 0 to ~300-400 mL, pressure remains relatively LOW and constant (~10-15 cm H2O)
• This represents receptive relaxation of the detrusor muscle (stress relaxation)
• The bladder accommodates increasing volume with minimal pressure rise - this is high compliance
• First sensation of fullness is felt at ~150-200 mL; urgency at ~400 mL

• At a volume of ~300-400 mL, the voiding reflex is triggered
• A sharp, steep rise in pressure occurs as the detrusor contracts (can reach 40-60 cm H2O)
• If micturition occurs, this pressure spike leads to voiding
• If the external sphincter is voluntarily contracted, pressure may briefly rise further

Clinical Significance

• Overactive bladder/detrusor instability: Involuntary pressure rises (uninhibited contractions) during filling phase
• Hypotonic bladder (neurogenic): Very high compliance, no voiding contraction (seen in sacral nerve lesions, tabes dorsalis)
• Low compliance bladder: Pressure rises steeply with small volumes (seen in fibrosis, radiation cystitis)
• Normal first sensation at ~150 mL; normal voiding reflex at ~400-500 mL

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Q.2(e) - Ventilation-Perfusion (V/Q) Ratio (6 Marks)

Definition

Normal Values

• Normal alveolar ventilation (V̇A) = ~4 L/min
• Normal pulmonary blood flow (Q̇) = ~5 L/min
• Normal overall V̇A/Q̇ ratio = 0.8 (4/5)

Extremes of V/Q Ratio

• Alveolus is perfused but not ventilated (e.g., collapsed alveolus, mucus plug)
• Alveolar gases equilibrate with venous blood (PO2 = 40 mmHg, PCO2 = 45 mmHg)
• Blood passes through without picking up O2 or losing CO2
• This is called a "true shunt" - causes hypoxemia not correctable by supplemental O2

• Alveolus is ventilated but not perfused (e.g., pulmonary embolism)
• Air reaches the alveolus but cannot exchange gas with blood
• This is "alveolar dead space" - wasted ventilation
• Alveolar gas has high PO2 (150 mmHg) and very low PCO2 (approaching inspired air values)

• Alveolar PO2 ≈ 100 mmHg, PCO2 ≈ 40 mmHg
• Efficient gas exchange

Regional V/Q Distribution in the Lung (Upright position)

• Apex: High V/Q (>1) - ventilation exceeds perfusion; approaches alveolar dead space
• Base: Low V/Q (<1) - perfusion exceeds ventilation; approaches shunt
• Both ventilation and perfusion increase from apex to base, but perfusion increases more steeply (due to gravity), so V/Q is lowest at the base

Clinical Significance

• V/Q mismatch is the most common cause of hypoxemia in lung disease (COPD, asthma, pneumonia, pulmonary embolism)
• Unlike diffusion impairment, moderate V/Q mismatch can be partly corrected with supplemental O2 (by increasing alveolar PO2 in low V/Q units)
• Measurement of V/Q ratio using radionuclide scanning (V/Q scan) is used to diagnose pulmonary embolism

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Sources: Costanzo Physiology 7th Ed, Guyton & Hall Medical Physiology, Ganong's Review of Medical Physiology 26th Ed, Brenner & Rector's The Kidney, Fishman's Pulmonary Diseases and Disorders

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SENDUP EXAMINATION - AUG 2025

Physiology Paper I

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SECTION - A

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Q.1 - Erythropoiesis (1 + 2 + 4 + 3 = 10 Marks)

Definition (1 Mark)

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Sites of Erythropoiesis (2 Marks)

• Infants/Children: All bones have active red marrow (sternum, ribs, vertebrae, long bones)
• Adults: Active red marrow limited to flat bones - sternum, ribs, vertebrae, skull, pelvis, and proximal ends of femur and humerus
• In pathological conditions (severe hemolytic anemia, myelofibrosis): liver and spleen can resume erythropoiesis - called extramedullary hematopoiesis

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Stages of Erythropoiesis with Characteristic Features (4 Marks)

Pluripotent Stem Cell (Hemocytoblast)
          ↓
    Committed Progenitor (BFU-E → CFU-E)
          ↓
    Proerythroblast (Rubriblast)
          ↓
    Early Normoblast (Basophilic erythroblast)
          ↓
    Intermediate Normoblast (Polychromatic erythroblast)
          ↓
    Late Normoblast (Orthochromatic erythroblast)
          ↓
    Reticulocyte
          ↓
    Mature Erythrocyte (RBC)

• Cell size decreases
• Nucleus becomes smaller and denser → finally extruded
• Cytoplasm changes from basophilic → polychromatic → acidophilic (pink)
• Hemoglobin content increases progressively
• RNA decreases progressively

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Factors Regulating Erythropoiesis (3 Marks)

• Glycoprotein hormone produced mainly by peritubular interstitial cells of the renal cortex (90%) and liver (10%)
• Stimulated by hypoxia (e.g., high altitude, anemia, chronic lung disease)
• Acts on CFU-E progenitors → promotes proliferation, differentiation, and survival
• Increases Hb synthesis and accelerates reticulocyte release

• Iron - essential for heme synthesis; deficiency causes iron deficiency anemia
• Vitamin B12 (Cobalamin) - required for DNA synthesis; deficiency → megaloblastic anemia
• Folic acid - required for thymidine synthesis for DNA; deficiency → megaloblastic anemia
• Vitamin C - helps iron absorption and protects against oxidative damage
• Protein - globin chain synthesis requires amino acids
• Copper - needed for iron incorporation into heme

• Androgens (testosterone) - stimulate EPO production → explains higher Hb in males
• Thyroid hormones - increase basal metabolic rate and O2 demand → stimulate erythropoiesis
• Growth hormone / IGF-1 - promote early erythroid progenitor development
• Cortisol - mild stimulatory effect

• Transforming Growth Factor-β (TGF-β) - inhibits erythroid progenitor proliferation
• Estrogens - mild inhibitory effect (lower Hb in females)
• Hepcidin - antimicrobial peptide; blocks iron release from stores → inhibits erythropoiesis

• Stromal cells, growth factors, and the extracellular matrix provide the erythroid niche essential for normal erythropoiesis

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Q.2 Short Notes (6 Marks Each)

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Q.2(a) - Baroceptors in Blood Pressure Regulation

• Carotid sinus baroreceptors: Located in the wall of the internal carotid artery just above the carotid bifurcation; afferent signals travel via the Hering's nerve (branch of glossopharyngeal nerve - CN IX)
• Aortic arch baroreceptors: Located in the wall of the aortic arch; afferent signals travel via the aortic depressor nerve (branch of vagus nerve - CN X)

1. Arterial wall stretches → baroreceptors are activated → increased firing rate
2. Afferent impulses travel to the nucleus tractus solitarius (NTS) in the medulla
3. NTS activates the Cardio-inhibitory center (CIC) and inhibits the vasomotor center
4. Increased parasympathetic outflow (vagus) → decreased HR (bradycardia)
5. Decreased sympathetic outflow → vasodilation (decreased peripheral resistance)
6. Result: BP returns to normal (negative feedback)

1. Reduced stretch → decreased baroreceptor firing
2. Vasomotor center is activated; CIC is inhibited
3. Increased sympathetic outflow → tachycardia + vasoconstriction
4. Result: BP rises back to normal

• Respond to both mean pressure and pulse pressure
• Most sensitive in the range of 60-180 mmHg
• Show adaptation with sustained pressure - less effective in chronic hypertension (reset to a higher level)
• Respond more to rapidly changing pressures than sustained ones

• Baroreceptors are the most important short-term regulators of blood pressure
• They buffer acute changes (e.g., on standing up - orthostatic hypotension)
• They do NOT provide long-term BP regulation (kidneys do that via RAAS and pressure natriuresis)

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Q.2(b) - Cardiac Output Measurement - Fick's Principle

O2 consumed by body = Cardiac output × (O2 content of arterial blood - O2 content of venous blood)

• V̇O2 = O2 consumption of the whole body (mL O2/min) - measured by spirometry
• [O2]arterial = O2 content of pulmonary venous / systemic arterial blood (mL O2/mL blood) - sampled from peripheral artery
• [O2]venous = O2 content of mixed venous / pulmonary arterial blood - sampled by right heart catheterization

• O2 consumption = 250 mL O2/min
• Arterial O2 content = 0.20 mL O2/mL blood
• Mixed venous O2 content = 0.15 mL O2/mL blood

• Steady state condition (O2 consumption is constant)
• No intracardiac shunts (pulmonary and systemic cardiac outputs are equal)

• Gold standard for CO measurement
• Can be applied to measure blood flow to individual organs (e.g., renal blood flow using renal O2 or PAH extraction)

• Requires cardiac catheterization (invasive)
• Errors in sampling mixed venous blood if not truly mixed
• Not ideal in rapidly changing hemodynamic states

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Q.2(c) - Facilitated Diffusion and Factors Affecting It

1. Substance binds to specific receptor on carrier protein
2. Carrier undergoes conformational change (shape change)
3. Pore opens to opposite side of membrane
4. Substance released due to weak binding
5. Carrier returns to original state

1. Concentration gradient - Greater gradient → faster rate (up to Vmax)
2. Number/density of carrier proteins - More carriers → higher Vmax (e.g., insulin upregulates GLUT4 density 10-20x in muscle/adipose)
3. Carrier specificity - Each carrier accepts only structurally similar molecules
4. Competitive inhibition - Structurally similar molecules compete for the same carrier binding site
5. Saturation of carriers - Once all carriers occupied, further increase in substrate concentration does NOT increase rate
6. Temperature - Higher temperature → increased molecular kinetic energy → faster diffusion
7. Membrane fluidity - Affects carrier protein mobility within the membrane

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Q.2(d) - Homeostasis

• Temperature (~37°C)
• Blood pH (7.35-7.45)
• Blood glucose (70-100 mg/dL fasting)
• PO2, PCO2
• Osmolality (~285-295 mOsm/kg)
• Blood pressure (~120/80 mmHg)
• Electrolyte concentrations (Na+, K+, Ca2+)

• The response opposes (reverses) the initial disturbance
• Maintains stability around a set point
• Example: When body temperature rises → sweating + vasodilation → heat loss → temperature returns to 37°C
• Example: When blood glucose rises → insulin secreted → glucose uptake → glucose normalizes
• Components: Receptor (sensor) → Afferent pathway → Control center (integrator) → Efferent pathway → Effector

• The response amplifies the initial disturbance
• Used when a process must be driven to completion
• Examples: LH surge causing ovulation; blood clotting cascade; parturition (oxytocin release); nerve action potential (Na+ influx)

• Anticipatory mechanism; responds to a stimulus before the disturbance actually occurs
• Example: Increased salivation and gastric secretion in the cephalic phase before food reaches the stomach

• Disruption = disease state
• All organ systems work together to maintain homeostasis
• Claude Bernard (1857) - first described the concept of milieu interieur (internal environment)

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Q.2(e) - Wallerian Degeneration

• Immediately (0-24 hrs): Disruption of axoplasmic transport; calcium and sodium influx through disrupted axonal membrane
• Day 1-3: Axon begins to swell and fragment; Schmidt-Lanterman clefts in myelin separate
• Day 3 onwards: Schwann cells retract from nodes of Ranvier; Schwann cells and macrophages are recruited and begin to phagocytose myelin and axon debris
• ~1 week: The entire distal axonal process degenerates completely
• 2-3 weeks: The endoneurial tube (Schwann cell tubes) remains - forms Bands of Büngner - a scaffold for future regeneration

• Limited retrograde axon breakdown up to the first node of Ranvier
• Cell body undergoes chromatolysis:
  • Dissolution and dispersal of Nissl substance (rough ER)
  • Eccentric displacement of nucleus toward cell membrane
  • Cell body swells
  • Increased protein synthesis (switch from axon maintenance → regeneration mode)

• Axon sprouts grow from the proximal stump at ~1-3 mm/day
• Schwann cells in Bands of Büngner guide the growing axon to its target
• Eventually remyelination occurs (new myelin is thinner with more, shorter internodes)
• Collateral sprouting from intact neighboring axons can also reinnervate denervated muscle

• Explains why peripheral nerve injuries can recover (unlike CNS injuries)
• EMG shows fibrillation potentials after Wallerian degeneration of motor nerves
• Rate of recovery = ~1 inch (2.5 cm) per month
• Furosemide and bumetanide (and other factors) may modulate recovery

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SECTION - B

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Q.1 - Clinical Case: 40-year-old man with epigastric pain

a) Most Likely Diagnosis (1 Mark)

• Epigastric pain that worsens on empty stomach and improves after eating is classic for duodenal ulcer (food buffers acid)
• NSAID use (for chronic back pain) damages the gastric mucosal barrier → a major risk factor for both gastric and duodenal ulcers
• Bloating and nausea are common dyspeptic symptoms

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b) Mechanism of HCl Secretion (3 Marks)

1. Inside the parietal cell:
   • CO2 + H2O → H2CO3 (catalyzed by carbonic anhydrase)
   • H2CO3 → H+ + HCO3-
2. Apical (luminal) side:
   • H+/K+-ATPase (proton pump) actively pumps H+ into the canalicular lumen in exchange for K+
   • Cl- is simultaneously secreted into the lumen through Cl- channels
   • H+ + Cl- → HCl in the lumen
3. Basolateral side:
   • HCO3- (generated from Step 1) exits the cell in exchange for Cl- via the Cl-/HCO3- antiporter
   • This enriches the blood with HCO3- → causes the "alkaline tide" in portal blood postprandially
   • Na+/K+-ATPase maintains intracellular ion gradients
4. Stimulants of the proton pump activate parietal cells via:
   • Acetylcholine (ACh) via M3 receptors → IP3/Ca2+ pathway
   • Gastrin via CCK-B receptors → IP3/Ca2+ pathway
   • Histamine (from ECL cells) via H2 receptors → cAMP/PKA pathway
   • All three act synergistically - blocking any one (e.g., H2 blocker, PPI) reduces acid output

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c) Protective Mechanisms of Gastric Mucosa (2 Marks)

• Surface mucous cells secrete a 2-3 mm thick gel of mucus (mucin glycoproteins)
• Subepithelial cells secrete HCO3- which is trapped in the mucus layer
• Creates a pH gradient - pH 7 at the cell surface vs pH 1-2 in the lumen
• PGE2 and PGI2 (prostaglandins) stimulate both mucus and HCO3- secretion - this is why NSAIDs (which inhibit COX → reduce prostaglandins) damage the mucosa

• The gastric epithelium has a very rapid turnover (~3-5 days)
• Rapid regeneration replaces damaged cells before ulceration can occur
• EGF (epidermal growth factor) stimulates this renewal

• Epithelial cells are connected by tight junctions that prevent back-diffusion of H+ from lumen into mucosa

• Rich submucosal blood flow delivers O2, nutrients, and HCO3- to maintain epithelial integrity
• Helps clear any acid that penetrates the mucosa

• "Cytoprotective" molecules that:
  • Stimulate mucus and bicarbonate secretion
  • Maintain mucosal blood flow
  • Enhance epithelial proliferation
  • Inhibit acid secretion

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d) Physiological Basis of Treatment of Peptic Ulcer Disease (4 Marks)

• Basis: Irreversibly inhibit the H+/K+-ATPase (proton pump) of parietal cells
• Most potent acid suppressants; reduce gastric pH to >4 for >18 hrs/day
• Promote ulcer healing by eliminating the acid environment
• First-line treatment

• Basis: Competitively block H2 receptors on parietal cells → reduce histamine-stimulated acid secretion
• Less potent than PPIs; effective for healing duodenal ulcers and prophylaxis

• Basis: H. pylori colonizes the gastric antrum, produces urease → NH3 + HCO3- → damages mucosa, disrupts mucosal defense
• H. pylori stimulates gastrin secretion and increases acid production
• Triple therapy: PPI + Amoxicillin + Clarithromycin for 14 days
• Eradication cures ~90% of duodenal ulcers and prevents recurrence

• Basis: Chemically neutralize HCl in the stomach (Al(OH)3 + HCl → AlCl3 + H2O)
• Provide immediate symptom relief; do not promote healing on their own
• Also stimulate mucosal prostaglandin synthesis

• Basis: Synthetic prostaglandin that:
  • Inhibits acid secretion (via Gi-coupled inhibition of adenylyl cyclase)
  • Stimulates mucus and HCO3- secretion
  • Maintains mucosal blood flow
• Used specifically for NSAID-induced ulcers (restores prostaglandins inhibited by NSAIDs)

• Basis: Polymerizes in acidic environment → forms a protective coating over the ulcer base, binding to exposed proteins
• Stimulates prostaglandin and mucus production
• Physical barrier against acid and pepsin

• NSAIDs inhibit COX-1/COX-2 → reduce prostaglandins → impair mucosal defense
• Stopping NSAIDs (or switching to selective COX-2 inhibitors + PPI) is mandatory for NSAID-induced ulcers

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Q.2 Short Notes (6 Marks Each)

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Q.2(a) - Oxygen-Hemoglobin Dissociation Curve

• P50 = 26.5 mmHg (PO2 at 50% Hb saturation)
• Arterial blood (PO2 = 100 mmHg): ~97% saturation
• Mixed venous blood (PO2 = 40 mmHg): ~75% saturation
• At rest, only ~25% of delivered O2 is extracted

• Flat upper portion (>60 mmHg): Hb stays nearly saturated despite significant PO2 fall - protects in lung disease/altitude
• Steep middle portion (20-60 mmHg): Large O2 release with small PO2 drop at tissues - efficient delivery

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Q.2(b) - Nervous Regulation of Respiration

• Primarily active during inspiration
• Receives afferents from peripheral chemoreceptors (via CN IX and X) and stretch receptors
• Drives basic inspiratory rhythm
• Sends impulses to phrenic nerve (diaphragm) and external intercostal muscles

• Contains both inspiratory and expiratory neurons
• Mainly active during forced/increased breathing
• Contains the pre-Bötzinger complex - considered the primary rhythm generator
• Drives accessory inspiratory and expiratory muscles
• The Bötzinger complex contains expiratory neurons that inhibit inspiration

• Sends inhibitory signals to the DRG → terminates inspiration
• Controls the duration and depth of inspiration
• Damage → prolonged inspiratory gasps (apneusis)

• Stimulates the DRG to sustain inspiration
• Normally inhibited by the pneumotaxic center
• If pneumotaxic center is destroyed → apneustic breathing (prolonged gasping inspiration with short expiratory pauses)

• Cerebral cortex: Voluntary control of breathing (speaking, breath-holding, voluntary hyperventilation)
• Hypothalamus: Modulates breathing with temperature and emotional states
• Limbic system: Breathing changes with fear, anxiety

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Q.2(c) - Factors Affecting Glomerular Filtration Rate (GFR)

• Pcap = glomerular capillary hydrostatic pressure (~60 mmHg)
• Pbs = Bowman's space hydrostatic pressure (~18 mmHg)
• πcap = glomerular capillary oncotic pressure (~32 mmHg)
• πbs = Bowman's space oncotic pressure (~0 mmHg)
• Net filtration pressure = 60 - 18 - 32 = ~10 mmHg
• Normal GFR = ~120 mL/min/1.73 m²

1. ↑ Glomerular capillary pressure (Pcap):
   • ↑ Afferent arteriolar dilation (prostaglandins, bradykinin, ANP)
   • ↑ Efferent arteriolar constriction (angiotensin II - maintains GFR when renal perfusion falls)
   • ↑ Systemic blood pressure (within autoregulatory range)
2. ↓ Plasma oncotic pressure (πcap):
   • Hypoproteinemia (e.g., liver disease, nephrotic syndrome before adaptive changes)
   • Leads to ↑ filtration fraction
3. ↓ Bowman's capsule pressure:
   • Ureteral patency (obstruction raises Pbs and decreases GFR)
4. ↑ Kf (filtration coefficient):
   • Increased glomerular surface area or permeability
   • Mesangial cell relaxation (ANP increases Kf)

1. ↓ Renal perfusion pressure:
   • Hypotension, shock, heart failure, dehydration
   • Below the autoregulatory range (<80 mmHg)
2. ↑ Afferent arteriolar resistance:
   • Sympathetic stimulation, catecholamines, angiotensin II → afferent constriction
   • NSAIDs (inhibit prostaglandins that dilate afferent arteriole)
3. ↑ Bowman's capsule pressure:
   • Ureteral obstruction, bladder outlet obstruction (back-pressure)
4. ↑ Plasma oncotic pressure:
   • Hyperproteinemia, severe dehydration
5. ↓ Kf:
   • Glomerulonephritis (reduced filtering surface), mesangial cell contraction (angiotensin II)
6. Hormones/mediators causing vasoconstriction:
   • Angiotensin II (reduces GFR in afferent constriction; but maintains it via efferent)
   • Endothelin, vasopressin (high doses)

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Q.2(d) - Juxtaglomerular Apparatus (JGA)

• A plaque of specialized tall, closely packed epithelial cells in the wall of the thick ascending limb of Henle where it contacts the glomerulus
• Named for their densely packed nuclei ("macula densa" = dense spot)
• Function: Sense NaCl concentration in tubular fluid
• High NaCl → signals to JG cells → adjusts GFR (tubuloglomerular feedback) and renin release
• Contain neuronal NOS and COX-2 (important for signaling)
• Have prominent lateral intercellular spaces that change width with functional state

• Modified smooth muscle cells in the wall of the afferent arteriole (and to lesser extent efferent arteriole)
• Contain cytoplasmic granules storing renin
• Function: Synthesize and secrete renin in response to:
  • ↓ Renal perfusion pressure (stretch receptor mechanism)
  • ↓ NaCl at macula densa (via adenosine/PGE2 signaling)
  • ↑ Sympathetic stimulation (via β1 adrenergic receptors → cAMP → renin release)
  • Low plasma sodium

• Located between the macula densa, afferent, and efferent arterioles
• Heavily branched cells interconnected by gap junctions
• Function: Relay signals between macula densa and JG cells; may modulate GFR by altering glomerular capillary surface area

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Q.2(e) - Professional Qualities and Roles of a Physician

• Thorough, up-to-date knowledge of biomedical sciences and clinical medicine
• Commitment to lifelong learning and continuing medical education (CME)
• Evidence-based practice

• Proficiency in history taking, physical examination, and diagnostic reasoning
• Skill in procedures appropriate to one's specialty
• Sound clinical judgment and decision-making

• Clear, empathetic communication with patients and families
• Ability to break bad news sensitively
• Effective communication with colleagues (oral, written, electronic)
• Active listening

• Respect for patient autonomy and dignity
• Maintaining confidentiality (doctor-patient privilege)
• Honesty and integrity in all professional dealings
• Accountability to patients, society, and the profession
• Punctuality and reliability

• Understanding the patient's suffering, fears, and psychosocial context
• Treating the whole person - not just the disease

• Adherence to the four principles of medical ethics:
  • Beneficence (do good)
  • Non-maleficence (do no harm)
  • Autonomy (respect patient's right to decide)
  • Justice (fair distribution of resources)
• Informed consent for all procedures
• Avoiding conflicts of interest

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Sources: Costanzo Physiology 7th Ed, Guyton & Hall Medical Physiology, Ganong's Review of Medical Physiology 26th Ed, Brenner & Rector's The Kidney, National Kidney Foundation Primer on Kidney Diseases 8e, Bradley & Daroff's Neurology in Clinical Practice, Comprehensive Clinical Nephrology 7th Ed