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Respiratory System Physiology - 1st Year BHMS Complete Notes

(Exam-Oriented | Based on Costanzo Physiology & Ganong's Review)

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UNIT 1: OVERVIEW OF THE RESPIRATORY SYSTEM

• Upper airway (nose, mouth, pharynx, larynx)
• Conducting airways (trachea, bronchi, bronchioles)
• Gas-exchanging unit (alveoli)
• Chest wall and respiratory muscles
• Brainstem control centers
• Pulmonary circulation

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UNIT 2: LUNG VOLUMES AND CAPACITIES ⭐ (Most Important for Exams)

Static Lung Volumes (Measured by Spirometry)

Lung Capacities (Each = sum of 2 or more volumes)

Exam Tip: RV cannot be measured by spirometry. Capacities that include RV (FRC, TLC) also cannot be measured by spirometry. FRC is measured by helium dilution or body plethysmograph.

• Larger body size
• Male gender
• Physical conditioning

• Increasing age

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UNIT 3: MECHANICS OF BREATHING

Pressures in the Respiratory System

Sequence of Normal Inspiration

1. Diaphragm and external intercostals contract
2. Thoracic cavity volume increases
3. Intrapleural pressure becomes more negative (from -5 to -8 cmH₂O)
4. Transpulmonary pressure increases
5. Lungs expand, alveolar pressure falls below atmospheric
6. Air flows INTO lungs (down pressure gradient)

Expiration (Passive at rest)

• Diaphragm relaxes
• Lungs recoil (elastic recoil)
• Alveolar pressure rises above atmospheric
• Air flows OUT of lungs

Forced expiration uses internal intercostals and abdominal muscles.

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UNIT 4: COMPLIANCE AND SURFACTANT ⭐

Lung Compliance

• Compliance (C) = ΔVolume / ΔPressure
• High compliance = lungs expand easily (e.g., emphysema - lungs too compliant)
• Low compliance = stiff lungs, require more pressure to expand (e.g., fibrosis, neonatal RDS)

Surfactant ⭐⭐

• Produced by Type II pneumocytes (alveolar cells)
• Main component: Dipalmitoyl phosphatidylcholine (DPPC)
• Function: Reduces surface tension in alveoli, prevents alveolar collapse (atelectasis)
• Works on the principle: small molecules attract each other via surface tension; surfactant's amphipathic DPPC molecules break these forces

• Small alveoli have HIGH collapsing pressure without surfactant
• Surfactant reduces T, so collapsing pressure stays manageable

• Seen in premature infants (< 35 weeks gestation)
• Surfactant synthesis begins at week 24, almost always present by week 35
• Without surfactant: increased surface tension → alveolar collapse → atelectasis → hypoxemia
• Decreased lung compliance → increased work of breathing
• Treatment: Exogenous surfactant administration

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UNIT 5: AIRWAY RESISTANCE AND AIRFLOW

Air Flow Equation (Analogous to Ohm's Law)

• Q = air flow (L/min)
• ΔP = pressure gradient (cmH₂O or mmHg)
• R = airway resistance

No pressure difference = No airflow. Between breaths, Palv = Patm → no flow.

Factors Affecting Airway Resistance

• Radius is the most important factor: R ∝ 1/r⁴ (Poiseuille's Law)
• Bronchoconstriction → increased resistance (e.g., asthma)
• Bronchodilation → decreased resistance
• Parasympathetic stimulation → bronchoconstriction (ACh, M receptors)
• Sympathetic stimulation → bronchodilation (epinephrine, β₂ receptors)
• Lung volume: at high lung volume, airways are pulled open → lower resistance

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UNIT 6: DEAD SPACE ⭐

Only alveolar ventilation participates in gas exchange, NOT dead space ventilation.

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UNIT 7: GAS EXCHANGE - DIFFUSION ⭐

Dalton's Law of Partial Pressures

Normal Partial Pressures

Fick's Law of Diffusion

• A = cross-sectional area of membrane
• D = diffusion coefficient (solubility / √molecular weight)
• ΔP = partial pressure difference
• T = membrane thickness

In normal lungs, O₂ diffusion is perfusion-limited (not diffusion-limited). CO₂ is always perfusion-limited.

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UNIT 8: OXYGEN TRANSPORT IN BLOOD ⭐⭐

Two Forms of O₂ in Blood

1. Dissolved O₂ (~2%): PaO₂ × 0.003 = 0.3 mL O₂/100 mL blood (at PaO₂ = 100 mmHg)
   • Insufficient alone to meet tissue demand (only delivers 15 mL/min vs need of 250 mL/min)
2. O₂ bound to Hemoglobin (~98%)

Hemoglobin Structure

• 4 subunits, each with a heme (iron-binding porphyrin) + polypeptide chain
• Adult Hb (HbA) = α₂β₂
• Each subunit binds 1 O₂ molecule (total 4 O₂ per Hb)
• Iron must be in ferrous state (Fe²⁺) to bind O₂

Hemoglobin Variants

O₂-Hemoglobin Dissociation Curve ⭐⭐

• ↑ Temperature
• ↑ PCO₂ (Bohr effect)
• ↑ Acidity (↓ pH)
• ↑ 2,3-DPG
• Memory: "CADET, face RIGHT" - CO₂, Acid, 2,3-DPG, Exercise, Temperature

• ↓ Temperature
• ↓ PCO₂
• ↑ pH (alkalosis)
• ↓ 2,3-DPG
• Fetal Hb (HbF)
• Carbon monoxide (CO)

Bohr Effect: CO₂ produced in tissues lowers pH → shifts curve right → O₂ unloaded at tissues. In lungs, CO₂ is expelled → pH rises → curve shifts left → O₂ loaded onto Hb.

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UNIT 9: CO₂ TRANSPORT IN BLOOD ⭐

• HCO₃⁻ leaves RBC → plasma (chloride shift: Cl⁻ enters RBC)

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UNIT 10: VENTILATION-PERFUSION RATIO (V/Q) ⭐⭐

• Apex of lung: V/Q > 0.8 (relatively over-ventilated, under-perfused)
• Base of lung: V/Q < 0.8 (relatively under-ventilated, over-perfused)

V/Q Extremes

Hypoxic Vasoconstriction

• Low alveolar PO₂ → pulmonary arteriolar constriction → diverts blood away from poorly ventilated alveoli
• Maintains V/Q matching
• At high altitude: widespread hypoxic vasoconstriction → pulmonary hypertension → right ventricular hypertrophy

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UNIT 11: THREE ZONES OF THE LUNG ⭐

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UNIT 12: CONTROL OF BREATHING ⭐⭐

Components of Respiratory Control

1. Chemoreceptors (for O₂, CO₂, H⁺)
2. Mechanoreceptors (in lungs and joints)
3. Brain stem control centers (medulla and pons)
4. Respiratory muscles

Brainstem Centers

Cortex can override brainstem voluntarily (breath-holding, voluntary hyperventilation).

Chemoreceptors ⭐⭐

• Located in medulla (ventral surface)
• Respond to: CO₂ and H⁺ (primarily CO₂)
• CO₂ crosses blood-brain barrier → combines with H₂O → H⁺ → stimulates receptors
• Most important for minute-to-minute regulation of breathing
• Do NOT respond directly to O₂

• Located in carotid bodies (CN IX - glossopharyngeal) and aortic bodies (CN X - vagus)
• Respond to: ↓ PO₂, ↑ PCO₂, ↑ H⁺, ↓ arterial pressure
• Carotid bodies are more important in humans
• Only receptors that respond to hypoxemia (↓ PO₂)
• Stimulated by ↓ PaO₂ below 60 mmHg

Response to CO₂

• Most potent stimulus for breathing
• ↑ PaCO₂ → ↑ ventilation (via central chemoreceptors mainly)
• ↓ PaCO₂ (hypocapnia) → ↓ ventilation (apnea)

Response to O₂

• ↓ PaO₂ (below 60 mmHg) → stimulates peripheral chemoreceptors → ↑ ventilation
• Normal PaO₂ = 100 mmHg (not a strong drive at this level)

Hering-Breuer Reflex

• Stretch receptors in lungs (via vagus nerve)
• Lung inflation → inhibits inspiration (prevents over-inflation)
• Acts as a protective mechanism

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UNIT 13: PULMONARY CIRCULATION

• Low-pressure, low-resistance system (normal pulmonary artery pressure: 25/8 mmHg)
• Mean pulmonary artery pressure: ~15 mmHg (vs systemic ~100 mmHg)
• Hypoxic vasoconstriction = unique feature (opposite of systemic circulation)
  • Low PO₂ → vasoconstriction (to divert blood from poorly ventilated areas)

• Highest at base, lowest at apex
• Supine: nearly uniform distribution

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UNIT 14: HYPOXIA VS. HYPOXEMIA ⭐

Types of Hypoxia

Causes of Hypoxemia (↓ PaO₂)

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UNIT 15: ACID-BASE AND RESPIRATORY SYSTEM ⭐

Henderson-Hasselbalch Equation:

Respiratory Acid-Base Disorders

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UNIT 16: NON-RESPIRATORY FUNCTIONS OF THE LUNGS

1. Filtration of blood clots and emboli
2. Metabolism of vasoactive substances:
   • Angiotensin I → Angiotensin II (by ACE in lung capillaries)
   • Inactivation of bradykinin, serotonin, norepinephrine
   • Prostaglandins (E, F) inactivated
   • Prostacyclin (vasodilator) synthesized
   • Thromboxane A₂ (vasoconstrictor) synthesized
3. Reservoir for blood (acts as blood reservoir for left heart)
4. Vocalization (speech)
5. pH regulation (adjusting ventilation changes PaCO₂ → adjusts pH rapidly)

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IMPORTANT NORMAL VALUES QUICK TABLE ⭐⭐

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LIKELY EXAM QUESTIONS (Short/Long Answers)

1. Define and differentiate lung volumes and capacities with normal values.
2. What is surfactant? Name its composition and clinical importance.
3. Describe the O₂-Hb dissociation curve with factors causing shifts.
4. What is the Bohr effect? What is the Haldane effect?
5. Enumerate the functions of the carotid body.
6. What is FRC? How is it measured?
7. Describe Fick's law of diffusion as applied to lungs.
8. What is hypoxic vasoconstriction? What is its significance?
9. Differentiate between anatomical and physiological dead space.
10. What is the alveolar gas equation?

1. Describe the mechanics of breathing with reference to pressures involved.
2. Explain the regulation/control of respiration.
3. Discuss the transport of O₂ and CO₂ in blood.
4. What is V/Q ratio? Describe its regional variations and clinical significance.
5. Classify and explain causes of hypoxia.

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🫁 RESPIRATORY SYSTEM PHYSIOLOGY

1st Year BHMS - Complete Notes

In the Style of GK Pal | Point-wise & Topic-wise | Simple & Easy to Understand

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TOPIC 1: INTRODUCTION TO RESPIRATORY SYSTEM

• Respiration = the process of taking in O₂ and removing CO₂ from the body
• The lungs are the main organ of respiration

1. External respiration - exchange of O₂ and CO₂ between lungs and blood
2. Internal/Tissue respiration - exchange of O₂ and CO₂ between blood and tissue cells
3. Regulation of blood pH - by controlling CO₂ levels
4. Vocalization - speech, voice production
5. Defence - filtering dust, microbes from inspired air
6. Metabolism of vasoactive substances - e.g., converting Angiotensin I → Angiotensin II

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TOPIC 2: FUNCTIONAL ANATOMY OF THE RESPIRATORY SYSTEM

• Nose, nasal cavity, pharynx, larynx

• Trachea → 2 main bronchi → lobar bronchi → segmental bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli (site of gas exchange)

• Total number of alveoli in adult lungs = 300-500 million
• Total surface area of alveoli = 70 m² (size of a tennis court)
• Alveolar wall has Type I pneumocytes (flat, for diffusion) and Type II pneumocytes (cuboidal, produce surfactant)
• The alveolar-capillary membrane thickness = 0.5 micrometers (very thin for easy diffusion)

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TOPIC 3: RESPIRATORY MUSCLES

1. Diaphragm - MOST IMPORTANT muscle of inspiration
   • Contraction → diaphragm moves down → thoracic cavity length increases
2. External intercostal muscles - elevate ribs → increase anteroposterior diameter of chest

• Sternocleidomastoid (SCM) - lifts sternum upward
• Scaleni muscles - elevate first two ribs
• Anterior serratus muscles - lift many ribs

• At rest: expiration is PASSIVE (elastic recoil of lungs, no muscle needed)
• During forced/active expiration:
  1. Abdominal muscles (rectus abdominis) - push diaphragm upward
  2. Internal intercostal muscles - depress rib cage

Key Point: Normal quiet expiration = passive (no muscle needed). Only forced expiration needs muscles!

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TOPIC 4: PRESSURES IN THE RESPIRATORY SYSTEM ⭐⭐

• The lung has elastic recoil (wants to collapse inward)
• The chest wall has elastic recoil (wants to spring outward)
• They pull against each other → creates a subatmospheric pressure in the pleural space
• This negative pressure keeps the lungs "inflated" against the chest wall

1. Diaphragm contracts → chest cavity expands
2. Intrapleural pressure decreases (more negative)
3. Lungs expand → alveolar pressure falls below atmospheric
4. Pressure gradient created → air flows INTO lungs

1. Diaphragm relaxes
2. Lungs recoil (elastic recoil)
3. Alveolar pressure rises above atmospheric
4. Air flows OUT of lungs

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TOPIC 5: COMPLIANCE OF THE LUNGS ⭐

• Compliance = change in lung volume per unit change in pressure
• Formula: C = ΔV / ΔP
• Unit: L/cmH₂O
• Normal lung compliance = 0.2 L/cmH₂O

1. Lung compliance - ability of lungs alone to expand
2. Chest wall compliance - ability of chest wall to expand
3. Total (lung + chest wall) compliance - combined = 0.1 L/cmH₂O

1. Elastic recoil of lung tissue (due to elastin fibres)
2. Surface tension of alveolar lining fluid

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TOPIC 6: SURFACTANT ⭐⭐ (Very Important for Exam!)

• A lipoprotein substance lining the alveolar surface
• Produced by Type II alveolar (pneumocyte) cells
• Main component: Dipalmitoyl Phosphatidylcholine (DPPC)
• Also contains: SP-A, SP-B, SP-C, SP-D proteins

1. Reduces surface tension of alveolar fluid → prevents alveolar collapse
2. Increases lung compliance → easier to expand lungs
3. Equalizes alveolar size → prevents large alveoli from expanding at the expense of small ones
4. Prevents pulmonary oedema → reduces fluid transudation into alveoli

P = 2T / r

• P = pressure needed to keep alveolus open
• T = surface tension
• r = radius of alveolus

• Occurs in premature babies (< 35 weeks gestation)
• Surfactant production starts at 24 weeks, reliably present by 35 weeks
• Without surfactant:
  • Alveoli collapse (atelectasis)
  • Hypoxemia (low blood O₂)
  • Decreased lung compliance → increased work of breathing
  • Ground glass appearance on X-ray
• Treatment: Exogenous surfactant instillation + mechanical ventilation

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TOPIC 7: PULMONARY VOLUMES AND CAPACITIES ⭐⭐ (Must Know!)

• By Spirometry using a Spirometer
• Subject breathes into the spirometer; the displacement of a bell is recorded

Lung VOLUMES (4 volumes)

Memory trick for volumes: TIRE → Tidal, IRV, RV, ERV

Lung CAPACITIES (4 capacities - each = 2 or more volumes added together)

• RV, FRC, and TLC cannot be measured by spirometry (they all include RV)
• FRC and RV are measured by helium dilution method or body plethysmograph
• FRC = equilibrium/resting volume of lungs (lung volume after a normal quiet expiration)

• ↑ VC in: males, tall people, athletes, deep breathing
• ↓ VC in: females, old age, short stature, obstructive/restrictive lung diseases

Minute Respiratory Volume:

MRV = Respiratory Rate × Tidal Volume = 12-15 × 500 = 6000-7500 mL/min (~6-7.5 L/min)

Alveolar Ventilation:

VA = (TV - Dead Space) × Respiratory Rate = (500 - 150) × 12 = 4200 mL/min

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TOPIC 8: DEAD SPACE ⭐

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TOPIC 9: AIRWAY RESISTANCE AND AIRFLOW

Q = ΔP / R

• Q = Airflow (L/min)
• ΔP = Pressure difference between mouth and alveoli
• R = Airway resistance (cmH₂O/L/sec)

R = 8ηL / πr⁴

• Radius (r) is the MOST important factor
• Doubling radius → decreases resistance 16 times!

• Bronchoconstriction (asthma, histamine, parasympathetic stimulation)
• Mucus secretion (bronchitis)
• Foreign body
• Decreased lung volume (airways narrow)

• Bronchodilation (sympathetic, adrenaline, β₂ agonists)
• Deep inspiration (airways pulled open)
• Normal surfactant

• Parasympathetic (vagus nerve): Bronchoconstriction (via ACh → M₃ receptors)
• Sympathetic: Bronchodilation (via adrenaline → β₂ receptors)
• This is why adrenaline is used in acute asthma!

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TOPIC 10: GAS LAWS (Applied to Respiration)

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TOPIC 11: PARTIAL PRESSURES OF GASES ⭐⭐

• O₂ = 21%, N₂ = 79%, CO₂ = 0.04%

Water vapour pressure at body temperature (37°C) = 47 mmHg (always subtract when calculating)

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TOPIC 12: DIFFUSION OF GASES (GAS EXCHANGE) ⭐

Rate of diffusion = (A × D × ΔP) / T

• A = Surface area of membrane
• D = Diffusion coefficient = solubility / √molecular weight
• ΔP = Partial pressure difference
• T = Thickness of membrane

• Thickness = 0.5 micrometers (very thin)
• Surface area = 70 m²

• For O₂ at rest = 21 mL/mmHg/min (can increase 3× during exercise)
• CO₂ diffuses 20 times faster than O₂ despite a smaller gradient (because CO₂ is much more soluble)

• ↑ Membrane thickness (pulmonary fibrosis, oedema)
• ↓ Surface area (emphysema, pneumonia, surgical removal of lung)
• ↓ Pressure gradient (high altitude, hypoventilation)

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TOPIC 13: OXYGEN TRANSPORT IN BLOOD ⭐⭐⭐

Form 1: Dissolved O₂ (~2-3%)

• Dissolved directly in plasma
• Amount = PaO₂ × 0.003 = 100 × 0.003 = 0.3 mL O₂/100 mL blood
• Very small amount - insufficient alone to sustain life

Form 2: Bound to Haemoglobin (~97-98%)

• O₂ + Haemoglobin → Oxyhaemoglobin (HbO₂)
• 1 gram Hb can carry 1.34 mL O₂ (Hüfner's constant)
• Normal Hb = 15 g/dL
• O₂ carrying capacity = 15 × 1.34 = 20.1 mL O₂/100 mL blood

• 4 subunits: each has a globin chain + haem group
• Haem = iron-containing porphyrin ring
• Iron in ferrous state (Fe²⁺) to bind O₂
• Adult Hb (HbA) = α₂β₂
• Fetal Hb (HbF) = α₂γ₂ → higher affinity for O₂ than HbA
• 1 molecule of Hb can carry 4 molecules of O₂

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TOPIC 14: OXYHAEMOGLOBIN DISSOCIATION CURVE ⭐⭐⭐ (MOST IMPORTANT!)

• A graph showing the relationship between PO₂ (x-axis) and % Hb saturation (y-axis)
• S-shaped (sigmoid) curve due to cooperative binding of O₂ to Hb

• At PO₂ = 100 mmHg (lungs): Hb is 97-98% saturated ← Loading O₂ in lungs
• At PO₂ = 40 mmHg (tissues): Hb is 75% saturated ← Unloading O₂ to tissues
• P50 = 26 mmHg = PO₂ at which Hb is 50% saturated

CO₂/H⁺ shift the curve to the RIGHT → reduced O₂ affinity → more O₂ unloaded at tissues

• ↑ CO₂ (Bohr effect)
• ↑ H⁺ (acidosis, ↓ pH)
• ↑ Temperature
• ↑ 2,3-Diphosphoglycerate (2,3-DPG)
• Memory tip: "CADET face RIGHT" = CO₂, Acidity, 2,3-DPG, Exercise, Temperature

• ↓ CO₂
• ↓ H⁺ (alkalosis, ↑ pH)
• ↓ Temperature
• ↓ 2,3-DPG
• Fetal Hb (HbF)
• CO poisoning
• Methemoglobin

Right shift = Good at tissues (releases O₂ more easily) Left shift = Good at lungs (loads O₂ more easily)

• Flat upper part = Loading zone in lungs (Hb stays saturated even if PO₂ drops slightly)
• Steep lower part = Unloading zone at tissues (small drop in PO₂ → large O₂ release)

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TOPIC 15: CARBON DIOXIDE TRANSPORT ⭐⭐

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (catalyzed by Carbonic Anhydrase enzyme - speeds reaction 5000 times!)

• HCO₃⁻ moves out of RBC into plasma
• Cl⁻ moves into RBC to maintain electrical balance
• This is called the CHLORIDE SHIFT (Hamburger's shift)

Deoxygenated Hb (at tissues) binds MORE CO₂ and H⁺ than oxygenated Hb

• Helps load CO₂ in tissues and unload CO₂ in lungs
• Opposite of Bohr effect - both work together to facilitate gas exchange!

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TOPIC 16: VENTILATION-PERFUSION RATIO (V/Q RATIO) ⭐⭐

• V = Ventilation (VA) = 4.2 L/min
• Q = Perfusion (cardiac output to lungs) = 5 L/min
• Normal V/Q = 0.8

• When alveolar PO₂ is low → pulmonary arterioles constrict
• Blood diverted away from poorly ventilated alveoli
• Purpose: Improves V/Q matching, reduces hypoxemia
• This is the OPPOSITE of what happens in systemic circulation!
• At high altitude: widespread hypoxic vasoconstriction → pulmonary hypertension

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TOPIC 17: THREE ZONES OF THE LUNG (West's Zones) ⭐

In supine position, there is no Zone 1 - blood flow is nearly uniform throughout the lung.

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TOPIC 18: CONTROL OF RESPIRATION ⭐⭐⭐

18A: Respiratory Control Centres (in Brainstem)

• Basic breathing rhythm is generated in the Medulla (even if pons is cut, breathing continues)
• Cortex can voluntarily override the brainstem (e.g., breath-holding, singing)

18B: Chemoreceptors ⭐⭐⭐ (Most Important!)

Central Chemoreceptors:

• Location: Ventral surface of Medulla (not in respiratory centre itself)
• Stimulus: CO₂ and H⁺ ions (mainly CO₂)
• Mechanism: CO₂ crosses blood-brain barrier → combines with H₂O → H₂CO₃ → H⁺ → stimulates receptors
• Most important for minute-to-minute regulation of breathing
• Do NOT respond directly to hypoxia (low O₂)

Peripheral Chemoreceptors:

• Location: Carotid bodies (at bifurcation of common carotid artery) + Aortic bodies (in aortic arch)
• Nerve supply: Carotid body → CN IX (Glossopharyngeal); Aortic body → CN X (Vagus)
• Stimuli: ↓ PaO₂, ↑ PaCO₂, ↑ H⁺, ↓ arterial pressure
• Carotid bodies are more important in humans
• ONLY receptors that respond to hypoxemia (low PO₂)
• Stimulated when PaO₂ falls below 60 mmHg

CO₂ is the most powerful stimulus for breathing!

18C: Pulmonary Reflexes

• Stretch receptors in bronchial walls activated by lung inflation
• Send signals via vagus nerve → inhibit inspiratory centre → stops further inspiration
• Prevents over-distension of lungs
• Active mainly at large tidal volumes (> 1.5 L); not very active in normal quiet breathing in adults

• Stimulus: Dust, mucus, foreign body, chemical irritants in trachea, bronchi, carina
• Afferents via vagus nerve to medulla
• Steps:
  1. Deep inspiration (~2.5 L)
  2. Vocal cords close tightly; glottis closes
  3. Forceful contraction of expiratory muscles → intrapulmonary pressure rises to 100 mmHg
  4. Glottis suddenly opens → explosive outflow of air at 75-100 miles/hr
  5. Mucus and foreign material expelled

• Similar to cough but receptors in nasal mucosa
• Afferents via trigeminal nerve (CN V)
• Uvula depresses → air expelled mainly through nose

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TOPIC 19: PULMONARY CIRCULATION

• Low pressure system: Pulmonary artery pressure = 25/8 mmHg (mean = 15 mmHg)
• Compare with systemic: 120/80 mmHg
• Low resistance (vessels are thin-walled, distensible)
• Large compliance (acts as blood reservoir)

• Low alveolar PO₂ → pulmonary arteriole constriction
• Opposite of systemic circulation (where hypoxia causes vasodilation)
• Purpose: Diverts blood to better-ventilated alveoli
• Clinical: At high altitude → widespread pulmonary vasoconstriction → pulmonary hypertension → cor pulmonale (right heart failure)

• In standing: Maximum at BASE, Minimum at APEX (gravity effect)
• In supine: Nearly uniform (no gravitational effect)

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TOPIC 20: MUCUS AND CILIA - RESPIRATORY DEFENCE

• All respiratory passages lined with ciliated epithelium with ~200 cilia per cell
• Cilia beat at 10-20 times/sec toward pharynx (mucociliary escalator)
• Mucus traps dust particles, bacteria, allergens
• Trapped particles → swept upward → swallowed or coughed out
• This is called the Mucociliary Clearance Mechanism

• Cigarette smoke damages cilia → impairs clearance → chronic bronchitis

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TOPIC 21: HYPOXIA AND HYPOXEMIA ⭐

• Hypoxemia = low PaO₂ (↓ O₂ in arterial blood)
• Hypoxia = low O₂ supply to tissues (or inability to use O₂)
• Hypoxemia can cause hypoxia, but not always (e.g., cyanide poisoning = hypoxia without hypoxemia)

• Cyanosis (bluish discoloration), headache, restlessness, confusion, tachycardia, tachypnoea

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TOPIC 22: NON-RESPIRATORY FUNCTIONS OF THE LUNGS

1. ACE action: Angiotensin I → Angiotensin II (by ACE in pulmonary capillaries)
2. Inactivation: Bradykinin, serotonin, norepinephrine inactivated in lungs
3. Synthesis: Prostacyclin (PGI₂ = vasodilator) synthesized
4. Prostaglandin metabolism: PGE₁, PGF₂α removed
5. Thromboxane A₂ synthesis (vasoconstrictor)
6. Blood reservoir for left heart
7. Vocalization (speech, singing)
8. pH regulation (rapid, by adjusting ventilation → changes PaCO₂ → changes pH)
9. Filtration of microemboli and clots from venous blood

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QUICK REFERENCE: IMPORTANT NUMBERS TO MEMORIZE ⭐⭐

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IMPORTANT EXAM QUESTIONS FROM EACH TOPIC

1. Define tidal volume. List all lung volumes and capacities with normal values.
2. What is surfactant? Name its type and functions.
3. Describe the oxyhaemoglobin dissociation curve and factors causing shifts.
4. What is Bohr effect? What is Haldane effect?
5. Explain the mechanics of inspiration.
6. What is FRC? How is it measured?
7. Define dead space. What are its types?
8. What is V/Q ratio? What are its regional variations?
9. Classify types of hypoxia with examples.
10. Describe the cough reflex.

1. Describe the regulation/control of respiration with a neat diagram of respiratory centres.
2. Discuss the transport of O₂ in blood. Draw and explain the oxyhaemoglobin dissociation curve.
3. Discuss the transport of CO₂ in blood.
4. Describe lung volumes and capacities with a spirogram diagram.
5. Describe the mechanics of breathing - inspiration and expiration.
6. What is surfactant? Describe its production, composition and clinical significance.

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🫁 RESPIRATORY SYSTEM PHYSIOLOGY

1st Year BHMS — Complete Flowchart Notes

GK Pal Style | Simple & Easy to Understand

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FLOWCHART 1: OVERVIEW OF RESPIRATION

RESPIRATION
     │
     ├──────────────────────────────────────┐
     ▼                                      ▼
EXTERNAL RESPIRATION                INTERNAL RESPIRATION
(Lungs ↔ Blood)                    (Blood ↔ Tissues)
O₂ enters blood                    O₂ delivered to cells
CO₂ leaves blood                   CO₂ collected from cells

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FLOWCHART 2: DIVISIONS OF RESPIRATORY TRACT

RESPIRATORY TRACT
        │
        ├──────────────────────┬──────────────────────┐
        ▼                      ▼                      ▼
 UPPER AIRWAY           CONDUCTING ZONE          RESPIRATORY ZONE
        │               (No gas exchange)         (Gas exchange)
   Nose/Mouth                 │                       │
      │                  Trachea                Respiratory
   Pharynx              Bronchi                 bronchioles
      │                 Bronchioles             Alveolar ducts
   Larynx               Terminal                 Alveoli ← GAS
      │                 bronchioles             EXCHANGE HERE
      └──────────────────┘
         Air enters here
         (Warmed, humidified,
          filtered here)

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FLOWCHART 3: RESPIRATORY MUSCLES

RESPIRATORY MUSCLES
         │
         ├──────────────────────────┐
         ▼                          ▼
 MUSCLES OF INSPIRATION       MUSCLES OF EXPIRATION
         │                          │
         ├─ PRIMARY:                ├─ AT REST:
         │   Diaphragm ←            │   PASSIVE
         │   MOST IMPORTANT         │   (elastic recoil only)
         │                          │
         ├─ SECONDARY:              └─ FORCED EXPIRATION:
         │   External intercostals       Internal intercostals
         │                               Abdominal muscles
         └─ ACCESSORY:                   (Rectus abdominis)
             (only in heavy breathing)
             SCM (lifts sternum)
             Scaleni (lift 1st 2 ribs)
             Serratus anterior

---

FLOWCHART 4: MECHANICS OF BREATHING

┌─────────────────────────────────────────────────────┐
│               INSPIRATION (Active)                  │
└─────────────────────────────────────────────────────┘

Diaphragm contracts + External intercostals contract
                      │
                      ▼
        Thoracic cavity volume INCREASES
                      │
                      ▼
    Intrapleural pressure ↓ (−5 → −8 cmH₂O)
                      │
                      ▼
        Lung expands → Alveoli expand
                      │
                      ▼
        Alveolar pressure ↓ (below atmospheric)
                      │
                      ▼
      Pressure gradient (atmosphere > alveoli)
                      │
                      ▼
           AIR FLOWS INTO LUNGS ✓


┌─────────────────────────────────────────────────────┐
│               EXPIRATION (Passive at rest)          │
└─────────────────────────────────────────────────────┘

         Diaphragm relaxes
                │
                ▼
   Elastic recoil of lungs & chest wall
                │
                ▼
      Thoracic cavity volume DECREASES
                │
                ▼
   Intrapleural pressure ↑ (less negative)
                │
                ▼
    Alveolar pressure ↑ (above atmospheric)
                │
                ▼
      Pressure gradient (alveoli > atmosphere)
                │
                ▼
         AIR FLOWS OUT OF LUNGS ✓

---

FLOWCHART 5: PRESSURES SUMMARY

                PRESSURES IN RESPIRATORY SYSTEM
                             │
          ┌──────────────────┼──────────────────┐
          ▼                  ▼                  ▼
  ATMOSPHERIC          INTRAPLEURAL          ALVEOLAR
   PRESSURE             PRESSURE             PRESSURE
  760 mmHg           −5 cmH₂O (rest)      0 cmH₂O (rest)
  (reference)        −8 cmH₂O (insp)      −3 cmH₂O (insp)
                     −5 cmH₂O (exp)       +3 cmH₂O (exp)

           TRANSPULMONARY PRESSURE
                = Palv − Pip
                = 0 − (−5) = +5 cmH₂O
                    │
                    ▼
        Keeps lungs OPEN (prevents collapse)

---

FLOWCHART 6: SURFACTANT

TYPE II PNEUMOCYTES (Alveolar cells)
              │
              │ Produces
              ▼
         SURFACTANT
         (Main component: DPPC -
          Dipalmitoyl Phosphatidylcholine)
              │
              │ Functions
              ├──────────────────────────────────────┐
              ▼                                      ▼
   REDUCES SURFACE TENSION              INCREASES LUNG COMPLIANCE
   of alveolar lining fluid             (lungs expand more easily)
              │
              ▼
   Prevents alveolar COLLAPSE (atelectasis)
   [By Laplace's Law: P = 2T/r
    ↓T → ↓P → alveoli stay open]

              │ If ABSENT (premature baby)
              ▼
   NEONATAL RESPIRATORY DISTRESS SYNDROME
              │
              ├── Alveoli collapse (atelectasis)
              ├── Hypoxemia (↓ PaO₂)
              ├── ↓ Lung compliance
              ├── ↑ Work of breathing
              └── Treatment: Exogenous surfactant + O₂

---

FLOWCHART 7: LUNG VOLUMES & CAPACITIES

MAXIMUM INSPIRATION LEVEL ─────────────────────────┐
         │                                          │
         │← IRV = 3000 mL                          │
         │  (Inspiratory Reserve Volume)            │
         │                                          │ TLC
NORMAL BREATH LEVEL ─────────────────────────      │ = 5900 mL
         │← TV = 500 mL (Tidal Volume)             │
         │                                     (Total Lung
NORMAL EXPIRATION LEVEL ────────────────────── Capacity)
         │← ERV = 1200 mL                          │
         │  (Expiratory Reserve Volume)             │
         │                                          │
MAXIMUM EXPIRATION LEVEL ──────────────────────────┘
         │← RV = 1200 mL
         │  (Residual Volume)
         │  CANNOT BE MEASURED BY SPIROMETRY
         ▼
      ZERO VOLUME


           LUNG CAPACITIES
           ════════════════
  IC  = TV + IRV          = 500 + 3000 = 3500 mL
  FRC = ERV + RV          = 1200 + 1200 = 2400 mL  ← cannot spirometry
  VC  = IRV + TV + ERV    = 3000 + 500 + 1200 = 4700 mL
  TLC = VC + RV           = 4700 + 1200 = 5900 mL  ← cannot spirometry


           HOW TO MEASURE FRC?
                   │
        ┌──────────┴──────────┐
        ▼                     ▼
  HELIUM DILUTION        BODY PLETHYSMOGRAPH
  METHOD                 (uses Boyle's Law: P×V = constant)

---

FLOWCHART 8: DEAD SPACE

INSPIRED AIR
(500 mL tidal volume)
        │
        ├──────────────────────────────┐
        ▼                              ▼
DEAD SPACE (150 mL)          ALVEOLAR AIR (350 mL)
No gas exchange              GAS EXCHANGE occurs here
        │                              │
Conducting airways             Alveoli ↔ Capillaries
(nose to terminal
 bronchioles)

TYPES OF DEAD SPACE
        │
        ├──────────────────────────────────────┐
        ▼                                      ▼
ANATOMICAL DEAD SPACE               ALVEOLAR DEAD SPACE
~150 mL                             ~0 mL (in health)
Conducting airways                  Ventilated but
(no exchange regardless)            NOT perfused alveoli
        │                                      │
        └──────────────┬───────────────────────┘
                       ▼
           PHYSIOLOGICAL DEAD SPACE
           = Anatomical + Alveolar
           = 150 mL (in health)
           ↑ INCREASES in pulmonary embolism!

ALVEOLAR VENTILATION
= (TV − Dead Space) × Rate
= (500 − 150) × 12
= 350 × 12
= 4200 mL/min = 4.2 L/min

---

FLOWCHART 9: GAS EXCHANGE (DIFFUSION)

       FICK'S LAW OF DIFFUSION
               │
               ▼
  Rate of diffusion = (A × D × ΔP) / T

  A = Surface area (70 m² in lungs)
  D = Diffusion coefficient (solubility/√MW)
  ΔP = Pressure difference
  T = Membrane thickness (0.5 micrometers)

  Diffusion ↑ when:            Diffusion ↓ when:
  ↑ Surface area               ↓ Surface area (emphysema)
  ↑ Pressure difference        ↑ Thickness (fibrosis, oedema)
  ↓ Thickness                  ↓ Pressure difference (altitude)


      PARTIAL PRESSURES (mmHg)
              │
    ┌─────────┼──────────┬─────────┬──────────┐
    ▼         ▼          ▼         ▼          ▼
 Dry air  Humidified  Alveolar  Arterial   Venous
          inspired    air       blood      blood
  PO₂=159  PO₂=150   PO₂=100   PO₂=100   PO₂=40
  PCO₂=0.3 PCO₂=0.3  PCO₂=40   PCO₂=40   PCO₂=46

  ← O₂ moves: Alveoli (100) → Blood (40) → O₂ enters blood
  ← CO₂ moves: Blood (46) → Alveoli (40) → CO₂ leaves blood

---

FLOWCHART 10: OXYGEN TRANSPORT IN BLOOD

           O₂ IN BLOOD
                │
     ┌──────────┴──────────┐
     ▼                     ▼
DISSOLVED (2-3%)      BOUND TO Hb (97-98%)
  in plasma                │
  0.3 mL/100 mL       Hb + O₂ → HbO₂
  (insufficient!)      (Oxyhaemoglobin)
                           │
                    1g Hb × 1.34 mL O₂
                    15g Hb × 1.34 = 20.1 mL/100 mL
                           │
              TOTAL O₂ CONTENT = 20.1 + 0.3 = ~20.4 mL/100 mL


HAEMOGLOBIN STRUCTURE
         │
         ▼
  4 Subunits
  Each = 1 Globin chain + 1 Haem group
         │
         ▼
  Iron in FERROUS state (Fe²⁺) → binds O₂
  If Fe³⁺ (Ferric) → Methaemoglobin → CANNOT bind O₂
         │
         ▼
  HbA (Adult)  = α₂β₂
  HbF (Fetal)  = α₂γ₂  ← Higher O₂ affinity than HbA
  HbS (Sickle) = Abnormal β chains

---

FLOWCHART 11: OXY-Hb DISSOCIATION CURVE

% Hb SATURATION
    │
100%┼──────────────────╮
    │              ╭───╯  ← FLAT PART
 97%┼─────────────╮│       (Lungs: PO₂ 100 mmHg → 97% sat.)
    │             ││       Small drop in PO₂ → no big change
    │         ╭──╯│        → Hb STAYS LOADED in lungs
 75%┼────────╮│   │
    │        ││   │← STEEP PART
    │     ╭──╯│   │  (Tissues: PO₂ 40 mmHg → 75% sat.)
 50%┼─────│  │   │  Small drop in PO₂ → BIG O₂ release
    │     │  │   │  → Hb UNLOADS O₂ at tissues
    │     │  │   │
    └─────┴──┴───┴─── PO₂ (mmHg)
          26  40  100
          │
          P50 = 26 mmHg (PO₂ where Hb is 50% saturated)


SHIFT OF CURVE
         │
    ┌────┴────────┐
    ▼             ▼
RIGHT SHIFT       LEFT SHIFT
(↑ P50)           (↓ P50)
↓ O₂ affinity     ↑ O₂ affinity
More O₂           Hb holds O₂
released          more tightly
         │                │
  Caused by:       Caused by:
  ↑ CO₂            ↓ CO₂
  ↑ H⁺/↓ pH        ↓ H⁺/↑ pH
  ↑ Temperature     ↓ Temperature
  ↑ 2,3-DPG         ↓ 2,3-DPG
  Exercise          Fetal Hb (HbF)
                    CO poisoning

  "CADET face RIGHT"
  CO₂, Acidity, 2,3-DPG, Exercise, Temperature

---

FLOWCHART 12: CO₂ TRANSPORT IN BLOOD

CO₂ produced at TISSUES
           │
           ▼
     Enters BLOOD
           │
  ┌────────┼────────────────────────┐
  ▼        ▼                        ▼
DISSOLVED  CARBAMINO-Hb         BICARBONATE
in plasma  (CO₂ + Hb protein)   (Most important!)
 7-10%         20-23%              ~70%

                BICARBONATE FORMATION (in RBC):
                          │
          CO₂ + H₂O ──Carbonic Anhydrase──▶ H₂CO₃
                                                │
                                                ▼
                                        H⁺ + HCO₃⁻
                                        │         │
                                        ▼         ▼
                                    Binds Hb   Moves to PLASMA
                                    (buffered) (Chloride Shift)
                                               Cl⁻ enters RBC
                                               = HAMBURGER'S SHIFT

HALDANE EFFECT:
Deoxygenated Hb (at tissues) binds MORE CO₂
Oxygenated Hb (at lungs) releases MORE CO₂
→ Facilitates CO₂ loading at tissues & unloading at lungs

---

FLOWCHART 13: V/Q RATIO

V/Q RATIO = Ventilation / Perfusion = 4.2/5.0 = 0.8 (Normal)

         V/Q VALUES
              │
    ┌─────────┼─────────┐
    ▼         ▼         ▼
  V/Q = 0   V/Q = 0.8  V/Q = ∞
     │       Normal        │
     │                     │
 Shunt          Dead Space
 (airway blocked)  (vessel blocked)
 Alveolus perfused  Alveolus ventilated
 but NOT ventilated but NOT perfused
     │                     │
 Blood like           Air like
 venous blood         inspired air
     │                     │
 Example:            Example:
 Atelectasis         Pulmonary Embolism


REGIONAL V/Q VARIATION (standing person)
              │
    ┌─────────┴──────────┐
    ▼                    ▼
  APEX                 BASE
V/Q > 0.8           V/Q < 0.8
Over-ventilated,    Over-perfused,
under-perfused      under-ventilated
Low blood flow      High blood flow
(gravity pulls      (gravity increases
 blood away)         blood pressure)

---

FLOWCHART 14: WEST'S THREE ZONES OF LUNG

STANDING POSITION — LUNG ZONES
                    │
        ┌───────────┼───────────┐
        ▼           ▼           ▼
     ZONE 1       ZONE 2      ZONE 3
      APEX        MIDDLE       BASE
        │           │           │
PA > Pa > Pv   Pa > PA > Pv   Pa > Pv > PA
        │           │           │
  Capillaries   Capillaries   Capillaries
   collapse     intermittent   always
   (no flow)      flow          open
        │           │           │
 LEAST blood    MODERATE      MOST blood
   flow           flow           flow

PA = Alveolar pressure
Pa = Pulmonary Arterial pressure
Pv = Pulmonary Venous pressure

---

FLOWCHART 15: CONTROL OF RESPIRATION

CONTROL OF RESPIRATION
           │
    ┌──────┴──────────────────┐
    ▼                         ▼
BRAINSTEM CENTRES         CHEMORECEPTORS
    │                         │
    │                    ┌────┴───────┐
    │                    ▼            ▼
    │               CENTRAL       PERIPHERAL
    │               CHEMO-        CHEMO-
    │               RECEPTORS     RECEPTORS
    │                    │             │
    │               Ventral        Carotid bodies
    │               medulla        (CN IX)
    │                    │        Aortic bodies
    │               Responds       (CN X)
    │               to CO₂/H⁺          │
    │               (NOT O₂!)      Responds to:
    │                    │         ↓PO₂ (<60mmHg)
    │                    │         ↑PCO₂
    │                    │         ↑H⁺
    │                    │         ↓BP
    │                    │
    ▼
BRAINSTEM RESPIRATORY CENTRES
           │
   ┌───────┼────────────────────┐
   ▼       ▼                    ▼
MEDULLA                       PONS
   │                            │
   ├─ DRG (Dorsal               ├─ PNEUMOTAXIC centre
   │   Respiratory Group)       │   (Upper pons)
   │   Controls INSPIRATION     │   Inhibits inspiration
   │   Sets basic rhythm        │   Regulates rate & depth
   │   Output → Phrenic nerve   │
   │   → Diaphragm              └─ APNEUSTIC centre
   │                                (Lower pons)
   └─ VRG (Ventral                  Prolongs inspiration
       Respiratory Group)           (opposed by pneumotaxic)
       Controls EXPIRATION
       (active only in
       forced breathing)

          ↕ Voluntary override
      CEREBRAL CORTEX
      (breath holding,
       singing, talking)

---

FLOWCHART 16: STIMULI FOR BREATHING

RESPIRATORY STIMULI
          │
  ┌───────┼──────────────────┐
  ▼       ▼                  ▼
↑ CO₂   ↓ pH (↑ H⁺)        ↓ O₂
(most    (acidosis)      (hypoxemia)
powerful)     │               │
  │           │               │
  ▼           ▼               ▼
Central   Central &       Peripheral
Chemorecp Peripheral      Chemorecp
 BOTH     Chemorecp         ONLY
  │           │               │
  └─────────┬─┘               │
            ▼                 ▼
     Stimulate INSPIRATORY   Only when
     CENTRE (DRG)            PaO₂ < 60 mmHg
            │
            ▼
     ↑ Rate & Depth
     of Breathing

---

FLOWCHART 17: COUGH REFLEX

STIMULUS
(Dust, mucus, irritants in
Trachea / Bronchi / Carina)
         │
         ▼
AFFERENT: Vagus nerve (CN X)
         │
         ▼
MEDULLARY COUGH CENTRE
         │
         ▼
STEP 1: Deep inspiration (~2.5 L air)
         │
         ▼
STEP 2: Glottis closes tightly
(vocal cords shut)
         │
         ▼
STEP 3: Forceful contraction of
expiratory muscles
→ Intrapulmonary pressure rises
→ up to 100 mmHg!
         │
         ▼
STEP 4: Glottis suddenly OPENS
         │
         ▼
Air EXPLODES outward
(75-100 miles/hr)
         │
         ▼
Mucus/Foreign material expelled ✓

---

FLOWCHART 18: HERING-BREUER REFLEX

Lung INFLATES (during inspiration)
         │
         ▼
Stretch receptors in bronchi/bronchioles
activated
         │
         ▼
Afferent impulses via VAGUS nerve (CN X)
         │
         ▼
INSPIRATORY CENTRE INHIBITED (DRG)
         │
         ▼
Inspiration STOPS (switches to expiration)
         │
         ▼
Lung DOES NOT OVER-INFLATE ✓

Note: Only active at large tidal volumes (>1.5 L)
Not very active in normal quiet breathing in adults

---

FLOWCHART 19: HYPOXIA TYPES

HYPOXIA
(↓ O₂ supply/use by tissues)
         │
    ┌────┴──────────────────────┐
    ▼                           ▼
HYPOXEMIA                    NORMOXEMIA
(↓ PaO₂)                    (Normal PaO₂)
    │                            │
    ▼                       ┌────┴────────┐
HYPOXIC HYPOXIA             ▼             ▼
(low PaO₂)             ANAEMIC       STAGNANT    HISTOTOXIC
Causes:                HYPOXIA       HYPOXIA     HYPOXIA
- High altitude        (↓ Hb)       (↓ blood     (cells can't
- Lung disease         Anaemia       flow)         use O₂)
- Hypoventilation      CO poisoning  Heart failure Cyanide
- V/Q mismatch         MetHb         Shock         poisoning
- R→L shunt

---

FLOWCHART 20: PULMONARY CIRCULATION

RIGHT HEART
     │
     ▼
PULMONARY ARTERY
Pressure: 25/8 mmHg (LOW!)
(Compare systemic: 120/80 mmHg)
     │
     ▼
PULMONARY CAPILLARIES
(surround alveoli)
     │
     ├─── O₂ moves: Alveoli → Blood (PaO₂ rises to 100 mmHg)
     └─── CO₂ moves: Blood → Alveoli (PaCO₂ falls to 40 mmHg)
     │
     ▼
PULMONARY VEINS
(Oxygenated blood)
     │
     ▼
LEFT HEART


SPECIAL FEATURE: HYPOXIC VASOCONSTRICTION
          │
          ▼
Low alveolar PO₂
          │
          ▼
Pulmonary arterioles CONSTRICT
(Opposite of systemic vessels!)
          │
          ▼
Blood diverted to better-ventilated alveoli
          │
          ▼
V/Q matching improved ✓

At HIGH ALTITUDE:
Low PO₂ everywhere → widespread vasoconstriction
→ Pulmonary hypertension
→ Right heart hypertrophy → Cor pulmonale

---

FLOWCHART 21: NON-RESPIRATORY FUNCTIONS

LUNGS
  │
  ├──── ACE: Angiotensin I ──→ Angiotensin II
  │           (in pulmonary capillaries)
  │
  ├──── INACTIVATION: Bradykinin, Serotonin,
  │                   Norepinephrine removed
  │
  ├──── SYNTHESIS: Prostacyclin (PGI₂) = vasodilator
  │               Thromboxane A₂ = vasoconstrictor
  │
  ├──── FILTER: Microemboli, clots filtered from blood
  │
  ├──── BLOOD RESERVOIR: for left heart
  │
  ├──── pH REGULATION: ↑ Ventilation → ↓ CO₂ → ↑ pH
  │                    ↓ Ventilation → ↑ CO₂ → ↓ pH
  │
  └──── VOCALIZATION: Speech, singing

---

MASTER SUMMARY FLOWCHART: COMPLETE RESPIRATORY CYCLE

OUTSIDE AIR (PO₂=159 mmHg)
          │
          ▼
UPPER AIRWAY (Nose/Mouth)
Warmed, humidified, filtered
PO₂ = 150 mmHg (after humidification)
          │
          ▼
CONDUCTING AIRWAYS
(Trachea → Bronchi → Bronchioles)
Dead Space = 150 mL
          │
          ▼
ALVEOLI (PO₂=100 mmHg, PCO₂=40 mmHg)
          │
     ┌────┴────┐
     ▼         ▼
  O₂ in     CO₂ out
(diffusion) (diffusion)
     │         │
     ▼         ▼
PULMONARY CAPILLARIES
PaO₂=100 mmHg → O₂ binds Hb → HbO₂
PaCO₂=40 mmHg ← CO₂ from blood
          │
          ▼
PULMONARY VEINS → LEFT HEART
          │
          ▼
SYSTEMIC ARTERIES
Carry oxygenated blood (PaO₂=100)
          │
          ▼
SYSTEMIC CAPILLARIES (at tissues)
PO₂ of tissue = 40 mmHg
CO₂ of tissue = 46 mmHg
     │              │
     ▼              ▼
  O₂ unloads     CO₂ loads
  to tissues     from tissues
     │              │
     └──────┬───────┘
            ▼
VENOUS BLOOD (PvO₂=40, PvCO₂=46)
            │
            ▼
RIGHT HEART → LUNGS → cycle repeats ✓

---

QUICK RECALL FLOWCHART: EXAM MEMORY AIDS

LUNG VOLUMES (memory: TIRE)
T = Tidal Volume = 500 mL
I = IRV = 3000 mL
R = RV = 1200 mL
E = ERV = 1200 mL

LUNG CAPACITIES (memory: IFVT)
I = IC  = 3500 mL (TV + IRV)
F = FRC = 2400 mL (ERV + RV) ← needs helium/plethysmograph
V = VC  = 4700 mL (IRV+TV+ERV)
T = TLC = 5900 mL (VC + RV)  ← needs helium/plethysmograph

SHIFT OF Oxy-Hb CURVE (memory: CADET)
C = CO₂ ↑ → Right shift
A = Acidity ↑ → Right shift
D = 2,3-DPG ↑ → Right shift
E = Exercise → Right shift
T = Temperature ↑ → Right shift

CO₂ TRANSPORT (memory: 7-20-70)
7% = Dissolved
20% = Carbamino-Hb
70% = Bicarbonate (HCO₃⁻) ← most important

TYPES OF HYPOXIA (memory: HASH)
H = Hypoxic hypoxia (↓ PaO₂)
A = Anaemic hypoxia (↓ Hb)
S = Stagnant hypoxia (↓ blood flow)
H = Histotoxic hypoxia (cells can't use O₂)

BRAINSTEM CENTRES (Top to Bottom)
Pneumotaxic (Upper Pons) — STOPS inspiration
Apneustic (Lower Pons) — PROLONGS inspiration
DRG/VRG (Medulla) — GENERATES rhythm

---