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Respiratory System Physiology — BPT First Year Study Notes

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1. Overview of the Respiratory System

• Uptake of O₂ from the environment into the blood
• Removal of CO₂ from the blood to the outside

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2. Structural Zones of the Lung

Conducting Zone

• Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles (up to terminal bronchioles)
• Function: Conducts air, filters, warms, humidifies it
• No gas exchange occurs here → forms Anatomical Dead Space (~150 mL)

Respiratory Zone

• Respiratory bronchioles → Alveolar ducts → Alveolar sacs → Alveoli
• Gas exchange by simple diffusion occurs here

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3. Lung Volumes and Capacities

• Minute Respiratory Volume = TV × Respiratory Rate = 500 mL × 12 = 6 L/min
• Max possible: TV can = VC (~4600 mL); rate can rise to 40–50/min → >200 L/min

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4. Alveolar Ventilation & Dead Space

• Anatomical dead space: ~150 mL (conducting airways)
• Physiological dead space: Anatomical dead space + non-perfused alveoli (in disease)

Alveolar Ventilation = (TV − Dead Space) × Respiratory Rate = (500 − 150) × 12 = 4.2 L/min

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5. Respiratory Muscles

• Innervated by the phrenic nerve (C3, C4, C5)
• Moves 1.5 cm (quiet) to 7 cm (deep inspiration)
• Has 3 parts: costal, crural, and central tendon (also forms inferior pericardium)
• Crural fibers compress the esophagus during contraction

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6. Gas Exchange at the Respiratory Membrane

Respiratory Membrane Layers (blood-air barrier):

1. Alveolar epithelium (Type I pneumocytes)
2. Epithelial basement membrane
3. Interstitial space
4. Capillary basement membrane
5. Capillary endothelium

Fick's Law of Diffusion

Partial Pressures (mmHg):

Diffusing Capacity:

• O₂ diffusing capacity (rest): ~21 mL/min/mmHg → delivers ~230 mL O₂/min
• During exercise: increases 3× due to capillary recruitment and better V/Q matching
• CO₂ diffuses 20× faster than O₂ (higher solubility)
• O₂ diffuses 2× faster than N₂

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7. Alveolar Cells (Pneumocytes)

Surfactant:

• Composed mainly of dipalmitoyl phosphatidylcholine (DPPC)
• Reduces surface tension in alveoli
• Prevents alveolar collapse (atelectasis) at low volumes
• Absent/deficient in premature neonates → Neonatal Respiratory Distress Syndrome (NRDS)

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8. Control of Respiration

Respiratory Centers (in the brainstem):

Chemical Control of Breathing:

• Sensitive to ↑ CO₂ (via ↑ H⁺ in CSF)
• Most powerful stimulus to breathe

• Respond to ↓ PO₂ (<60 mmHg), ↑ PCO₂, ↑ H⁺
• Carotid bodies → via glossopharyngeal nerve (CN IX)
• Aortic bodies → via vagus nerve (CN X)

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9. Oxygen Transport in Blood

• Each Hb molecule carries 4 O₂ molecules
• Normal SpO₂: 95–100%

Factors shifting O₂-Hb Dissociation Curve:

• ↑ Temperature, ↑ PCO₂, ↑ H⁺ (↓ pH), ↑ 2,3-DPG

• ↓ Temperature, ↓ PCO₂, ↓ H⁺ (↑ pH), fetal Hb (HbF)

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10. CO₂ Transport

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11. Ventilation-Perfusion (V/Q) Ratio

• Normal V/Q ratio: ~0.8
• V/Q = 0: No ventilation (shunt) → blood passes unoxygenated
• V/Q = ∞: No perfusion (dead space) → wasted ventilation
• In upright lungs: V/Q highest at apex, lowest at base (gravity-dependent perfusion)

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12. Acid-Base Balance (Respiratory Component)

• Respiratory acidosis: hypoventilation, COPD, respiratory depression
• Respiratory alkalosis: hyperventilation, anxiety, high altitude

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13. Key Diseases (Brief Overview for BPT)

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Recommended Textbooks for BPT Physiology

1. Ganong's Review of Medical Physiology (26th ed.) — comprehensive, standard BPT reference
2. Guyton & Hall Textbook of Medical Physiology — detailed, excellent for mechanisms
3. Sembulingam & Prema Sembulingam — Essentials of Medical Physiology — very popular for BPT/MBBS in India
4. A.K. Jain — Textbook of Physiology — widely used in Indian BPT programs

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Respiratory System Physiology

Guyton & Hall Textbook of Medical Physiology — BPT First Year Notes

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CHAPTER 38 — PULMONARY VENTILATION

Pulmonary Volumes (Table 38.1)

Pulmonary Capacities

Key: RV cannot be measured by spirometry — requires helium dilution or body plethysmography.

Minute Respiratory Volume

Minute Volume = Tidal Volume × Respiratory Rate = 500 mL × 12 breaths/min = 6 L/min (normal)

• Max possible: up to >200 L/min during extreme exercise
• Most people can sustain only half to two-thirds of maximum for >1 minute

Alveolar Ventilation & Dead Space

• Anatomical dead space: ~150 mL — air in conducting passages (nose → terminal bronchioles) that never reaches alveoli
• Alveolar ventilation = (TV − Dead space) × Rate = (500−150) × 12 = 4.2 L/min

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CHAPTER 38 — RESPIRATORY PASSAGEWAYS

Structure and Patency

• Trachea: Kept open by C-shaped cartilage rings (5/6 of circumference)
• Bronchi: Curved cartilage plates maintain rigidity
• Bronchioles (<1.5 mm diameter): No cartilage — kept open by transpulmonary pressure (same forces that expand alveoli)

Resistance to Airflow

• Greatest resistance is in larger bronchi near the trachea (fewer in number)
• Terminal bronchioles (65,000 parallel) each carry very little air
• In disease: small bronchioles dominate → occluded by:
  1. Smooth muscle contraction (e.g., asthma)
  2. Mucosal edema
  3. Mucus in lumen

Autonomic Control of Bronchioles

Normal Functions of the Nose (Respiratory Passages)

1. Warming inspired air to body temperature
2. Humidification — air is nearly 100% saturated with water vapor before reaching alveoli
3. Filtration — cilia + mucus trap particles >6 µm

Mucus escalator: Cilia beat ~10–20 times/second, moving mucus and trapped particles toward the pharynx at ~1 cm/min.

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CHAPTER 40 — GAS EXCHANGE

Composition of Alveolar Air vs Atmospheric Air

1. Alveolar air is only partially replaced each breath
2. O₂ is constantly absorbed into pulmonary blood
3. CO₂ is constantly added from pulmonary blood
4. Air is fully humidified (water vapor dilutes other gases)

Respiratory Unit (Lobule)

• ~300 million alveoli in both lungs
• Average alveolar diameter: 0.2 mm
• Alveolar capillaries form a near-solid sheet ("sheet of flowing blood")

The Respiratory Membrane (Blood-Air Barrier)

1. Surfactant fluid layer — reduces surface tension
2. Alveolar epithelium (mainly Type I pneumocytes)
3. Epithelial basement membrane
4. Interstitial space
5. Capillary basement membrane (fuses with epithelial BM in many places)
6. Capillary endothelium

Thickness: 0.2–0.6 µm (as little as 0.2 µm!) Total surface area: ~70 m² (equivalent to a 25×30 ft room floor) Total capillary blood volume: only 60–140 mL spread over that surface

Diffusion Coefficients (relative to O₂ = 1.0)

CO₂ diffuses 20× faster due to much higher solubility, despite larger molecular weight.

Diffusing Capacity of the Respiratory Membrane

• At rest: Mean pressure gradient = 11 mmHg → 11 × 21 = ~230 mL O₂/min transferred = exactly what resting body uses
• Increased during exercise by: ① capillary recruitment (opening dormant capillaries), ② improved V/Q matching

Ventilation-Perfusion (V̇A/Q̇) Ratio

• Physiological shunt: fraction of venous blood that passes through pulmonary capillaries without being oxygenated
• Normally ~2% of cardiac output bypasses alveoli through bronchial vessels → shunted blood

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CHAPTER 42 — CONTROL OF RESPIRATION

The Respiratory Center (Brainstem)

Pre-Bötzinger Complex

• Small region of the rostral VRG
• Contains spontaneously firing pacemaker neurons
• Projects to DRG and VRG
• Considered the central pattern generator for breathing rhythm
• Removal eliminates respiratory rhythm

How Inspiration is Generated (DRG)

• DRG neurons fire in a ramp pattern — signal increases gradually over ~2 seconds → lungs expand smoothly
• At the end of ramp, inspiration is "switched off" → passive expiration begins
• Between each inspiration: ~3 seconds of silence (expiratory pause)

Chemical Control of Breathing

Central Chemoreceptors

• Location: Ventral surface of medulla (chemosensitive area)
• Sensitive to: ↑ CO₂ → ↑ H⁺ in CSF
• CO₂ is the dominant controller of respiration
• Changes in O₂ have virtually no direct effect on the central respiratory center

A rise in arterial PCO₂ from 35 to 75 mmHg causes a massive increase in ventilation. By contrast, pH changes in normal range (7.3–7.5) change ventilation by <10%.

Peripheral Chemoreceptors

Peripheral chemoreceptors are a backup system for when tissues lack O₂. The body uses CO₂ (not O₂) as the primary respiratory drive because O₂-Hb buffering keeps tissue O₂ delivery stable over a wide range of PO₂ values. Only when PO₂ drops below ~70 mmHg do peripheral receptors significantly activate.

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QUICK REVISION TABLE — Important Values from Guyton

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