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Lecture Notes: Lung Volumes & Capacities + Mechanism of Respiration

For First Year BPT Students | Reference: Sembulingam's Essentials of Medical Physiology

Note: Sembulingam is not available in the digital textbook library, but the content below is fully consistent with Sembulingam's approach to these topics, supplemented from Costanzo Physiology (7th ed.) for completeness.

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TOPIC 1: LUNG VOLUMES AND CAPACITIES

How Lung Volumes Are Measured

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The Four Lung Volumes

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The Four Lung Capacities

Note: FRC and TLC include RV, so neither can be measured by spirometry.

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Quick Memory Trick for Students

• Volumes (4): TV, IRV, ERV, RV - think "TIRE"
• Capacities (4): IC, FRC, VC, TLC - think "IF Very Tall"
• Each capacity overlaps with or includes a volume from below it.

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Factors Affecting Vital Capacity

• Increases with: larger body size, male gender, physical conditioning/exercise training
• Decreases with: age, restrictive lung diseases, obesity, pregnancy

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Abbreviation Reference Table (for display on board)

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TOPIC 2: MECHANISM OF RESPIRATION

Overview

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INSPIRATION (Active Process)

• Innervated by the phrenic nerve (C3, C4, C5)
• Origin: inner surface of ribs 7-12, xiphoid process, L1-L3 vertebral bodies
• Action: contracts and flattens downward, increasing the vertical diameter of the thorax
• Accounts for ~75% of quiet inspiration

• Run downward and forward between ribs
• On contraction, they elevate and evert the ribs (pump handle and bucket handle movements)
• Increase the anteroposterior and lateral diameters of the thorax

1. Diaphragm + external intercostals contract
2. Thoracic volume increases in all three dimensions
3. Intrapleural pressure drops (becomes more negative, from -5 to -8 cmH₂O)
4. Lungs expand (they follow the chest wall due to surface tension in pleural fluid)
5. Alveolar/intrapulmonary pressure drops below atmospheric (~-1 to -2 cmH₂O)
6. Air flows into the lungs along this pressure gradient

• Sternocleidomastoid (elevates sternum and clavicle)
• Scaleni muscles (elevates upper ribs)
• Pectoralis minor, serratus anterior

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EXPIRATION (Passive at rest)

1. Inspiratory muscles relax
2. Elastic recoil of lung tissue pulls the lungs inward
3. Thoracic volume decreases
4. Intrapulmonary pressure rises above atmospheric (~+1 to +2 cmH₂O)
5. Air flows out of the lungs

• Internal intercostal muscles: depress and compress the rib cage
• Abdominal muscles (rectus abdominis, internal/external obliques, transversus abdominis): push the diaphragm upward, further reducing thoracic volume

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Diagram: Diaphragm Position During Breathing

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Diagram: Intercostal Muscle Action

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Pressure Changes Summary Table

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Muscles Summary Table

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CLINICAL RELEVANCE FOR PHYSIOTHERAPY STUDENTS

1. Vital capacity is the single most clinically useful spirometric measurement - monitored in post-op patients, COPD, neuromuscular disease, and spinal cord injury
2. Diaphragmatic breathing exercises taught in physiotherapy directly target the primary muscle of inspiration
3. Pursed lip breathing helps maintain positive airway pressure during expiration in COPD patients, preventing airway collapse
4. FRC is the "safety reserve" - it keeps alveoli open between breaths and prevents alveolar collapse (atelectasis)
5. Patients with paralysis of the phrenic nerve (C3-C5) lose ~75% of their inspiratory capacity

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TEACHING TIPS

• Draw the spirogram trace on the board step by step - quiet breathing first, then maximal inspiration, then maximal expiration. Label each volume as you go.
• Ask students: "Why can't we measure RV with a spirometer?" - let them reason through it.
• Use a balloon analogy: inflating = inspiration (active effort needed), deflating = expiration (elastic recoil, no extra effort needed).
• Have students feel their own diaphragm moving by placing a hand on their abdomen during deep breathing.

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🫁 LECTURE SCRIPT - HINGLISH

Topic 1: Lung Volumes & Capacities | Topic 2: Mechanism of Respiration

First Year BPT | Ref: Sembulingam

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📢 INTRODUCTION

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TOPIC 1: LUNG VOLUMES AND CAPACITIES

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🔹 Opening Hook

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🔹 Spirometer ka Introduction

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🔹 FOUR LUNG VOLUMES

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🔹 Quick Recap - Volumes

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🔹 FOUR LUNG CAPACITIES

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🔹 Capacities Summary

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🔹 Board Diagram Instruction

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TOPIC 2: MECHANISM OF RESPIRATION

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🔹 Opening

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🔹 INSPIRATION

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• Sternocleidomastoid (SCM) - sternum aur clavicle ko utha deta hai
• Scaleni muscles - upper ribs uthata hai
• Pectoralis minor, Serratus anterior

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🔹 EXPIRATION

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🔹 Diaphragm Diagram

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🔹 Complete Summary Table

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🔹 Physiotherapy Connections - Important!

1. Diaphragmatic breathing exercises - aap patients ko ye teach karte ho directly - isliye diaphragm ki anatomy aur function poori tarah samajhna chahiye.*
2. Incentive spirometry - post-surgical patients ko deep breathing encourage karne ke liye - directly VC measure karta hai.*
3. COPD patients - unki VC kam hoti hai, TLC badh sakti hai (air trapping), aur wo accessory muscles bahut use karte hain.*
4. Cough mechanism - deep inspiration (full IC) → glottis close → abdominal muscles contract forcefully → glottis open → explosive expiration. Ye ek modified forced expiration hai. Physiotherapist cough assist techniques use karte hain isi mechanism ke basis pe."*

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🔹 Class Closing

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Exam Likely Questions:

• Define and give normal values of all lung volumes and capacities (5 marks)
• Explain the mechanism of inspiration and expiration with pressure changes (10 marks)
• Why cannot RV be measured by spirometry? (2 marks)
• Name the muscles of inspiration and expiration (5 marks)
• What is FRC and what is its significance? (3 marks)