Respiratory Physiology

Pulmonary
Mechanics

How pressure, compliance and resistance move air.

Guyton & Hall · 15th ed. · pp. 501–507 Lecture by Dr Rajeev Bajaj
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Learning objectives

Six ideas. One breath.

Pressures & the breathing cycle
Objectives 6 · 7
Compliance & elastic recoil
Objectives 8 – 11
Surface tension & surfactant
Objectives 12 · 13
Lung volumes & spirometry
Objectives 1 – 4
Airway resistance & work
Objectives 5 · 14 · 15
Obstructive vs restrictive
Objective 16
The bottom line

Three questions explain every breath.

1
What produces airflow?
A pressure difference
PA ≠ Patm
2
What changes alveolar pressure?
Lung volume
Muscles → chest shape → volume
3
How fast?
Flow = ΔP ÷ R
Pressure gradient vs resistance
Pressures

Four pressures

  • PaoAirway opening
  • PplPleural space
  • PAAlveoli
  • PbsBody surface
Air movement

Air flows from high to low pressure

Alveolar pressure (PA)
0.0mm Hg
−10 = Pao+1
Breathing cycle

One quiet breath, 4 seconds

Resting
PA0mm Hg
Ppl−2.5cm H₂O
VolumeFRCVT = 0
Transmural pressure

What keeps airways open?

PTM = Pairway − Ppl
  • Positive → airway held open
  • Forced expiration → negative → compressed
Ventilation

What influences air movement?

1
Size
Lung volume
2
Compliance
Ease of expanding
3
Elastance
Recoil to rest
4
Resistance
Ease of airflow
1 · Size

Volume sets pressure

  • Lung volume: sex, race, height
  • Boyle: P₁V₁ = P₂V₂ (T constant)
  • P and V vary inversely
Flow = PA − PatmR
Volume
1.00V
Pressure
1.00P
P × V
1.00
2 & 3 · Compliance & Elastance

Stretch vs recoil

  • Lungs are elastic → recoil
  • Compliance = ΔV ÷ ΔP
  • Compliance ∝ 1 ÷ elastance
Elastic recoil

Two forces pull the lung inward

Elastic fibres
Elastin & collagen
Surface tension
Alveolar lining fluid
Laplace  P = 2Tr
Pulmonary surfactant

Surface-active agent

  • ↓ Surface tension of alveolar fluid
  • Type II pneumocytes → lamellar bodies
  • Spreads as a monolayer
  • From week 25 · needs glucocorticoids
Pulmonary surfactant

Composition & turnover

Why a monolayer?
Pulmonary surfactant

Keeps small alveoli open

  • More concentrated in small alveoli
  • Prevents collapse → stable alveoli
  • ↓ Recoil → easier breathing
  • Deficient → atelectasis
Small alveolus
—P
Large alveolus
—P
Compliance · lungs alone

Compliance is a slope

  • Steep near rest → high compliance
  • ≈ 200 mL / cm H₂O (both lungs)
  • Flat at high volume → stiff
Compliance at the cursor
High
Compliance · chest wall alone

The chest wall wants to spring out

  • Holds volume at zero pressure
  • Hard to compress (negative P)
  • Easier to expand
Compliance · whole system

A tug-of-war sets FRC

  • Lung recoils in
  • Chest wall springs out
  • Balance = FRC
Intrapleural pressure

Why pleural pressure is negative

  • Opposite pulls on a sealed space
  • → Sub-atmospheric Ppl at FRC
  • Air enters → lung collapses, chest springs out
FRC in disease

Change one recoil, FRC moves

Functional residual capacity
Normal
Think it through

Tap a question to reveal

Lung mechanics

Static vs dynamic

Static
No airflow · time given
MeasuresLung volumes
Tells usCompliance
Dynamic
Airflow · forced effort
MeasuresRate of airflow
Tells usResistance
Volume spirometry

A bell that weighs your breath

  • Breathe into a bell over water
  • Exhale → bell up → pen down
  • Trace = volume vs time
Spirogram

4 volumes · 4 capacities

Spirometry's blind spot

Air you can never breathe out

  • RV — after maximal exhalation
  • FRC — after normal exhalation
  • TLC — after maximal inhalation
All contain RV
Lung volumes

One volume, two consequences

Compliance
Airway resistance
4 · Airflow resistance

Radius rules

R = 8 L ηπ r⁴
  • Narrow → ↑↑ R (1/r⁴)
  • Longer → ↑ R
  • More viscous → ↑ R
Resistance
×1.0
Flow
×1.00
Airway calibre

What changes airway resistance?

Constriction
Radius
×1.00
Resistance
×1.0
Dilation
Radial traction

Bigger lungs, wider airways

  • ↑ Volume → elastin pulls airways open
  • ↓ Recoil (emphysema) → ↑ resistance
Turbulence & density

Chaos costs pressure

  • Turbulence → ↑ resistance
  • Large airways: turbulent / mixed
  • Small bronchioles: laminar
  • Less dense gas → ↓ resistance
Relative resistance · illustrative
×2.5
Pressure–volume loop

Hysteresis

  • Inflation ≠ deflation path
  • Same pressure → more volume on expiration
  • Loop = extra force vs airway resistance
Work of breathing

Work = area

  • Compliance work — elastic
  • Resistance work — airflow
  • ↑ with ↓ compliance or ↑ resistance
Total work
×1.00
Dynamic spirometry

The flow–volume loop

  1. 1Inhale fully → TLC
  2. 2Blast out, all the way → RV
  3. 3Inhale hard → TLC
Effort

Push harder — up to a point

  • PEFR — effort-dependent
  • Late flow, FEF25–75% — effort-independent
  • Why? Dynamic airway compression
Spirometry in disease

Two patterns

↓ Compliance

Stiff lungs

Fibrosis · oedema · ↓ surfactant

TLCsmall ↓
FRClarge ↓
RVunaffected
↑ Compliance

Floppy lungs

Severe emphysema · loss of elastic tissue

TLCsmall ↑
FRClarge ↑
RV↑ air trapping
Clinical scenarios

What would happen if…

Summary

Remember five things

1
Flow = ΔP ÷ R — volume change creates ΔP
2
FRC = lung recoil in ⇄ chest wall out
3
Surfactant stabilises small alveoli
4
Radius⁴ dominates airway resistance
5
Obstructive ↓ FEV₁/FVC · Restrictive ↓ volumes
Assessment

Test yourself

Exam-style MCQs on the whole chapter — instant feedback, explanations and a topic-by-topic score.

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References

Thank you

Hall JE, Hall ME. Guyton & Hall Textbook of Medical Physiology, 15th ed. — Pulmonary Ventilation, pp. 501–507
Ninja Nerd — youtu.be/uYm4l_alVV0
Lecturio Medical — youtu.be/TsGAfub2wxA
Dr Matt & Dr Mike — youtu.be/RQb9GdJ7uQs

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