Part 2 mechanical properties of the lung and chest wall static and dynamic
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Part 2 Mechanical Properties of the Lung and Chest Wall: Static and Dynamic. Section I STATIC LUNG MECHNICS: The mechanical properties of a lung whose volume is not changing with time. 1. Pulmonary Volume and Capacity. Pulmonary Volumes. Tidal volume ( 潮气量)

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Part 2 Mechanical Properties of the Lung and Chest Wall: Static and Dynamic

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Part 2Mechanical Properties of the Lung and Chest Wall: Static and Dynamic

Section I


The mechanical properties of a lung whose volume is not changing with time

1. Pulmonary Volume and Capacity

Pulmonary Volumes

  • Tidal volume (潮气量)

    • Volume of air inspired or expired during a normal inspiration or expiration (400 – 500 ml)

  • Inspiratory reserve volume (补吸气量)

    • Amount of air inspired forcefully after inspiration of normal tidal volume (1500 – 2000 ml)

  • Expiratory reserve volume (补呼气量)

    • Amount of air forcefully expired after expiration of normal tidal volume (900 – 1200 ml)

  • Residual volume (残气量,RV)

    • Volume of air remaining in respiratory passages and lungs after the most forceful expiration (1500 ml in male and 1000 ml in female)

Pulmonary Capacities

  • A Capacity is composed of two or more volumes

  • Inspiratory capacity (深吸气量)

    • Tidal volume plus inspiratory reserve volume

  • Functional residual capacity (功能残气量, FRC)

    • Expiratory reserve volume plus the residual volume

  • Vital capacity (肺活量, VC)

    • Sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume

  • Total lung capacity (肺总量, TLC)

    • Sum of inspiratory and expiratory reserve volumes plus the tidal volume and residual volume

  • RV/TLC

  • Normally less than 0.25

  • Increase by the obstructive pulmonary disease (RV)

  • Increase during the restrictive lung disease (TLC)

Minute and Alveolar Ventilation

  • Minute ventilation: Total amount of air moved into and out of respiratory system per minute

  • Respiratory rate or frequency: Number of breaths taken per minute

  • Anatomic dead space: Part of respiratory system where gas exchange does not take place

  • Alveolar ventilation: How much air per minute enters the parts of the respiratory system in which gas exchange takes place

Dead Space

  • Area where gas exchange cannot occur

  • Includes most of airway volume

  • Anatomical dead space (=150 ml)

    • Airways

  • Physiological dead space

    • = anatomical + non functional alveoli

A tube = Airway (Trachea – Bronchi – Bronchioles)


A thin walled Sac = Alveolus




Blood Vessels

Basic Structure of the Lung



Formula: Total Ventilation = Dead Space + Alveolar Space VT = VD + VA


  • Anatomical Dead Space = Airways (constant)


Physiological =Anatomical Dead Space Dead Space +

Additional Dead Space


Similar Concept: Physiological Dead Space

Healthy Lungs:

Diseased lungs:

2. Lung Compliance

  • Lung compliance (CL)

    • is a measure of the elastic properties of the lung,

    • is a reflection of lung distensibility


  • Tendency to return to initial size after distension.

  • High content of elastin proteins.

    • Very elastic and resist distension.

      • Recoil ability.

  • Elastic tension increases during inspiration and is reduced by recoil during expiration.


  • Distensibility (stretchability):

    • Ease with which the lungs can expand.

    • The compliance is inversely proportional to elastic resistance

  • Change in lung volume per change in transpulmonary pressure.

  • DV/DP

  • 100 x more distensible than a balloon.

  • Specific compliance (比顺应性): the compliance per unit volume

Static lung compliance














Transpulmonary pressure (cmH2O)


Aspects of mechanics

that studies the lung in motion

1. Dynamic Compliance

Normal (with surfactant)

Saline Filled

Volume L



Without surfactant


Pleural Pressure



- 15

- 30 cm H2O

Volume-pressure curves of lungs filled with saline and with air (with or without surfactant)

Dynamic lung compliance

  • Is always less than static compliance

  • Increase during exercise

  • Increase during sighing and yawning

2.Airflow in Airways

Types of Flow

Laminar flow

  • … is when concentric layers of gas flow parallel to the wall of the tube.

  • The velocity profile obeys Poiseuille’s Law

Poiseuille and Resistance

  • Airway Radius or diameter is KEY.

  •  radius by 1/2  resistance by 16 FOLD - think bronchodilator here!!

  • The gas flow in the larger airways (nose, mouth, glottis, and bronchi) is turbulent

  • Gas flow in the smaller airway is laminar

  • Breath sounds heard with a stethoscope reflect the turbulent airflow

  • Laminar flow is silent

3. Gas Flow Resistance

  • Elastic Resistance

  • Inelastic Resistance

Elastic Resistance

  • Caused by

    • the elastic tissue of the lung and the thoracic wall

    • surface tension of the fluid that lines the inside wall of the alveoli

  • The elastic resistance caused by surface tension

    • are much more complex.

    • accounts for about two thirds of the total elastic resistance

  • Inelastic Resistance

  • comprises

    • airway resistance (friction)

    • pulmonary tissue resistance (viscosity, and inertia).

  • the airway resistance account for 80%-90% of the inelastic resistance

    • the more important both in health and disease.

Airway Resistance

  • Airway resistance is the resistance to flow of air in the airways

  • due to :

    • internal friction between gas molecules

    • 2) friction between gas molecules and the walls of the airways

Factors that influence airway resistance

  • Airway diameter

    • asthma (哮喘)and parasympathetic stimulation: Narrowing airways.

    • Emphysema (肺气肿): decreases small airway diameter during forced expiration

  • Turbulence air flow

    • Rapid breathing:

  • Density and viscosity of the inspired gas

Control of Airway Smooth Muscle

  • Neural control

    • Adrenergic beta receptors causing dilatation

    • Parasympathetic-muscarinic receptors causing constriction

    • NANC nerves (non-adrenergic, non-cholinergic)

      • Inhibitory release VIP and NO  bronchodilitation

      • Stimulatory  bronchoconstriction, mucous secretion, vascular hyperpermeability, cough, vasodilation “neurogenic inflammation”

Control of Airway Smooth Muscle

  • Local factors

    • histamine binds to H1 receptors-constriction

    • histamine binds to H2 receptors-dilation

    • slow reactive substance of anaphylaxis (过敏反应)- constriction-allergic response to pollen

    • Prostaglandins (前列腺速) E series - dilation

    • Prostaglandins (前列腺素)F series - constriction

Control of Airway Smooth Muscle


  • Environmental pollution

    • smoke, dust, sulfur dioxide, some acidic elements in smog

  • Elicit constriction of airways

    • mediated by:

      • parasympathetic reflex

      • local constrictor responses

4.Measurement of Expiratory Flow - FVC

FVC - forced vital capacity

  • Defines maximum volume of exchangeable air in lung (vital capacity)

    • forced expiratory breathing maneuver

    • requires muscular effort and some patient training

  • Initial (healthy) FVC values approx 4 liters

    • slowly diminishes with normal aging

FVC - forced vital capacity(cont)

  • Significantly reduced FVC suggests damage to lung tissue

    • restrictive lung disease (fibrosis,纤维化)

    • constructive lung disease

    • loss of functional alveolar tissue

  • Intra-subject variability factors

    • age

    • sex

    • height

    • ethnicity

FEV1 - forced expiratory volume

(1 second)

  • maximum air flow rate out of lung in initial 1 second interval

    • forced expiratory breathing maneuver

    • requires muscular effort and some patient training

  • FEV1/FVC ratio

    • normal FEV1 about 3 liters

    • FEV1 needs to be normalized to individual’s vital capacity (FVC)

    • typical normal FEV1/FVC ratio = 3 liters/ 4 liters = 0.75

FEV1 - forced expiratory volume (1 second)

  • Standard screening measure for obstructive lung disease

    • FEV1/FVC reduction trend over time (years) is key indicator

    • calculate % predicted FEV1/FVC (age and height normalized)

  • Reduced FEV1/FVC suggests obstructive damage to lung airways

    • episodic, reversible by bronchodilator drugs

      • probably asthma (哮喘)

    • continual, irreversible by bronchodilator drugs

      • probably COPD (chronic obstructive pulmonary disease,慢性阻塞性肺病)


1 sec

Volume (litres)

Forced Expiratory Volume in 1 sec - FEV1

Time (sec)

Total Lung Capacity

Forced Vital Capacity - FVC

Residual Volume

Normal Lung Volume

Lung Volume in Restrictive Disease

Assessment of RESTRICTIVE Lung Diseases

These are diseases that reduce the effective surface area available for gas exchange

eg fibrosis / pulmonary oedema


RESTRICTIVE lung disease

Total Lung Capacity

Vital Capacity


Volume (litres)

Residual Volume

Time (sec)

Normal Airway Calibre

Airway Calibre in Obstructive Disease

Assessment of OBSTRUCTIVE Lung Diseases

These are diseases that reduce the diameter of the airways and increase airway resistance -

remember Resistance increases with 1/radius 4

eg asthma / bronchitis

Forced Vital Capacity - FVC

FEV1 > 80% of FVC

is Normal

or in words - you should be able to forcibly expire more than 80% of your vital capacity in 1 sec.

Forced Expiratory Volume in 1 sec - FEV1


1 sec

Total Lung Capacity

Volume (litres)

Forced Expiratory Volume in 1 sec - FEV1

Forced Vital Capacity - FVC

Residual Volume

Time (sec)

OBSTRUCTIVE lung disease

FEV1 < 80% of FVC

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