Respiratory System: Pressure-Volume Relationships and Lung Compliance

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Questions and Answers

What is the forced vital capacity?

The amount of air that enters the lungs but does not participate in gas exchange

The largest amount of gas that can be moved into and out of the lungs in 1 minute

The amount of air that normally moves into (or out of) the lungs with each respiration

The amount of air expired after maximal inspiratory effort (correct)

Where is surfactant produced?

In the lungs of heavy smokers

In the elastic tissue resistance

In alveolar type I cells and secreted into the alveolus (correct)

In the airway during bronchial smooth muscle contraction

How does airway resistance behave?

Does not affect the work of breathing

Makes up 80% of the work of breathing

Is increased following bronchial smooth muscle contraction (correct)

Is increased in paraplegic patients

What happens to dynamic compliance in obstructive disease?

Decreases as respiratory rate and expiratory flow increase Signup and view all the answers

Where is the pressure outside the airway downstream from the equal pressure point?

Less than the driving pressure inside the airway Signup and view all the answers

What does static compliance represent?

The ease with which lungs can be inflated when no airflow is present Signup and view all the answers

What does high compliance in the respiratory system indicate?

Easier inflation of the lungs Signup and view all the answers

In the respiratory system, what is the most likely cause of low compliance?

High elastance Signup and view all the answers

Why are alveoli more compliant at low volumes in the respiratory system?

Decreased elastic recoil Signup and view all the answers

How does lung compliance change with increased breathing frequencies in obstructive diseases?

Increases dramatically Signup and view all the answers

Which statement is true about variations in compliance in the respiratory system?

Lungs are more difficult to stretch with decreased compliance. Signup and view all the answers

Which condition results in decreased lung volume due to excess fibrous tissue?

Fibrosis Signup and view all the answers

What is the major impact of emphysema on lung compliance?

Increases distensibility Signup and view all the answers

How is static compliance measured in the respiratory system?

$Cstat = Vt/(Pplat - PEEP)$ Signup and view all the answers

Which term describes the tendency of tissues to oppose stretch and increase at higher lung volumes?

Elastance Signup and view all the answers

What is the primary function of pulmonary surfactant in the respiratory system?

Stabilize alveoli and prevent collapse Signup and view all the answers

What is the significance of the functional residual capacity (FRC)?

It is the volume where outward recoil of the chest wall equals the inward recoil of the lungs Signup and view all the answers

What happens during dynamic compression of small airways?

Collapsing of small airways after the equal pressure point Signup and view all the answers

In emphysema, why is there great difficulty in achieving high airflow rates?

Less alveolar elastic recoil and less traction to oppose dynamic compression Signup and view all the answers

What does the term 'equal pressure point' refer to during forced expiration?

When driving pressure equals surrounding peribronchial pressure Signup and view all the answers

How does airway resistance vary with lung volume?

Airway resistance increases at higher lung volumes Signup and view all the answers

What happens to small airways during forced expiration with an open glottis?

They undergo dynamic compression and collapse Signup and view all the answers

Why is negative pressure breathing a concern regarding upper airway obstruction?

It can obstruct airflow by pulling upper airway inward Signup and view all the answers

How does distribution of airway resistance differ in upper airways compared to lower airways?

(35-50%) in both regions, but lower resistance in parallel small airways Signup and view all the answers

Study Notes

Pressure-Volume Relationships in the Respiratory System

Compliance (C) measures the ease of lung distension: C = ΔV/ΔP
- High compliance indicates loss of elastance; low compliance indicates high elastance
- Compliance is affected by elastic tissue recoil and surface tension recoil
Non-linear aspects of compliance:
- Compliance decreases at high lung volumes (alveoli become less compliant)
- Compliance increases at low lung volumes (alveoli become more compliant)
- At FRC (functional residual capacity), the compliance curve becomes linear

Types of Compliance

Static compliance (measured during no airflow)
Dynamic compliance (measured during airflow, during inspiration and expiration)
Hysteresis: difference between inspiration and expiration curves

Variations in Compliance

Increased compliance: lungs are easily stretched (e.g., obstructive disease)
Decreased compliance: lungs are difficult to stretch (e.g., restrictive disease)
Units of compliance: L/cm H2O or ml/cm H2O; normal value: 0.2 L/cm H2O

Histology of Abnormal Compliance

Conditions affecting compliance:
- Fibrosis: decreased compliance due to excess fibrous tissue
- Emphysema: increased compliance due to destruction of elastic septa

Static compliance: measured in the absence of gas flow (Cstat = Vt/(Pplat - PEEP))
Dynamic compliance: measured in the presence of gas flow (Cdyn = Vt/(Ppeak - PEEP))

Clinical Evaluation of Lung Compliance - Specific Compliance

Specific compliance: compliance relative to lung volume (C/FRC)
Used to standardize compliance for lung size
Example: calculating specific compliance for two lungs and one lung

Elastance

Tendency to oppose stretch
Elastic recoil of alveolar walls increases at higher lung volumes
Increases elastance compresses alveolar gas, raising pressure above atmospheric pressure (during exhalation)

Elastic Recoil of Lungs - Inward

Recoil due to elastic tissue follows Hooke's law (F = k * x)
Elastic tissue in lungs: F = ΔIPP, x = Δlung volume

Surface Tension

Accounts for 2/3 of total elastic recoil forces in normal lungs
Surface tension forces attempt to collapse lungs
La Place's law: P = 4T/r (for spherical bubbles) or P = 2T/r (for alveoli with one air-liquid interface)
Surface tension affects alveolar stability

Surfactant

Surface active agent at the fluid surface of the inner lining of alveoli
Decreases surface tension
Derived from type II alveolar epithelial cells
Composition: phospholipids (80%), cholesterol (10%), and surfactant proteins (10%)
Surfactant:
- Increases alveolar compliance
- Prevents atelectasis
- Aids in keeping alveoli dry
Surfactant deficiency:
- Conditions: atelectasis, failure of normal lung expansion in premature neonates, pulmonary edema, respiratory distress syndrome
- Causes: immature lung, hypoxia, interrupted blood supply

Alveolar Interdependence

Mechanical interdependence stabilizes alveoli and opposes collapse
Elastic septa and capillaries

Atelectasis

Caused by respiratory changes during anesthesia
Decreased FRC, compliance, and increased resistance
Prevention methods: positive end-expiratory pressure, recruitment maneuvers, minimizing gas resorption, maintaining muscle tone

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Description

Learn about the pressure-volume relationships in the respiratory system, focusing on lung compliance. Understand how compliance affects the distensibility of the lungs, and the impact of elastance on high and low compliance levels.