Respiratory Mechanisms Flashcards
Tidal Volume
Volume of air inspired or expired with each normal breath
Inspiratory Reserve Volume (IRV)
Volume that can be inspired over and above tidal volume
Used during exercise
Expiratory Reserve Volume
Volume that can be expired after the expiration of tidal volume
Residual Volume
Volume that remains after maximal expiration
Cannot be measured by spirometry
Inspiratory Capacity
The sum of tidal volume and inspiratory reserve volume
Functional Residual Capacity (FRC)
Sum of ERV and residual volume
Volume remaining in the lungs after tidal volume expiration
Includes residual volume so cannot be measured with spirometry
Vital Capacity or Forced Vital Capacity (FVC)
- Sum of tidal volume, ERV, and IRV
- Volume of air that can be forcefully expired after a maximal inspiration
Total Lung Capacity
- Sum of all 4 lung volumes
- Volume of air in the lung after maximal inspiration
- Includes residual volume so cannot be measured with spirometry
Define the mechanisms that determine the clinically important boundaries of lung volume (TLC, FRC, & RV).
- TLC: volume where static balance b/t maximal inspiratory force that can be generated by respiratory muscles and the expiratory force generated by the inward directed elastic recoils of the lung and chest wall
- FRC: volume at which the elastic recoil of the lung and chest wall are equal but opposite
- RV: volume where static balance achieved b/t max expiratory force that can be generated by respiratory muscles and the force generated by outward-directed elastic recoils of the lung and chest wall
Forced Expired Volume (FEV1)
- Volume of air that can be expired in the 1st second of forced maximal expiration
- FEV1 is normal 80% of FVC: FEV1/FVC=0.8
- In obstructive lung disease, such as asthma, FEV1is reduced more than FVC, so that overall ratio decreases
- In restrictive lung disease, such as fibrosis, FEV1 and FVC are reduced such that the ratio is either normal or increased
Surface Tension of the Alveoli
-Results from attractive forces between liquid molecules lining the alveoli
-Creates a collapsing pressure (LaPlace Law)
-Directly proportional to the surface tension
-Inversely proportional to alveolar radius
P=2T/r
-Large alveoli: low collapsing pressure and easy to keep open
-Small alveoli: high collapsing pressure & hard to keep open
-In the absence of surfactant, small alveoli have a tendency to collapse (atalectasis)
Surfactant
- Lines the alveoli and is synthesized by type 2 alveolar cells
- Reduces surface tension by disrupting intermolecular forces between water molecules
- Prevents small alveoli from collapsing
- increases compliance
- Consists of dipalmitoyl phosphatidylcholine
- Fetal surfactant production by week 35
- Sign of mature lungs: lectin:sphingomyelin ratio >2:1
- Neonatal respiratory distress syndrome can occur due to lack of surfactant
- atelectasis, difficulty re inflating, and hypoxemia
Causes and characteristics of obstructive lung disease (Emphysema) including the abnormalities in lung volumes
- tissue is distensible
- increased compliance
- But it is difficult to expel air from the alveoli
- Increased chest wall elastic recoil
- Decrease in lung elastic recoil due to degradation
- Barrel chested
- Increased TLC
Causes and characteristics of restrictive lung disease (Fibrosis), including the abnormalities in lung volumes are
- tissue is stiff
- increased elastic recoil
- Lung collapses and it is difficult to force air into the alveoli
- Decreased TLC
Define compliance (lung and chest wall)
C=V/P where compliance=volume/pressure
-Is the change in volume for a given change in pressure
-Pressure refers to transpulmonary pressure: the pressure difference across pulmonary structures
-Describes distensibility of the lung and chest wall
-Inversely related to elastance (amount of elastic tissue)
-Inversely related to stiffness
-Is the slope in the pressure volume loop
-
Determining Factors in compliance of the Lung
- Transmural pressure is alveolar pressure - intrapleural pressure
- When intrapleural pressure is negative, the lungs expand
- When pressure outside the lung is positive, the lung collapses and volume decreases
Hysteresis
- Inspiration follows a different volume-pressure curve than expiration
- In the middle range of pressures, compliance is greatest and lung is most distensible
- At high expanding pressures, compliance is lowest, the lungs are least distensible, and the curve flattens
- Identify the forces that generate the negative intra-pleural pressure,
- predict the direction that the lung and chest wall will move if air is introduced into the pleural cavity (pneumothorax) when the lung is at functional residual volume.
- Negative intra-pleural pressure generated by Transpulmonary pressure: alveolar pressure out - pleural pressure (chest wall) in
- If air introduced, lung collapses and chest wall expands
Compliance of the combined lung-chest system
- Compliance of the lung-chest system is less than that of the lungs alone or the chest alone
- At rest, lung volume is at FRC and airway pressure is equal to atmospheric pressure (0)
- Collapsing force on lung and expanding force on chest wall
- Equal and opposite
- As a result, intrapleural pressure is negative
- Pneumothorax causes lung to collapse and chest to expand
Changes in Lung Compliance in disease states
- Emphysema: Lung compliance increased and tendency of lung to collapse decreases
- At FRC lung collapse forcechest expand force
- Seek new lower FRC to reach equilibrium
Describe how airway resistance alters dynamic lung compliance.
- Resistance increase at lung volumes below FRC (less compliant)
- Resistance low with lung volumes above FRC (more compliant)
Driving force for Airflow
-Pressure difference between the nose/mouth and the alveoli
-Airflow is inversely related to airway resistance
Q=P/V
Resistance in AIrways
R=8nl/pi*r^4
- Main site of airway resistance is medium sized bronchi
- Not smallest airways b/c they add in parallel
- Bronchial smooth muscle affects:
- parasympathetics, asthma, anaphylaxis, and irritants decrease radius and increases resistance
- Sympathetics and agonists dilate airway via B2 receptors, increase radius and decrease resistance
- Lung volume alters resistance via radial traction on airways
- High volumes: greater traction and decreased resistance
- Low volumes: less traction and increased resistance
- Viscosity of inhaled gas
- turbulent flow:
- laminar flow has less resistance than turbulent flow.
- If turbulent, the pressure difference is increased to maintain flow, this response itself increases resistance.
Describe how FRC and residual volumes and FEV, FEV1, FVC, TLC, and flow volume curves are affected in restrictive and obstructive lung disease
Restrictive Lung Disease: pulmonary fibrosis -Decreased FVC -Decreased FEV1 -Decreased TLC -FRC decreased -RV decreased Obstructive: asthma, bronchiectasis, bronchitis and COPD -Decreased FEV1 -Increased TLC -FRC is increased -RV Increased
Define dynamic airway compression, and use this principle to explain the shift in the shape of flow volume curves that occur with COPD (chronic obstructive pulmonary disease).
- Dynamic airway compression is the result of the equal pressure point
- Equal pressure point (EPP) is the point where Intrapleural pressure and Alveolar pressure are equal.
- Large airways with cartilage only collapse partially
- Smaller airways without support will collapse when higher pressures are exerted
- Higher pressures during exhalation move air at great velocities at larger lung volumes
- In aging patients and patients with emphysema, elastic recoil of the lung is reduced.
- Pressures within the lung are lower than normal
- EPP is reached in small, collapsible airways
- Expiratory airflow is limited
Important Lung Products
- Surfactant: made by type II pneumocytes
- decrease alveolar surface tension
- increases compliance
- decreases work of inspiration
- Prostaglandins
- Histamine increases bronchoconstriction
- Angiotensin-converting enzyme (ACE)
- angiotensin I to angiotensin II
- inactivates bradykinin (ACE inhibitorys increases-cough)
- Kallikrein- activates bradykinin
