Mechanical Properties of the Lung (B2: W6) Flashcards
Describe the pulmonary tree
A series of airways, and each divides into two daugher airways
- Approximately 23 generations total
- Conducting airways - first 16 generations or so
When does the respiratory zone begin in the pulmonary tree?
At generation 17 and above
- Gas exchange occurs here
The diameter of the bronchioles decreases as they branch; what happens to the total cross sectional area?
Total cross sectional area increases dramatically
What is the volume of the conducting zone?
150 mL
- Anatomical dead space - no gas exchange
- Trachea, bronchi, bronchioles, and terminal bronchioles
- First 16 generations
What is the volume of the respiratory zone?
3 Liters (3,000 mL)
- Respiratory bronchioles, alveolar ducts, alveolar sacs
- Functional unit is called an acinus
- DIstance from terminal bronchiole to distal alveolus is only a few mm
What is the total lung capacity?
TLC = volume following maximal inspiration
- Vital capacity + Residual volume
What is residual volume?
RV = volume left after maximal expiration
What is vital capacity?
VC = Total lung capacity - Residual volume
- Inspiring as much as possible and expiring as much as possible
- Beyond peaks of tidal volume
- Upper limit for total capacity
What is the tidal volume?
VT = volume inspired under normal resting conditions
What is the functional residual capacity?
FRC = volume remaining at the end of normal tidal expiration
What is the expiratory reserve volume?
EVR = volume expelled during maximal forced expiration starting at the end of normal tidal inspiration
What is the inspiratory reserve volume?
IRV = volume inspired during maximal inspiratory effort starting at the end of normal tidal inspiration
What is the inspiratory capacity?
IC = volume inspired duing maximal inspiration starting at the end of normal tidal expiration
What comprises the total lung capacity in a person standing upright?
Half is inpiratory capacity (3.0 L) and half is functional residual capacity (3.0 L)
What happens to the distribution of the total lung capacity when a person lies down supine?
Functional residual capacity decreases
- Inspiratory capacity increases
- Inspiratory reserve volume: larger
- Expiratory reserve volume: smaller
- Tidal volume doesn’t really change
What is the reason for the changes in lung volumes while lying supine?
Abdominal contents push up more on the diaphragm → change in distribution
Which lung volumes cannot be measured using spirometry?
We can’t define zero, so can’t measure
- Functional residual capacity
- Once you calculate FRC via another method, you can calculate others
- Residual volume
- Total lung capacity
How is functional residual capacity calculated?
- Nitrogen washout (nitrogen dilution method)
- Start with air in the lungs and goes to the compartment
- Helium dilution
- Start with air in the container and go to the lungs
-
Plethymography
- Uses Boyle’s law
- During inspiration in a closed box: ∆Vbox = ∆Vlungs
- ∆Vlungs = FRC
What are the basics of pulmonary mechanics?
To understand the mechanics of breathing, one must understand
- The forces that affect the movement of air into and out of the lungs
- The resistances that must be overcome
What are the forces that move air in and out of the lungs?
- Positive pressure breathing
- Negative pressure breathing
Boyle’s law: Pressure (P) x Volume (V) = a constant (k)
How does positive pressure breathing work?
- Create a gradient: pressure outside is greater than pressure inside
- Ex: ventilator
- Not what we use normally
How does negative pressure breathing work?
- Pressure gradient: greater outside than inside
- Create negative pressure in the alveolar space
- Intrapleural space needs to be more negative than the alveolar space (< -10 cm H2O)
What is the most important muscle involved in respiration?
Diaphragm
- When it contracts, volume of the chest cavity increases
- Abdominal contents are forced down and forward
- Decrease in intrapleural pressure
- Inspiration is active, expiration is passive
How are the external intercostal muscles involved in inspiration?
They pull the ribs upward, which expands the chest cavity
What are the most important muscles for expiration?
Expiration is passive through relaxation
Can become active during exercise
- Abdominal
- Rectus abdominus
- Internal and external obliques
- Transversus abdominus
- Internal intercostals - pull the ribcage down
What two components are essential for creating the pressure gradient for breathing?
Atmosphere and alveolar space (driving pressure)
- Intrapleural space has to be negative
Describe the elastic properties to lung and to chest wall
- Lungs tend to recoil inwards
- Chest wall is typically going the opposite direction - springs outward with elastic recoil
- These properties create negative intrapleural pressure
How is transpulmonary pressure calculated?
Transpulmonary pressure = alveolar pressure - intrapleural pressure
- Positive transpleural pressure
- Due to elastic recoil - referred to as the elastic recoil pressure
- Changes in lung volume are due to changes in the transpulmonary pressure
When is the alveolar pressure (PA - pressue inside the lungs) is equal to the atomospheric or barometric pressure (PB)
At functional residual capacity (FRC)
What is the intrapleural pressure?
PIP - the pressure in the space between the lungs and the chest wall
- Intrapleural pressure is negative relative to the atmospheric pressure (PB)
What additional factors are associated with changes in the transpulmonary pressure?
Transrespiratory pressure (PRS) and transthoracic pressure (PCW)
- Transrespiratory pressure = alveolar pressure (PA) - atmospheric pressure (PB)
- Transthoracic pressure = intrapleural pressure (PIP) - atmospheric pressure (PB)
What happens to the intrapleural pressure during inspiration and expiration
- Inspiration
- Intrapleural pressure gets more negative
- Expiration
- Intrapleural pressure increases
What happens to the alveolar pressure during inspiration and expiration?
Alveolar pressure gets more negative
- As air moves into the lung, the pressure gradient becomes equilibrated
What does the slope of the pressure-volume relationship reflect?
Slope reflects compliance
- Change in volume for a given change in pressue
- Starting from residual volume and going to total lung capacity
How is compliance related to elastance?
Compliance is the inverse of elastance
- Properties of lung due to connective tissue and elastic fibers
- Compliance: ∆V/∆P
- Elastance: ∆P/∆V
What happens to compliance at higher lung volumes?
Slope/compliance becomes less at higher lung volumes
- As you inflate lungs, the fibers get more stretched
- Relationship gets flatter
What happens to the compliance of the lungs in the event of emphysema?
There is a breakdown of connective tissue of the lungs
- Compliance increases
- Lungs become more distensible - slope becomes steeper
What happens to the compliance of the lungs in the event of pulmonary fibrosis?
Compliance decreases with pulmonary fibrosis
- Compliance can vary in different disease states
What components are responsible for the elastic properties of the lungs?
Elastin and collagen fibers that surround the bronchi and alveoli
- The pressure in the intrapleural space is less than atmospheric because of the elastic recoil properties of the lungs
Explain the phenomenon of hysteresis
- The relationship between lung volume and intraplerual pressure differs between inspiration and expiration - known as hysteresis
- The lung volume at any given intrapleural pressure is greater during deflation (expiration) than it is during inlation (insparation)
- Compliance is greater during expiration
- Even when there is no trasnpulmonary pressure gradient, there is still some air in the lung (the volume is not zero)
How does inflating the lung with saline rather than air increase compliance and eliminate hysteresis?
Surface tension!
- Surface tension is due to the liqud film lining the alveoli
- Reflects the attractive forces that exist between adjacent molecules of liquid
- Greater than the forces that exist between liquid and gas molecules
- Creates a force that contributes tot the elastic recoil pressure of the lung
What defines the relationship between pressure, surface tension, and radius?
Laplace’s Law
- Pressure generated by surface tension is directly proportional to surface tension
- Pressure is indirectly proportional to the radius of the alveolus
What does Leplace’s law indicate about pressure and radius as alveoli get smaller?
Pressure created by surface tension is greater the smaller the radius
- Surface tension is greater in small alveoli
- Greater elastic recoil
Why do small alveoli not collapse, causing larger alveoli to get bigger until they burst?
Surfactant lines the alveoli
- Main component: dipalitoyl phosphatidylcholine (DPPC)
- Amphipathic: hydrophobic and hydrophillic
- These interactions create opposing forces that counteract the attractive forces of surface tension
- In small alveoli, the molecules are closer together and thus the repulsive forces are greater
- Surfactant tends to reduce the pressure created by surface tension
Which cells secrete surfactant?
Type II alveolar cells
- Consists of lipids (85-90%) and proteins (10-15%)
- Amphipathic
What is the relationship between surfactant and hysteresis?
Hysteresis is due to surface tension, NOT surfactant
- By reducing surface tesion, surfactant actually reduces hysteresis
- Surfactant also increases compliance
What is the significance of surfactant in premature infants?
Don’t produce enough surfactant
- Infant respiratory distress syndrome (RDS)
- Common in infants born more than 6 weeks prematurely
- Affects nearly all infants born more than 12 weeks prematurely
- Alveoli can collapse
- Decrease in compliance so it is harder to get air in
What is the cause of acute respiratory distress syndrome?
Hypoxia/hypoxemia leads to a decrease in surfactant
- Increases effort requried to inflate lungs because of decreased compliance
- Increases tendency for alveoli to collapse
What happens in the event of a pneumothorax?
There is a hole in the chest wall
- Lung tends to contract inward, so it shrinks
- Chest tends to contract outward, so the volume of the chest cavity will tend to increase and expand
- Equalizes the atmospheric and intrapleural pressures
How is functional residual capacity determined?
FRC is determined by the balance between outward elastic recoil properties of the chest wall and the inward elastic recoil properties of the lung
- An increase in airway pressure expands the lung
- It’s natural state is to collapse
- Chest wall is the opposite
- At equillibrium, it expands and volume is high
How do emphysema and fibrosis affect the compliance of the lung?
- Emphysema can increase compliance
- Fibrosis can decrease compliance
- More pressure required for a change in volume
What happens to the functional residual capacity with emphysema?
- Lung can’t compete with the elastic properties of the chest wall, and the chest wall will win out
- Upward shift in curve
- Functional residual capacity increases dramatically
- Vital capacity will be reduced
Too inflated
What happens to the functional residual capacity with fibrosis?
- The lung becomes less compliant
- Equilibrium point shifted down
- Functional residual capacity is decreased
- Vital capacity is reduced
Cannot fully inflate
What is the cause of regional differences in ventillation of the lungs?
The effect of gravity contributes to a gradient of intrapleural pressure
- At the base, where the effect of the weight is the greatest, the intrapleural pressure is less negative than it is at the apex
- As a consequence, the alveoli in the base of the lung are more compressed
Why are alveoli at the base of the lung ventilated better than those in the apex?
- Alveoli at base are operating at a low volume where the lung is very compliant
- Small changes in transmural pressure tend to cause a greater change in volume
- Alveoli at the apex are operating at a higher volume where compliance is lower
- Small changes in transmural pressure tend to cause less of a change in volume
What happens to regional ventilation at lower than normal lung volumes
- Intrapleural pressures are different
- Gradient of pressure is different
- Intrapleural pressure at the base can exceed atmospheric pressure
- Alveoli can collapse
- Transpulmonary pressure may not be sufficient to inflate them
- Alveoli in the apex may be operating at a volume where compliance is high, so small changes in pressure cause significant changes in volume
- Ventilation is better at the apex at low pressure!
What determines regional ventilation?
Regional ventilation epends on conditions that you are breathing under, and the volume of the lung to begin with
- Has to do with graidents of interapleural pressure and the compliance
- When things are collapsed, there is no exchange
What factors are involved in the mechanics of breathing?
- The forces that affect the movement of air into and out of the lungs
- The resistances tha must be overcome
- Elastic resistance (65%)
- Non-elastic resistance (35%)
- Airflow (30%)
- Viscous (5%)
Describe airflow through the lungs during normal circumstances
- Airflow is typically laminar, especially at lower flow rates
- Turbulent flow can occur at higher flow rates
- Harder to move air in and out when air is turbulent
What is one of the most important ways of affecting airflow?
Changing the radius
- Pressure-flow relationship for laminar airflow is described by Poiseuille’s law:
V = ∆Pπr4 / 8nl
- Ex: doubling the radius increases airflow 16 fold
What is the most important factor for altering turbulent airflow?
Pressure gradient
- Flow is proportional to the square root of the change in pressure (∆P)
WHat determines if airflow is laminar or turbulent?
Calculate Reynold’s number (Re):
Re = 2rvd / n
d = density
r = radius
v = velocity (major player)
n = viscosity
Where and when is turbulent flow more likely to occur?
- Turbulent flow is most likely to occur when
- Velocity is high
- Radius is large
- Gas is dense
- Turbulent flow tends to occur in the trachea at high flow rates
- Radius is large
- Ex: during exercise
- Laminar flow is most likely to occur in smaller airways, like terminal bronchioles
Why does airway resistance not increase along the bronchial tree as airways become smaller?
- Greatest resistance to airflow is not just determined by radius
- Total cross sectional area increases with continued branching
- Large number of airways
Where is the point of greatest airway resistance?
Resistance peaks at the medium sized bronchioles (5-7th generation)
- Decreases with further branching
- Cross sectional area increasing
Dow does lung volume affect airway resistance?
- Bronchi are supported by the radial traction of the surrounding lung tissue, and their diameter increases as the lung expands
- At low lung volues, small airways can completely close
- Especially at the base of the lung
- Patients with increased airway resistance often breathe at high lung volumes in an attempt to reduce that resistance
How do nerve innervation and inflammation affect airway resistance?
Bronchial smooth muscle contraction regulates airway radius and thus resistance
- Sympathetic ß2 adrenergic stimulation (Epi) - causes relaxation of smooth muscle and increases airway diameter
- Decreases resistance to airflow
- Parasympathetic muscarinic stimulation (ACh) - causes contraction of the smooth muscle and decreases diameter
- Increases resistance to airflow
- Inflammatory mediators (leukotrienes and histamine) - released during asthma attacks or allergic responses and cause bronchial smooth muscle constriction
- Increases resistance to airflow
How does the respiratory effort affect airway resistance?
- Increased amount of expiratory effort causes changes in rate of flow out of lungs
- Effort dependence happens early on
- There can be a decrease in airway diameter and an increase in airway resistance
- Due to dynamic compression of airways
- There is a return to a point of effort independence
What is the limiting factor for dynamic compression of the airways?
The limiting factor is the effect that the increase in intrapleural pressure has on the transpulmonary pressure along the way
What are the conditions pre-inspiration regarding dynamic airway compression?
- Intrapleural pressure is negative (-5)
- Alveolar pressure is at equilibrium with atmospheric pressure (0)
- Transpulmonary pressure is positive (+5)
- Transpulmonary pressure gradient is uniform (0-0)
What are the conditions during inspiration in regards to dynamic airway compression?
- Intrapleural pressure becomes more negative (-7)
- Alveolar pressure is not at equilibrium with atmospheric pressure
- Transpulmonary pressure gradient is no longer uniform
- More positive towards the mouth, increasing airway diamter and reducing resistance
- This is why the effor dependence of inspiration always increases with the amount of effort you apply
What are the conditions at the end of inspiration in regards to dynamic compression of airways?
- Intrapleural pressure is still more negative that preinspiration
- Alveolar pressure is again at equilibrium with atmospheric pressure
- Transpulmonary pressure gradient is uniform once again
What happens during forced expiration due to dynamic compression of the airways?
- Intrapleural pressure increases dramatically
- Alveolar pressure is not at equilibrium with atmospheric pressure
- Transpulmonary pressure gradient is no longer uniform
- Moving closer to the mouth, the transpulmonary pressure drops
- Differences in pressure causes airways to collapse
- Increase in resistance to airflow
- This is the reason for the effort independent component of the forced expiration curve
What does the effort independent component of the forced expiration curve signify?
No matter how hard you try, you cannot increase airflow because you are starting to collapse the airways
What happens during forced expiration in the case of chronic obstructive pulmonary disease and emphysema?
- Elastic recoil is reduced and compliance is increased
- Pressures inside airways are different, even with the same intrapleural pressures, because the lungs are more compliant and less elastic
- Transpulmonary gradient (from alveolus to mouth) may become negative and obstruct airflow
- Can’t get all of the air out of the lungs
