Pulmonary Mechanics Flashcards
total lung capacity (TLC)
sum of all lung volumes
MAXIMUM volume - after forced inspiration
tidal volume
volume moved in a normal breathing cycle
resting breath - NOT forced
small amount of TLC
residual volume (RV)
smallest volume possible in the lung inside of an intact chest
MINIMUM volume - after forced expiration
maintained by coupling of lung to chest wall
minimum volume (MV)
smallest volume possible in the lung of an open chest
absolute minimum value - requires uncoupling of lung to chest wall
inspiratory reserve volume (IRV)
the volume that can be moved during a forced inspiration up to total capacity
expiratory reserve volume (ERV)
volume that can be moved during a forced expiration down to residual volume
functional residual capacity (FRC)
volume left in the lungs at the end of a passive respiration
normal resting lung volume
maintained by equilibrium between inward lung recoil and outward chest wall recoil pressures (creates negative pressure)
what is the purpose of functional residual capacity
acts as a buffer of oxygen for gas exchange - prevents immediate hypoxia if you stop breathing
TLC equation
TLC = RV + ERV + VT + IRV
vital capacity (VC)
sum of all volumes in the lung that can be moved
IRV equation
IRV = TLC - VT
ERV equation
ERV = VT - RV
FRC equation
FRC = RV + ERV
VC equation
VC = VT + ERV + IRV
what drives air movement
pressure gradients (moves from high to low)
generated by the contraction/relaxation of inspiratory muscles
what are the inspiratory muscles
diaphragm and external intercostal muscles
inspiration
active process initiated by. the contraction of inspiratory muscles
steps of inspiration
- diaphragm and intercostal muscles contract, which increases the volume in the thoracic cavity
- increasing volume causes pressure inside thoracic cavity to drop, which decreases pressure in the pleural space and alveoli
- air moves from outside (high PB) to inside alveoli (low PA)
expiration
passive process initiated by the relaxation of inspiratory muscles
steps of expiration
- diaphragm and intercostal muscles relax, causing volume to decrease in thoracic cavity
- decreasing volume causes pressure to increase inside pleural space and alveoli
- air moves from inside (high PA) to outside (low PB)
how does expiration differ in horses
horses have ACTIVE end-expiration (contraction of expiratory muscles) during normal breaths
artificial ventilation
use of an external ventilator to generate positive pressure to force air into the lungs, causing an increase in pleural/alveolar pressure during inspiration
how does artificial ventilation differ from normal ventilation
pressure gradient is generated by an outside machine NOT by contraction of inspiratory muscles
causes pressure to increase in pleural space/alveoli during inspiration
what are the two mechanical properties of the airways and lungs
- elastic (compliance)
- resistance
compliance
the ease by which the elastic structures of the respiratory system stretch
what two components of the respiratory system determine compliance
chest wall
lungs
compliance equation
C = deltaV / deltaP
delta P: the pressure difference from beginning to end of breath
if C is high –> low pressure required to expand lungs
if C is low –> high pressure required to expand lungs
resistance
obstructions to air movement
USUALLY related to airway diameter
high resistance = more pressure required to inflate lungs
what two components of the respiratory system determine resistance
airways
tissues
elastic properties of the lungs
strong inward recoil caused by:
1. elastic composition (elastin + collagen)
2. alveolar surface tension
elastic properties of the chest wall
outward recoil due to relaxation at rest
why do the lungs and chest wall have opposing elastic properties
coupling of the chest wall and the lungs generates a positive pressure gradient across the alveoli, which keeps the alveoli open
what happens if the lung and chest wall become uncoupled
lung retracts/collapses, chest wall expands
causes a loss of the pressure gradient leading to inability to breath
atelectasis
collapse of alveoli that causes a decrease in the amount of tissue available for gas exchange
LaPlace’s law
P = 2T / r
(T = tension; constant
r = alveoli radius)
THEORY that low radius (small) alveoli should be higher pressure than high radius (large) alveoli, causing air to flow from small to large alveoli, which collapses the small and overinflation of the large alveoli
does NOT happen in normal lungs due to coupling of lungs to chest wall and production of surfactant
surface tension
the tendency of fluid-lined surfaces to “contract” due to the greater attraction of liquid molecules to each other than to air molecules
present at all fluid-gas interfaces
THEORY - should cause alveoli to collapse, but does not happen due to surfactant production
surfactant
phospholipid-protein-CHO complex that covers alveolar walls to reduce surface tension
acts as a detergent that lowers surface tension and minimizes the effects of La Place’s law by creating equal pressures in large and small alveoli
is surfactant more effective at high or low lung volumes
lower lung volumes
what would happen without surfactant
- alveolar collapse
- decreased compliance
- pulmonary edema
requires external ventilation
pressure-volume (PV) curves
graphs the pressure required to generate a certain amount of volume into the lung
what is slope on a PV curve
compliance (C = deltaV / deltaP)
steep slope = higher compliance
shallow slope = lower compliance
hysteresis
the difference between the inspiratory and expiratory path on the PV curve
caused by greater surfactant efficacy at lower lung volumes
function of PV curves
asses changes in compliance
if slope is low: low compliance; more pressure needed to achieve normal lung volumes
if slope is high: high compliance; less pressure needed to achieve normal lung volumes
what is total airway resistance determined by
- airway resistance
- tissue resistance
where does the majority of airway resistance come from
upper airways; especially medium sized bronchi
small bronchioles have smaller radius BUT larger total cross sectional area, so less resistance
in what scenario does tissue resistance affect the lungs
lung disease - makes the lungs heavier and more resistant to air flow
what are the three airflow patterns
- laminar
- turbulent
- transitional
laminar flow
- normal, straight flow
- LOW resistance
- inner air is faster than outer air
turbulent flow
- disorganized flow
- present in LARGE, HIGH VELOCITY airways and bifurcations/bends
- HIGH resistance
transitional flow
combination of laminar and turbulent flow
common in airways due to bifurcations
ohm’s law
R = deltaP / Q
calculates resistance as a function of pressure gradient and volume
poiseuille’s law
Q = (deltaP x pi x r^4) / 8nL
calculates flow as a function of pressure gradient, radius, viscosity, and airway length
what is the main determinant of air flow
RADIUS (airway diameter)
Reynold’s number
Re = 2rvd / n
determines when flow changes from laminar to transitional to turbulent
what causes an increase in reynolds number
increases in radius, velocity and density
more likely to become TURBULENT
what causes a decrease in reynolds number
increases in viscosity
less likely to become turbulent
how does the ANS affect airflow
controls airway radius by innervating bronchial smooth muscle
SNS: B2 receptors –> bronchodilation (dec. resistance)
PNS: muscarinic receptors –> bronchoconstriction (inc. resistance)
how does lung volume affect airflow
increases in lung volume = expands lungs = opens/dilates airways = decreases resistance = increases flow
what are the 5 factors affecting resistive properties
- radius of airways and gas viscosity
- artificial airway radius (ET tube size)
- flow pattern - turbulent vs laminar (reynolds number)
- lung volume
- ANS innervation
dynamic airway obstruction
obstructions to airflow that change based on inspiration or expiration
ex. collapsing trachea
what part of respiration is affected by an extra thoracic collapsed trachea
inspiration
what part of respiration is affected by an intra thoracic collapsed trachea
expiration
pressure gradients throughout respiratory cycle
- pre-inspiration (FRC): negative pressure in pleural space, 0 pressure in airways - POSITIVE PRESSURE GRADIENT
- inspiration: air flow follows positive pressure gradient from airways –> alveoli
- forced expiration: contraction of expiratory muscles increases pleural pressure (positive pleural pressure > airway pressure), leads to a normal decrease in airway diameter
total work of breathing
elastic work + resistive work
increased WOB = high elastic work (low compliance) + high resistance
what is elastic work
compliance
influenced by LUNG VOLUME
(high vol = high elastic work = dec. compliance)
examples of states with decreased compliance (increased elastic work)
obestiy
pneumonia
what is restrictive work
airway resistance
influenced by RESPIRATORY RATE - tries to decrease total WOB
(high RR = high RW = high resistance)
breathing at resting state
animal breathes at the rate and volume that results in the lowest WOB
breathing in states of increased elastic work (decreased compliance)
animal will increase RR and decrease lung volume to achieve the lowest total WOB
breathing in states of increased resistive work (increased resistance)
animal will decrease RR and increase lung volume to achieve the lowest total WOB
(ex. lower airway disease)
