overview of pulmonary function Flashcards
total ventilation definition
the total volume of air leaving the lung each minute, measured in ml per minute
is the volume inspired greater than the volume expired?
yes, as more oxygen is taken in than carbon dioxide given out
dead space definition
the volume of air that is inhaled that does not take part in gaseous exchange
name two different types of dead space
anatomical and alveolar
explain anatomical dead space
the portion of airways, such as the mouth and trachea that conducts gas to the alveoli but no gaseous exchange occurs here as there is no perfusion
alveolar dead space explained
the sum of the volumes of those alveoli that have little or no blood flowing through their adjacent pulmonary capillaries, so alveoli that are ventilated but not perfused, and thus where no gas exchange can occur
alveolar ventilation definition
the volume of fresh gas entering the respiratory zone each minute (removing the volume that remains in the anatomical dead space)
partial pressure definition
the pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature
why is the partial pressure of gases important?
it is a measure of the gas molecule’s thermodynamic activity
gases dissolve and diffuse and react according to their partial pressures, not their concentration
unit of partial pressure
pascals, atm, kPa, units of gas per volume
resting oxygen consumption
250ml/min
maximum oxygen consumption
3000ml/min
respiratory quotient definition
number used in calculations of the basal metabolic rate, estimated from carbon dioxide production relative to the oxygen consumed by the body
normal alveolar partial pressure of oxygen
90 mmHg, 12kPa, range 80-100mmHg, 11-13 kPa
normal alveolar partial pressure of carbon dioxide
40mmHg 5.3kPa, range 35-45mmHg, 4-6kPa
what is the normal alveolar partial pressure similar to?
normal arterial values
alveolar gas equation definition
an equation that allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable
tidal volume definition
the volume of air that moves into and out of the lungs in one normal breath, 500ml
vital capacity definition
amount of gas that can be exhaled by maximal inspiratory effort, following maximal inhalation, 5L
residual volume definition
volume of gas in the lung after maximal expiration, 3L
functional residual capacity definition
the volume of gas in the lung after a normal expiration
total lung capacity definition
the total volume of the lungs at the end of a maximal inspiratory effort
how is tidal volume measured?
using a spirometer
- patient breathes into a sealed container
- water level moves up and down as the gas is exhaled and inhaled
- the changes in volume can be recorded by a pen or recording the apparatus of a computer
what cannot be measured using spirometry? + why?
functional residual capacity and residual volume
as the air remains in dead space
3 methods that can measure the functional residual capacity
gas dilution technique
nitrogen washout
body plethysmography
explain the gas dilution technique
- the subject is connected to a spirometer containing a known concentration of helium, which is virtually insoluble in blood
- after some breaths, the concentration of helium in the lungs will equal that of the spirometer
- as no helium is lost, the amount of helium before equilibration is the same as after equilibration
- can use C1 x V1 = C2 x( V1 + V2)
in practice, oxygen is added to the spirometer to make up for lost oxygen and carbon dioxide absorbed by soda lime
only measures communicating gas of the ventilated lung volume
explain nitrogen washout
- subject connected to spirometer breathing 100% oxygen
- expired gases are collected until the expired nitrogen is zero
- then the N2 content of all the expired gas is measured, since the nitrogen content of the lungs is 80% the total lung volume can be determined
explain body plethysmography
- subject sits in a large airtight box
- at the end of normal expiration, a shutter closes the mouthpiece
- when the subject makes inspiratory effects against a closed airway, the volume of the lungs slightly increases
- as the subject tries to inhale, the lung volume increases and the pressure inside the lungs decrease. The box pressure rises as its gas volume decreases
- Boyle’s Law states that pressure x volume is constant
- the total volume of gas in the lung, including any trapped behind closed airways, can then be measured.
intra pleural pressure definition
the pressure within the pleural cavity, between the lungs and chest wall
is intrapleural pressure positive or negative? +why?
intra pleural pressure is slightly negative, less than atmospheric to enable a transpulmonary pressure to expand the lungs, preventing the lungs from collapsing in on themselves
transpulmonary pressure definition
the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity
pneumothorax definition
an abnormal collection of air in the pleural space, between the lung and the chest wall which can cause a collapsed lung
compliance definition
the volume change per unit of pressure, or the slope of the pressure-volume curve. A measure of the lungs ability to stretch and expand
normal compliance
200ml/cm H2O
explain normal compliance
measure of the pressure required to inflate the lung by a certain volume
during normal breathing, low airway resistance and other frictional forces reduce the compliance of the lungs, however this is small
explain reduced compliance
can be caused by an increase in fibrous tissue, pulmonary fibrosis, increase in pulmonary venous pressure, alveolar oedema, preventing the expansion of alveoli
this makes inhalation difficult
at higher expanding pressures, lung becomes stiffer and compliance is lower
what else can alter compliance + why is this a negative?
increased compliance in pulmonary emphysema and age, the alveoli are damaged and enlarged
not elastic enough to exhale so exhalation becomes difficult
total compliance definition
the compliance of the lung and chest wall
surface tension definition
the tendency of a liquid surface to shrink to its minimum surface area where possible
it arises because the attractive forces between adjacent liquid molecules are much stronger than those between liquid and gas
cohesion is greater than adhesion
where does surface tension occur in the lungs?
the film which lines the alveoli
what does surface tension cause?
tends to draw the surface molecules into the bulk of the liquid, minimising SA and producing a collapsing force
what is Laplace’s law?
used to predict the pressure inside spheres, relating it to the surface tension of the liquid (T) and its radius
pressure= 2T/r
what is used to overcome surface tension?
pulmonary surfactant
pulmonary surfactant definition?
a surface active lipoprotein complex secreted by type II alveolar epithelial cells which greatly lowers the surface tension of the alveolar fluid lining
surfactant structure and how this aids function
proteins and lipids making up surfactants have hydrophobic and hydrophilic regions
the phospholipids adsorb to the air liquid interface in the alveoli with the hydrophilic heads interacting with the water and the hydrophobic tails interacting with the air
the intermolecular repulsive forces oppose the normal attractive forces between the liquid surface molecules which are responsible for surface tension
3 roles of surfactant in lungs
- low surface tension produced by surfactant increases the compliance of lungs and reduces the work needs for inflation
- increases the stability of alveoli, without surfactant, upon inhalation the alveoli would expand, however their surface tension surface area would also increase, causing them to collapse
- keeps alveoli dry, reduces hydrostatic pressure in the tissue outside the capillaries, increasing transudation and thus prevents fluid accumulation
what is the main lipid in surfactant?
dipalmitoyl phosphatidylcholine DPPC
evidence for the link between compliance and surface tension
saline filled lungs have a much greater compliance than air filled lungs
saline abolishes the surface tension forces in the alveoli , not the lung, showing how important the surface tension in the alveoli is for the elastic recoil force of the lung
infant respiratory distress syndrome explained
believed to be caused by insufficient pulmonary surfactant
should be secreted from type II alveolar cells from 30 weeks gestation in humans
loss of surfactant results in stiff lungs, areas of atelectasis (collapse) and alveoli filled with transudate pulmonary oedema, due to the increase in surface tension
now possible to treat infants by instilling synthesised surfactant into the lungs
