Mechanics of Breathing II Flashcards
Impaired airway function will lead to what?
insufficient ventilation
wha does the rate of airflow depends on?
pressure gradient and level of airway resistance
state Ohm’s Law:
Airflow (V) = Pressure (P) / Resistance (R)
what is the Hagen–Poiseuille equation?
resistance = 1/radius^4
As an airway’s radius decreases, the resistance increases (and the airflow decreases) dramatically
Very small changes in radius produce very big changes in resistance, because it’s r^4
what is airflow proportional to?
Airflow is therefore proportional to the size of the airway lumen
having a relaxed, larger smooth muscle cell does what to the lumen?
- increases lumen diameter and luminal area
- decreases resistance and increases flow
having a contracted, smaller smooth muscle cell does what to the lumen?
-decreases lumen diameter
and luminal area
-increases resistance and decreases flow
what effect does turbulent flow have on airway resistance?
Airway resistance is further increased by turbulent flow
in what situation is there laminar flow and what does this mean?
- in a normal situation, where someone is breathing passively
- the air moves with a single front and is uni-directional
what happens when air is turbulent?
- air starts to move in multiple directions
- this generates friction and decreases airflow
what are the 2 situations where you get turbulent flow of air?
- When you breathe extremely hard, turbulence of air flow increases the faster the air goes. When you do a forced expiration you start to generate turbulent flow.
- Areas of obstruction within the airway (eg. areas where the airway starts to constrict a little bit, where you have a change in diameter). Obstructive airways disease. Increase the level of resistance.
what type of flow generates noise and how can this noise be detected?
Where you have turbulent flow as opposed to laminar flow, you start to generate noise as you have vibration of air, and you can pick up the noise using a stethoscope eg. if you have asthma/chest infection, you make a wheezing sound.
what is patency?
a state of being open
how can a loss of airway patency cause airway obstruction?
the degradation of structure causes airway obstruction
so, what does a a loss of airway patency relate to?
the airways going from a nice open state to collapsing-not necessarily constricting, but flopping shut because they don’t have the structural integrity to maintain the nice open structure so you can have a nice big lumen
How can the level of airway obstruction be investigated?
Spirometry- the individual breathes through a spirometer that uses a transducer to measure the level of airflow going through the equipment
The person does a maximum expiration (breathe out for as long and fast as they can)
what 2 things does a spirometer measure when looking at airway obstruction?
FVC and FEV1 - look at the ratio between the 2 values
what is FVC?
Forced vital capacity
- looks at the movement of air through the machines and out of the lungs over time
- total change in volume gives you an idea of the FVC
what is FEV1?
Forced expiratory volume in 1 second (i.e. what volume of air can the person expire in the first second)
what would you not just look at FEV1 on its own?
because someone with a larger lung capacity will be able to get more out
how do you then work out the ratio between FVC and FEV1, and what does this give you?
100 x FEV1/FVC
gives you a % of the total lung capacity an individual can exhale in the first second
state how airway obstruction affect FVC?
no effect
how will airway obstruction affect FEV1?
decrease it
therefore explain the effect airway obstruction on FVC and FEV1:
FEV1 is decreased because the speed at which the patient can get air out of the lungs is impaired, FVC is the same because their overall lung capacity is the same
what percentage is indicative of the patient having an obstructive airways disease?
a ratio that is less than 70% is indicative of obstructive airways disease
explain the difference between obstructive and restrictive:
restrictive means FVC is reduced, less than 80%
give an example of a disease that causes obstruction and the consequences:
asthma
causes increased resistance
give an example of a situation that is restrictive, and the consequences:
fibrosis
causes decreased compliance
what is fibrosis?
thickening and scarring of connective tissue, usually as a result of injury
- scarring and deposition of collagen
- causes decreased compliance, makes the lungs stiff
what is transpulmonary pressure?
the level of force acting to expand the lung
what are the consequences of the lungs being really stiff?
it will be harder to us to breathe and you’ll need to effort to generate the force needed to expand them to generate the changes you need to for breathing-lung compliance quantifies this and gives it a number which you can compare.
In order to think about how stiff the lungs are what 2 things should we look at
- how much expansionary force you need to apply to lung tissue in order to get a specific volume change, stiff=more force
- trans pulmonary pressure, the force required to expand the lungs
what is lung compliance
the relationship between trans-pulmonary pressure and lung volume (how much force is required to distend the lungs)
relates to the elastic properties of the lungs, so having a decreased compliance means the lungs are stiffer
what does having a higher lung compliance mean?
means there is a steeper gradient (between volume and pressure)
-you get a greater amount of volume change for a certain amount of pressure change
what happens when lung compliance goes down?
(compliance goes down when the lungs are stiffer)
means applying a certain pressure change will only get you a certain amount of the volume change, not as much as normal
How is lung compliance affected by disease?
chest wall mechanics
- scoliosis
- muscular dystrophy
- obesity (affects position of lungs and spine, decreases compliance)
alveolar surface tension
concentration density of elastic proteins in the lungs (elastin collagen)
so compliance will decreases in instances where the lungs become stiffer, but when will compliance increase?
emphysema
inflammatory processes will degrade the level of elastin within the lungs so that the lungs expand really easily
- might seem like a good thing but those proteins that have been degraded are needed for recoil as well, not just expansion- expiration will therefore become a problem.
how is the changes in lung compliances illustrated on a curve of lung volume (y axis) vs transpulmonary pressure (x axis)
lung compliance increases gives you a steeper curve, steeper gradient
lung compliance decreasing gives you a shallower curve , less steep gradient
-less volume change for a certain pressure change
what to air liquid interfaces (e.g. alveoli) generate?
surface tension, which resists inflation
describe what alveoli are lined with, and the structure of an alveoli:
all alveoli are lined with a physiological fluid (similar to saline or water)
so, you have air in alveoli, a wall and then a lining of physiological fluid. You’ve essentially got a bubble.
how does the fluid lining help collapse the alveoli?
When you have an interface - air on one side and liquid on the other - you start to generate an inwardly acting surface tension because the interaction between the water molecules within the bubble develop more attraction than the air, and they want to occupy the smallest space possible which generates inward pressure-collapses the alveoli.
the collapsing force in alveoli generates what?
pressure
state the law of Laplace
P= 2T/r
p is pressure, t is surface tension and r is radius of bubble (alveoli)
therefore, if T remains constant:
The smaller the alveoli, the larger the pressure gradient
more pressure causing collapse is generated in a smaller or larger bubble?
smaller
what is the consequence of more pressure being generated in a smaller bubble?
creates pressure gradients, resulting in smaller alveoli emptying into bigger ones
-this would make inflating the lungs extremely difficult
so, how is the problem over smaller alveoli emptying into bigger ones overcome?
physiological adaptation to help lungs fill in an efficient manner
PULMONARY SURFACTANT
where is pulmonary surfactant secreted and what is it secreted by?
secreted by type II pneumocytes (a particular type of cell within the wall of the alveoli) into the alveoli
-sits in the air-liquid interphase
how do the properties of the surfactant influence where it goes in the alveoli
some of the molecules within the surfactant has hydrophobic and hydrophilic regions
this means it will naturally want to sit in the air-liquid interphase so half can be in contact with the air and half with the fluid
how does the surfactant reduce the level of surface tension and therefore generate less collapsing pressure?
it sits between the water molecules and disrupts some of the interactions between water molecules
what is the benefit of the surfactant generating less collapsing pressure?
the alveoli are less likely to collapse and it’s easier for them to inflate
role of surfactant
pulmonary surfactant acts to equalise pressure and volume across varying alveoli by reducing the surface tension at the air liquid interface, increasing compliance and preventing alveolar collapse and oedema
the surfactant reverses the previous relationship, so now larger bubbles empty into smaller ones - how?
each alveoli has a certain amount of pulmonary surfactant present as the alveoli starts to inflate the surfactant is now stretched across a larger surface area, so there is a decrease local concentration in pulmonary surfactant, so surface tension starts to rise
so the larger an alveoli gets, the greater the surface tension.
larger filling into smaller means that the lungs will fill up in an even and efficient manner
how does pulmonary surfactant helps to prevent pulmonary oedema
there is a decrease in hydrostatic pressure in tissue surrounding capillary, so fluid is pulled from the capillary into the alveolus
what diseases is caused by insufficient production of pulmonary surfactant?
Neonatal respiratory distress syndrome
what is neonatal respiratory distress syndrome?
- babies are born prematurely before their lungs are sufficiently developed and before they have enough pulmonary surfactant
- mothers can take glucocorticoid before the baby is born
- infant can have supplementation
airflow via airways is determined by what?
- the pressure gradient at each end
2. the level of airways resistance
give an example of when lung compliance increases and decreases
increase - emphysema
decrease - pulmonary fibrosis
