lecture 32 factors affecting pulmonary ventilation
TRANSCRIPT
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Biology 336 - Human Physiology
Lecture 32 Friday, December 3rd, 2012
Factors affecting pulmonary ventilation
Reading: Widmaier, 12th Ed. Chapter 13
Pages 443 - 448
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Learning objectives
At the end of this lecture, you will be able to:
1) Describe the contributions of lung elasticity and surfactants uponlung compliance.
2) List and describe the factors that affect airway resistance.3) Differentiate between an obstructive and a restrictive pulmonary
disorder based on changes in lung capacity.
4) Contrast minute ventilation with alveolar ventilation.
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Two factors affect pulmonary ventilation:
2) Airway resistance refers to the resistance of theentire system of airways in the respiratory tract.
1) Lung compliance lungs are elastic and recoilafter being stretched. Compliance is a measure of the easewith which they can be stretched.
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Having larger lung compliance is advantageous because:
-A smaller change in transpulmonary pressureneeded to bring in a given volume of air.
- Thus less work or muscle contraction is required.
Ease with which lungs can be stretchedV
Lung compliance =
(Palv Pip)
1) Lung compliance is defined as the change in
lung volume (V) that results from a given
change in transpulmonary pressure (Palv Pip)
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Factors affecting lung compliance
a) Elasticity the lungs are elastic because of the presence ofelastic connective tissue fibers. Forces exerted by these elastic fibers
generally oppose lung expansion since as the lungs stretch, the
fibers tend to recoil.
More elastic less compliant
Emphysemaresults in thedestruction of elastin fibersnormally found in lung tissue.
As a result, the lungs exhibit
high compliance and stretcheasily during inspiration;howeverthey do not recoil totheir resting position duringexpiration.
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Factors affecting lung compliance
Surface tension of lungs is created by the air-liquid interfaceformed by the thin layer of fluid lining the internal surface of thealveoli. As lung tissue expands, work is required not only to stretchthe elastic tissue but also to increase the surface area of the fluidlayer.
Greater tension less compliant
b) Surface tension of a liquid is a measure of the work requiredto increase its surface area by a certain amount. The greater the
surface tension, the more work needed to spread the fluid out.
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Pulmonary surfactant (surface active agents) decreases
the surface tension in alveoli. Surfactant is a detergent secreted from type II alveolar cells that
decreases surface tension by interfering with hydrogen bonding between
water molecules.
Surfactant increases lung compliance making inspiration easier. If surface tensionwere equalbetween two alveolisharing a duct, thepressure would behigher in b, and airin b would move tothe region of lower
pressure in acausing b tocollapse.
Surfactant stabilizes alveoli of different sizes by differentially alteringsurface tension allowing the alveoli to have the same pressure.
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Babies who are born prematurely can develop newbornrespiratory distress syndrome (NRDS).
Treatment includes administration ofsteroid hormones to help stimulate
surfactant production, aerosol
administration of artificial surfactant,
and artificial ventilation.
Normally, surfactant synthesis begins about the 25th week of fetaldevelopment and reaches adequate levels by the 34th week, about 6weeks before normal delivery.
In addition to having stiff (low-
compliance) lungs, too little surfactant
allows the alveoli to collapse and then
they have to re-inflate every time. This
is a huge energy drain.
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The effect of airway resistance on breathing:
When resistanceincreases, alarger pressuregradient isrequired to
produce a givenrate of air flow.
2) Airway resistanceis the second major factor influencing thework of breathing.
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2. Based on what you know about resistance in the circulatorysystem...
Consider the following:
1. What three parameters contribute to resistance to flow?
...or from personal experience
...make a list of all of the factors you canthink of that increase airway resistance.
3. In a normal person, what contributes more to the work ofbreathing: airway resistance or lung and chest wall elastance?
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Three parameters contribute to resistance (R): the systemslength (L), the viscosity of the substance flowing through the
system (), and the radius (r) of the tubes in the system.
Resistance (R) =8 L
r4
1.2.
3. In healthy lungs, resistance toair flow into and out of the lungs islow, because the radii of the tubes inthe conducting zone are large and, inthe respiratory zone, the total cross-sectional area of the smaller tubesincreases due to extensivebranching.
Consequently, lung and chest wallelastance provides more of the workof breathing.
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The bronchioles have a total cross sectional area about 2000times that of the trachea and do not normally contribute
significantly to airway resistance.
The bronchioles, however, are collapsible tubes and a decrease in theirdiameter (bronchoconstriction) can contribute significantly to their resistance.
Bronchioles, like arterioles, are subject to reflexcontrol by the autonomic nervous system and byhormones. Most minute to minute changes occurin response to paracrines.
For example, the paracrine signal Histamineacts as a powerful bronchoconstrictor. Histamineis released by mast cells in response to tissuedamage or contact by allergens.
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3. Respiratory volumes and capacities
Spirometryis a technique for measuringthe volumes of inspired and expired airusing a device called a spirometer.
An individual breathes into and out of atube connected to a transducer thatconverts the volume of air to an electricalsignal proportional to the volume.
Using spirometry, three of the four nonoverlapping lungvolumesthat together make up the total lung capacity canbe directly measured, including: tidal volume, inspiratory, andexpiratory reserve volumes.
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Tidalvolume (VT):the volumeof air that
moves intoand out ofthe lungsduring asingle,unforcedbreath (ave.
VT = 500ml)
Inspiratory reserve volume (IRV):the maximum volume of air thatcan be inspired from the end of a normal inspiration (ave. IRV = 3000ml)
Residual volume (RV):the volumeof air remaining remaining in thelungs following a maximal expiration(ave. RV = 1200ml).
Expiratory reserve volume(ERV):the maximum volumeof air that can be expired fromthe end of a normal expiration
(ave. ERV = 1000ml)
The 4 nonoverlapping lung volumes.
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Lung capacities are sums of two or more of the lung volumes.
Use of spirometry to measure lung volumes and calculate lung capacitiescan differentiate between obstructive and restrictive pulmonary disorders.
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For egchronic
obstructivepulmonarydisease (COPD)
refers to acombination of
two lungdiseases,
chronicbronchitis andemphysema.
Obstructive pulmonary diseases
involve increases in airway resistance.In these cases, residual volumeincreases becausean increase inresistance makes both expirationand inspiriation difficult.
The lungs become overinflated andultimately the functional residualcapacityandtotal lung capacityincrease.
Restrictive pulmonarydiseasesinvolve aninterference with lungexpansion.
Restrictive disorders ofteninvolve structural damage tothe lungs, plura, or chest wallthat decrease thetotal lungcapacityand vital capacity.
Use of spirometry to measure lung volumes and calculate lung capacitiescan differentiate between obstructive and restrictive pulmonary disorders.
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4. Minute ventilation is greater than alveolar
ventilation because of dead space
Minuteventilation
(ml/min)
Tidalvolume
(ml/breath)
Respiratoryrate (breath/
min)
= x
500 ml/breath 12 breath/min6000 ml/min
Note only a fraction of this air is available for exchange with the blood.The air that remains in the upper airways does not get to the alveoli. Theupper airways are thus referred to as dead space.
The upper conducting airways have a volume of ~150ml, therefore thevolume offresh airreaching the alveoli (or the alveolar ventilation) is:
= x
350 ml/breath 12 breath/min4200 ml/min = x
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Ventilation
Figure 17-14, step 1
1
1
150mL
2700 mL
RESPIRATORY
CYCLE IN
ADULT
Dead space filled
with fresh air
150 mm Hg (fresh air)100 mm Hg (stale air)
End of inspiration
KEY
PO2=PO2~~
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Ventilation
Figure 17-14, steps 12
1
2
2
1
150mL
2700 mL
2200 mL
150mL
The first exhaled
air comes out of
the dead space.
Only 350 mL
leaves the alveoli.
RESPIRATORY
CYCLE IN
ADULT
Dead space filled
with fresh air
150 mm Hg (fresh air)100 mm Hg (stale air)
End of inspiration
KEY
Exhale 500 mL
(tidal volume)
PO2=PO2~~
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Ventilation
Figure 17-14, steps 13
1
2
32
3
1
150mL
2700 mL
2200 mL
150mL
2200 mL
150mL
The first exhaled
air comes out of
the dead space.
Only 350 mL
leaves the alveoli.
RESPIRATORY
CYCLE IN
ADULT
Dead space filled
with stale air
Dead space filled
with fresh air
150 mm Hg (fresh air)100 mm Hg (stale air)
End of inspiration
At the end of expiration, the
dead space is filled withstale air from alveoli.
KEY
Exhale 500 mL
(tidal volume)
PO2=PO2~~
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Ventilation
Figure 17-14, steps 14
1
2
3
4
2
3
4
1
150
150mL
350
150
2700 mL
2200 mL
2200 mL
150mL
2200 mL
150mL
Only 350 mL
of fresh air
reaches alveoli
The first exhaled
air comes out of
the dead space.
Only 350 mL
leaves the alveoli.
Dead space is
filled with
fresh air.
The first 150 mL
of air into thealveoli is stale
air from the
dead space.
RESPIRATORY
CYCLE IN
ADULT
Dead space filled
with stale air
Dead space filled
with fresh air
150 mm Hg (fresh air)100 mm Hg (stale air)
End of inspiration
At the end of expiration, the
dead space is filled withstale air from alveoli.
Inhale 500 mL
of fresh air.
KEY
Exhale 500 mL
(tidal volume)
Atmospheric
air
PO2=PO2~~
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Ventilation
Figure 17-14
1
2
3
4
2
3
1
150mL
2700 mL
2200 mL
150mL
2200 mL
150mL
The first exhaled
air comes out of
the dead space.
Only 350 mL
leaves the alveoli.
RESPIRATORY
CYCLE IN
ADULT
Dead space filled
with stale air
Dead space filled
with fresh air
150 mm Hg (fresh air)100 mm Hg (stale air)
End of inspiration
Inhale 500 mL
of fresh air.
KEY
Exhale 500 mL
(tidal volume)
PO2=PO2~~
4
150
350
1502200 mL
Only 350 mL
of fresh air
reaches alveoli
Dead space is
filled with
fresh air.
The first 150 mL
of air into thealveoli is stale
air from the
dead space.
Atmospheric
air
At the end of expiration, the
dead space is filled withstale air from alveoli.
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During a time of increased oxygen demand such asduring exercise, alveolar ventilation must also
increase. Which of the two strategies below would be
the most efficientmethod to increase alveolar
ventilation?
1) Increase tidal volume2) Increase respiration rate
Consider the following: