lecture 32 factors affecting pulmonary ventilation

7/30/2019 Lecture 32 Factors Affecting Pulmonary Ventilation

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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


End of inspiration

KEY

Exhale 500 mL

(tidal volume)

PO2=PO2~~


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Ventilation


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


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


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


End of inspiration



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


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




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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:

lecture 32 factors affecting pulmonary ventilation

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