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Pulmonary Ventilation

Breakdown of Topics in Respiratory Physiology: Ventilation, Perfusion, Diffusion, Transport of Gases, Control of Respiration

Chap 37

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Functions of the Respiratory System

• Respiration• Acid-base balance• Enabling vocalization• Defense against pathogens

and foreign particles• Route for water and heat

losses• Enhancing venous return• Activation of certain plasma

proteins (e.g. Ang I)

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General Concepts of Respiration Respiratory System and CV System

• Ventilate: bring the air to the blood• Gas exchange: diffusion, alveolar

air with blood; get gases into/out of plasma water; to/from erythrocytes; to /from Hb

• CV system: deliver gases to/from cells

• O2 utilization: Mitochondria need the oxygen to metabolize fuels. (oxidative phosphorylation)

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General Concepts: Airway AnatomySurface area 70 sq meters- each lung!

Barrier/ thickness to diffusion 0.2 microns

Conductive airways vs. respiratory airways. Do you remember the difference?

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Conducting zone: air passageway (anatomical dead space 150ml)Cartilage- support necessary to prevent collapse during inspiration due to decline in air pressureBronchioles- no cartilage for support but do have elastic fibers-can change diameter! (increase or decrease resistance!)Terminal bronchioles

Respiratory zone –air exchange. starts with respiratory bronchioles to alveolar ducts to alveolar sacs with alveoli

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VentilationInspiration (inhalation)

Expiration (exhalation)

Normal inhalation, normal exhalation

Forced inhalation, forced exhalation

Concepts:1. Pressure gradient created by

volume changes (Boyle’s Law)

2. Anatomy of lung and chest wall

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Gas LawsEquation of State P V= n R T

• P = gas pressure• V= gas volume• T= gas temperature (Kelvin

scale)• N = number of moles of gas• R = universal gas constant

Boyle's Law P1V1=P2V2

• Pressure and volume are inversely related (if other variables are kept constant.)

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Gases move down pressure gradients

P atm = 760 torr

p alveolar = 758 torr

Air moves from high to low pressure

Flow Rule = Patm-Palv

ResistanceHow are the pressure gradients changed?

According to Boyle’s law we will need to create volume changes!

PROBLEM! THE LUNGS ARE NOT MUSCULAR STRUCTURES!

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Boyle’s LawIf working with a closed system, a fixed number of gas molecules,

and temperature is constant, then

Pressure is inversely related to volumeP = K V

Or, in other words –changes in lung volume will lead to changes in pressure

P

V P

V

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Mechanics of Ventilation• Normal Inspiration

• Is an active process (It’s work! ATP using)

– Contract Diaphragm and it moves inferiorly to increase thoracic volume -60-75% of volume change

– Contract external intercostalsForced Inspiration

Accessory muscles needed

SternocleidomastoidScalenesSerratus anteriorOthers (erector spinae)

Diaphragm

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Pleural Space or Cavity1. a vacuum (contains no

air)

2. pleural fluid (water) has surface tension

Result? Lung moves with the chest wall

When the chest wall moves, so do the lungs! Why are the lungs right up against the chest wall?

Lungs are not muscular organs, they cannot actively move. They move with the chest wall.

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Mechanics of Ventilation

• Normal Expiration- A Passive process• Simply relax the muscles of

inspiration• Rely on the elastic properties of

lung (like a balloon deflating on its own)

• Forced Expiration• Relax muscles of inhalation AND• Contract internal intercostals• Contract Abdominal muscles

• Internal and external obliques• Transverse abdominis • Rectus abdominis

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Both the Lung and Chest Wall are Elastic • Both lung and chest wall have the

tendency to recoil • What is recoil? Tendency to snap back

to resting position (like a stretched rubber

band recoils when youlet go of one end)

The chest wall recoils outward (springs out)The lung recoils inward (ie. it collapses!)

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• Increase in lung volume decreases intra-alveolar pressure (we now have a pressure gradient) = air goes in.

• Decrease in lung volume raises intra-alveolar pressure above atmosphere = air goes out.

inspiration expiration

Pal

v (m

mH

g)B

reat

h vo

l. (L

)

Palv=0Palv=0

Palv=0

Palv= -1

Palv=+1

0.5

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Flow Rule = Patm-Palv Resistance

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Pressures

Atmospheric Pressures (Patm)- pressure of the outside air (760mmHg=760 torr = 1 atm).

Intra-alveolar pressure (Palv) pressure within the alveoli of the lungs. Equal to Patm (0mmHg) at rest, but varies during phases of ventilation.

Intra-pleural pressure – (Pip) pressure in the intra-pleural space.• Pressure is negative because of the lack of air in the intrapleural space,

lymph drainage, and opposing forces of lung and chest wall.Transpulmonary (palv-Pip)

First two create the pressure gradient that drives ventilation

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What happens if the lung dissociates from the chest wall?

• Pneumothorax: air in the pleural cavity• Hemothorax: blood in the pleural cavity• How?

– Injury (Gun shot, stabbing) – Spontaneous (tissue erosion, disease lung)– Bleeding wound

• Chest wall recoils outward (barrel chest)• Lung recoils inward (atelectasis = alveolar, lung collapse)

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Positive vs. Negative Pressure Breathing

• Normally, the pressure gradient is produced by changing palv

• This is called negative pressure breathing

• If one changes patm, then this is positive pressure breathing

• Ex. bag, cpr, mouth to mouth

Air moves from high to low pressure

P atm = 760 torr

p alveolar = 758 torr

Is this positive or negative pressure breathing?

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Respiratory Cycle

• A single cycle of inhalation and exhalation• Tidal volume: amount of air you move into or out of your lungs during a single

respiratory cycle.• Respiratory rate: number of breaths per minute (12-18; children higher 18-20).

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Rates: How much air moves in one minute?

• Total Ventilation: Minute

Ventilation Volume (MVV)

– Tidal volume x respiratory rate

• Alveolar Ventilation– (Tidal volume-dead space)

x respiratory rate

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“Deadspace”: a volume of air that is not

involved in gas exchange with blood

• Anatomical dead space volume (based on size of one’s conductive airways)– Typically about 150 ml

• Physiological dead space volume (anatomical dead space plus any areas of lung where alveoli are not perfused with blood)– Depends on ventilation-perfusion

ratio– But also, in disease, depends on

blocked arteries

Anatomic Dead Space

Low Blood Flow

Physiologic Dead Space

Definitions of Dead Space

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Alveolar Ventilation vs. Total Minute Ventilation of the lung

Total ventilation rate is the volume of air that enters the airways (passes the lips) each min.

Total ventilation = Tidal volume x rate of breathing

= (500 ml/breath) x 12 breaths/min

= 6,000 ml/min

Alveolar ventilation is the volume of air that fills all the lung’s respiratory airways (alveoli) each min. In a normal, healthy lung, this might be:

Alveolar ventilation = (tidal volume – dead space volume) x rate of breathing

= (500 ml/breath – 150 ml) x 12 breaths/min

= (350 ml/breath) x 12 breath/ min

= 4, 200 ml/min

In a diseased, poorly perfused lung, this value may well be much lower.

Then, is panting an example of hyper, normal, or hypoventilation????

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Lung function tests

• Spirometry– Static lung tests

• Volumes and capacities• No element of time

involved, ie. How long does it take you to push the air out? Normal expiration takes 2-3 x longer than inspiration

– Dynamic lung tests• Time element, rate of

exhale• How much, how quickly?

• Lung volumes are assessed by spirometry.

• Subject breathes into a closed system in which air is trapped within a bell floating in H20.

• The bell moves up when the subject exhales and down when the subject inhales.

24Figure 37-6; Guyton & Hall

Pulmonary Volumes Static Lung Tests

500ml

1200ml

6000ml3500ml3100ml

1200ml

2400ml

4600ml

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Capacities are two or more volumes added together

•Residual Volume (* can’t be exhaled, requires indirect measurements, not simple spirometry)

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Forced Vital Capacity (FVC, FEV 1.0)

Dynamic Lung Tests

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Lung Capacity and Disease— Summary

• Obstructive Disease:– Decreased VC– Increased TLC, RV,

FRC.– FEV1/VC is less than

80%

• Restrictive Disease:– Decreased VC– Decreased TLC, RV,

FRC– So FEV1/VC ratio

normal

FRC: ERV +RV. Why is this important?It’s the volume of air in your lungs at the end of a normal exhale.It represents the normal equilibrium position of your chest wall trying to spring out and lung to recoil, but forced together due to pleural cavity.