Respiratory ventilation

1Dr.Aida Korish

Learning objectives By the end of the following 2 lectures you should be able to: - 1-Define the various Lung Volumes and capacities and provide

typical values for each. 2-Define ventilation rate, their typical values, and their

measurement. 3- Describe FEV1.o and its role in differentiating obstructive and

restrictive lung diseases 4- Understand air movement and airway resistance:

Definition, determinants, role of autonomic nervous system and mechanical factors

5- Describe the types of dead space. State a volume for the anatomical dead space.

6- Define the term minute ventilation and state a typical value. 7- Distinguish minute ventilation from alveolar ventilation. 8-Understand the work of breathing

2Dr.Aida Korish

Lung volumes and capacities 4 lung volumes:

Tidal volume (TV)

(~500 ml)

inspiratory reserve

(IRV)(~3000 ml)

expiratory reserve

(ERV)(~1100 ml)

residual volume (RV)

(~1200 ml)

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Spirometry

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pulmonary capacities

Two or more lung volumes are described as pulmonary capacity

Inspiratory capacity ( IC) IC= TV+ IRV= 500+ 3000= 3500 ml The functional residual capacity ( FRC) FRC= ERV+ RV= 1100+ 1200= 2300 ml Is the amount of air that remains in the

lungs after normal tidal expiration. Acts as a buffer against extreme changes in alveolar gas levels with each breath.

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Cont… lung capacities

The vital capacity ( VC)

= TV+IRV+ERV = 500+3000+1100 =4600 ml The total lung

capacity (TLC) = TV+IRV+ERV+RV = 500+3000+1100+ 1200= 5800ml.

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All lung volumes and capacities are 20-25% less in women than men , they are greater in large athletic people than in small asthenic people.

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**Determination of the FRC, RV, TLC Closed circuit

Helium Dilution Method

C1xV1= C2xV2

FRC =( Ci He - 1) Vi Spi

Cf He

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Air flow

-Air flow occurs only when there is a difference

between pressures -Air will flow from a region of high pressure to one of

low pressure-- the bigger the difference, the faster the

flow

-driving pressure

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Air Flow

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Factors affecting pulmonary ventilation

Lung compliance Elasticity:Surface tension of alveolar fluid.

Airway resistance

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Airway Resistance Airway resistance is the opposition to flow caused by the forces of friction .

It is defined as the ratio of driving pressure to the rate of air flow .

Resistance to flow in the airways depends on

whether the flow is laminar or turbulent ,on the dimensions of the airway, and

on the viscosity of the gas .

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Airway resistance decreases as lung volume increases

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Forced Vital Capacity (FVC) and FEV1

( Timed vital capacity) The person is asked to inspire as deeply as

possible and then to breath out as hard and as fast as he can. The expiration is continued until he expired all the air out and thus forced vital capacity is obtained. During this process the volume of air expired in the first second is collected and is known as FEV1.

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Obstructive Ventilatory Defect

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Restrictive Ventilatory Defect

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FEV1/FVC ratio

Normally it is about 80%. This ratio differentiate between

obstructive and restrictive lung diseases

is normal in restrictive lung diseases ( pulmonary fibrosis)

It decreases in obstructive ( bronchial asthma, emphysema)

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Minute respiratory volume MRV = Respiratory rate x Tidal volume = RR X TV

= 12 X 500 = 6L/min.

it could rise to 200 L/min or more than 30 times normal if RR = 40 TV= 4600 ml in young adults man

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Alveolar ventilation

Rate of alveolar ventilation per min

Is the total volume of new air entering the adjacent gas exchange area each minute.

It = (TV - Dead space volume) x RR = 12 (500-150) = 12x 350 = 4200ml/min

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Dead space and its effect on alveolar ventilation

the volume of air present in the conductive part of the respiratory passages= 150 ml

Anatomical versus physiological dead space: On occasion some of the alveoli are none

functioning or partially functioning due to absent or poor blood flow so when the alveolar dead space is included, this called physiologic dead space

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Work of breathing

During normal quiet respiration almost all respiratory muscles contraction occurs during inspiration, whereas expiration is a passive process caused by elastic recoil of the lungs and chest cage structures.

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The work of inspiration can be divided into three parts

Compliance work or elastic work (expand the lungs against the lung and chest elastic forces.

Tissue resistance work to overcome the viscosity of the lung and chest wall structures)

Airway resistance work (required to overcome airway resistance during the movement of air in the lungs.

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Energy required for respiration

3-5% of total energy expended by the body Can increase 50 folds during heavy

exercise. During pulmonary disease all the three

types of work are increased

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Gas Transfer(Diffusion of O2 and CO2)

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Objectives 1-Define partial pressure of a gas, how is influenced by altitude. 2- Understand that the pressure exerted by each gas in a mixture of

gases is independent of the pressure exerted by the other gases (Dalton's Law)

3- Understand that gases in a liquid diffuse from higher partial pressure to lower partial pressure (Henry’s Law)

4- Describe the factors that determine the concentration of a gas in a liquid.

5- Describe the components of the alveolar-capillary membrane (i.e., what does a molecule of gas pass through).

6- Knew the various factors determining gas transfer: -

Surface area, thickness, partial pressure difference, and diffusion coefficient of gas

7- State the partial pressures of oxygen and Carbon dioxide in the atmosphere, alveolar gas, at the end of the pulmonary capillary, in systemic capillaries, and at the beginning of a pulmonary capillary.

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After ventilation of the alveoli with fresh air the next step is the process called Diffusion of oxygen and carbon dioxide.

The energy required is provided from the kinetic motion of the molecules themselves.

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Diffusion

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The respiratory membrane.

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Gas Exchange Exchange of O2 and CO2 between alveolar air and blood

occurs via passive diffusion

Governed by

Dalton’s Law

○ Each gas in a mixture exerts own pressure

Partial pressure

Henry’s Law

○ Quantity of gas that dissolves in liquid proportional to partial

pressure and solubility coefficient

Solubility of CO2 greater than O2 (24x)

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In respiratory physiology we are dealing with a mixture of gasses O2, N2, CO2.

The rate of diffusion of each of these gases is directly proportional to the pressure caused by this gas alone which is called the partial pressure of the gas

Pressure is caused by the constant impact of kinetically moving molecules against a surface.

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For air 79% is N2 , 21% is O2 the total pressure is 760 mmHg , each gas contribute to this total pressure in direct proportion to its concentration independent. Of the pressure of other gases (Dalton’s law).

The partial pressure of individual gas is called PO2, PN2, PCO2 .

PO2= 160 mmHg , PN2= 600mmHg. Total pressure 760 mm Hg.

Gases in a liquid diffuse from area of high partial pressure to area of low partial pressure (Henry’s

law)Partial pressure of gases is affected by altitude.

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This is why we can give the “kiss of life”

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Pressure of gases in water and tissues

The vapor pressure of water: At complete humidification of the

inspired air with water vapor the partial pressure of water in this air is 47 mmHg (P H2O= 47 mmHg).

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Respiration Effective external and internal respiration depends on:

1. partial pressure differences

○ gases move from high to low partial pressures

2. surface area for gas exchange

3. diffusion distance

4. Molecular weight and solubility of gas

○ O2 has lower molecular weight than CO2

O2 would be expected to diffuse 1.2x faster

○ CO2 24x more soluble than O2

○ Net result: CO2 diffusion approx 20x faster than O2 diffusion

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Factors affecting diffusion across the respiratory membrane

D α ΔP x A xS D x √MW D is the diffusion rate; ΔP is the pressure

difference . A is the cross sectional area of the pathway, S is the solubility of the gas, D is the distance of the diffusion, and MW is the molecular weight of the gas,

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S/ √MW is called the diffusion coefficient of the gas.

The relative rates at which different gases at the same pressure level will diffuse are proportional to their diffusion coefficient.

Oxygen 1.0 carbon dioxide 20.0 nitrogen 0.53.

CO2 diffuses 20 times as rapidly as O2 because of its high solubility in tissue fluids.

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Composition of inspired air ( atmospheric) O2 20.84% N2 78.62% CO2 0.04% H2O 0.05%Alveolar air O2 13.6% N2 74.95 % CO2 5.3% H2O 6.2%Expired air O2 15.7% N2 74.5% CO2 3.6 (4%) H2O 6.2%

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O2 and CO2 concentration in various potions of normal expired air

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PO2 in various parts of the circulation

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Alveolar air does not have the same concentration of gases as does inspired air.

This is because The alveolar air is only partially

replaced by atmospheric air with each breath.

Oxygen is constantly being absorbed from the alveolar air.

Carbon dioxide is constantly diffusing from the pulmonary blood into the alveoli.

Dry atmospheric air that enters the respiratory passages is humidified even before it reaches the alveoli.

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Rate at which alveolar air is renewed by atmospheric air

FRC= 2300ml new air with each breath entering

the alveoli =350ml. Thus at normal alveolar ventilation

half the gas is exchanged in 17 seconds.

This slow replacement of the alveolar air is important to prevent sudden changes in gaseous concentrations in the blood.

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Oxygen concentration and pressure in the alveoli is controlled by1-The rate of absorption of oxygen into the blood

2- The rate of entry of new oxygen into the lungs by ventilatory process.

At resting condition 250ml of oxygen enter the pulmonary capillaries/min at ventilatory rate of 4.2 L/min,

during exercise 1000 ml of oxygen is absorbed by the pulmonary capillaries per minute, the rate of alveolar ventilation must increase four times to maintain the alveolar PO2 at the normal value of 104mmHg.

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Cont.… Extreme marked increase in

alveolar ventilation can never increase the alveolar PO2 above 149mmHg as long as the person is breathing atmospheric air, as this is the maximum PO2 of oxygen in humidified atmospheric air. However if the person breathes gases containing pressures of oxygen higher than 149 mmHg, the alveolar PO2 can approach these higher pressures.

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Carbon dioxide concentration and pressure in the alveoli: Normal rate of carbon dioxide

excretion of 200ml/min, at normal rate of alveolar ventilation of 4.2L/min.

The alveolar PCO2 increases directly in proportion to the rate of carbon dioxide excretion, and it decreases in inverse proportion to alveolar ventilation.

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