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Addis Ababa universityCollege of Health Science

Department of Medical Physiology

Presentation on blood gas exchange

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Presentation outline

1. Objectives

2. Introduction

3. Diffusion

3.1.Determinants of diffusion

3.2.Diffusion capacity

3.3.mesurement of diffusion capacity

4. Partial pressure

4.1 partial pressure of O2 and CO2 in the body

5. Blood gas exchange

6. References

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1.Objectives

At the end of this presentation students will able to:-

1. Define what diffusion and diffusion capacity is?

2. Explain the role of diffusion in gas exchange.

3. List the factors that affect diffusion.

4. Explain the mechanism of blood gas exchange.

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2.Introduction

Oxygen in the inspired air enters the alveoli and then diffuses across the alveolo capillary membrane in to the pulmonary capillary blood.

The respiratory membrane is the actual site for diffusion of alveolar gases and gases present in the blood in dissolved form.

The respiratory membrane consists of six layers. The gases have to diffuse through these.

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Introduction cont’d

The layers are:- 1. A layer of fluid lining the

alveolus.2. A layer of epithelial cells.3. The basement membrane of

the alveolar epithelial cells. 4. The interstitial space

between the epithelial and endothelial cells.

5. The basement membrane of the capillary endothelium.

6. capillary endothelial cells.

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3.Diffusion

The process of diffusion is simply random molecular motion of molecules down a concentration gradient .

Steps of diffusion in respiratory system

Diffusion across air-blood barrier Diffusion and chemical reaction within blood Diffusion with terminal respiratory unit

3.1 Determinants of Diffusion

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Ficks LawDiffusion = (P1-P2 ) * Area * Solubility*T

L *

Factors affecting diffusion: the thickness of the membrane the surface area of the membrane the diffusion coefficient of the gas in the substance of the membrane.Pressure difference of the gas.

D ∆P*S* A/d*√MWMW

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Determinants of Diffusion cont’d

According to Fick’s law, the rate of transfer of a gas through a sheet of tissue is directly proportional:-

to the tissue area the partial pressure difference

between the two sides. the fluid temperature. solubility coefficient of the gas.

inversely proportional to fluid viscocity. thickness of the tissue and square

root of molecular wt of the gas .

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Determinants of diffusion cont’d

The thickness of the respiratory membrane increase in conditions like pulmonary edema, fibrosis.

The surface area of the respiratory membrane can be greatly decreased by many conditions.

e.g. removal of an entire lung decreases the total surface area to one-half normal.

emphysema ,many of the alveoli coalesce, with dissolution of many alveoli walls.

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Determinants of diffusion cont’d

Pressure difference across the alveolo-capillary membrane Important factor determining the rate of diffusion.

- in case of oxygen, the pressure gradient is about 60 mmHg but it is only 6mmhg in case of CO2.however,because of the much greater diffusion coefficient of CO2 both gases take the same time (about 0.25 second ) to be equilibrated between alveoli and pulmonary capillary blood.

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Determinants of diffusion cont’d

Diffusion co efficient or diffusivity

it depends on two physical properties of gas. Solubility in membrane and molecular weight.

According to graham’s law

Diffusion coefficient is directly propertional to solubility

and inversely propertional to square root of molecular weight. Diffusivity of CO2 is much more about 20 times than O2 .

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Determinants of diffusion cont’d

Diffusion -limited and perfusion –limited transfer of gases:- transfer of gas will be diffusion-limited or perfusion limited

depends on the rate of equilibration of the gas with blood. Transfer of gases which quickly equilibrate with blood, will be

perfusion-limited.

e.g. N2O, quickly equilibrates with blood(in 0.1 second which is much lower than pulmonary circulation time of 0.75 sec).so, transfer of this gas will be limited by perfusion.

carbon mono oxide, which almost never equilibrate with blood, because carbon mono oxide rapidly taken up by hemoglobin.

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3.2 Diffusion Capacity

volume of gas that will diffuse through the membrane each minute for a pressure difference of 1 mmHg.

Different for different blood gases. Diffusion capacity of O2

21 ml /min/ mmHg under resting conditions and the mean pressure gradient across the respiratory membrane is 11 mmHg so that oxygen diffusion is 230 ml/min.

Strenuous exercise and factors which increase pulmonary blood flow and alveolar ventilation can increase the diffusion capacity to 65ml/min/mmHg.

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Diffusion capacity cont’d

This increase is caused by several factors

1.Opening up of a number of previously dormant pulmonary capillaries or extra dilatation of already open capillaries, thereby increasing the surface area of the blood into which the oxygen can diffuse.

2. A better match of Ventilation perfusion ratio In general during exercise, the oxygenation of the blood is

increased by alveolar ventilation and greater diffusing capacity of the respiratory membrane for transporting oxygen into blood.

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Diffusion capacity cont’d

Diffusion capacity of carbon dioxide

CO2 diffuse through the respiratory membrane so rapidly that the average PCO2 in pulmonary blood is not far different from pco2 in the alveoli the average difference is less than 1 mmHg.

Diffusion capacity 400 ml/min/mm Hg * gradient < 1 mmHg.

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3.3 Measurement of diffusing capacity

The oxygen-diffusing capacity can be calculated from measurement

1.Alveolar PO2

2.PO2 in pulmonary capillary blood

3.The rate of oxygen uptake by the blood.o Measuring the PO2 in the pulmonary capillary blood is so

difficult and so imprecise that it is not practical to measure oxygen diffusing capacity by such a direct procedure except on experimental basis.

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Measurement of diffusing capacity cont’d

The carbon monoxide methodThe principle of the CO method is the following

1. A small amount of CO is breathed into the alveoli

2. The PCO in the alveoli is measured from appropriate alveolar air samples.

3. The carbon monoxide pressure in the blood is essentially zero because hemoglobin combines with this gas so rapidly.

4. The pressure difference of CO across the respiratory membrane is equal to its partial pressure in the alveolar sample.

5. Measuring the volume of CO absorbed in short period of time.

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Measurement of diffusing capacity cont’d

DLCO=VCO/PA-PC=VCO/PA

To convert CO-diffusing capacity to oxygen-diffusing capacity

1.DLCO*1.23

the diffusion coefficient for oxygen is 1.23 times that for carbon monoxide.

DLCO is 17ml/min/mmHg,

DLO2 =1.23*17ml/min/mmHg=21ml/min/mmHg

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4. Partial Pressure

Dalton’s Law

the total pressure of a gas mixture is equal to the sum of the pressures that each gas in the mixture would exert independently.

Partial pressure

The pressure exerted by each type of gas in a mixture.

PAN2+PAO2+PACO2+PAH2O= PB PAO2PVO2loading of blood with

O2 PACO2PVCO2unloading of

excess CO2

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Partial Pressures of Gases in Inhaled Air

PN2 =0.786 x 760mm Hg = 597.4 mmHg

PO2 =0.209 x 760mm Hg = 158.8 mmHg

PH2O =0.004 x 760mm Hg = 3.0 mmHg

PCO2 =0.0004 x 760mm Hg = 0.3 mmHg

Pother gases =0.0006 x 760mm Hg = 0.5 mmHg

TOTAL = 760.0 mmHg

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4.1 Partial pressures of O2 and CO2 in the body

AlveoliPO2 = 104 mm HgPCO2 = 40 mm Hg

Alveolar capillariesEntering the alveolar

capillaries.PO2 =40mmHg,relatively low

because this blood has just returned from systemic circulation & has lost much of its oxygen.

PCO2 = 45 mm Hg (relatively high because the blood returning from the systemic circulation has pick up CO2 from the tissues

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5.Blood gas exchange

in the alveolar capillaries, the diffusion of gasses occurs: oxygen diffuses from the alveoli into the blood & carbon dioxide from the blood into the alveoli. Leaving the alveolar capillaries

PO2 = 104 mm Hg PCO2 = 40 mm HgBlood leaving the alveolar capillaries returns to the left atrium & is

pumped by the left ventricle into the systemic circulation. This blood travels through arteries & arterioles and into the systemic, or

body, capillaries. As blood travels through arteries & arterioles, no gas exchange occurs.

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Blood gas exchange

Entering the systemic capillaries PO2 = 95 mm Hg PCO2 = 40 mm Hg

Body cells (resting conditions) PO2 = 40 mm Hg PCO2 = 45 mm Hg

Because of the differences in partial pressures of oxygen & carbon dioxide in the systemic capillaries & the body cells, oxygen diffuses from the blood & into the cells, while carbon dioxide diffuses from the cells into the blood. Leaving the systemic capillaries.

PO2 = 40 mm Hg PCO2 = 45 mm Hg 24

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Blood gas exchange

Blood leaving the systemic capillaries returns to the heart (right atrium) via venules & veins (and no gas exchange occurs while blood is in venules & veins).

This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle.

Remember in a normal person alveolar PO2 = arterial PO2, and alveolar PCO2 = arterial PCO2 .

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Overview of Gas Exchange in the Lungs

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Adapted from: Costanzo, LS. Physiology, 1st ed. 1998.

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6.References

Guyton and hall: Text of medical physiology 11th edition.

Berne and levy physiology 6th edition. Internet websites.

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Thank you!