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GAS EXCHANGELecture – 5

Dr. Zahoor Ali Shaikh

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GAS EXCHANGE Gas Exchange takes place in alveoli and

then at tissue level. Why we are breathing? To provide a continuous supply of

oxygen to tissues.Gas Exchange in Alveoli

It is diffusion of O2 and CO2 through a permeable membrane of alveoli.

It is down the partial pressure gradients of O2 and CO2.

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PARTIAL PRESSURES First we will see Composition of

Atmospheric Air ( Inspired Air ) - Nitrogen (N2) – 79% - Oxygen (O2) – 21% - Carbon dioxide (CO2) – 0.04% [0% for practical purpose] - Water Vapor - Other gases and pollutants These gases exert an atmospheric

pressure of 760mmHg at sea level.

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PARTIAL PRESSURES The pressure exerted by a particular gas is

directly proportional to the percentage of that gas in a total air mixture.

Example: N2 is 79%, - Partial Pressure of N2 will be 79% of 760 = (79 * 760)/100 = 600mmHg - O2 is 21% = (21*760)/100 = 160mmHg

- Partial pressure of CO2 (PCO2), is negligible 0.23 mm Hg.

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PARTIAL PRESSURE What is Partial Pressure [Pgas] ? It is the pressure exerted by each gas in

a mixture of gases, is known as ‘Partial Pressure’ of gas. It is designated by Pgas e.g. PO2, PCO2.

NOTE – Gases dissolved in a liquid such as blood also exert a partial pressure. Greater the partial pressure of the gas in the liquid, more of that gas is dissolved.

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PARTIAL PRESSURE GRADIENT A difference in partial pressure between

capillary wall and surrounding structure is known as ‘Partial Pressure Gradient’.

Partial Pressure Gradient exist between alveolar air and pulmonary capillary blood.

NOTE – A gas always diffuses down its partial pressure gradient i.e. from area of high partial pressure to area of low partial pressure.

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PARTIAL PRESSURE We will see the Alveolar Air Composition

and Partial Pressure of O2 and CO2 in the alveolar air.

Alveolar Air Partial Pressure

O2 – 14% - Partial pressure 100mmHg CO2 – 6% - Partial pressure 40mmHg N2 – 80% - Partial pressure 573mmHg H2O – 47mmHg ( Total Pressure = 760mmHg )

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PO2 AND PCO2 GRADIENTS ACROSS PULMONARY CAPILLARIES As blood passes through the lungs, it

picks up O2 and gives CO2 by diffusion down the partial pressure gradient between blood and alveoli.

Pulmonary Capillary has Venous blood [Deoxygenated].

Pulmonary Capillary blood PO2 – 40mmHg PCO2 - 46mmHg

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PO2 AND PCO2 GRADIENTS ACROSS PULMONARY CAPILLARIES As blood flows through Pulmonary

capillaries, it is exposed to alveolar air. As alveolar air has PO2 at 100mmHg,

which is higher than PO2 of 40mmHg in the blood of Pulmonary capillaries [Venous blood].

Therefore O2 diffuses from alveoli to capillary blood till no further gradient exist i.e. PO2 in blood capillary becomes 100mmHg [oxygenated blood].

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PO2 AND PCO2 GRADIENTS ACROSS PULMONARY CAPILLARIES Partial Pressure of CO2 in Pulmonary

capillary [Venous blood] is 46mmHg, whereas alveolar PCO2 is only 40mmHg, therefore, CO2 diffuses from the blood into the alveoli.

Blood leaving the Pulmonary capillary has PCO2 of 40mmHg.

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PO2 AND PCO2 GRADIENTS ACROSS PULMONARY CAPILLARIES

SUMMARY Blood leaving the lungs [oxygenated] PO2 - 100mmHg PCO2 - 40mmHg Blood coming to the lungs from tissues

[Venous Blood] PO2 - 40mmHg PCO2 - 46mmHg

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FACTORS AFFECTING THE GAS TRANSFER ACROSS THE ALVEOLAR MEMBRANE

Diffusion constant means ‘solubility of gas’.

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GAS DIFFUSIONAPPLIED ASPECT In Emphysema – there is loss of alveolar walls,

resulting in large but less alveoli , therefore lung surface area decreases, therefore,

decreased gas exchange. Pulmonary fibrosis --Increased thickness of

alveolar membrane due to pulmonary fibrosis leads to decrease exchange of gases.

NOTE – Diffusion constant of CO2 is 20 times more than O2 in alveolar membrane as CO2 is more soluble, therefore, in diseased lung O2 transfer is more seriously affected than CO2 transfer.

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GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES Gas exchange across systemic

capillaries also occurs down partial pressure gradient.

In Systemic capillaries O2 and CO2 move to tissues by diffusion down their partial pressure gradients.

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GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES Arterial Capillary PO2 – 100mmHg Arterial Capillary PCO2 – 40mmHg Oxygen is given to tissues and CO2 diffuses from

tissues to blood. Tissue - PO2 – 40 - PCO2 – 46 Venous Capillary Blood - PO2 – 40 - PCO2 – 46 This blood [Venous] goes to right side of the

heart and then to lungs to get oxygen and give out CO2 and cycle repeats.

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COMPOSITION OF INSPIRED, ALVEOLAR, EXPIRED AIR

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Pathological conditions reducing pulmonary surface area:

(1) Emphysema (surface area is reduced , because many alveolar walls are lost) (2) Collapsed regions of the lung

(3) When part of the lung tissue is surgically removed—(for example, in treating lung cancer).

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EFFECT OF THICKNESS ON GAS EXCHANGE: The thickness increases in, (1) Pulmonary fibrosis involving replacement of delicate lung tissue with thick, fibrous tissue (2) Pulmonary edema an excess accumulation of interstitial fluid

(3) Pneumonia, which is characterized by inflammatory fluid accumulation within or around the alveoli.

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WHAT DO YOU KNOW FROM THIS LECTURE Composition of Inspired Air, Alveolar Air, and

Expired Air Partial Pressure of O2 and CO2 in Alveolar Air Partial Pressure of O2 and CO2 in Oxygenated

[Arterial] and in Deoxygenated [Venous] blood

Partial Pressure of O2 and CO2 in tissues Factors Affecting the Gas transfer (Diffusion)

across the Alveolar Membrane Gas Exchange across the Systemic Capillaries Solubility of O2 and CO2

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