Gas Exchange & Gas Transfer

Dr Taha Sadig Ahmed Physiology Department , College of Medicine , King Saud University ,

Riyadh

• Objectives1- Explain what is meant by diffusion .2. Define partial pressure of a gas 3- Understand that gases in a liquid ( e.g.,water) diffuse from higher partial pressure tolower partial pressure .4. State the partial pressures of oxygen andcarbon dioxide in the atmosphere, alveolus,pulmonary capillary & systemic capillary .4- Describe the factors that determine

diffusion and the concentration of a gas in a liquid .

Definitions• Diffusion is a process leading to equalization ofoxygen and carbon dioxide concentrations betweentwo compartments ( alveolus and blood and pulmonaryblood capillary .• Factors that determine the concentration of gas in aliquid phase (alveolus or capillary) . • Partial presure of a gas is the pressure of a

gaspresent in a mixture of gases . It is independent of thepressure exerted by the other gases (Dalton's Law) It depends upon :(1) Partial pressure differences of the gas ( O2 or CO2)between the two compartments .(2) Diffusivity or Diffusion Coefficient The higher thediffusivity of of a gas ( O2 or CO2) , the faster is thespeed of its diffusion . Diffusivity depends on the (a) molecular weight (MW) ( the smaller the MW , of a substance the faster is itsrate of diffusion ) , & and (b) its solubility ( in water , which lines our alveoli & occupies the interstitial space )• O2 has lower molecular weight than CO2 , and this , theoretically should make it more diffusible than CO2.However, in spite of that , CO2 is 24 times more soluble

inwater than O2 the net result is that CO2 diffusion is20 times faster than O2 diffusion

(3) Diffusion distance across the alveolar-capillary membrane, which

consists of (i) blood capillary endothelium , (& its basement membrane , and (ii) alveolar wall epithelium

(4) Surface area available for diffusion .

Composition of Inhaledand Exhaled Air

• Partial Pressures of O2 and CO2• Oxygen concentration in the

atmosphere is 21% • Atmospheric pressure = 760

mmHg • Hence oxygen partial pressure

( PO2 ) in atmosphere = 760 mmHg x 21 % = 160 mmHg. • This mixes with “old” air already present in alveolus to arrive at PO2of 104 mmHg in alveoli. • Carbon dioxide concentration in

the atmosphere is 0.04% • Therefore , PCO2 in atmosphere

=760 mmHg x 0.04% = 0.3 mm Hg

• This mixes with high CO2 levels from residual volume in the alveoli to arrive at PCO2 of 40 mmHg in the alveoli.

Oxygen Carbon Dioxide

Atmospheric air

160 mm Hg (21%?)

0.3 mmHg (0.04 %)

Alveolus 100 -104 mm Hg

40 mmHg

Pulmonary Capillary

PO2=104 40 mmHg

Pulmonary Artery

95 mmHg 40 mmHg

Pulmonary Vein 40 mm

Hg45 mmHg

Tissue capillary

PO2 = 95 mmHg

PCO2=40 mmHg

Interstitial Space

PO2 = 40 mmHg

PCO2=45 mmHg

Tissues PO2 = Less than 40 ( around 20 mmHg)

PCO2=46 mmHg

Figure 14-3a

Gas exchange in the Lung and in the Tissues:

40 mmHg

Est means estimated

45mmHg

Pulmonary

Artery 95

mmHg

Alveolus 104 mmHg

• Oxygen and CO2 Concentration in alveoli • At resting condition 250 ml 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 4 times to maintain the alveolar PO2 at the normal value of 104 mmHg.

• Normal rate of CO2 excretion is 200 ml/min, at normal rate of

alveolar ventilation of 4.2 L/min.

• How can you explain the fact that the PO2 in the expired air is higher than PO2 in the alveolar air?

• Diffusion of CO2 through the conducting air ways.

• Humidification of expired air with water vapor.• Mixing with the dead space air during expiration.(T)• Uptake of CO2 by pulmonary capillary blood during expiration.