Upload
allan-hamilton
View
217
Download
1
Embed Size (px)
Citation preview
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.