1. Gas Exchange & Gas Transfer Dr Taha Sadig Ahmed Physiology Department, College of Medicine, King Saud University, Riyadh

2. Objectives 1- 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 to lower partial pressure. 4. State the partial pressures of oxygen and carbon 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.

3. Definitions Diffusion is a process leading to equalization of oxygen and carbon dioxide concentrations between two compartments ( alveolus and blood and pulmonary blood capillary. Factors that determine the concentration of gas in a liquid phase (alveolus or capillary). Partial presure of a gas  is the pressure of a gas present in a mixture of gases. It is independent of the pressure 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 the diffusivity of of a gas ( O2 or CO2), the faster is the speed of its diffusion. Diffusivity depends on the (a) molecular weight (MW) ( the smaller the MW, of a substance  the faster is its rate 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 in water than O2  the net result is that CO2 diffusion is 20 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.

5. 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 PO2 of 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.

6. OxygenCarbon Dioxide Atmospheric air 160 mm Hg (21%?) 0.3 mmHg (0.04 %) Alveolus100 -104 mm Hg 40 mmHg Pulmonary Capillary PO2=10440 mmHg Pulmonary Artery 95 mmHg40 mmHg Pulmonary Vein40 mm Hg45 mmHg Tissue capillary PO2 = 95 mmHg PCO2=40 mmHg Interstitial Space PO2 = 40 mmHg PCO2=45 mmHg TissuesPO2 = Less than 40 ( around 20 mmHg) PCO2=46 mmHg

7. Figure 14-3a Gas exchange in the Lung and in the Tissues:

8. 40 mmHg Est means estimated 45mmHg Pulmonary Artery 95 mmHg Alveolus 104 mmHg

10. 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 CO 2 through the conducting air ways. Humidification of expired air with water vapor. Mixing with the dead space air during expiration.(T) Uptake of CO 2 by pulmonary capillary blood during expiration.