discussed EXCHANGE OF GASES BETWEEN THE ALVEOLI AND PULMONARY CAPILLARY BLOOD

Shunts 5 Applied Physiology 6. Summary Introduction Oxygen passes from the alveoli to the pulmonary capillary blood and C02 is disgorged from the capillary blood to the alveoli In ihis chapter, the mechanism and the factors invoked in such transfers will be discussed The gases, in order to be transferred, have to pass through the following bamers T. Lining epithelium of the alveolar wall 2. The capillary endoihelium and Ihe basement membrane. These two layers, laken together constitute the alveolocapilEary membrane Alveolocapillary membrane = alveolar epithelium + capillary endothelium + basement membrane lying between these Two layears. 3. Plasma of the capillary blood. 4. The erythrocyte membrane plus The intracellular fluid within the red blood corpuscle Therefore, 'resistance' against the passage of gases is due lo The (i) alveolocapillary membrane (ii) its passage through the capillary plasma, (iii) its passage through the RBC membrane plus ICF in RBC and (iv) 'reaction rale' The term reaction rale rs used to mean Ihe reaction rale of oxygen with Hb, to form Hb02 or breaking down of compounds of C02 to release C02. At this stage, another term. viz, membrane component should be understood. Membrane component = a Iveolo capillary membrane + plasma in the capillary of the lung + RBC membrane + the intracellular fluid within the RBC it is clear That the resistance against the gaseous transfer is due To The combined effects of (i) membrane component, and (ii) reaction rate However, the reaction rale accounts for less than 5% of the tolal resistance and hence very often ignored in routine discussions. Passage of gas through the membrane component occurs by purely physjcal diffusion and no chemical reaction is involved For this to occur, it will be seen in the subsequent paragraphs. that all the conditions are favoring thus- (i) the alveolo capillary membrane is extremely thin (ii) there is minimum tissue fluid in the lung (ni) the dtffusivrty of 02 or C02 is satisfactory, and (iv) the pressure gradient of the gases across the membrane is sufficient THE ALVEOLOCAPILLARV MEMBRANE Components of Ihe alveotocapillary membrane have already been stated It's thickness vanes from 0.36 gm to 2.5 0gm. the average being about 0.5 ym. The tissue fluid m the lung is absolutely minimum Presence of a grealer amount of tissue fluid (as occurs in other tissues elsewhere in our body) between alveolar epithelium and capillary wall would have obstructed diffusion Details of so as to why the lung parenchyma is almost free of Tissue fluid is little controversial, but the broad outlines are as follows. The mechanism of formal