BISC 307L 2nd Edition Lecture 35 Current Lecture Gas Exchange between Blood Air and Tissue Gases only move by diffusion down their concentration gradients there is no active transport So if the circulatory and respiratory systems are to work properly the system has to be designed to create the gradients in each location so the directions will move in the proper directions Start at the top The box at the top says dry air has total pressure of 760 mmHg and PO2 of 160 and PCO2 of 0 25 mmHg That is the ambient air surrounding it Alveolar air has a PO2 of 100 and PCO2 of 40 Why is that PO2 is less than the inhaled air because we have a one way ventilation system at the end of exhalation the rigid airways are full of exhaled air which has less oxygen in it and so when you take the next breath in the fresh air coming in mixes with stale air in the airways and you end up with something lower maybe 100 mmHg For the same reason there s a lot of CO2 in the alveolar air because the oxygen is diffusing out of the blood into the alveolar air that s how the body rids itself of CO2 and that CO2 is mixing with the incoming air At the end of exhalation the rigid airways are full of gas with high CO2 the incoming fresh air with low CO2 mixes and you end up with around 40 mmHg The 40 can change a little bit but it stays mostly constant over a wide variety of breathing rates and even during exercise and different physiological conditions And the maintenance of 100 mmHg in the alveolar air is so constant that it seems to be the point of all these mechanisms Incoming venous blood from the right side of the heart coming back form the systemic vein has a PO2 of 40 mmHg and a PCO2 of 46mmHg Remember that gas exchange and exchange of other materials between the blood and the tissues only happens in the capillaries not in the larger vessels So the P of the O2 and CO2 in the central vein stays the same as it goes into the lung its not until it gets to the capillaries of the alveolus that you begin to have movement of the gases And you can see that the concentration gradient is in the right direction Blood comes into the lungs with a PO2 of 40 and it comes close to air with a PO2 of 100 making a strong concentration gradient for oxygen to go out of the alveolar air into the blood The gradient for CO2 is less 46 in blood and 40 inside the alveolus Blood picks up oxygen and gets rid of CO2 and if you look at the pulmonary artery blood its PO2 is 100 mmHg and PCO2 is 40 So in one pass of the lungs the blood has come completely into equilibrium with the gas in the alveolar air That s what comes out of the pulmonary vein Because there are no capillaries the blood coming into the systemic arteries has that concentration of 100mmHg oxygen and 40 CO2 and that is what enters the tissue capillaries In the tissue cells on the bottom it says PO2 less than or equal to 40 mmHg which is easy to understand because the cells are using up oxygen for oxidative metabolism And PCO2 is greater than or equal to 46 which also makes sense because CO2 is being produced as a waste product And blood is coming in at 100 and the concentration gradient is in the right direction Blood gives up oxygen to the tissue because of the 60 mmHg gradient And CO2 comes in at 40 passing tissues that are producing 46 or more so gradient of CO2 is going into the blood Exchange of oxygen between the blood and systemic tissues is also very efficient In terms of concentration it looks like the conc of oxygen has fallen to less than half And it has fallen but that is misleading The concentration in these units has fallen but the actual content the amount of oxygen that has come out of the arteriole blood and gone into the tissue is more like 25 as opposed to 60 100 40 So then you end up with venous blood with a PO2 of 40 and PCO2 of 46 Pathological Ventilation and Gas Exchange The pathological conditions to the right all involve poor ventilation of the alveoli or poor gas exchange or both These conditions all involve hypoxia a deficiency of oxygen Most of them also involve hypercapnia excessive CO2 levels in body tissue Keep in mind Fick s law of diffusion on right Emphysema chronic progressive destruction of alveoli due mainly to cigarette smoking There is scar tissue in the lungs instead of health alveoli The destruction of the alveoli reduces the surface area for gas exchange In a normal healthy lung the elasticity of the lung tissue and the residual surface tension in the alveolar fluid keeps the bronchioles open Those two things create a lateral force that keeps the bronchioles open And as in other tubular organs that have smooth muscle constriction is due to contraction of the smooth muscle and bronchodilation is just due to relaxation of smooth muscles and these lateral forces pull open the blood vessels Well the destruction of the alveoli and the replacement of the elastic connective tissue between the alveoli with scar tissue reduce the forces that keep the bronchioles open This traps air in the lung and makes expiration very laborious ventilation is poor Alveolar PO2 may be normal or may be low because of the destruction of alveoli and reduction of surface area for gas exchange Fibrotic lung disease loss of lung compliance because you have stiff connective tissue where you should have elastic Fibrotic deposits in the alveoli thickens the membrane causing alveolar PO2 to be low Pulmonary edema fluid in the interstitial spaces in the lung Interstitial fluid comes from leakage out of capillaries and if there is excessive leakage of fluid out of the capillaries in the lungs edema And this fluid can actually infiltrate into the alveoli and to some extent fill the alveoli or increase the thickness of the alveolar fluid lining the epithelium This increases the diffusion difference membrane thickness and alveolar PO2 will be normal but PO2 of the vein will be low because of the increased thickness in the fluid This increased fluid increases the distance between the capillary purple and the alveolus Although PO2 goes down arterial PCO2 doesn t change much this reflects the fact that CO2 is much more soluble in water than O2 so this increased diffusion distance doesn t affect CO2 in the lungs as much as it does O2 The causes of pulmonary edema are the same causes of edema everywhere in the body The main cause in this case is high pulmonary blood pressure Remember that the pulmonary circulation has to take the entire cardiac output But the total resistance