Functions of the Respiratory System

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Functions of the Respiratory System

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University of Southern California
Bisc 307l - General Physiology
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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 ...

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