Welcome to Respiratory HQ. I'm Tonya Peel and today I want to talk to you a little bit about tissue oxygenation. And so using this picture, if you haven't seen it before, this is how I teach blood flow through the heart.
So let's get ourselves acclimated to this picture and then we'll talk about tissue oxygenation. There is also a free video on my YouTube channel that talks about blood flow through the heart and also my website. that you can sign up for free instructional resources and that video is there.
But basically what we've got is blood flow returning from systemic circulation through the inferior and superior vena cava dumping deoxygenated blood into the right side of the heart. And so it goes from the right atrium through the tricuspid valve to the right ventricle the right ventricle pumps pumping blood into the lung where co2 is dropped off. O2 is picked up that oxygenated blood comes back through the pulmonary vein to the left side of the heart going from the left atrium through the mitral valve to the left ventricle.
The left ventricle squeezes and pumps blood through the aortic valve through the into the aortic and into systemic circulation so continues in a round around motion. So what I want to do is apply this and start this concept of tissue oxygenation at the alveoli. Okay so when we breathe our diaphragm drops and air from the atmosphere floods into our alveoli until we get a pressure equilibrium.
But if we just look at that pressure we have in here the pressure called the P big A O2 alveolar oxygen tension and then that oxygen is driven into the pulmonary capillary bed where it is carried two ways, bound to hemoglobin and dissolved in plasma. And there is another instructional video that talks about O2 content. So go back and look at that. But basically what we have here is the combination of what's bound to hemoglobin and dissolved in plasma.
So this concept of O2 content starts right at the AC membrane. This blood, this good oxygenated blood, this O2 content if you will, is then sent to the left side of the heart. So what comes out of the left ventricle is CaO2 and that A stands for artery because this is the amount of oxygenated blood that is then being sent into arterial circulation. Alright so this blood travels to the tissues of the body The tissues extract the amount of oxygen that they need and then this blood continues to come back to the right side of the heart where it is deoxygenated.
Now when I say deoxygenated, that doesn't mean all of the oxygen is gone. There is still a little bit of oxygen, but we call this amount of oxygen mixed venous oxygen. So CVO2 and that V with a line on it means mixed.
venous oxygen. So where we get that is from the pulmonary artery. We can put a special type of catheter in there, draw blood from it, and then run it through a blood gas machine and it can tell us our mixed venous oxygen.
So this formula is remarkably similar to O2 content. So we still are talking about the hemoglobin and what's bound to it, which is the SVO2. That's 1.34 and to that we're adding the PV O2 times 0.003.
So it is the same formula as O2 content we're just substituting in the venous oxygen instead of arterial oxygen. So basically let's make simple simple math here okay. Let me get rid of the formula. Let's get rid of all the noise and just look at it simplistically.
Let's say for the sake of math that we calculate O2 content and it comes out 20 volume percent. So we start out with 20 and this oxygenated blood is passed to the tissues. The tissues extract and then what is coming back to the right side of the heart is measured. And let's say that comes back at 15 volume percent. So really if we want to know the amount of oxygen the tissues extracted all we have to do is take what we started with and subtract what we ended with and that's the amount.
that's coming out of the tissue. So we go CaO2 which was 20 minus CvO2 which was 15 and our difference then is is 5. Oops that was ugly. 5 volume percent.
Okay so this is the A to V difference. But how you will normally see this written is like this. Okay, so A to V difference, 20 minus 15 is the amount of oxygen the tissues have extracted.
So With this very simple concept you should be able to plug in all the components that make up each one of these and figure out how it affects tissue oxygenation. Hope this has helped.