09.04 Respiratory Functions of Blood

So, as we know, the main purpose of the respiratory system is to bring oxygen into the body and let CO2 out - but without blood circulation around the lungs and around the body - it’s meaningless. So we’re going to talk about how the bloodstream and red blood cells participate in the role of the respiratory system.

So, one of the main functions of blood when it comes to the respiratory system is the transport and exchange of oxygen and carbon dioxide. During this process, we also see another primary function of blood come into play which is that it serves as an alkaline reserve to help us maintain our acid-base balance. A few terms to be aware of here. The first is hypoxemia. Let’s break this word down - hypo, we know that means low - ox refers to oxygen, and -emia refers to the blood. So hypoxemia is low oxygen levels in the blood. Hypoxia - low oxygen - but where? What makes these two different. When we talk about hypoxemia, we are specifically talking about oxygen levels in the blood, but hypoxia is when there isn’t enough oxygen getting to the tissues or the body. So hypoxemia can lead to hypoxia. There are a few types - anemic hypoxia can occur if we don’t have enough blood or blood cells to carry the oxygen to the tissues. Stagnant hypoxia is when the blood isn’t actually flowing out to the tissues like it should - so in cases of low cardiac output. And histotoxic hypoxia - think ‘toxin’ - that’s when something is preventing our blood from carrying the oxygen. The 2 big examples here are carbon monoxide and cyanide - both of those will prevent oxygen from being carried by red blood cells. So hypoxemia - low oxygen in the blood - hypoxia - low oxygen in the tissues. Now, let’s look at how these gases are actually exchanged in the body.

Well, remember that the gas exchange occurs initially in the alveoli in the lungs - then the blood is carried out to the body tissues, right? Well the main thing that allows these gases to move between these different spaces is what’s known as partial pressures. This is basically a fancy way to measure the concentration of a gas. Since it’s a gas, we wouldn’t say it’s in milligrams, right? We use partial pressures instead, which are measured in millimeters of mercury. So a higher partial pressure means a higher concentration and vice versa. So - as the venous blood enters the capillaries around the alveoli, the partial pressure of oxygen is about 40 mmHg, and CO2 is about 45 - 50 mmHg. In the alveoli, Oxygen is at about 100 mmHg and CO2 is at about 40 mmHg. So - what we start to see is these gases begin to diffuse across from high to low. CO2 is higher in the capillaries, so it shifts into the alveoli to be exhaled. And oxygen is higher in the alveoli - so it shifts into the capillaries. Now the oxygenated blood can circulate out to the tissues. Then, we basically see the reverse process happening out here. The oxygen in the capillaries is at about 60-80 mmHg and in the tissues it’s at 30, so the oxygen shifts into the tissues. CO2 is at about 40 in the capillaries and 50 in the tissues, so it shifts out of the tissues into the bloodstream. Now - these are just some general concepts when it comes to the diffusion and exchange of gases, but there’s a lot more going on here than just partial pressures.

First thing you need to know is that as oxygen and carbon dioxide are being transported throughout the body, they are stored in certain chemical forms. For both oxygen and carbon dioxide, about 5% of it is dissolved in the plasma. The rest of the oxygen is attached to hemoglobin and we call that oxyhemoglobin. There’s a great lesson on hemoglobin in the labs course so make sure you check that out. For Carbon dioxide - we see only about 20 percent attached to the hemoglobin and we call that carboxyhemoglobin. The rest of the carbon dioxide actually goes through the carbonic acid reaction and converts to bicarb in the blood. Remember the carbonic acid reaction is CO2 plus H2O creates carbonic acid, which immediately breaks up into a Hydrogen ion and bicarbonate. We talk about this in the breathing control lesson because it regulates the chemical control of breathing, and you’ll also see it come into play with any kind of acid base balance situation. So - let’s look at the details of how gas exchange occurs both in the lungs and in the tissues.

So - in this image, these are our tissues, this is our blood stream, and this circle is our red blood cell. Remember that 95% of our oxygen is in the red blood cell attached to hemoglobin. And remember that there’s already CO2 in our tissues waiting to diffuse over because of those partial pressures. So - the oxygen breaks off the hemoglobin and diffuses across to the tissues, and the CO2 diffuses into our bloodstream. Then some of that CO2 will shift onto the hemoglobin for storage. The rest of the carbon dioxide will undergo the carbonic acid reaction we talked about in the blood plasma and create bicarb. That process also tends to cause more release of oxygen, so that will diffuse across to the tissues as well. And then the bicarb will combine with sodium that’s already in our blood plasma to create sodium bicarbonate. This is where our alkaline reserve comes in. This is what helps us maintain our blood at a slightly alkaline pH of 7.35 to 7.45.

Now - we see basically the opposite actions happening in the alveoli in the lungs. The CO2 that’s on the hemoglobin will break off and diffuse into the alveoli so we can exhale it. The oxygen that’s in the alveoli will diffuse across into the bloodstream and about 95% of it will jump into the red blood cells to attach to hemoglobin. Now - remember that we actually stored 75% of the CO2 as bicarb, right? So what we have to do now is reverse that carbonic acid - break the bicarb off the sodium and use it to create CO2 in the cells. Then that CO2 will be able to diffuse across to the alveoli to be exhaled. So, again, it’s basically the reverse process of what happened in the tissues.

Okay guys, let’s recap and simplify this a bit for you. Remember the primary functions of blood as it relates to the respiratory system is the transport and exchange of oxygen and carbon dioxide - both in the lungs and the tissues - and to function as our alkaline reserve. Gases like oxygen and carbon dioxide are measured in partial pressures and we see those gases moving from high to low based on those concentrations. In the tissues, we see oxygen break off of hemoglobin to diffuse into the tissues and CO2 comes into the bloodstream from the tissues to be stored mostly as bicarb. In the lungs, we see the oxygen diffuse into the bloodstream from the alveoli and the CO2 breaks off from hemoglobin and bicarb gets converted back to CO2 so that we can exhale it out of the lungs. And remember that when we are creating bicarb in the blood and attaching it to sodium - that’s what helps create our alkaline reserve to keep our pH where it needs to be.

Alright guys, that’s it for the respiratory functions of blood. Make sure you check out the hemoglobin lessons as well as the gas exchange lesson in the respiratory course. Now, go out and be your best selves today. And, as always, happy nursing!