98% of all the oxygen that comes into our blood will get transported by hemoglobin. You already know that hemoglobin is there to carry oxygen. (The remaining 2% is dissolved in the plasma.) The oxygen associates with a hemoglobin molecule through the iron atom in a heme group. Once oxygen is stuck to hemoglobin, we now call the hemoglobin oxyhemoglobin, and (as you know) this is a brighter red hemoglobin than when it has no oxygen and is called deoxyhemoglobin. Remember, oxygen is a nonpolar molecules, and prefers to be shielded from the plasma by hemoglobin than to have to interact with the plasma itself.
Transport is useful if the transported item can be released. I mean, if oxygen stayed stuck to the hemoglobin once it attached in the first place, that would not help us at all! We would suffocate while full with oxygen if the oxygen couldn't be released to the cells that need it for cellular respiration in their mitochondria! So, the new information on oxygen transport is actually how the transition from oxyhemoglobin to deoxyhemoglobin can occur. You will also learn how deoxyhemoglobin can once again pick up oxygen and become oxyhemoglobin.
Start by considering an oxyhemoglobin molecule, full of oxygen. (quick refresher question... how many O2 molecules are attached to oxyhemoglobin when it is full of oxygen? Do you remember? One is attached to each heme group, and there are four heme groups, one within each globin, in a hemoglobin molecule. So, 4 O2 molecules are attached to a full oxyhemoglobin).
What has to happen for oxygen to leave oxyhemoglobin? You know that oxygen will diffuse from high concentration in the blood to low concentration in the tissues. A better way to describe this concentration difference is by considering the partial pressure of oxygen in each environment. The partial pressure of oxygen in the blood is higher than the partial pressure of oxygen in the tissues. All materials move from higher pressures to lower pressures if they can.
So how can oxygen separate from oxyhemoglobin to run down its pressure gradient? Oxygen only associates with hemoglobin through weak chemical bonds. It stays on the hemoglobin because that environment is still better than the plasma outside the red blood cells. But, once oxyhemoglobin encounters an environment with a low partial pressure of oxygen, the oxygen molecule readily rips itself off the oxyhemoglobin molecule to move into the low partial pressure environment.
This figure from your book simply shows this point graphically. At high partial pressures of oxygen in the tissues (the far right of the graph), oxygen remains attached to hemoglobin. But as the partial pressure in the tissues drops (moving along the X axis toward the left), the oxygen leaves the oxyhemoglobin.
Finally, the willingness of oxyhemoglobin to give up oxygen can be modified by other environmental factors. Some examples of the environmental factors are: any increase in temperature, decrease in pH (becoming more acidic), or increase in blood carbon dioxide. All of these changes serve to make oxyhemoglobin even more willing to give up oxygen. This may seem odd at first, but if you think about it, you'll realize that it makes sense. You see, the most active tissues of the body (which use the most energy) tend to give off excess carbon dioxide (a product of cellular respiration). So if those tissues are doing a lot of cellular respiration, they will need more oxygen to keep it up. And an increase in carbon dioxide leads to more oxygen falling off the oxyhemoglobin. In fact, active skeletal muscles not only release extra carbon dioxide, but get hot as they use up energy and decrease the pH of the blood as they secrete lactic acid into the blood. So active muscles create a local blood environment conducive to releasing excess oxygen.
So, more carbon dioxide is released when energy is being actively made (for immediate use), thereby requiring more oxygen to come to make more energy. Got it?
When the partial pressure of oxygen is higher outside of the blood than inside of the blood, oxygen will attach to the hemoglobin in the blood. This situation is like the far right side of the graph shown above. As the partial pressure in the tissue increases, more and more oxygen will now move into the blood (thus associating with hemoglobin), until the hemoglobin is saturated with oxygen (meaning that all four oxygen molecules are attached). The only place where the partial pressure of oxygen outside of the blood is higher than inside of the blood is in the lungs. This will be explained in the alveolar gas exchange webpage.
Our atmosphere does not have much carbon monoxide in it. Hardly any. Our bodies are thus unprepared to deal with this gas. It just so happens that carbon monoxide readily bonds with hemoglobin. In fact, CO bonds so well with hemoglobin that it does NOT tend to ever dissociate. And hemoglobin cannot bind both oxygen and carbon monoxide. This is a one-or-the-other situation. Since carbon monoxide binds more strongly, it always wins.
Therefore, when a person encounters air filled to any real extent with carbon monoxide, as they breathe in the air, the carbon monoxide attaches to hemoglobin. With every breath, more and more CO permanently binds to hemoglobin, blocking oxygen from ever being able to be carried on those hemoglobin molecules. Eventually, the person suffocates (even though oxygen is still in the air, too) because they cannot transport oxygen to their tissues.
This is an especially dangerous predicament for us, since many modern machines (like cars) release CO, yet CO is not detectable by sight, smell or taste. The only way we have to detect this deadly gas is through store-purchased carbon monoxide detectors. I have one in my house... how about you?
© 2011 STCC Foundation Press