John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.
Most people are aware of the fact that we breathe to get oxygen into our body. What may not be so clear, however, is how oxygen is transported to the various cells of our body - that is, after we breathe it in. Our cells need oxygen to make the energy-containing molecule that we call ATP. There's no time like the present, so let's take a look at how oxygen is transported in our bodies.
First off, blood is the avenue by which oxygen is transported through the body. You see, blood flows through our lungs much like a stream flows through a summer camp. Just like kids jumping into the stream, oxygen diffuses into the blood as it flows through our lungs. Enough oxygen diffuses into our blood to load every 100 ml of blood with about 20 ml of oxygen. That's a lot of oxygen, and we call this oxygen-rich blood. We need oxygen to meet our metabolic needs, and oxygen transport refers to the different ways by which oxygen is delivered to our metabolizing tissues.
Okay, we've established that our blood carries oxygen, but we need to examine how the oxygen is transported in the blood. By far, most of the body's oxygen is delivered in the blood bound to hemoglobin. Relatively little oxygen is delivered dissolved in the plasma. Let me quickly note that very little oxygen is dissolved, because its solubility in water is low. In this lesson, we're going to discuss the significance of both hemoglobin and dissolved oxygen in terms of oxygen transport.
As I just stated, oxygen-rich blood contains 20 ml of oxygen in every 100 ml of total blood volume. Of that 20 ml, only 0.3 ml is dissolved in the plasma. That's not very much at all, and it begs the question, where's the rest of the oxygen? Well, the remaining 19.7 ml of oxygen is still in the blood, but it's bound to hemoglobin. Hemoglobin is a complex protein that is contained within our red blood cells. Approximately one-half of our blood volume is composed of red blood cells, and we've got between 12 and 18 grams of hemoglobin in every 100 ml of blood.
It's helpful to think of a red blood cell as being a bag of hemoglobin. Each hemoglobin molecule is made of four subunits, each containing a heme group that can bind one molecule of oxygen. Therefore, one molecule of hemoglobin can bind four molecules of oxygen. Oxyhemoglobin is hemoglobin bound to oxygen, and it gives oxygen-rich blood a red color. On the other hand, deoxyhemoglobin is hemoglobin without oxygen bound to it.
Loading of Hemoglobin in Lungs
The binding of oxygen to hemoglobin as it flows through the lungs is referred to as loading. It's helpful to think of hemoglobin as being like microscopic boats that pick up oxygen in the lungs as if they were a loading dock. The boats of hemoglobin then deliver oxygen to the metabolizing tissues, where the oxygen is used to make ATP.
It's important to note that before oxygen can bind hemoglobin in the red blood cell, it must first dissolve in the plasma. Therefore, while the amount of dissolved oxygen may be low compared to the amount bound to hemoglobin, dissolved oxygen is essential for oxygen transport. You see, the dissolved oxygen diffuses into the plasma first, and then it diffuses into the red blood cells.
So, in a sense, each oxygen molecule makes a voyage. It diffuses across the respiratory membrane, dissolves in the blood, and then enters the red blood cell, where it binds to hemoglobin. Once in the red blood cell, oxygen is removed from the plasma. This effectively leaves an empty space, so to speak, in the plasma, into which another oxygen molecule can dissolve.
Unloading of Hemoglobin in Metabolizing Tissue
Now that we understand loading of hemoglobin with oxygen in the lungs, we can talk about unloading of hemoglobin as it occurs in the tissues. Unloading refers to the removal of oxygen from the oxyhemoglobin. As blood flows through the capillaries in our metabolizing tissues, oxygen diffuses out of the red blood cells. Oxygen is released from oxyhemoglobin, diffuses out of the red blood cell and dissolves in the plasma. The dissolved oxygen can then diffuse into the tissues, where it's needed to make ATP.
Equation for Loading/Unloading
It's helpful to look at the equation for oxygen loading and unloading, so we can better understand how these processes work. So, here it is!
Hgb + 4 O2 <--> 4 HgbO2
So, what do those terms mean? Hgb refers to deoxyhemoglobin - that is, hemoglobin without oxygen. O2 is obviously molecular oxygen. And, HgbO2 refers to oxyhemoglobin.
Now, note the arrow in the middle. It's bidirectional. The bidirectional arrows indicate that this reaction can occur in either direction. Loading occurs to the right, and unloading occurs to the left. The overall direction of the reaction is determined by - and this is important - the relative concentration of oxygen.
Well, what does that mean? Let's take a look. The high concentration of oxygen that exists in our lungs pushes the reaction to the right, thus favoring loading and the formation oxyhemoglobin. Conversely, the low concentration of oxygen in the tissues pushes the reaction to the left, thus favoring unloading of oxygen.
In summary, oxygen transport refers to the different ways by which oxygen is transported from the lungs to the metabolizing tissues. Almost all of this oxygen is transported throughout the body in our red blood cells in the form of oxyhemoglobin. Relatively little oxygen is transported dissolved in the plasma; however, oxygen must first be dissolved in the plasma before it can enter the red blood cell.
The process by which hemoglobin binds oxygen to form oxyhemoglobin is called loading. That's what happens in the lungs. Once in the metabolizing tissues, oxyhemoglobin is unloaded as oxygen is released and diffuses into the plasma and ultimately our cells.
The equation for loading and unloading is Hgb + O2 <--> HgbO2, where Hgb is hemoglobin, O2 is oxygen and HgbO2 is oxyhemoglobin. Once again, the overall direction of the reaction is determined by the relative concentration of oxygen.
Following your viewing of this video, you'll be able to:
- Describe how oxygen is transported from the lungs to the tissues
- Dissect the equation for the loading and unloading of oxygen
- Explain why this equation is bidirectional
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