Hemoglobin Binding in Tissues & Lungs - Online Tutor, Practice Problems & Exam Prep

Hemoglobin's oxygen binding and release are influenced by environmental conditions. In tissues, low oxygen and high carbon dioxide (CO₂) levels promote the release of oxygen due to the Bohr effect, where CO₂ and protons act as allosteric inhibitors, stabilizing hemoglobin's T state. Conversely, in the lungs, high oxygen levels and low CO₂ concentrations favor oxygen binding, shifting hemoglobin to its R state. This dynamic process ensures efficient oxygen transport from the lungs to tissues, highlighting the importance of pH and CO₂ concentration in respiratory physiology.

Hemoglobin Binding in Lungs & Tissues

In this video, we're going to begin our discussion on hemoglobin binding in the tissues and in the lungs. At this point in our course, we already know that hemoglobin can bind to oxygen. However, what you may not have known is that hemoglobin can also bind to carbon dioxide and protons. This leads us directly into what's known as the Bohr effect, which we're going to talk about in a lot more detail later in our course. But for now, I can tell you that the Bohr effect describes the effect of the concentration of carbon dioxide (CO₂), and it describes the effect of pH, or in other words, it describes the effect of hydrogen ion concentration on hemoglobin's binding and release of oxygen.

It's important to note that under low oxygen conditions, hemoglobin is actually going to release its oxygen. When hemoglobin releases its oxygen, it's capable of binding and transporting both carbon dioxide and protons. When hemoglobin binds carbon dioxide, we're going to refer to it as HbCO₂. When hemoglobin binds protons, we're going to refer to it as HbH⁺. CO₂ and hydrogen ions both act as heterotrophic allosteric inhibitors to hemoglobin's oxygen-binding activity.

Recall from our previous lesson videos, "hetero" is referring to the fact that carbon dioxide and protons are different from oxygen. These are the allosteric effectors and the allosteric effectors are different than the ligand, and that's why it's heterotrophic. Inhibitors are going to decrease the activity, so both CO₂ and protons are going to decrease hemoglobin’s protonation and are going to stabilize hemoglobin's T-state, which is the tense state. The T-state binds ligand inefficiently. So when hemoglobin's T-state is being stabilized, it's going to cause hemoglobin to release its oxygen.

Now, if we take a look at our image down below (not shown), notice over here on the left-hand side, what we have is hemoglobin's T-state. Essentially, it can be bound to carbon dioxide as well as protons. Depending on whether we're specifically focusing on carbon dioxide or protons being bound, we're going to refer to this form of hemoglobin as either HbCO₂ or HbH⁺. Notice that when hemoglobin is in its T-state, it's going to be releasing its oxygen. These conditions exist mainly in the tissues.

However, in the lungs, the opposite reaction occurs. In the lungs, hemoglobin is mostly in its R-state. Here we have the R-state hemoglobin, which has a high affinity for its ligand. When it's in its R-state, it's going to be binding to oxygen as HbO₂. Then, of course, when it's in its R-state, it's going to have to release hydrogen ions or CO₂. Hemoglobin has an option: it can either bind carbon dioxide and protons but release oxygen, or it can bind oxygen and release protons and CO₂.

As we move forward in our course, we're going to talk about the exact conditions in the tissues that allow for the T-state to be stabilized, as we see here. We’ll also discuss the exact conditions in the lungs that allow for the R-state of hemoglobin to be stabilized. This concludes our introduction to hemoglobin’s binding to carbon dioxide and protons, and later in our course, we'll be able to focus even more on this particular binding. So, I'll see you guys in our next video.

Hemoglobin Binding in Lungs & Tissues

In this video, we're going to talk about the exact conditions in our muscle tissues that allow for hemoglobin to release oxygen. First, we need to note that our muscle tissues are constantly contracting and respiring. Recall from your previous biology courses that cellular respiration allows our muscle tissues to acquire the energy needed for muscle contraction. However, cellular respiration also consumes a lot of oxygen. This means that in our muscle tissues, oxygen is constantly being consumed and depleted through cellular respiration, leading to a relatively low partial pressure of oxygen in the tissues, around a value of about 20 tors, which is very low, especially in comparison to the partial pressure of oxygen in our lungs, which is 5 times greater at a value of about 100 tors.

Now, of course, in our respiring muscle tissues, cellular respiration is going to lead to lots of CO₂ production, produced by metabolism. This results in a relatively high concentration of CO₂ in our muscle tissues. These are the conditions in our muscle tissues that provide the background we need to understand the next five steps that lead to hemoglobin's release of oxygen in the tissues.

Starting with step number 1, the high concentration of CO₂ in our muscle tissues causes these high amounts of CO₂ produced by our respiring tissues to diffuse out of our muscle tissues into the capillaries and into our red blood cells (RBCs). Our muscle tissues have a quite high concentration of CO₂, and the high amounts of CO₂ are going to diffuse into the capillaries and into our red blood cells.

Step number 2 involves an enzyme inside of our red blood cells known as carbonic anhydrase. Carbonic anhydrase catalyzes a reaction where CO₂ can react with water to form carbonic acid, which is relatively acidic. Carbonic acid then breaks up into bicarbonate and a proton. The MathML representation for this reaction is: CO2 + H2O → H2CO3 , which has a pKa of 6.35.

Step number 3 is where the equilibrium shift towards hydrogen ion production is due to the high concentrations of CO₂ in the muscle tissues, along with lactic acid production by the tissues. This leads to an increased concentration of H⁺ ions, lowering the pH from about 7.4 to about 7.2 in the tissues, which is relatively acidic compared to the normal blood pH of 7.4.

Step number 4 describes that near the tissues, hemoglobin is going to bind to both CO₂ and protons. The presence of these molecules acts as heterotrophic allosteric inhibitors to hemoglobin’s oxygen binding, causing hemoglobin to release its bound oxygen and shift into its T state. This lowers its affinity for oxygen which then allows the release of oxygen.

In step number 5, myoglobin, which has a higher affinity for oxygen than hemoglobin, facilitates the diffusion of released oxygen into the tissues. Myoglobin's presence ensures that oxygen is effectively distributed to muscle tissues where it is critically needed.

Now that we understand how these five steps lead to hemoglobin's release of oxygen in the tissues, our next video will cover how hemoglobin binds oxygen in the lungs. See you in our next video.

Hemoglobin Binding in Lungs & Tissues

So now that we know that hemoglobin releases oxygen in our muscle tissues, in this video we're going to talk about the exact conditions in our lungs that allow hemoglobin to bind oxygen. And so really, these conditions in our lungs are going to be pretty much the exact opposite of the conditions in our tissues. We need to realize that in our lungs we are constantly and repetitively breathing in and out, inhaling lots of oxygen. Meaning that in our lungs, we're going to have a relatively high partial pressure of oxygen, right around a value of about 100 tors, which is fairly high considering that the partial pressure of oxygen in our muscle tissues is only about 20 tors. We also need to realize that in our lungs, with every breath that we exhale, we are exhaling lots of carbon dioxide or CO₂, meaning that we're going to have a relatively low concentration of CO₂ in our lungs. These are the conditions required in our lungs to understand the next five steps that allow hemoglobin to bind oxygen.

Just like we started our last story with the concentration of CO₂, in this video, we're also going to start with the concentration of CO₂. In our lungs, we know that there's a low concentration of CO₂ due to exhalation. Heading to the next diagram, we zoom in on the bloodstream by the lungs. On the far right, we have our lungs, and on the left, our blood capillary with our red blood cell shown in this circle. It's important to know that the equation we talked about in our last lesson video is being flipped, which is acceptable because we have equilibrium arrows allowing reactions to proceed forward and in reverse.

Continuing, the low concentration of CO₂ translates into our red blood cells where the low concentration, by Le Chatelier's principle, causes the equilibrium controlled by carbonic anhydrase to shift to the right, shifting towards the production of CO₂. This shift causes the concentration of hydrogen ions to decrease, raising the pH to about 7.6 in the lungs, slightly basic compared to the normal blood pH of 7.4 and the slightly acidic pH of 7.2 in the tissues.

This low concentration of CO₂ in our lungs, again via exhalation, causes CO₂ diffusion into the lungs. Diffusion goes from high to low concentration, causing CO₂ to diffuse into the lungs. This facilitates the breathing out of all the CO₂ in our lungs which exits our bodies and goes into the atmosphere. This is step number two, linking to the CO₂ diffusion into our lungs.

Moreover, in our lungs, we are inhaling a lot of oxygen, creating a high concentration of oxygen. The high oxygen concentration causes oxygen diffusion into the capillaries and into our red blood cells. Upon arrival to the lungs, hemoglobin in its deoxygenated T state will bind the oxygen, leading to the R state of hemoglobin which has a high affinity for oxygen. In this R state, hemoglobin releases carbon dioxide and hydrogen ions as it binds to oxygen.

All this oxygenated hemoglobin is then ready to deliver oxygen to the tissues. You can see that in the lungs, hemoglobin becomes oxygenated and is then transported to the tissues where it can deliver that oxygen before returning to the lungs to acquire more oxygen, continuing this cycle that enables hemoglobin to be an excellent oxygen transporter. One last note here about the lungs is that myoglobin does not come into play as it is significant only in the tissues, not in the lungs. This concludes our lesson on why exactly hemoglobin binds oxygen in the lungs, and we'll be able to get some practice utilizing all of these concepts as we move forward in our course. I'll see you guys in our next video.

In the lungs, the _________ pressure of CO₂ causes hemoglobin to _________ CO₂ and H⁺. The _________ pressure of O₂ causes O₂ to _________ hemoglobin, allowing for its transportation to the tissues.

• A. Low; Bind; Low; Release.
• B. High; Bind; Low; Release.
• C. High; Bind; High; Release.
• D. Low; Release; High; Bind.
• E. High; Release; Low; Bind.
• F. Low; Release; Low; Bind. Blood pH in Tissues ≈ 7.2 Blood pH in Lungs ≈ 7.6 Blood pH ≈ 7.4

Which of the following explains why hemoglobin in RBCs binds to O ₂ in the lungs but releases O₂ in muscle tissues? (Select all that apply).

• A. pH is lower in the lungs than in the tissues.
• B. pCO₂ is lower in the lungs than in the tissues.
• C. BPG is produced at higher levels in tissues compared to the lungs.
• D. The temperature of the lungs is generally higher than the temperature in actively contracting muscle tissues.
• E. None of the above are true.

How is the vast majority of carbon dioxide transported in the blood from the tissues to the lungs?

• A. As diffused gas bubbles of CO₂, just as in carbonated soft drinks.
• B. As bicarbonate ions (HCO₃-).
• C. As carbamate groups bound directly to deoxyhemoglobin.
• D. As carbon monoxide.
• E. As carbon compounds and oxygen radicals.

Here’s what students ask on this topic:

What is the Bohr effect and how does it influence hemoglobin's oxygen binding?

The Bohr effect describes how hemoglobin's oxygen binding affinity is inversely related to the concentration of carbon dioxide (CO₂) and hydrogen ions (H⁺). In tissues, high levels of CO₂ and H⁺ stabilize hemoglobin's T state, reducing its affinity for oxygen and promoting oxygen release. This is crucial for delivering oxygen to metabolically active tissues. Conversely, in the lungs, low CO₂ and H⁺ levels favor the R state of hemoglobin, enhancing its oxygen-binding capacity. This dynamic ensures efficient oxygen transport from the lungs to tissues, adapting to varying physiological conditions.

How does hemoglobin release oxygen in muscle tissues?

In muscle tissues, cellular respiration consumes oxygen, leading to low partial pressure of oxygen (~20 torr). High CO₂ levels from metabolism diffuse into red blood cells, where carbonic anhydrase converts CO₂ and water into carbonic acid, which dissociates into bicarbonate and H⁺. The increased H⁺ concentration lowers pH, stabilizing hemoglobin's T state. Hemoglobin binds CO₂ and H⁺, acting as allosteric inhibitors, causing hemoglobin to release its bound oxygen. Myoglobin in muscle tissues then facilitates oxygen diffusion, ensuring efficient oxygen delivery to active muscles.

What conditions in the lungs allow hemoglobin to bind oxygen?

In the lungs, the partial pressure of oxygen is high (~100 torr) due to constant inhalation, while CO₂ levels are low due to exhalation. This low CO₂ concentration shifts the equilibrium of the carbonic anhydrase reaction towards CO₂ production, reducing H⁺ concentration and increasing pH (~7.6). These conditions favor the R state of hemoglobin, which has a high affinity for oxygen. Hemoglobin binds oxygen, transitioning from the T state to the R state, and releases CO₂ and H⁺. This oxygenated hemoglobin is then ready to transport oxygen to tissues.

What role does myoglobin play in oxygen transport in muscle tissues?

Myoglobin, found in muscle tissues, has a higher affinity for oxygen than hemoglobin due to its lower dissociation constant (K_d). When hemoglobin releases oxygen in the tissues, myoglobin binds the oxygen, facilitating its diffusion into muscle cells. This ensures a steady supply of oxygen for cellular respiration, especially during intense muscle activity. Myoglobin's high oxygen affinity allows it to effectively capture and store oxygen, making it readily available for muscle cells when needed.

How does carbonic anhydrase facilitate hemoglobin's release of oxygen in tissues?

Carbonic anhydrase, an enzyme in red blood cells, catalyzes the reaction between CO₂ and water to form carbonic acid (H₂CO₃). Carbonic acid then dissociates into bicarbonate (HCO₃^-) and hydrogen ions (H⁺). The increase in H⁺ concentration lowers the pH, stabilizing hemoglobin's T state. This stabilization reduces hemoglobin's affinity for oxygen, promoting oxygen release. The high CO₂ levels in tissues drive this reaction, ensuring efficient oxygen delivery to metabolically active cells.