Bohr Effect: Videos & Practice Problems

The Bohr effect describes how hemoglobin's oxygen affinity changes in response to CO₂ and pH levels. In tissues, high CO₂ and H⁺ concentrations lower pH, promoting oxygen release as hemoglobin binds to these molecules, shifting the oxygen binding curve to the right. Conversely, in the lungs, low CO₂ and H⁺ levels increase pH, enhancing oxygen binding and shifting the curve to the left. This dynamic allows hemoglobin to maximize oxygen uptake in the lungs and optimize release in the tissues, facilitating efficient gas exchange.

Bohr Effect

So at this point in our course, we're already familiar with hemoglobin binding in the tissues and lungs as well as hemoglobin carbonation and protonation. And really, all of that was just background information to help us understand the Bohr effect, which is what we're going to talk about in this video. And so really, the Bohr effect is just giving a name to a process that we're already familiar with from our previous lesson videos. And so, really, most of the information in this video is just going to be review. And really, there's just a small bit of new information that we're going to reveal here. What we need to recall from our previous lesson videos is that the Bohr effect describes the effect of the concentration of CO₂, as well as the effect of pH or the concentration of hydrogen ions \( H^+ \) on both hemoglobin's binding and release of oxygen. Recall from our previous lesson videos that carbaminohemoglobin or \( HbCO_2 \) as well as protonated hemoglobin, \( HHb^+ \), both stabilize the T state of hemoglobin to promote oxygen release. If we take a look at this Bohr effect, what it says is that when the concentration of CO₂ and \( H^+ \) are both really high, as they are in the tissues, they both act as allosteric inhibitors, and three significant events occur at these high concentrations. Those three events are mentioned below, and again, they're really just review from our previous lesson videos. There's such a high concentration of CO₂ in the tissues that hemoglobin is bound to bind to some of that CO₂ as carbaminohemoglobin, \( HbCO_2 \). Similarly, there's also such a high concentration of \( H^+ \) in the tissues that hemoglobin is bound to bind to some of that \( H^+ \) and become protonated as \( HHb^+ \). If hemoglobin is bound to CO₂ and these protons, they act as an inhibitor and decrease hemoglobin's oxygen affinity. Hemoglobin's oxygen affinity is going to be decreased, leading to oxygen release. So again, everything that we have here in green is pretty much just review from our previous lesson videos. What we're saying that's new here is that because CO₂ and \( H^+ \) act as allosteric inhibitors, they're going to cause a shift of the oxygen-binding curve to the right. We will be able to see that down below once we take a look at our oxygen-binding curve. The last point I want to make here, also a review from our previous lesson videos, is that when the concentration of CO₂ and \( H^+ \) are low, as they are in the lungs, the complete opposite events occur. Instead of hemoglobin binding CO₂, it's going to release CO₂. Instead of being protonated, it's going to be deprotonated. Instead of decreasing \( O_2 \) affinity, it's going to increase \( O_2 \) affinity. If we take a look at our oxygen-binding curve down below, notice that we have the fractional saturation, theta or y, on the y-axis, as well as the partial pressure of oxygen in units of torr on the x-axis. We've got these three different curves here: the blue curve, the red curve, and the green curve. In the background, we have the partial pressure of oxygen in the lungs, which is right around 100 torr, and the partial pressure of oxygen in the tissues, which is right around 20 torr. Over here on the right, we have the Bohr effect. As mentioned above, there's going to be a shift to the right in the tissue. The oxygen binding_curve is going to shift to the right, and we can see this with the green curve, which is indeed shifted to the right with respect to the red curve. This shift in the tissues will lead to a high \( K_d \) and a low oxygen affinity. If there's a low oxygen affinity, that means that it's going to release more oxygen. This is all due to the fact that in the tissues, there's such a high concentration of CO₂. A high partial pressure of CO₂ also a high concentration of \( H^+ \), and of course, a high concentration of \( H^+ \) leads to a lower pH, with a pH value of about 7.2 in the tissues. потому что there's a lower oxygen affinity due to the CO₂ and \( H^+ \) acting as inhibitors, this is going to increase the oxygen release to the tissues. Now, if we take a look at the conditions in the lungs, that's going to be represented by this blue curve. The opposite events occur, and instead of having a shift to the right, there's going to be a shift to the left. Our blue curve is shifted to the left with respect to the red curve. We can say that there is a left shift in the lungs. The left shift in the lungs is going to cause the \( K_d \) to be low, and a low \( K_d \) corresponds with a high oxygen affinity. This is all due to the fact that in the lungs, we're constantly exhaling CO₂. The partial pressure of CO₂ is going to be really low in the lungs, and so is the concentration of hydrogen ions. A low concentration of hydrogen ions is going to lead to a relatively high pH, right around a value of about 7.6 in the lungs, as we can see in our graph. A higher oxygen affinity is going to lead to less oxygen release, so less oxygen is going to be released, and more oxygen is going to be bound. We notice that in the lungs, everything is the complete and exact opposite of the tissues in terms of the up and down arrows and the highs and lows, as well as the left and right shifts. This together all describes the Bohr effect. What you'll notice is that the Bohr effect allows hemoglobin to switch from the blue curve to the green curve when transitioning from the lungs to the tissues. Taking a look at this oxygen-binding curve up above, when hemoglobin is in the lungs, it's going to take on this the blue curve. When hemoglobin is in the tissues, it's going to take on the green curve. This transition is what's referred to as the Bohr effect. What the Bohr effect allows is for hemoglobin to maximize its oxygen binding when it's in the lungs and optimize its oxygen release when it's in the tissues. With the blue curve here, when it's in the lungs, you'll notice that it has the highest binding of oxygen. Hemoglobin is able to maximize its oxygen binding in the lungs when it takes on the shape of the blue curve. Notice that as it starts to make its way to the tissues, hemoglobin is going to switch from following the blue curve to following the green curve because of the decreased pH. In the green curve, notice that it has a lower fractional saturation, than at the same point as the blue curve. This represents more oxygen being released because there's a lower fractional saturation. This allows hemoglobin to maximize its binding in the lungs and optimize its release of oxygen to the tissues. That is the Bohr effect in a nutshell. This here concludes our lesson on the Bohr effect, and we'll be able to practice using these concepts as we move forward in our course. I'll see you guys in our next video.

Identify all the correct statements regarding the Bohr effect on hemoglobin.
i) The Bohr effect shifts the fractional O₂ saturation curve to the right as pH decreases.
ii) The Bohr effect shifts the fractional O₂ saturation curve to the right as pH increases.
iii) The Bohr effect favors O₂ release in respiring tissues.
iv) O₂ and H⁺ compete for the same binding site on hemoglobin.

• A. i & iii.
• B. i & iv.
• C. ii & iv.
• D. ii, iii, & iv.

Answer: (A)

Bohr Effect

In this video, we're going to recap and summarize the Bohr effect on hemoglobin. Notice in our table down below, on the left-hand side, we're going to recap the Bohr effect as it pertains to the tissues. On the right-hand side, we recap the Bohr effect as it pertains to the lungs. Of course, there's a lower pH in the tissues due to the production of hydrogen ions, whereas in the lungs, there's a higher pH. Hemoglobin is going to release oxygen in the tissues, whereas in the lungs, hemoglobin is going to bind oxygen. In the tissues, hemoglobin is going to bind to the hydrogen ions that it's producing, whereas in the lungs it's going to release those hydrogen ions that it bound.

Notice in our image below, we have our little circulatory system where you can see our hearts in the middle and the bloodstream as it leads to the lungs and the tissues. In the tissues, there's a high CO₂ concentration and a low oxygen concentration. Conversely, in the lungs, we have a low CO₂ concentration and a high oxygen concentration. We're zooming into the tissues on the left and into the lungs on the right. This image below is the same as our previous images except we are combining everything into one single cell for reviewing purposes to make it easy to combine everything into one image.

We start off with the high concentration of CO₂ produced in our muscle tissues. This high concentration of CO₂ is because our tissues are performing cellular respiration and all of this CO₂ is going to diffuse out into our bloodstream, into the red blood cell, where there's going to be a relatively high concentration of CO₂ as well. In our red blood cells, there's an enzyme, carbonic anhydrase, that will catalyze a reaction with CO₂ and water. The high CO₂ causes this equilibrium to shift to the right to compensate, reminding us of Le Chatelier's principle. This forms carbonic acid, which is relatively acidic here and it's going to break up into its conjugate base and the hydrogen ion in the tissues.

Producing hydrogen ions is going to lead to a lower pH, and the pH in the tissues is going to be slightly lower, around 7.2. In the lungs, this hydrogen ion is going to react to form water ultimately. This is because in our lungs, there's a low concentration of CO₂ since we are constantly exhaling CO₂. If there's a low concentration of CO₂, then this equilibrium, controlled by carbonic anhydrase, is going to shift to the right to respond to the low concentrations of CO₂. This is going to occur via Le Chatelier's principle and will cause hydrogen ions to produce water. A decrease in hydrogen ions is going to increase the pH slightly to a value of 7.6 in the lungs.

Also in the lungs, with every breath, we inhale a high concentration of oxygen and all of that oxygen is going to diffuse out of the lungs, into our blood capillaries, and into our red blood cells. Hemoglobin is going to encounter a high concentration of hydrogen ions and a high concentration of CO₂ in the tissues. It will bind to the hydrogen ions and CO₂, and when it does that, it will cause hemoglobin to release its oxygen in the tissues. Myoglobin in our tissues can help facilitate oxygen diffusion into the tissues. So, this completes our full cycle here, and that is the end of this video. We'll be able to get some practice in our next video. I'll see you guys there.

On the graph below, draw in the approximate shapes of the O ₂-saturation curves in the lungs & tissues after a shift due to the Bohr effect takes place.

So this practice problem says, on the graph below, draw on the approximate shapes of the oxygen saturation curves in the lungs and the tissues oxygen and units of torr on the x-axis. And so we need to draw in the 2 curves, oxygen saturation curves for the lungs and the tissues with respect to this red curve here. And so, of course, we know that in the lungs, the 'l' tells us there's going to be a left shift. And so there's going to be a shift, somewhat like this, for the lungs. And then, of course, that means that in the tissues, there's going to be a right shift somewhat like this. And so really that's all that this was asking for, associating the oxygen saturation curve in the lungs with a left shift and the oxygen saturation curve in the tissues here, with a right shift. So, that concludes this practice problem and I'll see you guys in our next video.

The Bohr effect describes the change in hemoglobin's affinity for oxygen under two different conditions. What are these two conditions and how do they impact hemoglobin's affinity for oxygen? Complete the table below:

So, this practice problem says, the Bohr effect describes the change in hemoglobin affinity for oxygen under 2 different conditions. What are these two conditions and how do they impact hemoglobin's oxygen affinity? Complete the table below and notice the table has the condition in the first column and the impact on hemoglobin's oxygen affinity in the second column. And so of course we know from our previous lesson videos that the Bohr effect describes the effect of both carbon dioxide or CO2 and the effect of pH on hemoglobin's oxygen affinity. And so this first condition here is going to be the conditions that we find in our muscle tissues, which corresponds with a high concentration of CO2 and a high concentration of hydrogen ions. And so of course we know from our previous lesson videos that both CO2 and hydrogen ions are going to act as heterotropic allosteric inhibitors. And so because they act as heterotropic allosteric inhibitors, they are going to inhibit Hemoglobin's oxygen affinity and essentially, cause Hemoglobin, to stabilize. So, they're going to stabilize, hemoglobin's, T state. And so, recall that the T state is going to be the tense state and that's going to bind, to oxygen inefficiently. And so, this is going to stabilize hemoglobin's T state and cause hemoglobin to release oxygen. And so really, this is the answer to this first row here. Now in the second row, the second set of conditions that we're going to talk about are the conditions in the lungs, which of course corresponds with the opposite conditions that we have in the tissues. A low concentration of CO2 due to exhalation, and of course, the low CO2 leads to a decreased hydrogen ion concentration. And so, of course, again, because CO2 and H+ act as heterotropic allosteric inhibitors, they are not going to be present if they're at low concentrations, and they're not going to be able to stabilize Hemoglobin's T state. So instead, what happens is, Hemoglobin is essentially going to transition from its T state, to its R state. And of course, that's going to cause Hemoglobin to bind oxygen rather than release oxygen and so really this is the answer, to the second row. And so this concludes this practice problem and I'll see you guys in our next lesson.

Here’s what students ask on this topic:

What is the Bohr effect in hemoglobin?

The Bohr effect describes how hemoglobin's oxygen affinity changes in response to CO₂ and pH levels. In tissues, high CO₂ and H⁺ concentrations lower pH, promoting oxygen release as hemoglobin binds to these molecules, shifting the oxygen binding curve to the right. Conversely, in the lungs, low CO₂ and H⁺ levels increase pH, enhancing oxygen binding and shifting the curve to the left. This dynamic allows hemoglobin to maximize oxygen uptake in the lungs and optimize release in the tissues, facilitating efficient gas exchange.

How does pH affect hemoglobin's oxygen affinity?

pH affects hemoglobin's oxygen affinity through the Bohr effect. In tissues, a lower pH (higher H⁺ concentration) decreases hemoglobin's oxygen affinity, promoting oxygen release. This is because H⁺ ions stabilize the T-state of hemoglobin, which has a lower affinity for oxygen. Conversely, in the lungs, a higher pH (lower H⁺ concentration) increases hemoglobin's oxygen affinity, enhancing oxygen binding. This is because fewer H⁺ ions stabilize the R-state of hemoglobin, which has a higher affinity for oxygen.

Why does the oxygen binding curve shift to the right in tissues?

The oxygen binding curve shifts to the right in tissues due to high concentrations of CO₂ and H⁺, which lower the pH. These conditions stabilize the T-state of hemoglobin, reducing its affinity for oxygen and promoting oxygen release. This rightward shift indicates that at any given partial pressure of oxygen, hemoglobin will release more oxygen in the tissues, where it is needed for cellular respiration.

What role does CO₂ play in the Bohr effect?

CO₂ plays a crucial role in the Bohr effect by influencing hemoglobin's oxygen affinity. In tissues, high CO₂ levels lead to the formation of carbaminohemoglobin (HbCO₂) and increase H⁺ concentration through the formation of carbonic acid. Both CO₂ and H⁺ stabilize the T-state of hemoglobin, decreasing its oxygen affinity and promoting oxygen release. In the lungs, low CO₂ levels result in the release of CO₂ from hemoglobin, increasing pH and enhancing oxygen binding.

How does the Bohr effect facilitate efficient gas exchange in the body?

The Bohr effect facilitates efficient gas exchange by allowing hemoglobin to adjust its oxygen affinity based on the local environment. In the lungs, low CO₂ and H⁺ levels increase pH, enhancing hemoglobin's oxygen binding capacity. This ensures maximum oxygen uptake. In tissues, high CO₂ and H⁺ levels lower pH, reducing hemoglobin's oxygen affinity and promoting oxygen release where it is needed for cellular respiration. This dynamic adjustment optimizes oxygen delivery to tissues and uptake in the lungs.