Question

Hemoglobin (abbreviated \(\mathrm{Hb}\) ) is a protein that is responsible for the transport of oxygen in the blood of mammals. Each hemoglobin molecule contains four iron atoms that serve as the binding sites for \(\mathrm{O}_{2}\) molecules. The oxygen binding is \(\mathrm{pH}\) dependent. The relevant equilibrium reaction is $$\mathrm{HbH}_{4}{ }^{4+}(a q)+4 \mathrm{O}_{2}(g) \rightleftharpoons \mathrm{Hb}\left(\mathrm{O}_{2}\right)_{4}(a q)+4 \mathrm{H}^{+}(a q)$$ Use Le Châtelier's principle to answer the following. a. What form of hemoglobin, \(\mathrm{HbH}_{4}^{4+}\) or \(\mathrm{Hb}\left(\mathrm{O}_{2}\right)_{4}\), is favored in the lungs? What form is favored in the cells? b. When a person hyperventilates, the concentration of \(\mathrm{CO}_{2}\) in the blood decreases. How does this affect the oxygenbinding equilibrium? How does breathing into a paper bag help to counteract this effect? (Hint: \(\mathrm{CO}_{2}\) reacts with water to produce carbonic acid.) c. When a person has suffered a cardiac arrest, an injection of a sodium bicarbonate solution is given. Why is this step necessary?

Short answer

In the lungs, Hb(O2)4 form is favored due to high oxygen concentration, while in the cells, HbH44+ form is favored due to low oxygen concentration and release of CO2. Hyperventilation decreases CO2 concentration, increasing oxygen binding to hemoglobin. Breathing into a paper bag restores CO2 concentration, helping oxygen release to cells. Sodium bicarbonate injection during cardiac arrest neutralizes low blood pH, improving oxygen supply to cells by shifting the equilibrium towards oxygen release.

Step-by-step solution

a. Hemoglobin forms in lungs and cells

In lungs, the concentration of O2 is high and the concentration of CO2 is low. According to Le Châtelier's principle, since O2 is a reactant, the equilibrium will shift in the forward direction (right) in response to high O2 concentration, forming more hemoglobin-oxygen complex, Hb(O2)4. Therefore, Hb(O2)4 is favored in the lungs. In cells, the concentration of O2 is low, and the release of CO2 increases the concentration of H+ (due to the formation of carbonic acid). Since H+ is a product of the forward reaction, the equilibrium shifts in the reverse direction (left), favoring the formation of HbH44+. So, HbH44+ is favored in the cells.

b. Effect of hyperventilation and paper bag

When a person hyperventilates, the concentration of CO2 in the blood decreases due to rapid expulsion of CO2 from the body. Lower CO2 concentration reduces the amount of carbonic acid, and thereby decreases the concentration of H+ ions in the blood. As H+ is a product of the forward reaction, Le Châtelier's principle states that the decrease in H+ concentration shifts the equilibrium towards the forward direction, increasing oxygen binding to hemoglobin. However, this is not necessarily a good thing as it can lead to reduced oxygen delivery to cells. Breathing into a paper bag helps counteract this effect by increasing the inhaled CO2 concentration. As the concentration of CO2 in the blood increases, more carbonic acid is formed, and the concentration of H+ ions increases. According to Le Châtelier's principle, this will shift the equilibrium in the reverse direction, releasing more oxygen from hemoglobin and delivering it to the cells.

c. Sodium bicarbonate during cardiac arrest

During a cardiac arrest, the heart's function is compromised, leading to reduced oxygen delivery to cells and therefore increased anaerobic respiration in the cells. This causes increased production and release of acidic byproducts like lactic acid, leading to a decrease in blood pH. As the concentration of H+ ions increases in the blood, this shifts the hemoglobin-oxygen binding equilibrium towards the forward direction, making it difficult for hemoglobin to release oxygen to the cells. Injecting a sodium bicarbonate solution helps neutralize the acidic environment of the blood. Sodium bicarbonate reacts with the excess H+ ions, lowering the H+ concentration in the blood. According to Le Châtelier's principle, this would shift the equilibrium in the reverse direction, allowing more oxygen to be released by hemoglobin and improving the supply of oxygen to the cells.

Key concepts

Hemoglobin

Hemoglobin, often abbreviated as Hb, is an essential protein in the blood responsible for transporting oxygen throughout the body.
It consists of four subunits, each containing an iron atom that can bind to an oxygen molecule. This configuration allows hemoglobin to carry four oxygen molecules per molecule of hemoglobin.

• In the lungs, where oxygen concentration is high, hemoglobin binds to oxygen readily.
• In tissues, where oxygen concentration is lower, hemoglobin releases oxygen to the cells.

This dynamic binding and releasing process is crucial for supplying oxygen to bodily tissues and organs, facilitating cell respiration and energy production.

Oxygen Binding

Oxygen binding by hemoglobin is central to its role of oxygen transport. This process is highly dependent on several factors, including pH levels.
The binding is described by the following equilibrium reaction:\[\text{HbH}_4^{4+}(aq) + 4\text{O}_2(g) \rightleftharpoons \text{Hb(O}_2\text{)}_4(aq) + 4\text{H}^+(aq)\]The reaction shows that oxygen binding to hemoglobin releases hydrogen ions (H^+).

• In the lungs: High (O_2) concentration shifts the equilibrium to the right, favoring oxygen binding.
• In tissues: Increased (H^+) due to carbon dioxide conversion shifts it to the left, prompting oxygen release.

Le Châtelier's principle explains how the equilibrium shifts in response to changes in concentration, such as when hyperventilating or during varying pH levels.

Carbon Dioxide

Carbon dioxide ( CO_2 ) plays a pivotal role in regulating the oxygen binding of hemoglobin. In the body, CO_2 is a byproduct of cellular respiration.
When CO_2 levels rise, it reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions.

• As CO_2 increases in tissues, more ( H^+ ) ions are produced, lowering pH, which promotes oxygen release.
• Reduced CO_2 levels, such as during hyperventilation, decrease ( H^+ ) concentration, making it harder to release bound oxygen.

Breathing into a paper bag increases CO_2 inhalation, restoring normal blood H^+ levels and helping to ensure oxygen is released to the tissues effectively.

Sodium Bicarbonate

Sodium bicarbonate ( NaHCO_3 ) is often used in medical situations to counteract acidosis, a condition where the blood becomes too acidic.
Acidosis can occur during cardiac arrest when lactic acid and other acidic byproducts accumulate due to insufficient oxygen supply.

• Injecting sodium bicarbonate helps neutralize excess hydrogen ions, raising the blood pH to a more normal level.
• This pH adjustment enables hemoglobin to release oxygen more readily to the cells, crucial during emergencies like cardiac arrest.

The use of sodium bicarbonate is a critical intervention to enhance the transfer of oxygen from blood to tissues, improving overall oxygenation.