Question

Hemoglobin (abbreviated 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 are the binding sites for \(\mathrm{O}_{2}\) molecules. The oxygen binding is pH- dependent. The relevant equilibrium reaction is $$ \mathrm{HbH}_{4}^{4+}(a q)+4 O_{2}(g) \rightleftharpoons \mathrm{Hb}\left(\mathrm{O}_{2}\right)_{4}(a q)+4 \mathrm{H}^{+}(a q) $$ Use Le Châtelier's principle to answer the following. a. What form of hemoglobin, 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 is decreased. How does this affect the oxygen-binding equilibrium? How does breathing into a paper bag help to counteract this effect? (See Exercise \(146 .\) ) c. When a person has suffered a cardiac arrest, injection of a sodium bicarbonate solution is given. Why is this necessary? (Hint: CO, blood levels increase during cardiac arrest.)

Short answer

In the lungs, the form of hemoglobin favored is \(\mathrm{Hb}\left(\mathrm{O}_{2}\right)_{4}\) due to a higher concentration of oxygen, while in cells, the favored form is HbH\(_{4}^{4+}\) due to lower oxygen concentration. Hyperventilation leads to decreased \(\mathrm{CO}_{2}\) concentration, increasing pH, and favoring oxygen-binding. Breathing into a paper bag helps restore \(\mathrm{CO}_{2}\) levels and balance oxygen binding and release. Sodium bicarbonate injection during cardiac arrest neutralizes acidity caused by increased \(\mathrm{CO}_{2}\) levels, shifting the oxygen-binding equilibrium, and improving oxygen supply to vital organs.

Step-by-step solution

a. Determine the favored form of hemoglobin

: In the lungs, the partial pressure of oxygen is higher, which means there is a large concentration of \(\mathrm{O}_{2}\). According to Le Châtelier's principle, the equilibrium will shift to the right to consume the excess oxygen. Therefore, the form favored in the lungs is \(\mathrm{Hb}\left(\mathrm{O}_{2}\right)_{4}\). In the cells, the partial pressure of oxygen is lower, which means the \(\mathrm{O}_{2}\) concentration is less than in the lungs. According to Le Châtelier's principle, the equilibrium will shift to the left to counteract the decrease in oxygen concentration. So, the form favored in the cells is HbH \(_{4}^{4+}\).

b. Effect of hyperventilation on oxygen-binding equilibrium

: When a person hyperventilates, the concentration of \(\mathrm{CO}_{2}\) in the blood decreases. This leads to an increase in pH (less acidic) because dissolved \(\mathrm{CO}_{2}\) forms carbonic acid in blood. An increase in pH will shift the equilibrium to the right (Le Châtelier's principle), which means oxygen-binding will be favored. However, this situation can decrease the oxygen delivery to tissues as oxygen binding is too strong. Breathing into a paper bag allows the person to inhale some of the exhaled \(\mathrm{CO}_{2}\) back into their bloodstream, which increases the \(\mathrm{CO}_{2}\) concentration in blood, lowering the pH, and shifting the equilibrium back toward the left. This helps counteract the effect of hyperventilation and restores the balance of oxygen binding and release.

c. Role of sodium bicarbonate injection

: During cardiac arrest, the blood \(\mathrm{CO}_{2}\) levels increase because the heart is unable to pump adequately, and the carbon dioxide removal process by the lungs is affected. The increased concentration of \(\mathrm{CO}_{2}\) in the blood results in a decrease in pH (more acidic). Injecting sodium bicarbonate solution into the bloodstream helps to neutralize this acidity, and it shifts the oxygen-binding equilibrium to the right, allowing more oxygen to be bound and transported by hemoglobin. This increased oxygen supply is essential to revive the patient's vital organs and prevent further damage.

Key concepts

Le Châtelier's Principle

Le Châtelier's principle is a fundamental concept in chemistry that helps us understand how equilibrium systems respond to changes. In simple terms, this principle states that if you disturb a system at equilibrium, the system will adjust itself to counteract the disturbance and try to re-establish equilibrium. In the context of hemoglobin and oxygen transport, this principle is pivotal.
In the lungs, where oxygen levels are high, the equilibrium shifts towards binding more oxygen to hemoglobin. This means that the \ \( \mathrm{Hb}(\mathrm{O}_{2})_{4} \ \) form is favored since it allows more oxygen to be carried through the bloodstream.
On the other hand, within body tissues where oxygen is consumed and its level drops, the equilibrium shifts to release oxygen from the hemoglobin. Consequently, the \ \( \mathrm{HbH}_{4}^{4+} \ \) form is predominant in tissues, allowing oxygen to be freed for cellular activities. This balance is crucial for efficient oxygen delivery throughout the body.

Oxygen Transport

Oxygen transport is a vital function of hemoglobin, ensuring every cell in the body receives necessary oxygen for metabolism. Hemoglobin molecules in red blood cells carry oxygen from the lungs to tissues and return carbon dioxide back to be expelled. The reversible binding and release of oxygen by hemoglobin depend heavily on environmental conditions such as partial pressure of oxygen, pH, and the presence of other molecules like carbon dioxide.
This process is highly efficient due to the structural nature of hemoglobin and its affinity for oxygen, which varies with its environment. At high oxygen levels in the lungs, hemoglobin binds oxygen readily, ensuring maximal transport capability. Once in tissues where oxygen is used up, hemoglobin releases oxygen easily, reacting to shifts in equilibrium and allowing oxygen to diffuse into cells, where it's needed the most.

pH Dependence

The pH level of the blood is a crucial factor influencing hemoglobin's oxygen-binding ability. A change in pH can significantly affect the position of the oxygen binding equilibrium. Under normal physiological conditions, as \ \( \mathrm{CO}_2 \ \) is produced by cells and dissolved in the blood, it forms carbonic acid, which lowers the pH of blood. A lower pH (more acidic) encourages the release of oxygen from hemoglobin, thus ensuring effective oxygen delivery to tissues.
In the opposite scenario, when pH levels rise due to decreased \ \( \mathrm{CO}_2 \ \), hemoglobin's affinity for oxygen increases, favoring the binding more firmly rather than releasing it. This phenomenon is a critical part of what is known as the Bohr effect, where changes in hydrogen ion concentration affect hemoglobin's ability to bind oxygen.
This pH-dependent mechanism allows the body to finely tune oxygen transport according to its metabolic needs, adjusting to varying levels of activity and environmental conditions.

Hyperventilation Effect

Hyperventilation is a condition where one breathes rapidly and deeply, causing an excessive loss of \ \( \mathrm{CO}_2 \ \) from the blood. This reduction in \ \( \mathrm{CO}_2 \ \) levels increases blood pH, making it less acidic. The shift to a higher pH influences hemoglobin's binding with oxygen, as dictated by Le Châtelier's principle, resulting in oxygen binding too firmly and reducing its availability to tissues.
This scenario presents a challenge, as oxygen, though bound in plenty, isn't released where it's needed. That's why breathing into a paper bag can be beneficial; it allows re-inhalation of \ \( \mathrm{CO}_2 \ \,\) restoring normal \ \( \mathrm{CO}_2 \ \) levels. This brings pH down to its usual state and helps reseat the balance, allowing hemoglobin to release oxygen appropriately.
Understanding hyperventilation and its effects on oxygen transport is critical, especially in medical situations, ensuring that interventions are appropriately applied to stabilize the patient's condition effectively.