Question

Hemoglobin plays a part in a series of equilibria involving protonation- deprotonation and oxygenation-deoxygenation. The overall reaction is approximately as follows $$ \mathrm{HbH}^{+}(a q)+\mathrm{O}_{2}(a q) \rightleftharpoons \mathrm{HbO}_{2}(a q)+\mathrm{H}^{+}(a q) $$ where Hb stands for hemoglobin, and \(\mathrm{HbO}_{2}\) for oxyhemoglobin. (a) The concentration of \(\mathrm{O}_{2}\) is higher in the lungs and lower in the tissues. What effect does high \(\left[\mathrm{O}_{2}\right]\) have on the position of this equilibrium? (b) The normal \(\mathrm{pH}\) of blood is \(7.4\). Is the blood acidic, basic, or neutral? (c) If the blood \(\mathrm{pH}\) is lowered by the presence of large amounts of acidic metabolism products, a condition known as acidosis results. What effect does lowering blood \(\mathrm{pH}\) have on the ability of hemoglobin to transport \(\mathrm{O}_{2}\) ?

Short answer

(a) High O₂ concentration shifts the equilibrium to the right, increasing the concentration of HbO₂ (oxyhemoglobin) and H⁺ and decreasing the concentration of HbH⁺ and dissolved oxygen. (b) Blood has a pH of 7.4, making it slightly basic (alkaline). (c) Acidosis lowers blood pH, increasing H⁺ concentration, shifting the equilibrium to the left, reducing oxyhemoglobin formation and thereby hindering hemoglobin's ability to transport oxygen.

Step-by-step solution

(a) Effect of high Oxygen concentration on equilibrium position

According to Le Chatelier's principle, if a system at equilibrium is disturbed by a change in concentration, temperature, or pressure, the system will shift its equilibrium position to counteract the change. In our case, the reaction is: \[ \text{HbH}^{+}(aq) + \text{O}_{2}(aq) \rightleftharpoons \text{HbO}_{2}(aq) + \text{H}^{+}(aq) \] If the concentration of O₂ increases (such as in the lungs), the reaction will shift to the right to consume the extra O₂ and re-establish equilibrium. Therefore, higher O₂ concentration will increase the concentration of HbO₂ (oxyhemoglobin) and H⁺ and decrease the concentration of HbH⁺ and dissolve oxygen.

(b) Blood pH and its nature

The pH of blood is given as 7.4. We know that a pH of 7 is considered neutral, while a pH below 7 is acidic and a pH above 7 is basic (alkaline). Since the blood's pH is 7.4, it is slightly basic (alkaline).

(c) Effect of acidosis on hemoglobin's ability to transport oxygen

Acidosis is a condition in which the blood pH is lowered due to the presence of large amounts of acidic metabolism products. When blood pH decreases, that means there is an increased concentration of H⁺ ions. According to Le Chatelier's principle, the equilibrium in the reaction will shift to counteract this change. In our case, the reaction will shift to the left to consume the excess H⁺ ions. As a result, the concentration of HbO₂ (oxyhemoglobin) will decrease, and the concentration of dissolved oxygen will also decrease. This means that the ability of hemoglobin to transport oxygen will be reduced during acidosis since the equilibrium will shift away from forming oxyhemoglobin.

Key concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that describes the behavior of a system at equilibrium when it experiences a change in concentration, temperature, or pressure. It dictates that the equilibrium will shift in a direction that helps to counteract the changes imposed upon the system.

For instance, when the concentration of a reactant is increased, the system responds by converting more of that reactant into a product until a new equilibrium is established. This principle is crucial in understanding how hemoglobin binds with oxygen in the lungs and releases it in tissues, as the reaction shifts according to the change in oxygen concentration in these environments. Consequently, at high oxygen levels, such as in the lungs, the equilibrium shifts to produce more oxyhemoglobin, enabling efficient oxygen transport throughout the body.

Blood pH Levels

Blood pH levels are an indicator of whether the blood is acidic, basic (alkaline), or neutral. The normal pH range for human blood is slightly alkaline, between 7.35 and 7.45. At this range, physiological processes function optimally. A pH level of exactly 7 is considered neutral, typical of pure water.

If the pH is below 7, the blood is acidic, and if it's above 7, it is basic. The body maintains the pH of the blood through mechanisms like respiration and renal function, which remove excess acid or base. The importance of the blood's pH cannot be overstated, as many of the body's critical functions, including oxygen transport by hemoglobin, are pH-sensitive.

Acidosis

Acidosis is a medical condition characterized by a decrease in blood pH, signifying an acidic environment. This condition can arise from an accumulation of acidic metabolic products. When acidosis occurs, the increased concentration of hydrogen ions (H⁺) in the blood causes hemoglobin's ability to bind and transport oxygen to be compromised.

Le Chatelier's Principle provides insight here: as the pH drops, the equilibrium of the hemoglobin binds less oxygen and shifts towards the release of oxygen and production of hydrogen ions. This reduction in the ability to carry oxygen, when not corrected, can lead to various systemic issues, as tissues rely on a steady supply of oxygen for metabolism. Understanding acidosis is vital in clinical settings to maintain proper oxygen delivery to the body's tissues.