Problem 109 - Chemistry: The Central Science

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 \(\mathrm{Hb}\) 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 \(\mathrm{O}_{2}\) ?

Step-by-Step Solution

Answer

(a) High [O2] shifts the equilibrium towards the formation of more oxyhemoglobin (HbO2) and H+. (b) The blood pH of 7.4 is slightly basic. (c) Lowering blood pH affects hemoglobin's ability to transport O2 by causing it to release more O2 to tissues.

Step 1: a) Effect of O2 concentration on equilibrium position

According to the given chemical reaction: \[ \mathrm{HbH}^{+}(a q)+\mathrm{O}_{2}(a q) \rightleftharpoons \mathrm{HbO}_{2}(a q)+\mathrm{H}^{+}(a q). \] If the concentration of O2 increases (higher in the lungs), the reaction will shift towards the right to counteract the increase in O2 according to Le Chatelier's principle. As a result, the position of this equilibrium will shift towards the formation of more oxyhemoglobin (HbO2) and H+.

Step 2: b) Blood pH classification

A pH of 7 is considered neutral. If the pH is lower than 7, the solution is acidic; if it is higher than 7, the solution is basic. Since the normal pH of blood is 7.4, it is considered slightly basic.

Step 3: c) Effect of blood pH on hemoglobin's ability to transport O2

If the blood pH is lowered due to the presence of acidic metabolism products (acidosis), it means that there is a higher concentration of H+ ions in the blood. Referring to the given chemical equation again: \[ \mathrm{HbH}^{+}(a q)+\mathrm{O}_{2}(a q) \rightleftharpoons \mathrm{HbO}_{2}(a q)+\mathrm{H}^{+}(a q). \] A higher concentration of H+ ions would shift the equilibrium to the left according to Le Chatelier's principle to counteract the increase in H+ ions. This causes the formation of more HbH+ and release of more O2. Consequently, the ability of hemoglobin to transport O2 is affected, with hemoglobin releasing more O2 to tissues in response to the lower blood pH.

Key Concepts

Le Chatelier's Principle

Le Chatelier's principle is a fundamental concept in chemistry that predicts how a system in equilibrium will respond to changes in concentration, temperature, volume, or pressure. Simply put, when a system at equilibrium is disturbed by an external change, the system will adjust itself in a way that counteracts the change and re-establishes equilibrium.

For the equilibrium involving hemoglobin oxygenation, when O₂ concentration is increased, such as in the lungs, Le Chatelier's principle suggests that the reaction will shift towards producing more oxyhemoglobin (HbO₂) and H⁺ ions to reduce the O₂ concentration. This shift facilitates efficient loading of oxygen onto hemoglobin in the lungs, where oxygen is abundant. On the other hand, in tissues where O₂ levels are low, the reaction shifts to release O₂ for use by the cells.

Blood pH

Blood pH is a measure of the acidity or alkalinity of blood, regulated to stay within a narrow range for proper physiological function. A pH of 7.0 is neutral, below 7.0 is acidic, and above 7.0 is basic (or alkaline). Normal blood pH is typically about 7.4, making it slightly basic.

It's important to maintain this pH because enzymes that facilitate biochemical reactions in the body, including those for oxygen transport, are sensitive to pH changes. If blood pH deviates significantly, it can impair these enzymes and thereby disrupt important physiological processes, leading to various health issues.

Oxygen Transport

Oxygen transport refers to the process by which oxygen is carried from the lungs to the tissues throughout the body. Hemoglobin, a protein in red blood cells, plays a crucial role in this process. It binds to oxygen in the lungs, where the oxygen concentration is high, forming oxyhemoglobin (HbO₂).

As red blood cells travel to tissues where the oxygen concentration is low, hemoglobin releases the oxygen, which is then used for cellular metabolism. This release is also influenced by other factors, such as temperature and pH, with alterations in these conditions affecting the hemoglobin's oxygen-binding affinity.

Acidosis

Acidosis is a condition characterized by an excessive accumulation of acid or depletion of alkaline (base) in the body, leading to a decrease in blood pH. There are different types of acidosis, but the one of interest here is metabolic acidosis, which often results from an increase in acidic substances due to improper metabolism or renal insufficiency.

When blood pH drops in acidosis, hemoglobin's affinity for oxygen decreases; this effect is known as the Bohr effect. As a result, hemoglobin tends to release more oxygen to the tissues even though they might not be utilizing it efficiently. This physiological response to acidosis aims to ensure that tissues maintain adequate oxygen levels despite the altered pH environment.