Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

Fear or excitement, generally causes one to breathe rapidly and it results in the decrease of \(\mathrm{CO}_{2}\) concentration in blood. In what way will it change the \(\mathrm{pH}\) of blood? (a) \(\mathrm{pH}\) will increase (b) \(\mathrm{pH}\) will decrease (c) no change (d) \(\mathrm{pH}\) will adjust to 7

Step-by-Step Solution

Answer

(a) pH will increase

Step 1: Understanding the Relationship Between CO2 and Blood pH

Carbon dioxide (\(\text{CO}_2\)) in the blood is involved in the formation of carbonic acid (\(\text{H}_2\text{CO}_3\)), which partially dissociates into hydrogen ions (\(\text{H}^+\)) and bicarbonate ions (\(\text{HCO}_3^-\)). This reaction is represented by:\[\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-\]The presence of \(\text{H}^+\) ions contributes to the acidity of the blood. Therefore, the concentration of \(\text{CO}_2\) affects blood pH.

Step 2: Effect of Rapid Breathing on CO2 Levels

When a person breathes rapidly, more \(\text{CO}_2\) is expelled from the body through the lungs. This results in a decrease in the concentration of \(\text{CO}_2\) in the blood.

Step 3: Impact of Reduced CO2 on Blood pH

With lower levels of \(\text{CO}_2\), there is less carbonic acid dissociation, resulting in fewer \(\text{H}^+\) ions available in the blood. A decrease in \(\text{H}^+\) ions leads to a rise in the blood pH, making it more alkaline.

Step 4: Conclusion

Since a decrease in \(\text{CO}_2\) concentration leads to a decrease in \(\text{H}^+\) ions and a subsequent increase in blood pH, the pH of the blood will increase. Therefore, the correct answer is (a) pH will increase.

Key Concepts

Carbon Dioxide and Blood Chemistry

Carbon dioxide (CO₂) plays a crucial role in maintaining the blood's chemical balance. When CO₂ is dissolved in blood, it forms carbonic acid (H₂CO₃). This reaction can be written as:\[\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3\]Once formed, carbonic acid can dissociate into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻):\[\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-\]This process is essential because it helps regulate the acid-base balance in our body.

• The more CO₂ in the blood, the more carbonic acid will form, increasing H⁺ ions.
• More H⁺ means a greater acidity level, lowering blood pH.

So, CO₂ concentration directly affects the blood's acidity, demonstrating why changes in breathing can impact blood chemistry.

Acid-Base Balance

The body's acid-base balance is about maintaining the right pH levels in the blood. Normally, blood pH is kept in a narrow range around 7.35 to 7.45, making it slightly alkaline.

Several mechanisms help maintain this balance:

• The kidneys regulate blood pH by excreting excess H⁺ ions and reabsorbing bicarbonate (HCO₃⁻).
• The respiratory system adjusts the CO₂ levels through changes in breathing, impacting how much carbonic acid is formed.

How Carbon Dioxide Impacts pH: When CO₂ levels drop (for example, from rapid breathing when excited or frightened), less H₂CO₃ forms:

• This reduction in carbonic acid decreases H⁺ concentration.
• With fewer H⁺ ions, blood becomes less acidic, so pH increases.

This shows how the body uses carbon dioxide levels to keep the crucial acid-base balance intact.

Respiratory System

The respiratory system is fundamental in regulating blood CO₂ levels and thus blood pH. It does this through breathing, which is more than just gas exchange for oxygen and CO₂.

Role of the Respiratory System in pH Regulation

• When you breathe faster and deeper, more CO₂ is expelled. This decreases CO₂ concentration in the blood.
• Lower CO₂ leads to reduced carbonic acid formation, decreasing H⁺ ion concentration and increasing pH.
• Conversely, slower breathing retains CO₂, increasing acidity and decreasing pH.

The respiratory system, therefore, helps the body quickly adjust pH levels by altering breathing rates to match the body's needs. It's an essential part of the homeostatic mechanisms that keep our internal environment stable.