Problem 6 - Clinical Chemistry

Question

When arterial blood from a normal patient is exposed to room air a. \(p \mathrm{CO}_{2}\) increases; \(p \mathrm{O}_{2}\) decreases b. \(p \mathrm{CO}_{2}\) decreases; \(p \mathrm{O}_{2}\) increases c. \(p \mathrm{CO}_{2}\) decreases; \(p \mathrm{O}_{2}\) decreases d. \(p \mathrm{CO}_{2}\) increases; \(p \mathrm{O}_{2}\) increases

Step-by-Step Solution

Answer

Option (b): \(p \mathrm{CO}_{2}\) decreases; \(p \mathrm{O}_{2}\) increases.

Step 1 - Understand the Variables

Identify what \(p \mathrm{CO}_{2}\) and \(p \mathrm{O}_{2}\) stand for. \(p \mathrm{CO}_{2}\) represents the partial pressure of carbon dioxide, and \(p \mathrm{O}_{2}\) represents the partial pressure of oxygen in the blood.

Step 2 - Knowledge of Gas Exchange

Recall that when blood is exposed to room air, gases diffuses between the blood and the air. The direction of gas movement is from higher to lower partial pressure. Room air has higher \(p \mathrm{O}_{2}\) (approximately 21% oxygen) and lower \(p \mathrm{CO}_{2}\) (approximately 0.04% carbon dioxide).

Step 3 - Effect on \(p \mathrm{O}_{2}\)

When exposed to room air, the \(p \mathrm{O}_{2}\) in the blood will increase because oxygen will diffuse into the blood from the room air.

Step 4 - Effect on \(p \mathrm{CO}_{2}\)

Because room air has much lower \(p \mathrm{CO}_{2}\), carbon dioxide will diffuse out of the blood into the air, causing the \(p \mathrm{CO}_{2}\) to decrease.

Step 5 - Determine the Correct Answer

Based on the changes to \(p \mathrm{O}_{2}\) and \(p \mathrm{CO}_{2}\), the correct answer is option (b), where \(p \mathrm{CO}_{2}\) decreases and \(p \mathrm{O}_{2}\) increases.

Key Concepts

partial pressure of carbon dioxide

The partial pressure of carbon dioxide, or \(p \mathrm{CO}_{2}\), is a way to measure how much carbon dioxide (CO₂) is in the blood. It's an important part of understanding gas exchange and respiratory function.

In the bloodstream, carbon dioxide is produced as a byproduct of cellular metabolism. Cells release CO₂, which then diffuses into the blood.

From the blood, CO₂ is transported to the lungs, where it is expelled during exhalation.

• The level of \(p \mathrm{CO}_{2}\) in arterial blood typically ranges between 35–45 mmHg in a healthy person.
• Changes in \(p \mathrm{CO}_{2}\) can indicate problems with ventilation or gas exchange.

When arterial blood is exposed to room air, which has a much lower CO₂ concentration, carbon dioxide diffuses out of the blood. This lowers the \(p \mathrm{CO}_{2}\) in the blood.

partial pressure of oxygen

The partial pressure of oxygen, or \(p \mathrm{O}_{2}\), refers to the concentration of oxygen in the blood. It is crucial for cellular respiration and overall tissue health.

Oxygen enters the body through the lungs and binds to hemoglobin in red blood cells, which transport it throughout the body.

• Normal \(p \mathrm{O}_{2}\) levels in arterial blood are between 75–100 mmHg.
• Low \(p \mathrm{O}_{2}\) can signal issues such as hypoxemia or respiratory disorders.

When arterial blood is exposed to room air, which has a higher oxygen concentration, oxygen diffuses into the blood. This increases the \(p \mathrm{O}_{2}\) level.

diffusion of gases

Diffusion of gases is a key process in respiration, enabling the exchange of oxygen and carbon dioxide between the bloodstream and the environment.

It occurs because gases move from areas of higher partial pressure to areas of lower partial pressure. Several principles guide this movement:

• Bronchial and alveolar air have high \(p \mathrm{O}_{2}\) and low \(p \mathrm{CO}_{2}\).
• Venous blood, returning to the lungs, has low \(p \mathrm{O}_{2}\) and high \(p \mathrm{CO}_{2}\).

This difference allows oxygen to diffuse into the blood and carbon dioxide to diffuse out. When arterial blood is exposed to the room air, oxygen from the room air diffuses into the blood, raising \(p \mathrm{O}_{2}\); carbon dioxide diffuses out, lowering \(p \mathrm{CO}_{2}\).