Question

If the blood lacked red blood cells but the lungs were functioning normally, a. the arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) would be normal. b. the oxygen content of arterial blood would be normal. c. both \(a\) and \(b\) would apply. d. neither \(a\) nor \(b\) would apply.

Short answer

The correct answer is a. the arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) would be normal. This is because the partial pressure of oxygen in arterial blood depends on the amount of oxygen in the alveoli and the amount of oxygen dissolved in plasma. Since the lungs are functioning normally, the arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) would be normal, even in the absence of red blood cells. However, the oxygen content of arterial blood would be significantly lower without red blood cells.

Step-by-step solution

Understanding arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) and oxygen content

Arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) is the partial pressure of oxygen in arterial blood. Oxygen binds to hemoglobin in red blood cells, and the oxygen content of blood refers to the amount of oxygen bound to hemoglobin in RBCs as well as the oxygen dissolved in plasma.

Option a: Analyzing arterial \(\mathrm{P}_{\mathrm{O}_{2}}\)

The partial pressure of oxygen in arterial blood depends on both the amount of oxygen in the alveoli (alveolar \(\mathrm{P}_{\mathrm{O}_{2}}\)) and the amount of oxygen that dissolves in plasma. Since the lungs are functioning normally, the alveolar \(\mathrm{P}_{\mathrm{O}_{2}}\) will be normal. This would imply that the arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) would also be normal, despite the absence of red blood cells. So, this statement is true.

Option b: Analyzing oxygen content of arterial blood

Oxygen content depends on the amount of oxygen bound to hemoglobin in red blood cells and the oxygen dissolved in plasma. Since there are no red blood cells, there will be no oxygen bound to hemoglobin; however, the oxygen dissolved in plasma will still be present. The oxygen content of arterial blood will be much lower. Therefore, this statement is false.

Conclusion

Based on our analysis, only option a is correct. The arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) would be normal, but the oxygen content of arterial blood would be significantly lower in the absence of red blood cells. Thus, the correct answer is: a. the arterial \(\mathrm{P}_{\mathrm{O}_{2}}\) would be normal.

Key concepts

Oxygen content

Oxygen content in blood is an important measure of how much oxygen is available for bodily functions. It reflects not just the oxygen dissolved in plasma, but more significantly, the oxygen bound to hemoglobin within red blood cells. As red blood cells play a critical role in transporting the majority of oxygen throughout the body, their absence drastically reduces the total oxygen content. When red blood cells are missing, only the small fraction of oxygen that dissolves directly into the plasma is available, which alone is insufficient to meet the body's needs. Thus, understanding oxygen content means appreciating both components: dissolved oxygen and hemoglobin-bound oxygen.

Red blood cells

Red blood cells (RBCs) are the body's primary transportation for oxygen. They are uniquely designed to carry oxygen due to the presence of hemoglobin, a special protein that binds oxygen molecules tightly. These cells travel through the bloodstream, delivering oxygen from the lungs to tissues and organs.

• They are abundant, making up about 40-45% of blood volume.
• Each RBC can carry approximately 1 billion molecules of oxygen.

Without red blood cells, only a minuscule amount of oxygen can dissolve directly in the blood plasma, and this is not enough for the body to function properly. Thus, RBCs are indispensable for effective oxygen delivery.

Lung function

Lung function refers to the ability of the lungs to take in oxygen and expel carbon dioxide. This process takes place in the alveoli, tiny air sacs where gas exchange occurs. Even if red blood cells are absent, the lungs can continue to function normally, maintaining a stable alveolar \\( \mathrm{P}_{\mathrm{O}_{2}} \).

• Lung health ensures the delivery of oxygen to the blood.
• Effective lung function is crucial for sustaining normal arterial \ \( \mathrm{P}_{\mathrm{O}_{2}} \), the partial pressure of oxygen.

Thus, normal lung function is fundamental for the initial intake and distribution of oxygen, although the presence of RBCs is necessary for efficient bodily oxygen supply.

Hemoglobin

Hemoglobin is a protein found in red blood cells, crucial for oxygen transport from the lungs to the rest of the body. It has a high affinity for oxygen, binding oxygen molecules tightly for transportation. When hemoglobin reaches bodily tissues with low oxygen levels, it releases the oxygen, allowing cells to use it for energy production.

• Hemoglobin can bind up to four oxygen molecules per homologous protein chain.
• It gives red blood cells their characteristic red color.

The role of hemoglobin is essential, as it greatly enhances the blood's capacity to carry oxygen. Without hemoglobin, the blood's oxygen content would be too low, even with normal lung function. Hemoglobin ensures that oxygen is delivered efficiently to where the body needs it most.