Select the correct statement about oxygen transport in blood and the behaviour of hemoglobin under different physiological conditions.

Introduction / Context:
Oxygen transport in the blood depends on hemoglobin, a protein in red blood cells that can reversibly bind oxygen. The affinity of hemoglobin for oxygen is influenced by factors such as partial pressure of oxygen, pH, carbon dioxide levels, temperature, and the concentration of 2,3 BPG. Understanding how these factors affect oxygen loading in the lungs and unloading in tissues is important for physiology and medical exams. This question tests your ability to select the statement that correctly reflects these physiological principles.

Given Data / Assumptions:

• The statements refer to oxygen saturation of blood returning to the lungs, hemoglobin saturation during normal activity, acidosis, and the effect of 2,3 BPG.
• We assume basic knowledge of the oxyhemoglobin dissociation curve and the Bohr effect.
• We also assume understanding that normal venous oxygen saturation is usually higher than 50 percent at rest.

Concept / Approach:
At rest, systemic arterial blood is almost fully saturated with oxygen, and venous blood returning to the lungs typically has an oxygen saturation around 70 to 75 percent, meaning tissues extract only about 25 percent of the available oxygen. During heavy exercise or high metabolic activity, tissues extract more oxygen, and venous saturation can drop closer to 50 percent or even lower. This indicates increased activity, not a problem with hemoglobin. In acidosis, low pH reduces hemoglobin affinity for oxygen, shifting the dissociation curve to the right, which facilitates oxygen unloading in tissues but makes binding in the lungs slightly less tight. Increased 2,3 BPG also lowers oxygen affinity, promoting release of oxygen in tissues rather than increasing binding in the lungs. Hemoglobin molecules have four binding sites for oxygen; in resting venous blood they typically still carry about three molecules of oxygen, not just one.

Step-by-Step Solution:
Step 1: Remember that normal mixed venous oxygen saturation at rest is around 70 to 75 percent. Step 2: Recognise that a fall in venous saturation towards 50 percent means tissues are extracting more oxygen, such as during intense exercise. Step 3: Evaluate statement A, which correctly links 50 percent saturation of blood returning to lungs with higher than normal activity level. Step 4: Consider statement B, which claims that hemoglobin returning to the lungs carries only one oxygen molecule, which contradicts the fact that venous hemoglobin is still roughly 75 percent saturated at rest. Step 5: Evaluate statement C about acidosis increasing hemoglobin efficiency. The Bohr effect shows that low pH decreases affinity, not increasing it, so this statement is incorrect. Step 6: Evaluate statement D, which suggests that increased 2,3 BPG enhances oxygen carrying capacity, whereas 2,3 BPG lowers affinity and promotes oxygen unloading, making this statement incorrect.

Verification / Alternative check:
Physiology diagrams of the oxyhemoglobin dissociation curve show that at a partial pressure of oxygen typical of venous blood at rest, saturation is high, around 75 percent, but shifts to lower values such as 50 percent when tissues are more active. Texts describing the Bohr effect and 2,3 BPG emphasise that both acidosis and increased 2,3 BPG shift the curve to the right, decreasing affinity and helping tissues obtain more oxygen. They do not claim that these factors increase oxygen binding in the lungs. These established principles support statement A as the correct one.

Why Other Options Are Wrong:

• During normal resting activity, a molecule of hemoglobin returning to the lungs usually carries only one molecule of oxygen: At rest, hemoglobin is still largely saturated and often carries about three oxygen molecules, so this is inaccurate.
• Under acidosis, when blood pH is low, hemoglobin binds oxygen more tightly and carries it more efficiently: In fact, low pH decreases affinity and favours oxygen unloading in tissues, not tighter binding.
• Increased 2,3 BPG levels in red blood cells increase the oxygen binding affinity of hemoglobin and its loading in the lungs: 2,3 BPG decreases affinity, shifting the curve to the right and aiding unloading in tissues.

Common Pitfalls:
Students often mix up the direction of shifts in the oxyhemoglobin dissociation curve and misinterpret them as always beneficial for oxygen carriage. It is important to separate loading in the lungs from unloading in tissues. A right shift, caused by acidosis, increased temperature, or increased 2,3 BPG, helps deliver more oxygen to tissues at a given partial pressure but slightly reduces affinity in the lungs. Remember also that venous blood is not almost empty of oxygen; it retains a significant amount that can be extracted if needed, which is why venous saturation changes with activity level.

Final Answer:
The correct statement about oxygen transport in blood is that a 50 percent oxygen saturation level of blood returning to the lungs may indicate a higher than normal activity level with increased tissue oxygen extraction.