Final answer:
Oxygen dissolved in blood plasma contributes to the partial pressure of oxygen and can be measured by a pulse oximeter. It represents a small portion (1.5%) of the blood's oxygen, with most of it bound to hemoglobin.
Step-by-step explanation:
Oxygen that is dissolved in the blood plasma makes up the partial pressure of oxygen. This dissolved oxygen is a small amount, only about 1.5% of the blood's oxygen content, since the majority (98.5%) is bound to hemoglobin within red blood cells. It can indeed be measured with a pulse oximeter, which estimates the saturation of oxygen in the blood, but not the actual volume of oxygen dissolved in the plasma. Dissolved carbon dioxide in the blood plasma, on the other hand, interacts with water under the enzyme carbonic anhydrase to form carbonic acid, which then dissociates into bicarbonate ions and hydrogen ions. This bicarbonate buffer system is crucial for maintaining blood pH. While the choice 'is quickly absorbed by bicarbonate ions' might cause confusion, it actually refers to carbon dioxide rather than oxygen and therefore doesn't apply to oxygen's behavior in blood plasma.