Which of the following factors does NOT control oxygen delivery?

Oxygen delivery to tissues is primarily determined by factors including hemoglobin concentration, cardiac output, and the amount of dissolved oxygen in plasma. Among these, heart rate itself does not directly control oxygen delivery, while it influences cardiac output indirectly.

Hemoglobin concentration is critical because hemoglobin in red blood cells carries the majority of oxygen in the bloodstream. The higher the concentration of hemoglobin, the more oxygen can be transported to tissues. Cardiac output, the volume of blood the heart pumps per minute, directly impacts how much oxygenated blood is delivered to tissues; therefore, an increase in cardiac output means more oxygen delivery.

Dissolved oxygen in plasma also plays a role, although it is much less significant than the oxygen bound to hemoglobin. Nevertheless, the presence of dissolved oxygen contributes to the overall oxygenation of blood.

In essence, while heart rate contributes to the dynamics of cardiac output, it is not a controlling factor for oxygen delivery like the actual volume of oxygen carried by hemoglobin or the pumping efficiency of the heart itself. Hence, it is the most fitting choice as the factor that does not control oxygen delivery directly.