Once hemoglobin reaches tissues, it undergoes a conformational change from ________ to ________.

Hemoglobin undergoes a significant conformational change when it releases oxygen to the tissues. In its "T" (tense) state, hemoglobin has a lower affinity for oxygen, which facilitates the release of oxygen to the tissues that need it. When hemoglobin binds to oxygen, it transitions to the "R" (relaxed) state, which has a higher affinity for oxygen, allowing it to pick up oxygen in the lungs.

When hemoglobin reaches the tissues, the environmental conditions—such as decreased oxygen concentration, increased carbon dioxide, and lower pH—promote the transition from the "R" state back to the "T" state. This switch is crucial for the efficient transport and delivery of oxygen throughout the body. Therefore, the correct characterization of this conformational change is the transition from the R state to the T state.