What does hemoglobin in red blood cells release?

Hemoglobin, the iron-containing protein present in red blood cells, plays a vital role in transporting oxygen throughout the body. As red blood cells travel through the bloodstream, hemoglobin binds to oxygen molecules in the lungs, where the partial pressure of oxygen is high. When the red blood cells reach tissues where the partial pressure of oxygen is lower, such as in active muscles or organs, hemoglobin releases its bound oxygen molecules, allowing them to diffuse into the surrounding tissue.

So, hemoglobin in red blood cells releases oxygen into the surrounding tissues as blood circulates within the body.