Unloading of oxygen to tissues cells by oxy-Hb is assisted by

Bohr effect: A shift of the oxygen-hemoglobin dissociation curve to the right in response to increases in blood carbon dioxide and hydrogen ions enhances the release of oxygen from blood in the tissues and enhances oxygenation of the blood in the lungs. This is called the Bohr effect. 2,3 - DPG: In hypoxic conditions that last longer than a few hours, the quantity of 2,3-BPG in the blood increases considerably, shifting the oxygen-hemoglobin curve to the right. This causes oxygen to be released to tissues. Low PO2 and high PCO2 in tissues: As blood passes through tissues, CO2 diffuses from the cells to blood. This increases blood PCO2 and decreases blood PO2. This, in turn, increases blood carbonic acid and hydrogen ion concentration. These effects shift the oxygen dissociation curve to the right, forcing oxygen away from hemoglobin, delivering oxygen to the tissues.Ref: Guyton and Hall textbook of medical physiology, 12th edition(South Asia), page 356