Final answer:
Deoxyhemoglobin is formed during internal respiration, where oxygen dissociates from hemoglobin due to the lower partial pressure of oxygen in body tissues.
Step-by-step explanation:
Deoxyhemoglobin is formed during internal respiration, which is the gas exchange that occurs at the level of body tissues. This process occurs due to a partial pressure gradient where the partial pressure of oxygen in the tissues is low (about 40 mm Hg) because oxygen is continuously used for cellular respiration. Consequently, oxygen dissociates from hemoglobin, and the hemoglobin without oxygen (deoxyhemoglobin) becomes more burgundy in color as the blood returns to the heart.