Final answer:
The diffusion distance is key for the transfer of oxygen from capillaries to tissues due to the typically slow nature of diffusion which is most efficient over short distances. The oxygen-hemoglobin dissociation curve regulates oxygen delivery to tissues with varying metabolic rates.
Step-by-step explanation:
The diffusion distance is the effective distance for oxygen to travel from the capillary into the surrounding tissue. This distance is crucial because diffusion is generally slow and thus more effective over small distances. As oxygen molecules detach from hemoglobin within the red blood cells (RBCs), they diffuse through the blood's plasma and eventually into the tissue cells.
During internal respiration, O₂ diffuses out of the capillaries and into the cells, while CO₂ diffuses into the capillaries from the cells. Oxygen binds to hemoglobin in RBCs and is transported to tissues where it dissociates due to the oxygen partial pressure difference between the capillaries and the tissues. For highly active tissues like muscle, the oxygen partial pressure is much lower, thus creating a steep gradient and promoting more oxygen to dissociate from hemoglobin and diffuse into the tissues.