Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?

Diffusion is a process where molecules move from an area of high concentration to an area of low concentration. While this process works well for single-celled organisms, it is insufficient for multi-cellular organisms like humans for several reasons:

1. Size: Multi-cellular organisms are much larger, so oxygen would take a long time to reach cells in the middle or deep within the body if diffusion was the only process at work.

2. Complexity: Multi-cellular organisms have specialized cells and tissues that perform specific functions. These cells may have different oxygen requirements and may not be directly exposed to the external environment. Therefore, a more complex system is needed to deliver oxygen to these cells.

3. High Energy Demand: Multi-cellular organisms like humans have a high metabolic rate and therefore require a large amount of oxygen. Diffusion alone is not efficient enough to meet this high demand.

4. Oxygen Concentration: The concentration of oxygen in the air is not high enough to rely solely on diffusion to meet the oxygen requirements of multi-cellular organisms.

Therefore, multi-cellular organisms like humans have developed specialized systems, like the respiratory and circulatory systems, to ensure that oxygen is efficiently delivered to all cells in the body.