In multicellular organisms like humans the body design is very complex and the oxygen demand is quite high. The oxygen requirements of an animal cannot be met by the process of diffusion as diffusion is a very slow process.To overcome this, multicellular organisms have specialized cells and tissues for performing various necessary functions of the body such as intake of oxygen and food. This helps for the higher animals like humans to meet the sufficient oxygen requirement. Unlike unicellular organisms, multicellular cells are not in direct contact with the outside environment. Therefore,diffusion simply cannot meet their oxygen requirements.

Diffusion Insufficiency in Meeting Oxygen Demands

Limited Surface Area
- Diffusion relies on a large surface area for efficient gas exchange. In the case of oxygen entering the body, the surface area available for diffusion is limited to the lung tissue. This restricted area may not be sufficient to meet the body's oxygen demands, especially during times of increased physical activity or stress.

Distance to Travel
- Diffusion is a passive process that relies on the movement of molecules from an area of high concentration to an area of low concentration. The distance that oxygen molecules need to travel from the lungs to the cells throughout the body can be significant. This distance can result in a slower rate of diffusion, making it challenging for oxygen to reach all tissues efficiently.

Thick Membranes
- In some cases, such as in individuals with respiratory conditions like emphysema or pulmonary fibrosis, the membranes involved in gas exchange can become thickened or damaged. These thickened membranes can impede the diffusion of oxygen, further reducing the efficiency of oxygen delivery to tissues.

High Metabolic Demand
- The body's oxygen demands can vary depending on activity level, stress, or illness. During times of high metabolic demand, such as during intense exercise or illness, the body requires more oxygen than can be efficiently delivered through diffusion alone. This mismatch between supply and demand can lead to oxygen insufficiency in meeting the body's needs.
In conclusion, while diffusion plays a crucial role in oxygen transport, it may not always be sufficient to meet the body's oxygen demands under certain circumstances. Other mechanisms, such as circulation and ventilation, work in conjunction with diffusion to ensure adequate oxygen delivery to tissues throughout the body.