A circulatory system is very essential in mammals but not in smaller organisms like Amoeba because

The circulatory system is essential in larger organisms, such as mammals, because their bodies are made up of many cells, and diffusion alone is not sufficient to transport materials to all parts of the body. On the other hand, smaller organisms, like Amoeba, are unicellular, meaning they are made up of only one cell. This allows materials, such as oxygen and nutrients, to diffuse easily across the cell membrane, without the need for a circulatory system. Additionally, Amoeba doesn't require blood containing haemoglobin for oxygen transport, as it absorbs oxygen directly from the surrounding water through its cell membrane. Moreover, as Amoeba doesn't have complex organs and tissues, anaerobic respiration is sufficient to produce energy for its needs. Therefore, Amoeba doesn't require a circulatory system like larger organisms, as diffusion is sufficient to transport materials across its cell membrane.