Circulatory systems compensate for the slow rate at which diffusion occurs over large distances.
Diffusion is the process of molecules moving from a place of high concentration to a place of low concentration, which happens thanks to the chemical potential gradient, such as the concentration gradient (the difference in the concentrations). This process is used by our cells, for example, in the transport of oxygen, since it can diffuse through the cell membrane and into the cell, when the concentration of oxygen is higher in the intercellular space.
Diffusion is generally a slow process when it comes to large distances, so for example, oxygen would take a lot of time to get from one place in the body to another, since it would have to diffuse through millions of cells.
This is where the circulatory system comes into play. Thanks to the circulatory system, the oxygen can simply enter the blood flow, and be transported by the hemoglobin in the red blood cells from our lungs to all the cells in our body much faster. Same goes for different molecules that the circulatory system transports, like nutrients, amino acids, carbon dioxide, hormones. These are all transported throughout our body at a rate much faster than the rate of diffusion, and diffusion is used in the capillaries, at the target cells after the molecules have already been transported by the blood.
