In early embryological stages, the cardiovascular system appears much earlier than any other major organ system. The primitive heart begins to beat regularly as early as four weeks after fertilisation. The cardiovascular system maintains homeostasis and an environment that is favourable for cellular development. In the continuous and controlled movement of blood through thousands of miles of capillaries that permeate every type of tissue and reach every cell in the body, can the cardiovascular system respond to the cellular need and maintain homeostasis. In the microscopic capillaries the blood performs its primary function. In the capillaries, nutrients and other essential materials pass from the capillary blood into the fluid surrounding the cells as waste products are removed. As the different tissue and cells under different circumstances have different needs, numerous control mechanisms help regulate and integrate blood supply to specific body areas.