ABSTRACT
The evolutionary leap to multicellular animals enabled huge increases in organismic size, structural and functional specialization, and complexity of cells into tissues, organs, and organ systems. However, it also required development of accurate (usually rapid) communication among these disparate cells, tissues, and organs. Three such communication systems evolved:
1. The nervous system (with the brain as the coordinating and communications center) that provides very rapid transfer of information from sensory organs and response information to effector organs, via afferent and efferent neuronal pathways, respectively, to provide rapid responses
2. The circulatory system (with the heart as the central pump and the arteries, capillaries, and veins as distribution conduits) that provides O2/CO2 exchange, nutrient/waste exchange, and delivery of signaling molecules such as hormones, etc.
