FUNCTION it is to exchange oxygen and carbon dioxide molecules to and from the bloodstream.These tiny, balloon-shaped air sacs sit at the very end of the respiratory tree and are arranged in clusters throughout the lungs.The respiratory system is the network of organs and tissues that help you breathe.It includes your airways, lungs, and blood vessels. The muscles that power your lungs are also part of the respiratory system.These parts work together to move oxygen throughout the body and clean out WASTE gases like carbon dioxide. The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion.Digestion involves the breakdown of food into smaller and smaller components until they can be absorbed and assimilated into the body.The circulatory system also CALLED the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino acids and ELECTROLYTES), oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis.The nervous system is the part of an animal's body that coordinates its behavior and transmits signals between different body areas.Invertebrates CONSIST of two main parts, called the central nervous system (CNS) and the peripheral nervous system (PNS).The CNS contains the brain and spinal cord.