Starfish take in oxygen and dispel carbon dioxide through their tube feet, thin skin and a hole located on top of their body, and circulate the gases throughout their body using a water-vascular system. Their respiratory system is rudimentary and sometimes nonexistent.
Starfish rely on osmosis, the process of particles moving across a membrane, in order to gather oxygen from the surrounding water and dispel carbon dioxide. The hole on the top of their bodies is called madreporite. Water is gathered through the madreporite and fills a cavity in the center of the starfish's body called the coelom. The coelom is intersected by a number of body canals that run from the center of the starfish to the tips of each arm. These canals fill with the water from the coelom. The tube feet that are connected to these canals also fill with water. Carbon dioxide then flows through the thin skin of the starfish's tube feet and body into the surrounding water while oxygen travels through the skin membrane and into the starfish's body. The water-vascular system, the network of water-filled canals that intersect the starfish's body, then transport the oxygen throughout the starfish. The water-vascular system does the same thing with nutrients from the starfish's meals and therefore acts as the starfish's respiratory, circulatory and digestive system.