As the shell closes, surplus water is ejected and what remains keeps the organs constantly bathed. As long as the oyster keeps its shells closed and retains its shell liquor, it is able to stave off predators and also survive long periods of exposure to unfavourable water conditions and to air at low tide. Other organs become visible when the covering half of the mantle is removed. The gills function both as a respiratory and a food-collecting organ. Covering them are tiny, lashing hairs which create an incoming current of water from which food particles are filtered and oxygen absorbed.
Oysters breathe much like fish, using both gills and mantle.
The mantle is lined with many small, thin-walled blood vessels which extract oxygen from the water and expel carbon dioxide. A small, three-chambered heart, lying under the adductor muscle, pumps colorless blood, with its supply of oxygen, to all parts of the body. At the same time a pair of kidneys located on the underside of the muscle purify the blood of any waste products it has collected. Incoming food is sorted by the lips and accepted morsels pass down a narrow throat into a pouch-shaped stomach surrounded by a digestive gland. Waste passes through a long, coiled intestine to be emptied by the rectum into an exhalant chamber.