what is the difference between valve and sphincter

Valve is a general term for any structure that allows something to pass through only when it is open. A sphincter is a circular muscle that functions as a valve by letting stuff pass through when it is relaxed (valve open), but stopping the passage when it is contracted (valve closed). 

But all valves are not sphincters. The valves in the heart and all through the veins of our bodies are not sphincters, but rather they are just passive flaps or sacs. In addition, these passive valves are one-way - allowing blood to pass through in only one direction. Such valves keep the blood flowing in the same direction all the time: from atria to ventricles, from ventricles to arteries, and from veins to the atria of the heart. 

Several sphincters are found in the digestive tract, between the stomach and small intestine, between the small intestine and colon, and between the rectum and the outside world. The are also sphincters in the urinary and reproductive tracts. 

The circulatory system also has sphincters in addition to those "passive" valves, before many capillary beds, that regulate the flow of blood into particular areas of the body..
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Sphincter can open backwards but valves do not open
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