Name and describe the functions of the four major sphincter muscles that divide the GI tract into its principal regions
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The esophagus has a sphincter muscle at each end. During a swallow, the upper esophageal sphincter opens. The bolus then slides down the esophagus, which passes through a hole in the diaphragm to the stomach. The lower esophageal sphincter at the entrance to the stomach closes behind the bolus so that it proceeds forward and doesn't slip back into the esophagus.
The stomach retains the bolus for a while in its upper portion. Little by little, the stomach transfers the food to its lower portion, adds juices to it, and grinds it to a semiliquid mass called chyme. Then, bit by bit, the stomach releases the chyme through the pyloric sphincter, which opens into the small intestine and then closes behind the chyme.
Having traveled the length of the small intestine, the remaining contents arrive at another sphincter: the ileocecal valve, located at the beginning of the large intestine (colon) in the lower right side of the abdomen.
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