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The process of digestion begins in the mouth. As you chew, your teeth crush large pieces of food into smaller ones, and fluids blend with these pieces to ease swallowing. Fluids also help dissolve the food so that you can taste it; only particles in solution can react with taste buds. The tongue allows you not only to taste food, but also to move food around the mouth, facilitating chewing and swallowing. Also, in the mouth the salivary enzyme amylase starts to work, hydrolyzing starch to shorter polysaccharides and to maltose. Fat digestion starts off slowly in the mouth, with some hard fats beginning to melt when they reach body temperature. A salivary gland at the base of the tongue releases an enzyme lingual lipase that plays a small role in fat digestion in adults and an active role in infants. In infants, this enzyme efficiently digests the short- and medium-chain fatty acids found in milk. When you swallow a mouthful of food, it passes through the pharynx, a short tube that is shared by both the digestive system and the respiration system. To bypass the entrance to your lungs, the epiglottis closes off your air passages so that you don’t choke when you swallow. After a mouthful of food has been swallowed, it is called a bolus. 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 juice 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. The swallowed bolus mixes with the stomach’s acid and protein-digesting enzymes, which inactivate salivary amylase. Thus amylase cannot complete its job of starch digestion. To a small extent, the stomach’s acid continues breaking down starch, but its juices contain no enzymes to digest carbohydrate.
Approximate Word count = 1515
Approximate Pages = 6.1
(250 words per page double spaced)
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