Digestion
<p><b>Digestion</b> is the mechanical and chemical breakdown of <a href="http://en.wikipedia.org/wiki/Food" title="Food">food</a> into smaller components that are more easily <a href="http://en.wikipedia.org/wiki/Small_intestine#Absorption" title="Small intestine">absorbed</a> into a <a href="http://en.wikipedia.org/wiki/Blood_stream" title="Blood stream" class="mw-redirect">blood stream</a>, for instance. Digestion is a form of <a href="http://en.wikipedia.org/wiki/Catabolism" title="Catabolism">catabolism</a>: a breakdown of large food molecules to smaller ones.</p>
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When food enters the mouth, its digestion starts by the action of <a href="http://en.wikipedia.org/wiki/Mastication" title="Mastication">mastication</a>, a form of mechanical digestion, and the contact of <a href="http://en.wikipedia.org/wiki/Saliva" title="Saliva">saliva</a>. </p>
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Saliva, which is secreted by the <a href="http://en.wikipedia.org/wiki/Salivary_glands" title="Salivary glands" class="mw-redirect">salivary glands</a>, contains <a href="http://en.wikipedia.org/wiki/Salivary_amylase" title="Salivary amylase" class="mw-redirect">salivary amylase</a>, an enzyme which starts the digestion of starch in the food. </p>
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After undergoing mastication and starch digestion, the food will now be in the form of a small, round mass, called a <a href="http://en.wikipedia.org/wiki/Bolus_%28digestion%29" title="Bolus (digestion)">bolus</a>. It will then travel down the <a href="http://en.wikipedia.org/wiki/Esophagus" title="Esophagus">esophagus</a> and into the <a href="http://en.wikipedia.org/wiki/Stomach" title="Stomach">stomach</a> by the action of <a href="http://en.wikipedia.org/wiki/Peristalsis" title="Peristalsis">peristalsis</a>. </p>
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<a href="http://en.wikipedia.org/wiki/Gastric_juice" title="Gastric juice" class="mw-redirect">Gastric juice</a> in the stomach starts protein digestion. Gastric juice mainly contains <a href="http://en.wikipedia.org/wiki/Hydrochloric_acid" title="Hydrochloric acid">hydrochloric acid</a> and <a href="http://en.wikipedia.org/wiki/Pepsin" title="Pepsin">pepsin</a>. </p>
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As these two chemicals may damage the stomach wall, <a href="http://en.wikipedia.org/wiki/Mucus" title="Mucus">mucus</a> is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. </p>
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At the same time protein digestion is occurring, mechanical mixing occurs by <a href="http://en.wikipedia.org/wiki/Peristalsis" title="Peristalsis">peristalsis</a>, which are waves of muscular contractions that move along the stomach wall. </p>
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This allows the mass of food to further mix with the digestive enzymes. </p>
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After some time (typically an hour or two in humans, 4–6 hours in dogs, somewhat shorter duration in house cats), the resulting thick liquid is called <a href="http://en.wikipedia.org/wiki/Chyme" title="Chyme">chyme</a>. </p>
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When the <a href="http://en.wikipedia.org/wiki/Pylorus" title="Pylorus">pyloric sphincter valve</a> opens, chyme enters the <a href="http://en.wikipedia.org/wiki/Duodenum" title="Duodenum">duodenum</a> where it mixes with digestive enzymes from the <a href="http://en.wikipedia.org/wiki/Pancreas" title="Pancreas">pancreas</a>, and then passes through the <a href="http://en.wikipedia.org/wiki/Small_intestine" title="Small intestine">small intestine</a>, in which digestion continues. </p>
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When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. </p>
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Water and minerals are reabsorbed back into the blood in the <a href="http://en.wikipedia.org/wiki/Colon_%28anatomy%29" title="Colon (anatomy)">colon</a> (large intestine). </p>
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Some vitamins, such as <a href="http://en.wikipedia.org/wiki/Biotin" title="Biotin">biotin</a> and <a href="http://en.wikipedia.org/wiki/Vitamin_K" title="Vitamin K">vitamin K</a> (K<sub>2</sub>MK7) produced by bacteria in the colon are also absorbed into the blood in the colon.<sup id="cite_ref-0" class="reference"><a href="http://en.wikipedia.org/wiki/Digestion#cite_note-0">[1]</a></sup> </p>
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Waste material is eliminated during <a href="http://en.wikipedia.org/wiki/Defecation" title="Defecation">defecation</a>.<sup id="cite_ref-Maton_1-0" class="reference"><a href="http://en.wikipedia.org/wiki/Digestion#cite_note-Maton-1">[2]</a></sup></p>