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The Digestive System

The Digestive System - Digestive System Overview The...

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The Digestive System P A R T A Digestive System: Overview The alimentary canal or gastrointestinal (GI) tract digests and absorbs food Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas Figure 23.1 Digestive Process The GI tract is a “disassembly” line Nutrients become more available to the body in each step There are six essential activities: Ingestion, propulsion, and mechanical digestion Chemical digestion, absorption, and defecation Figure 23.2 Gastrointestinal Tract Activities Ingestion – taking food into the digestive tract Propulsion – swallowing and peristalsis Peristalsis – waves of contraction and relaxation of muscles in the organ walls Mechanical digestion – chewing, mixing, and churning food
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Peristalsis and Segmentation Figure 23.3 Gastrointestinal Tract Activities Chemical digestion – catabolic breakdown of food Absorption – movement of nutrients from the GI tract to the blood or lymph Defecation – elimination of indigestible solid wastes GI Tract External environment for the digestive process Regulation of digestion involves: Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen Extrinsic control by CNS centers Intrinsic control by local centers Receptors of the GI Tract Mechano and chemoreceptors respond to: Stretch, osmolarity, and pH Presence of substrate, and end products of digestion They initiate reflexes that: Activate or inhibit digestive glands Mix lumen contents and move them along Nervous Control of the GI Tract Intrinsic controls Nerve plexuses near the GI tract initiate short reflexes Short reflexes are mediated by local enteric plexuses (gut brain) Extrinsic controls Long reflexes arising within or outside the GI tract CNS centers and extrinsic autonomic nerves Nervous Control of the GI Tract Figure 23.4
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