Digestive System Anatomy ch 21 lecture

Anatomy of the Digestive System

• The gastrointestinal system includes the oral cavity and the gastrointestinal tract.
• The oral cavity consists of the mouth and pharynx.
• The gastrointestinal tract (GI tract) includes the esophagus, stomach, small intestine, and large intestine.
• The gut refers to the region from the stomach through the anus.
• The gut is primarily involved in chemical and mechanical digestion.
• The GI tract is divided by sphincters.

The Digestive System

• Accessory glandular organs secrete materials that aid in digestion, such as the salivary glands, pancreas, and liver.
• Chyme is the mixture of food and secretions.
• Products of digestion are absorbed across the epithelium into the interstitial fluid, then distributed from the interstitial fluid to the blood or lymph.
• Waste is excreted from the GI tract via the anus.
• The digestive system is exposed to the external environment and is home to commensal microorganisms.

Digestion in the GI Tract

• Digestion begins in the oral cavity with chewing and salivary glands.
• Food passes through the esophagus to the stomach.
• The stomach is divided into the fundus, body, and antrum.
• The pylorus has a pyloric valve.
• The small intestine consists of the duodenum, jejunum, and ileum.
• The large intestine is divided into the colon, rectum, and anus, which includes an external anal sphincter.
• Feces are eliminated.

GI Tract Wall Layers

• Modifications to increase surface area include rugae and plicae, villi, and gastric, crypts, and submucosal glands.
• Mucosa consists of mucosal epithelium, lamina propria with Peyer's patches (GALT), and muscularis mucosae.
• Submucosa is a middle layer containing connective tissue and submucosal plexus of the enteric nervous system.
• The muscularis externa is the outer wall comprised of two layers of smooth muscle and contains the myenteric plexus of the enteric nervous system.
• Serosa is the outer covering of the digestive tract and a continuation of the peritoneal membrane, forming sheets of mesentery.

Digestive Function and Processes

• Four basic processes of the digestive system include digestion, absorption, secretion, and motility.

Gastrointestinal Motility

• Motility serves two purposes: moving food through the tract and mechanically mixing food into uniformly small particles.
• Different regions exhibit different types of contractions.
• Tonic contractions last for minutes or hours.
• Phasic contractions last for seconds.
• Slow-wave potentials originate in interstitial cells of Cajal.
• Contractions occur in three patterns: migrating motor complex, peristalsis, and segmental contractions.
• Peristalsis moves the bolus forward.
• Segmental contractions primarily serve a mixing function.

Regulation of GI Function

• The enteric nervous system can act independently to control motility, secretion, and growth of the digestive system.
• The enteric nervous system (ENS) shares features with the CNS, including intrinsic neurons, and extrinsic neurons from the CNS.
• The (ENS) also features neurotransmitters, neuromodulators, glial support cells, a diffusion barrier, and an integrating center.
• Short reflexes integrate in the enteric nervous system, whereas long reflexes integrate in the CNS.
• Cephalic reflexes originate outside the digestive system.

GI Hormones

• Gastrin, secreted by G cells in the stomach, is stimulated by peptides, amino acids, and neural reflexes.
• Gastrin’s primary targets are ECL cells and parietal cells, stimulating gastric acid secretion and mucosal growth.
• Somatostatin inhibits gastrin release.
• Cholecystokinin (CCK), secreted in the intestine, is stimulated by fatty acids and some amino acids.
• CCK targets the gallbladder, pancreas, and stomach.
• CCK stimulates gallbladder contraction and pancreatic enzyme secretion, and it inhibits gastric emptying and acid secretion, also promoting satiety.
• CCK may act as a neurotransmitter.
• Secretin released by the intestine is stimulated by acid in the small intestine.
• Secretin targets the pancreas and stomach, stimulating HCO3- secretion and inhibiting gastric emptying and acid secretion.
• Motilin released by the intestine is stimulated by fasting with effects every 1.5-2 hours.
• Motilin targets gastric and intestinal smooth muscle and stimulates the migrating motor complex, which is inhibited by eating.
• Gastric inhibitory peptide (GIP), secreted by the intestine, is stimulated by glucose, fatty acids, and amino acids in the small intestine.
• GIP targets beta cells of the pancreas, stimulating insulin release and inhibiting gastric emptying and acid secretion.
• Glucagon-like peptide-1 (GLP-1), secreted by the intestine, is stimulated by a mixed meal that includes carbohydrates or fats in the lumen.
• GLP-1 targets the endocrine pancreas to stimulate insulin release and inhibit glucagon release and gastric function, promoting satiety.

Digestive Function Overview

• In the oral cavity and esophagus, saliva secretion starts, carbohydrates are digested, and chewing and swallowing cause motility with no absorption occurring..
• In the stomach, HCl, pepsinogen and gastric lipase, mucus, bicarbonate, gastrin, and histamine secretion occur, leading to the digestion of proteins and some fats.
• Absorption of lipid-soluble substances like alcohol and aspirin occur, along with peristaltic mixing and propulsion.
• In the small intestine, enzymes and mucus are secreted, as well as the hormones CCK, secretin, and GIP.
• The exocrine pancreas releases enzymes and bicarbonate, and the liver (stored in the gallbladder) releases bile.
• Polypeptides, carbohydrates, fats, and nucleic acids are digested, and a variety of things are absorbed, including; amino acids, small peptides, monosaccharides, fatty acids, water, ions, minerals, and vitamins..
• Mixing and propulsion occur primarily by segmentation, with some peristalsis.
• In the large intestine, mucus secretion occurs with no digestion.
• The absorption of ions, minerals, vitamins, water, and small organic molecules made by gut bacteria is common, with segmental mixing and mass movement causing motility for propulsion.

The Cephalic Phase

• The cephalic phase is initiated by smelling, seeing, or thinking about food, beginning with a reflex in the brain.
• Anticipation or presence of food in the oral cavity activates neurons in the medulla, initiating the cephalic phase.
• Chemical and mechanical digestion begins in the mouth.
• Salivary secretions serve to soften and lubricate food, digest starch (chemical), facilitate taste, and provide defense.
• Mechanical digestion begins with chewing, also known as mastication.
• Three pairs of salivary glands (exocrine glands with acini) secrete saliva.
• Parotid glands produce a watery solution of enzymes, sublingual glands produce mucus-rich saliva, and submandibular glands are mixed.
• Salivary secretion is under autonomic control.

Deglutition

• Swallowing is integrated in the medulla oblongata, with sensory afferents in cranial nerve IX and somatic motor and autonomic neurons mediating the reflex.
• Initially, the tongue pushes the bolus against the soft palate and back of the mouth, triggering the swallowing reflex.
• The soft palate elevates, closing off the nasopharynx.
• Breathing is inhibited as the bolus passes the closed airway.
• The larynx moves up and forward, and the epiglottis folds down to help keep swallowed material out of the airways.
• The upper esophageal sphincter relaxes.
• The bolus moves downward into the esophagus, propelled by peristaltic waves and aided by gravity.

The Gastric Phase

• The stomach’s three functions include storage, digestion, and defense.
• The upper stomach is primarily for storage of food.
• The lower stomach is involved in digestion of lipids and proteins through acid, enzymes, paracrine signal molecules, and hormones.
• The stomach provides defense against swallowed pathogens.
• Digestive activity in the stomach begins with the long vagal reflex of the cephalic phase, then food in the stomach initiates short reflexes of the gastric phase.
• Gastric glands are found in the gastric pits.
• G cells secrete gastrin, which is stimulated by gastrin-releasing peptide (GRP).
• Parietal cells secrete gastric acid (HCl).
• Chief cells secrete pepsinogen (activated to pepsin) and gastric lipase.
• Enterochromaffin-like (ECL) cells secrete histamine.
• Parietal cells secrete intrinsic factor.
• D cells secrete somatostatin (SS).
• The stomach protects itself with a mucus-bicarbonate barrier, preventing autodigestion.
• Mucous cells are located on the luminal surface and neck of gastric glands, secreting mucus and bicarbonate.

The Intestinal Phase

• Motility in the small intestine is controlled via segmental and peristaltic contractions.
• The small intestine is not overwhelmed by chyme, and needs to complete the digestive process and allow for absorption.
• Absorption is maximized in the small intestine through villi and crypts, and the brush border.
• Absorbed nutrients from the intestinal epithelium primarily go to the circulatory system for distribution via the hepatic portal system.
• Fats use lymphatic circulation to enter the circulatory system.
• Intestinal secretions promote digestion.
• Digestive enzymes, bile, bicarbonate, and mucus are intestinal secretions.
• Isotonic NaCl secretion occurs via the cystic fibrosis transmembrane conductance regulator, or CFTR channel.

Role of the Pancreas

• The endocrine portion (islets) secretes insulin and glucagon.
• The exocrine portion secretes digestive enzymes (acini) and sodium bicarbonate (cuct cells).
• Brush border enteropeptidase converts trypsinogen to trypsin for enzyme secretion.
• Bicarbonate secretion neutralizes gastric acid.

The Liver

• Bile is secreted by hepatocytes and is composed of bile salts, bile pigments, and cholesterol.
• Bile aids in fat digestion.
• Bile is stored and concentrated in the gall bladder.
• The gall bladder sends bile to the duodenum through the common bile duct.

Digestion and Absorption

• Bile salts facilitate fat digestion.
• Lipases and colipases are enzymes for digestion.
• Micelles are important components of fat digestion
• Fat absorption occurs with chylomicrons.
• Carbohydrates are absorbed as monosaccharides.
• Pancreatic enzymes and brush border enzymes are involved.
• Digestion of carbohydrates polymers (starch, glycogen), and disaccharides (sucrose, maltose, lactose) results in monosaccharides (galactose, glucose, fructose).
• Lactose intolerance occurs when lactose can't be broken down.
• Proteins are digested into small peptides and amino acids by endopeptidases (proteases)
  • Includes zymogens: pepsin (stomach) and trypsin, chymotrypsin (pancreas)
  • Exopeptidases include aminopeptidases and carboxypeptidases
• Peptide absorption primarily involves amino acids.
• Some larger peptides can be absorbed intact, for example: dipeptides and tripeptides.

The Large Intestine

• Approximately 1.5 L (of 9 L) remains after passing through the ileum, with the majority of water absorbed in colon.
• Only about 0.1 L of water is lost in feces.
• The ileocecal valve marks the boundary between the small and large intestine.
• The large intestine consists of Cecum with appendix, ascending colon, transverse colon, descending colon, sigmoid colon, tenia coli, and haustra.

Motility in the Large Intestine

• Segmental contractions continue to mix chyme.
• Mass movement moves the chyme forward and triggers defecation.
• Defecation is triggered via the gastrocolic reflex and defecation reflex.
• The internal anal sphincter controls the excretory process.
• Bacteria inhabiting colon breakdown lots of undigested materials
• Lipophilic materials are absorbed
• Fatty acids are the energy substrate for colonpcytes
• Other products include vitamins and flatus.
• Diarrhea can cause dehydration.