The Digestive System

Organ groups of digestion Alimentary organs Mouth Pharynx Esophagus Stomach Small and large intestine Accessory digestive organs Teeth and tongue Liver Pancreas Salivary glands gallbladder

The processes of digestion Ingestion Secretion Mixing and propulsion Mechanical and chemical digestion Absorption Defecation

Peristalsis and segmentation

Layers of the GI tract Mucosa Submucosa Muscularis Serosa Layer of epithelium Lamina propria (connective tissue) Thin layer of smooth muscle (muscularis mucosae) Submucosa Muscularis Serosa

ANS input Vagus nerve supplies parasympathetic input to most parts of the GI tract. Preganglionic parasympathetic neurons synapse on postsynaptic neurons in the submucosal and myenteric plexuses. Parasympathetic stimulation increases motility and GI secretion. Sympathetic postsynaptic neurons in the plexuses decrease GI section and decrease motility by inhibiting neurons of ENS

Mouth Buccal cavity Oral mucosa Vestibule Gingivae Hardpalate Soft palate Uvula

Salivary glands Cleanse mouth Dissolve food chemicals Moisten food Enzymes break down carbs Fluid is excreted into the mouth by several different salivary glands, but the majority comes from three Parotid – at the base of the ear – secretes through the parotid duct Sublingual – below the tongue – secretes through the sublingual duct Submandibular – below the jaw – secretes into the submandibular duct Saliva – mixture of different fluids secreted by these glands—average output is 1-1.5 liters a day. 99.5% is water Mucus – responsible for the lubricating action of the saliva Buffers – keep the pH of your mouth around 7 to prevent the buildup of acids produced by bacteria Salivary amylase – enzyme that breaks down carbohydrates as they enter the mouth Lingual lipase – produced by glands on the tongue – begins the digestion of lipids, enzyme is activated in the stomach. Lysozyme – enzyme that attacks bacteria and inhibits their growth Immunoglobulins – IgA antibodies to attack bacteria – keeps them from attaching to the epithelium so they cannot penetrate Defensins—granules that contain a brew of antibacterial proteins

Teeth Crown, neck and root Peridontal ligament Enamel Dentin Root canal Periodontal ligament – teeth are embedded in sockets in the jaw bones Sockets are lined with this membrane of dense fibrous connective tissue (gomphosis) Anchors the teeth and absorbs shock while chewing Crown, neck, and root – crown is the exposed part of the tooth, neck is the point where the crown and root join, and the root is the part that is embedded in the jaw Enamel – outermost part of the tooth – hardest biologically manufactured molecule Dentin – similar to bone – does not contain cells – projections from dentin extend into the cavity below Pulp cavity – interior chamber that receives nerves and blood vessels Root canal – tunnel located at the base of the tooth that allows vessels and nerves to enter the tooth Apical foramen – opening at the base of the tooth that allows them in

Tooth varieties Incisors Cuspids Bicuspids Molars Mastication Bolus

Tongue Bolus moves from the mouth into the different parts of the pharynx – various muscles are involved in adjusting the position of the pharynx to facilitate swallowing.

Swallowing Deglutition 3 phases Peristalsis Buccal Pharyngeal Esophogeal Peristalsis Secondary peristaltic waves

Deglutition

Stomach Fundus Body Pylorus Pyloric sphincter Rugae

Stomach histology

Gastric Glands Mucous cells G cells Parietal cells Chief cells Surface & neck G cells Parietal cells Chief cells Enteroendocrine cells

Stomach and chemical digestion Cephalic (Reflex) phase Triggered by sensory signals surrounding food. Activates vagal nerve and ANS to enhance secretions Hydrochloric Acid Production in Parietal Cells

Stomach and chemical digestion Gastric Phase Activation of local myeteric reflexes and vagovagal reflexes Gastrin Release of HCl from parietal cells

Stomach and chemical digestion Intestinal Phase Excitatory component Inhibitory component

Gastric Secretion: 3 Phases

Stomach and mechanical digestion Stomach filling Contractile activity Peristalsis Gastric emptying

Small intestine and associated organs

Small intestine histology

Small intestine histology

Liver Digestive function is to produce bile. Metabolic function is to process venous blood from digestive tract Functional unit is a lobule

Liver

Liver Functions Carbohydrate metabolism Lipid metabolism Protein metabolism Processing of drugs and hormones Excretion of bilirubin Synthesis of bile salts Storage Phagocytosis Activation of vitamin D

Cholesterol

Accessory organs

Bile Alkaline solution that includes bile salts, phospholipids and electrolytes. Bile salts derived from cholesterol Bile salts emulsify fats and solubilize cholesterol, increasing absorption. Bile is recycled Fatty chyme and bile returning to liver stimulates more production of bile.

Gallbladder The gallbladder stores and concentrates bile. Bile release into duodenum Fatty chyme stimulates release of CKK and secretin. CKK enters bloodstream Bile salts and secretin in the blood stimulate liver to rapidly produce bile. Vagal stimulation leads to weak contractions. CKK causes gallbladder to contract and the hepatopancreatic sphincter to relax; bile enters duodenum Stores that which is not immediately needed and concentrates bile 10X

Pancreatic juice Insulin & Glucose Regulation 1200 to 1500 mL produced daily Composed of water, salts, sodium bicarbonate and enzymes (pH 7.1-8.2) Pancreatic juices buffer acidic gastric juices in chyme, stops the action of pepsin, and creates the proper pH for intestinal digestive enzymes. Enzymes Pancreatic amylase—starches Trypsin—proteins, secreted as trypsinogen, pancrease also makes trypsin inhibitor; trypsinogen is cleaved by enterokinase to make trypsin. Trypsin acts on the other protein enzyme precursors Chymotrypsin—proteins Carboxypeptidase—proteins Elastase—proteins Pancreatic lipase—lipids Ribonuclease—RNA Deoxyribonuclease—DNA

1200 to 1500 mL produced daily Composed of water, salts, sodium bicarbonate and enzymes (pH 7.1-8.2) Pancreatic juices buffer acidic gastric juices in chyme, stops the action of pepsin, and creates the proper pH for intestinal digestive enzymes. Enzymes Pancreatic amylase—starches Trypsin—proteins, secreted as trypsinogen, pancrease also makes trypsin inhibitor; trypsinogen is cleaved by enterokinase to make trypsin. Trypsin acts on the other protein enzyme precursors Chymotrypsin—proteins Carboxypeptidase—proteins Elastase—proteins Pancreatic lipase—lipids Ribonuclease—RNA Deoxyribonuclease—DNA

Digestion in Small intestine: chemical Digestion of carbohydrates Pancreatic amylase Brush-border enzymes Digestion of proteins Enzymes from pancreatic juice Brush-border peptidases releases single AAs Digestion of lipids Digestion & Absorption of Fats Digestion of nucleic acids

Digestion in Small intestine: mechanical Segmentation leads to chyme being moved backwards and forwards. Parasympathetic input increases intensity of contractions; sympathetic input decreases intensity. Peristalsis occurs after nutrients are absorbed; regulated by motilin. Migrating motility complex Movement through ileocecal sphincter is enhanced by gastrin. Once meal has passed through sphincter back pressure keeps the valve closed

Water Water moves in both directions and follows flow of electrolyhtes

Large intestine (5 ft) Mechanical digestion—haustral churning, peristalsis, mass peristalsis Chemical digestion Absorption Feces formation Reflexes in the Colon

Ulcerative Colitis

Defecation reflex Mass peristaltic movements Voluntary contractions of diaphragm and abdominals Parasympathetic input Voluntary input

Resources Human Digestion Heart Burn Video Peristalsis Animation Digestive System