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 Peristalsis

Layers of the GI tract Mucosa – 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

Teeth Crown, neck and root Peridontal ligament Enamel Dentin Root canal

Tooth varieties Incisors Cuspids Bicuspids Molars Mastication Bolus

Tongue

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

Deglutition

Stomach Fundus Body Pylorus – Pyloric sphincter Rugae

Stomach histology

Gastric Glands Mucous 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 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

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 1.Fatty chyme stimulates release of CKK and secretin. 2.CKK enters bloodstream 3.Bile salts and secretin in the blood stimulate liver to rapidly produce bile. 4.Vagal stimulation leads to weak contractions. 5.CKK causes gallbladder to contract and the hepatopancreatic sphincter to relax; bile enters duodenum

Pancreatic juice Insulin & Glucose Regulation Insulin & Glucose Regulation

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 & 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 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