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
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 duodenum1. Fatty chyme stimulates release of CKK and
secretin.2. CKK enters bloodstream3. 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
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