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