DIGESTIVE SYSTEM consists of: Muscular, hollow tube (= digestive tract ) + Various accessory organs
FUNCTION Individual parts function in: ingestion mechanical digestion chemical and enzymatic digestion secretion absorption compaction excretion and elimination The function of the system as a whole is processing food in such a way that high energy molecules can be absorbed and residues eliminated.
HISTOLOGICAL ORGANIZATION Tube made up of four layers. 2 1 Modifications along its length as needed. 3 Muscularis externa 4
4 LAYERS OF THE DIGESTIVE TRACT 1) Mucosa Epithelium simple columnar, stratified squamous if needed Lamina propria - connective tissue deep to epithelium Muscularis mucosae -produces folds - villi (small intestine) 2) Submucosa loose connective tissue contains submucosal plexus (neural tissue- controls contractions) and blood vessels 3) Muscularis externa smooth muscle, two layers outer layer: longitudinal inner layer: circular 4) Serosa visceral peritoneum layer of the mesentery or adventitia depending on location
DIGESTIVE TRACT
MEMBRANES Mesentery - double sheet of peritoneum, surrounding and suspending portions of the digestive organs Greater omentum - "fatty apron", hangs anteriorly from stomach, double layer encloses fat Lesser omentum - between stomach and liver Mesentery proper - suspends and wraps the small intestine Mesocolon - suspends and wraps the colon
MOVEMENT OF DIGESTIVE MATERIALS Peristalsis waves of muscular contractions Moves a bolus along the length of the digestive tract Segmentation Cycles of contraction: Churn and fragment the bolus Mix contents with intestinal secretions Does not follow a set pattern
PERISTALTIC MOTION - 3 STEPS 1. Circular muscles contract behind bolus: circular muscles ahead of bolus relax 2. Longitudinal muscles ahead of bolus contract 3. Wave of contraction in circular muscles: Forces bolus forward
ORAL CAVITY buccal cavity - lined with mucosa- (stratified squamous) Hard and soft palates - form roof of mouth Tongue mechanical processing, assist chewing, sensory analysis Salivary glands - three pairs Teeth
THREE PAIRS OF SALIVARY GLANDS Parotid lateral side of face, anterior to ear Submandibular medial surface of mandible Sublingual in floor of mouth Salivary amalyse (from parotid) breaks down starches Produce saliva (1-1.5 liters/day) 99.4% water.6% mucins, buffers, ions, metabolites and waste products Controlled by ANS
STRUCTURE OF TEETH Teeth perform mastication (chewing) Crown - exposed surface of tooth Neck - boundary between root and crown Enamel - outer surface Dentin bone-like, but noncellular Pulp cavity - hollow with blood vessels and nerves Root canal - canal length of root
TYPES AND NUMBERS OF TEETH Dental succession Deciduous (baby, milk) teeth - 20, replaced by Permanent teeth - 32 teeth
THE PHARYNX & ESOPHAGUS Pharynx-common passageway (3 divisions) Esophagus- hollow muscular tube, transports solid food and liquids to the stomach Upper and lower esophageal sphincters Deglution (swallowing) - there are 4 phases Initiated voluntarily, proceeds automatically
SWALLOWING PROCESS
STOMACH STRUCTURE Cardia Fundus Lesser Omentum Greater Omentum Pylorus Rugae
HISTOLOGY OF STOMACH Simple columnar epithelium lines all portions of stomach Epithelium is a secretory sheet: Produces mucus that covers interior surface of stomach Gastric pits shallow pits, external half rapidly reproduces for replacement Gastric glands deep in lamina propria, produce gastric juice Gastric juice produced by: 1. Parietal cells (produce HCl-lowers ph to 1.5 2.0 and intrinsic factor- facilitates absorption of Vit B 12 ) 2. Chief cells (produce pepsinogen- protein digestion)
REGULATION OF DIGESTION Three Phases: Cephalic Gastric Intestinal
DIGESTION IN THE STOMACH Stomach performs preliminary digestion of proteins by pepsin Produces partially digested food called chyme Stomach contents Become more fluid ph approaches 2.0 Pepsin activity increases Protein disassembly begins Although digestion occurs in the stomach, nutrients are not absorbed there
SMALL INTESTINE SI is longest part of dig. Tube Functions: Duodenum (short, 12 inches) fixed shape & position Mixing bowl for chyme Jejunum (2.5 m long) Most of digestion Ileum (longest at 3.5 m) Most of absorption Ileocecal valve slit valve into large intestine (colon)
Structure of Small Intestinal Wall Plicaecirculares circular pleats around the interior of the small intestine Villi minute finger-like projections, contain capillaries & lacteals Microvilli sub-microscopic size, projections on single cells Intestinal glands (crypts) intestinal juice production Cell regeneration
ACTIVITIES OF DIGESTIVE TRACT HORMONES
Regions of Large Intestine Cecum pocket at proximal end with Appendix - begins compaction Colon absorption of water and vitamins Ascending colon - on right, between cecum and right colic flexure Transverse colon - horizontal portion Descending colon - left side, between left colic flexure and Sigmoid colon - S bend near terminal end Fig 25-17 Rectum terminal end is anal canal - ending at the anus - which has internal involuntary sphincter and external voluntary sphincter storage of fecal matter prior to digestion
1. Mucosa - abundant goblet cells, stratified squamous epithelium near anal canal 2. No villi Histology of Large Intestine 3. Longitudinal muscle layer incomplete, forms three bands or taenia coli 4. Circular muscle - forms pockets or haustra between bands
Pancreas Function: Provides digestive enzymes & buffers to neutralize chyme though pancreatic juice Retroperitoneal
LIVER On right under diaphragm, largest organ made up of 4 lobes Functions: 1. Metabolic regulation 2. Hematological regulation 3. Bile production hepatic triad underside entry" point Gall bladder stores and concentrates bile Fig 25.20
DEFACATION Distension of the rectal wall triggers defecation reflex: Two positive feedback loops Both loops triggered by stretch receptors in rectum Elimination of Feces - Requires relaxation of internal and external anal sphincters Reflexes open internal sphincter and close external sphincter Opening external sphincter requires conscious effort