The Digestive System Chapter 25
Introduction Structure of the digestive system A tube that extends from mouth to anus Accessory organs are attached Functions include Ingestion Movement Digestion Absorption Excretion Two types of digestion Mechanical and Chemical Occur at different points in the digestive tract, sometimes both types at once!
Overview of Digestive System
Same basic arrangement of tissues from esophagus to anal canal Four layers (from innermost to outermost) Mucosa Submucosa Muscularis Serosa Histological Organization
Movement and Mixing of Digestive Materials Peristalsis Coordinated motion of two muscular layers Circular muscles contract, then longitudinal muscles Segmentation Mixing of food Circular muscles in two areas contract Longitudinal muscles alternately contract & relax
Structure Lined with stratified squamous epithelium Lips surround the opening Roof is formed from the hard & soft palate Tongue dominates the floor Functions Take in food Prepare food for digestion The Oral Cavity
Structure Skeletal muscle covered with mucosa Surface Functions Papillae/ taste buds Maneuvers food Forms bolus The Tongue
Found outside mouth Ducts carry saliva to mouth 3 pairs Parotid glands Submandibular glands Sublingual glands Saliva (salivary amylase) Functions Keeps mucous membranes moist Lubricates food Dissolves food Begins carbohydrate digestion Salivary Glands
2 sets Deciduous (20) Permanent (32) Held in sockets Gingiva = gums Structure Crown Root Neck Composition Dentin Enamel Cementum Teeth
Food enters the esophagus from the pharynx The esophagus is a muscular tube behind the trachea Food is moved by peristalsis from the pharynx to the stomach Cardiac sphincter separates esophagus from stomach The Pharynx and Esophagus
External Anatomy of the Stomach
Same 4 basic layers When the stomach is empty, the mucosa lies in large folds Rugae- folds Pyloric sphincter separates stomach from small intestine The Stomach
Mucosa is simple columnar epithelium with goblet cells Mucosa is folded to form gastric pits Gastric glands secrete gastric juice Histology of the Stomach
Gastric Glands Several kinds of cells produce substances that form gastric juice Mucous cells- protects stomach from acid pepsinogen- breaks down proteins intrinsic factor helps with absorption of vit. B12 HCl ph of 2 to help break down food and kill bacteria
Functions of the Stomach Mechanical digestion Food reaches pylorus Chemical digestion Digestion of proteins- pepsin Digestion of carbohydrates- salivary amylase Absorption No food too much mucous
The Small Intestine About 18 feet long The duodenum About 8 inches long Common bile duct & pancreatic duct empty here The jejunum About 8 feet long Most chemical digestion occurs here The ileum About 9.5 feet long Most absorption occurs here Ends in the ileocecal valve
Histology of the Small Intestine The lining is folded into circular pleats The mucosal surface is folded into villi The epithelial cell membranes are highly folded into microvilli Intestinal glands Secrete intestinal juice
A Villus
Functions of the Small Intestine Intestinal juice- buffers acid, moistens contents, breaks down digestive enzymes Chyme is further broken down Proteins Carbohydrates Fats Most absorption is in the small intestine Hormones: Secretin increase secretion of buffers and bile Cholecystokinin secretion of enzymes and bile Gastric inhibitory peptide slows down gastric activity
The Large Intestine (Colon) About 4.5 feet long Begins with the cecum Appendix is attached 1. Ascending colon 2. Transverse colon 3. Descending colon Sigmoid colon Colon connects to rectum Rectum connects to anal canal Empties to the exterior through the anus
Histology and Functions of the Large Intestine Mucosa - simple columnar epithelium Completion of absorption Formation of feces Lots of mucus glands Expulsion of feces from the body
Functions of the large intestine Movement: Regulated by the ileocecal valve Mixing and peristalsis Mass peristalsis Chemicals: Mucus secreted No enzymes Bacteria prepare chyme for elimination Absorption: WATER Bile salts
Feces Formation & Defecation Chyme is now solid or semi-solid - feces Large intestine absorbs any more water and electrolytes from feces Defecation Mass peristalsis pushes fecal material into rectum Rectum stretches Defecation reflex
Liver Pancreas Gall bladder Accessory Organs
The Liver Performs many lifesustaining functions Location under the diaphragm on the right Connected to the diaphragm by the falciform ligament Divided into lobes Right lobe Left lobe Caudate lobe Quadrate lobe
Outside is a capsule Composed of tiny lobules Each lobule is surrounded by liver cells and sinusoids capillaries that run through liver Hepatocytes release bile breakdown fats Bile ducts run between liver cells Histology of the Liver
Functions of the Liver Produces bile the primary digestive function Composition Water Bile salts Cholesterol Pigments Bilirubin Digestive function Emulsification of fats
Other Functions of the Liver Absorbs and stores iron, vitamins A, D, E, B7, K Detoxifies toxins and hormones Metabolizes proteins, carbohydrates, and lipids Removes bacteria from the blood Produces plasma proteins Removes worn-out and damaged red blood cells
The Gallbladder Location underside of right lobe of liver Function concentrate and store bile Collected from liver Hepatic ducts Adds bile to duodenum Cystic duct Common bile duct Gallstones salts precipitate and form salt chunks
Location in the curvature of the duodenum Connected to the duodenum by the pancreatic duct Produces pancreatic juice Functions Exocrine - digestion of all nutrient groups Endocrine control blood glucose level The Pancreas