Introduction to the Digestive System

Digestion

Animals cannot ingest macromolecules made by other animals and use them directly

2 kinds:Mechanical digestionChemical digestion

Vertebrate Digestive System

Food moves through digestive tract by peristalsis:

Mouth pharynx esophagus stomach

small intestine large intestine anus

Accessory organs: Salivary glands Liver Gall bladder Pancreas

Layers of digestive tract

MouthStart of food processing

Mechanical digestion – teeth IncisorsCaninesPremolarsMolars

Chemical digestion:

Salivary glands secrete 1 L saliva/day

Salivary amylase

Pharynx and Esophagus

Bolus moves through pharynx Epiglottis covers trachea during swallowing

Esophagus leads to stomachPeristalsis moves food along

Stomach

Ring of sphincter muscle relaxes and allows bolus to enter stomach

As stomach fills, rugae of stomach smooth out

Chemical Digestion in Stomach

Gastric glands in mucosa secrete:HCl Intrinsic factorPepsinogen (converts to pepsin in presence of acidic

gastric juice)

Pepsin, main digestive enzyme of stomach, breaks down large proteins into short polypeptides

Food spends 3-4 hours in stomach

Muscle contraction churns food

Salivary amylase works until stomach becomes too acidic

Partly digested food becomes chyme

Peristalsis releases chyme in spurts through pylorus into small intestine

Protection of StomachLining of stomach secretes large amounts of alkaline mucus

Epithelial cells are tightly joined to prevent leaking

Small Intestine

In human, approx. 17 feet long

Divided into 3 regions: duodenum, jejunum, and ileum

Lining is folded and composed of villi and microvilli

Digestion in Small IntestineBile, secreted by liver, mechanically digests fats

Enzymes secreted by small intestine and pancreas:

Polypeptides and peptidesTrypsin, chymotrypsin, peptidases

Completion of carbohydrate digestionPancreatic amylase, maltase, sucrase, lactase

Digestion of fat droplets lipase

Absorption of NutrientsNutrients must pass through epithelium to reach blood or lymph

Simple diffusion, facilitated diffusion, active transport

Large IntestineApprox. 4 feet long

Includes cecum and appendix, a blind pouch

Sections include ascending, transverse, descending & sigmoid colon

Ends at rectum & anus

Functions of Large Intestine

Undigested material and unabsorbed chyme passes slowly through large intestine

Water and sodium are absorbed

Bacteria in large intestine feed and produce vitamin K and some B vitamins for use by host