The Digestive System
The Digestive System
• Irregular tube called alimentary canal or gastrointestinal (GI) tract
• Food must first be digested, then absorbed, and later metabolized
Wall of the Digestive Tract
The wall of the digestive tract is formed by four layers of tissue:
• Mucosa—mucous epithelium
• Submucosa—connective tissue
• Muscularis—2-3 layers of smooth muscle
• Serosa—serous membrane that covers the outside of abdominal organs
• Mesenteries - folds connecting digestive tract to walls
Mouth
• Roof
• Hard palate- parts of maxillary and palatine bones
• Soft palate- muscle separating mouth from pharynx
• Uvula- projection of soft palate
• Floor
• Tongue and its muscles
• Papillae- elevated mucosa of tongue
• Taste buds - in papillae
• Lingual frenulum - fold of membrane anchoring tongue to floor of mouth
Teeth
• Names —incisors, cuspids, bicuspids, and tricuspids
• 20 teeth in temporary set
• cut tooth at 6 months and complete at 2 years
• 32 teeth in permanent set
• First permanent tooth at 6 years
• Complete set between 17 and 24
Salivary Glands
• Parotid glands
• Submandibular glands
• Sublingual glands
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Stomach
• Size—expands after meal; size of large sausage when empty
• Pylorus—lower part of stomach
• Pyloric sphincter muscle- pylorus into duodenum
• Wall—smooth muscle fibers
• Peristalsis - contractions
• Lining—mucous membrane
• microscopic glands secrete gastric juice and HCl into stomach
• Rugae – Folds in mucous membrane
Small Intestine
• Size— 7 m (20 feet) ; 2 cm or so in diameter
• Divisions – Duodenum & Jejunum
• Wall—peristaltic smooth muscle fibers
• Lining—mucous membrane
• Microscopic intestinal glands secrete intestinal juice
• Villi -microscopic projections into intestinal cavity contain blood and lymph capillaries
Liver and Gallbladder
• Liver
▪ Size and location—liver largest gland
▪ upper right section of abdominal cavity & extends into left side
▪ Secretes bile
▪ Ducts
▪ Hepatic—drain bile from liver
▪ Cystic—duct by which bile enters and leaves gallbladder
▪ Common bile—union of hepatic & cystic ducts
• drains bile from hepatic or cystic ducts into duodenum
Liver and Gallbladder
• Gallbladder
▪ Location—under surface of the liver
▪ Function—concentrates and stores bile produced in the liver
Pancreas
• Location—behind stomach
• Functions
▪ Secretes pancreatic juice into ducts
▪ main duct empties into duodenum
▪ Islets of Langerhans—cells not connected with pancreatic ducts
▪ secrete hormones glucagon and insulin into the blood
Large Intestine
• Divisions
• Opening to exterior—anus
▪ Cecum
▪ Colon-ascending, transverse, descending, and sigmoid
▪ Rectum
• Wall—peristaltic smooth muscle fibers produce churning and defecation
• Lining—mucous membrane
Appendix
Blind tube off cecum; no important digestive function in humans
Peritoneum
• Peritoneum- Serous membrane
• Parietal - lines abdominal cavity
• Visceral - covers abdominal organs
• Peritoneal space - between parietal and visceral layers
• Extensions
• Mesentery - extension of parietal peritoneum attaches small intestine to posterior abdominal wall
• Greater omentum (“lace apron”)- hangs down from stomach and transverse colon over intestines
Digestion
Changing foods so that they can
be absorbed and used by cells
• Mechanical digestion
• Chewing, swallowing, and peristalsis break food into tiny particles
• mix them with digestive juices
• move them along the digestive tract
• Chemical digestion
• Breaks up large food molecules into compounds of smaller molecules
• Uses digestive enzymes
• Carbohydrate digestion—mainly in small intestine
• Pancreatic amylase—breaks polysaccharides down to disaccharides
• Intestinal juice enzymes
• Maltase—changes maltose to glucose
• Sucrase—changes sucrose to glucose
• Lactase—changes lactose to glucose
• Protein digestion—starts in stomach and completes in small intestine
• Pepsin – digestive enzyme that partially digests proteins
• Trypsin - pancreatic enzyme continues digestion of proteins
• Peptidases - intestinal enzymes complete digestion of partially digested proteins to amino acids
• Fat digestion
• Bile contains no enzymes but emulsifies (breaks down) fats
• Pancreatic lipase changes emulsified fats to fatty acids and glycerol in small intestine
Absorption
• Digested food moves from intestine into blood or lymph
• Foods and most water from small intestine
• Some water absorbed in large intestine
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