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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