Digestive System Quiz

Digestive System Overview

• The digestive system comprises the digestive tract (also known as the gastrointestinal tract) and associated organs.
• The digestive tract is a continuous tube extending from the mouth to the anus.
• The primary functions of the digestive system include ingestion, mastication, propulsion, mixing, digestion, secretion, absorption, and elimination.

Topic Learning Outcomes

• List the major functions of the digestive system.
• Describe the general histology of the digestive tract.
• Describe the parts and their major functions of the digestive tract.
• Describe the anatomy, physiology, histology, and ducts of the liver and pancreas.
• Describe the digestion, absorption, and transport of carbohydrates, lipids, and proteins.
• Discuss water movement into and out of the digestive tract.
• Describe the effects of aging on the digestive system.

Digestive System Functions

• Ingestion and mastication of solids and liquids.
• Propulsion and mixing.
• Digestion and secretion.
• Absorption.
• Elimination.

Digestive System Components (Organs)

• Oral Cavity (Mouth)
• Pharynx (Throat)
• Esophagus
• Stomach
• Small Intestines
• Large Intestines
• Rectum
• Anus
• Salivary Glands
• Liver
• Gallbladder
• Pancreas

Layers of Digestive Tract Wall

• Mucosa (innermost layer; secretes mucus)
• Submucosa (above mucosa; contains blood vessels, nerves, and small glands)
• Muscularis (above submucosa; contains longitudinal, circular, and oblique muscles)
• Serosa/Adventitia (outermost layer; serosa when covered with peritoneum, adventitia otherwise)

Peritoneum and Mesenteries

• Peritoneum: smooth epithelial layer (serous membranes: visceral and parietal peritoneum)
• Mesenteries: connective tissue of organs within the abdominal cavity
• Specific Mesenteries:
  • Lesser Omentum: mesentery connecting lesser curvature of stomach to liver and diaphragm.
  • Greater Omentum: mesentery connecting greater curvature of stomach to transverse colon and posterior body wall.

Oral Cavity

• First part of the digestive system.
• Contains stratified squamous epithelium.
• Salivary glands produce saliva containing enzymes to breakdown carbohydrates into glucose.
• Saliva cleanses the mouth and moistens food.
• Amylase: salivary enzyme breaking down carbohydrates.
• Lysozyme: salivary enzyme active against bacteria.
• Tongue: houses taste buds and mucus.

Teeth

• Normal adults have 32 teeth.
• Primary teeth (baby teeth) number 20.
• Teeth are composed of enamel, dentin, pulp cavity, and root.
• Teeth are anchored in alveolar bone.
• Cavities are caused by the breakdown of enamel by acids from bacteria.
• The palate is the roof of the oral cavity. It consists of a hard palate and a soft palate.

Salivary Glands

• Parotid, submandibular, and sublingual glands.
• Produce saliva (a mixture of serous and mucous fluids) with enzymes for food breakdown.
• Parotid glands are the largest.
• Submandibular glands secrete more serous than mucus.
• Sublingual glands primarily secrete mucus.
• Mumps is an inflammation of the parotid gland.

Secretions of Oral Cavity

• Saliva is secreted about 0.5-1.5 liters per day.
• Salivary amylase works on starch and other polysaccharides, producing maltose and isomaltose (sweet taste).
• Humans lack enzymes to digest cellulose.
• Mastication is the chewing and breaking down of food into smaller particles by teeth.

Pharynx (Throat)

• Connects the mouth to the esophagus.
• Three parts: nasopharynx, oropharynx, and laryngopharynx.

Esophagus

• Tube connecting the pharynx to the stomach.
• Transports food to the stomach.
• Heartburn occurs when gastric juices regurgitate into the esophagus, often caused by caffeine, smoking, or excess eating/drinking.
• Swallowing involves three phases: voluntary, pharyngeal, and esophageal.

Swallowing

• Voluntary phase: bolus forms in the mouth and is pushed into the oropharynx.
• Pharyngeal phase: swallowing reflex initiated when bolus stimulates receptors in oropharynx.
• Esophageal phase: food moves from pharynx to stomach via peristalsis.
• Peristalsis: wave-like contractions pushing food through the digestive tract.

Stomach

• Located in the abdomen.
• Storage tank for food (up to 2 liters).
• Produces mucus, hydrochloric acid (HCl), and protein-digesting enzymes.
• Contains a layer of mucus to protect tissues from acidic environment.
• Three muscular layers facilitate churning.
• Rugae are folds that enable expansion.
• Chyme is the paste-like substance formed when food begins to be broken down.
• Pyloric sphincter controls passage of chyme into small intestine.
• Hunger pangs are contractions related to low blood glucose
• Stomach is involved in the three phases (Cephalic, Gastric, Intestinal) of stomach secretions regulation (using hormones and neurons)

Secretion of the Stomach

• Acidic chyme (containing mucus, HCl, intrinsic factor, pepsinogen, and gastrin) is produced during the gastric phase when partially digested proteins and stomach distention prompt the process

Regulation of Stomach Secretions

• Cephalic phase: initiated by sight, smell, taste, or thought of food.
• Gastric phase: partially digested proteins and stomach distension prompt secretion.
• Intestinal phase: acidic chyme stimulates neural reflexes and hormone release (e.g., secretin and cholecystokinin) that inhibit further gastric secretions.

Movement in the Stomach

• Mixing waves: weak contractions mix food to form chyme.
• Peristaltic waves: stronger contractions force chyme toward and through the pyloric sphincter.

Small Intestine

• Measures 6 meters in length.
• Major absorptive organ.
• Chyme takes 3-5 hours to pass through.
• Contains enzymes to further breakdown food.
• Contains secretions to protect against the acidity of chyme.
• Three parts: duodenum (first part; 25 cm), jejunum (second part; 2.5 meters), ileum (third part; 3.5 meters).

Parts of Small Intestine

• Duodenum: first part (25 cm); contains absorptive cells, goblet cells, granular cells, endocrine cells, bile and pancreatic ducts.
• Jejunum: second part (2.5 meters); primary site for nutrient absorption.
• Ileum: third part (3.5 meters).

Mucosa of the Small Intestine

• Simple columnar epithelium with four major cell types.
• Absorptive cells: have microvilli, produce digestive enzymes, and absorb digested food.
• Goblet cells: produce protective mucus.
• Granular cells: potentially help protect intestinal epithelium from bacteria.
• Endocrine cells: produce regulatory hormones.

Mucosa of the Small Intestine (Specialized)

• Intestinal glands (crypts of Lieberkühn): tubular glands in the mucosa at the base of villi
• Granular and endocrine cells are at the bottom of the glands
• Duodenum has duodenal glands in the submucosa, opening into the base of intestinal glands

Secretions of the Small Intestine

• Secretion of small intestine consists primarily of mucus, ions, and water.
• Peptidases, disaccharidases are enzymes breaking down proteins into amino acids, disaccharides into monosaccharides

Movement in Small Intestine

• Mixing and propulsion of chyme occur through peristalsis(along the length of the intestine) and segmental contractions (mixing intestinal contents)

Ileocecal Sphincter(Small and Large Intestines)

• Muscle at the junction of small and large intestines keeps it mildly contracted most of the time.
• Peristaltic contractions from small intestine cause the sphincter to relax, and chyme moves into the cecum.
• Prevents movement from large intestine back into small intestine

Segmental Contractions in Small Intestine

• Secretion introduced to the digestive tract begins in one spot.
• Segments of tract alternate between contraction and relaxation.
• Material spreads out in both directions from introduction point.
• Results in more diffuse mixture within digestive tract.

Liver Anatomy

• Located in the upper right quadrant of the abdomen.
• Divided into lobes: right, left, quadrate, and caudate.
• Covered by visceral peritoneum (serous membrane).
• Contains hepatic ducts, hepatic portal vein, hepatic artery, and gallblader

Liver Ducts

• Hepatic ducts: transport bile out of the liver
• Common hepatic duct: created from left and right hepatic ducts.
• Cystic duct: joins common hepatic duct; extends from gallbladder
• Common bile duct: formed from common hepatic duct and cystic duct

Bile and Pancreatic Secretions

• Hepatic ducts, combining to form a common hepatic duct that merges with the cystic duct to form a common bile duct which reaches the duodenum
• Accessory pancreatic duct empties into duodenum
• Pancreatic secretions can also enter duodenum via pancreatic duct

Functions of Liver

• Digestive and excretory functions.
• Stores and processes nutrients.
• Detoxifies harmful chemicals.
• Synthesizes new molecules.
• Secretes 700 milliliters of bile daily.
• Bile neutralizes stomach acid, breaks down fats.

Control of Bile Secretion and Release

• Vagus nerve stimulation causes gallbladder contraction and bile release into duodenum.
• Secretin from duodenum stimulates liver bile secretion.
• Cholecystokinin from duodenum stimulates gallbladder contraction and sphincter relaxation, releasing bile into duodenum.
• Bile salts also stimulate bile secretion.

Pancreas

• Located posterior to the stomach, inferior to the upper left quadrant.
• Consists of head, body, and tail.
• Tail touches the spleen.
• Endocrine tissues: pancreatic islets produce insulin and glucagon. Insulin increases glucose uptake, glucagon releases glucose (primarily in liver).
• Exocrine tissues: produce digestive enzymes (e.g., trypsin, chymotrypsin, carboxypeptidase, amylase, lipase, nucleases) that travel through ducts.

Pancreatic Secretions

• Major protein-digesting enzymes: trypsin, chymotrypsin, carboxypeptidase
• Pancreatic amylase continues polysaccharide digestion (started in mouth)
• Pancreatic lipase is a lipid-digesting enzyme
• Pancreatic nucleases degrade DNA/RNA to their component nucleotides.

Large Intestine

• Function: absorbs water from indigestible food.
• Components: cecum, colon (ascending, transverse, descending, sigmoid), rectum, anal canal.
• Cecum: joins small intestine at ileocecal junction. Appendix attached.
• Colon: 1.5 meters long
• Rectum: straight tube from sigmoid colon to anal canal.
• Anal canal: last 2-3 cm of digestive tract.
• Food takes 18-24 hours to pass through.
• Feces are product of water, indigestible food, and microbes.
• Microbes synthesize vitamin K inside large intestine.

Digestive Process

• Digestion: food breakdown in stomach and mouth
• Propulsion: moves food through digestive tract (swallowing and peristalsis)
• Absorption: primarily in duodenum and jejunum of small intestine.
• Defecation: elimination of waste (feces).

Digestion

• Food is broken down into carbohydrates (monosaccharides), lipids (fatty acids and monoglycerides), and proteins (amino acids).

Protein Digestion

• Pepsin: protein-digesting enzyme secreted by the stomach.
• Trypsin, chymotrypsin, carboxypeptidase: protein-digesting enzymes secreted by pancreas (in inactive form) and activated in the small intestine.
• Peptidases: small intestine enzymes breaking down peptides into tripeptides.
• Absorption of tripeptides, dipeptides, or individual amino acids occurs through intestinal epithelial cells (via cotransport).

Water and Minerals

• Water can move across the intestinal wall in either direction depending on osmotic pressures.
• 99% of water entering the intestine is absorbed.
• Minerals are actively transported across the small intestine wall.

Fluid Volumes in Digestive Tract

• Varies among substances and parts of the digestive tract (e.g., salivary glands secrete 1L, bile 0.7L, small intestine secretions measure 2L, etc.)
• About 92% of absorbed fluids are in small intestines, 6%-7% in large intestine
• Fecal water amount is 1% or less of total fluid consumed / secreted

Summary

• The presented notes summarize a wide variety of information about the human digestive system. Each part including major functions, components, and processes throughout the anatomical structures.