LAB 9 NOTES

FUNCTIONS OF THE DIGESTIVE SYSTEM

• Mechanical Digestion: Physical breakdown of food.

  • Mixing in the mouth by the tongue.

  • Churning in the stomach.

  • Segmentation in the small intestine.

  • Prepares food for enzyme degradation.

• Chemical Digestion: Enzyme breakdown of food molecules into building blocks.

  • Different enzymes for each major food group.

    • Carbohydrates broken down into simple sugars (e.g., amylase, lactase, fructase).

    • Proteins broken down into amino acids by proteases (e.g., trypsin).

    • Fats broken down into fatty acids and glycerol by lipases.

ORGANS OF THE DIGESTIVE SYSTEM

• Alimentary Canal (Gastrointestinal Tract):

  • Mouth

  • Pharynx

  • Esophagus

  • Stomach

  • Small Intestine

  • Large Intestine

  • Anus

• Accessory Organs:

  • Salivary Glands

  • Teeth

  • Pancreas

  • Liver

  • Gallbladder

LAYERS OF THE ALIMENTARY CANAL

• Mucosa

• Submucosa

• Muscularis Externa

• Serosa (Visceral Peritoneum)

Mucosa

• Innermost, moist membrane.

  • Surface epithelium

  • Small amount of connective tissue (lamina propria)

  • Small smooth muscle layer

Submucosa

• Beneath the mucosa.

• Soft connective tissue with blood vessels, nerve endings, and lymphatics.

Muscularis Externa

• Two layers of smooth muscle.

  • Circular layer

  • Outer longitudinal layer

Serosa

• Outermost layer with fluid-producing cells.

  • Visceral peritoneum (innermost layer that wraps canal organs)

  • Parietal peritoneum (outer layer that lines the abdominopelvic cavity)

MOVEMENT OF DIGESTIVE MATERIALS

• Visceral smooth muscle shows rhythmic cycles due to pacemaker cells.

• Peristalsis creates waves to move a bolus.

• Segmentation churns and fragments a bolus.

ANATOMY OF THE MOUTH (ORAL CAVITY)

• Lips (labia) protect the anterior opening.

• Cheeks form the lateral walls.

• Uvula directs food.

• Hard palate forms the anterior roof.

• Soft palate forms the posterior roof.

SALIVARY GLANDS

• Three pairs empty secretions into the mouth:

  • Parotid glands

  • Submandibular glands

  • Sublingual glands

• Saliva:

  • Mixture of mucus and serous fluids

  • Helps form a food bolus

  • Contains salivary amylase to begin starch digestion

  • Dissolves chemicals for taste

DEGLUTITION (SWALLOWING) & ESOPHAGUS

Esophagus Anatomy

• About 10 inches long.

• Runs from pharynx to stomach through the diaphragm.

Esophagus Physiology

• Conducts food by peristalsis.

• Passageway for food only (respiratory system branches off after the pharynx).

Pharynx and Esophagus Activities

• No chemical digestive function.

• Serve as passageways to the stomach.

REGIONS OF A TOOTH

Crown

• Exposed part.

  • Enamel: Hardest substance in the body.

  • Dentin: Deep to enamel, forms the bulk of the tooth.

  • Pulp cavity: Contains connective tissue, blood vessels, and nerve fibers.

  • Root canal: Pulp cavity extends into the root.

Neck

• In contact with the gum.

• Connects crown to the root.

Root

• Cementum covers outer surface and attaches roots to the periodontal membrane.

ANATOMY OF THE STOMACH

• Located on the left side of the abdominal cavity.

• Food enters via the cardioesophageal sphincter.

• Food empties into the small intestine at the pyloric sphincter (valve).

Regions of the Stomach

• Cardiac region: Near the heart.

• Fundus: Expanded portion lateral to the cardiac region.

• Body: Mid portion.

• Pylorus: Funnel-shaped terminal end.

• Rugae: Internal folds of the mucosa.

External Regions

• Lesser curvature: Concave medial surface.

• Greater curvature: Convex lateral surface.

FUNCTIONS OF THE STOMACH

• Bulk storage of undigested food.

• Mechanical breakdown of food.

• Disruption of chemical bonds via acids and enzymes.

• Production of intrinsic factor.

• Temporary storage tank for food.

• Chemical breakdown of protein begins.

• Delivers chyme (processed food) to the small intestine.

• Site of some food breakdown.

FOOD BREAKDOWN IN THE STOMACH

• Gastric juice is regulated by neural and hormonal factors.

• Food or rising pH causes the release of gastrin.

• Gastrin causes stomach glands to produce:

  • Protein-digesting enzymes

  • Mucus

  • Hydrochloric acid

• Hydrochloric acid makes the stomach contents very acidic.

• Acidic pH:

  • Activates pepsinogen to pepsin for protein digestion.

  • Provides a hostile environment for microorganisms.

• Protein-digesting enzymes:

  • Pepsin: active protein-digesting enzyme

  • Rennin: Digests milk proteins in infants.

• Alcohol and aspirin are absorbed in the stomach.

SMALL INTESTINE: THE BODY'S MAJOR DIGESTIVE ORGAN

• Important digestive and absorption functions.

  • The majority of chemical digestion begins in the small intestine.

  • Secretions and buffers provided by the pancreas, liver, and gallbladder.

  • Enzymes are produced by intestinal cells and the pancreas.

  • Pancreatic ducts carry enzymes to the small intestine.

  • Bile formed by the liver enters via the bile duct.

  • Site of nutrient absorption into the blood.

• Muscle tube extending from the pyloric sphincter to the ileocecal valve with three subdivisions:

  • Duodenum: Attaches to the stomach, curves around the head of the pancreas.

  • Jejunum: Attached anteriorly to the duodenum.

  • Ileum: Extends from the jejunum to the large intestine.

• Ileocecal sphincter: Transition between the small and large intestine.

STRUCTURES OF THE SMALL INTESTINE THAT INCREASE SURFACE AREA FOR DIGESTION AND ABSORPTION

• Microvilli: Tiny projections of the plasma membrane (create a brush border appearance).

• Villi: Finger-like structures formed by the mucosa.

• Circular folds (plicae circulares): Deep folds of the mucosa and submucosa.

LARGE INTESTINE

• Larger in diameter, but shorter in length than the small intestine.

• Frames the internal abdomen.

• Cecum: Sac-like first part of the large intestine.

Colon

• Ascending: Travels up the right side of the abdomen.

• Transverse: Travels across the abdominal cavity.

• Descending: Travels down the left side.

• Sigmoid: Enters the pelvis.

• Rectum and anal canal (also in pelvis).

Appendix

• Accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis).

• Hangs from the cecum.

MOVEMENT IN THE LARGE INTESTINE

• Sluggish peristalsis is the major means of moving food.

• Mass movements: Slow, powerful movements, occurring one to four times per day.

• Presence of feces in the rectum causes a defecation reflex:

  • Internal anal sphincter is relaxed.

  • Defecation occurs with relaxation of the voluntary (external) anal sphincter.

• Segmentation movements mix chyme with digestive juices.

• Gastroenteritis reflexes are initiated by stretch receptors in the stomach.

• Gastroileal reflex triggers relaxation of the ileocecal valve.

PANCREAS

• Pancreatic duct penetrates the duodenal wall.

Endocrine Functions

• Secretion of insulin and glucagon.

Exocrine Secretions

• The majority of pancreatic secretions.

  • Pancreatic juice is secreted into the small intestine.

  • Enzymes included are:

    • Carbohydrases

    • Lipases

    • Nuclease

    • Proteolytic enzymes

LIVER

• Largest gland in the body.

• Located on the right side of the body, under the diaphragm.

• Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament.

• Connects to the gallbladder via the common hepatic duct.

• Performs metabolic and hematological regulations and produces bile.

Histological Organization

• Lobules containing single cell thick plates of hepatocytes.

• Lobules unite to form the common hepatic duct; the duct meets the cystic duct to form the common bile duct.

FUNCTIONS OF THE LIVER

• Fats and fatty acids are picked up by the liver.

  • Some are oxidized to provide energy for liver cells.

  • The rest are broken down into simpler compounds and released into the blood.

Several Roles in Digestion

• Manufactures bile: emulsifies fats by breaking up large fat globules.

• Detoxifies drugs and alcohol.

• Degrades hormones.

• Produces cholesterol, blood proteins (albumin and clotting proteins).

• Plays a central role in metabolism.

• Can regenerate if part of it is damaged or removed.

GALLBLADDER

• Sac found in the hollow fossa of the liver hollow, pear-shaped organ.

• When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder.

• When digestion of fatty food is occurring, bile is introduced into the duodenum from the gallbladder.

• Stores, modifies, and concentrates bile.

• Gallstones are crystallized cholesterol which can cause blockage.

CARBOHYDRATE DIGESTION

• Glucose and galactose are absorbed via cotransport with sodium ions.

• Fructose passes via facilitated diffusion.

• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

• site of action: mouth, small intestine,

PROTEIN DIGESTION

• Amino acids are absorbed via cotransport with sodium ions.

• Some dipeptides and tripeptides are absorbed via cotransport with H* and hydrolyzed to amino acids within the cells.

• Infrequently, transcytosis of small peptides occurs.

• Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

• site of action: stomach, small intestine

FAT DIGESTION

• Fatty acids and monoglycerides enter the intestinal cells via diffusion.

• Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells. The resulting chylomicrons are extruded by exocytosis.

• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.

• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.

• mouth, stomach, small intestine

NUCLEIC ACID DIGESTION

• Units enter intestinal cells by active transport via membrane carriers.

• Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.

• site of action: small intestine