Chapter 22 – The Digestive System

2 main components• Alimentary canal– Continuous

passageway from mouth to anus

• Accessory organs/structures– Assists with the

physical and/or chemical breakdown of food

– Teeth, salivary glands, pancreas, liver

Digestive processes• Ingestion

– Bringing food into body’s interior • Digestion

– Mechanical/physical • Breaking down into smaller components

– Chewing/mastication – Stomach churning – Segmentation

» Back and forth movement in small intestine

– Chemical/enzymatic • Breaking of chemical bonds in nutrients into their smaller building blocks

• Absorption – Nutrients pass through intestinal wall into bloodstream

• Elimination – Removal of waste/undigested material – Defecation

Histology of alimentary canal• 4 tunics

– Mucosa – deepest• Secretes mucus, enzymes • Contains lymphoid cells to fight

infection – Submucosa

• Blood and lymphatic vessels, nerve fibers

– Muscularis externa – smooth muscle • Deep – circular layer • Superficial – longitudinal layer

– Serosa – most superficial• Visceral peritoneum • Reduces friction from organ

movement

Peristalsis

• Waves of smooth muscle contraction that propels the contents through the alimentary canal/GI tract

Mouth • Roof

– Anterior hard palate• Maxillary and palatine bones

– Posterior soft palate• Underlying muscle only; no bone• Uvula

– Elicits gag response– Blocks entrance into nasopharynx

when swallowing

• Tongue – Moves food around during

mastication to form bolus, and to push bolus to back of throat• Bolus – chewed food mixed with

saliva• Deglutition = swallowing

– Covered with taste buds • Also scattered in walls or oral cavity

and oropharynx

Salivary glands • Produce saliva

– Moistens food – Contains lysozyme to kill

bacteria– Dissolves chemicals for

sense of taste– Contains enzyme

amylase• Begins chemical digestion

of carbohydrates

• 3– Parotid – Sublingual – Submandibular

Tooth structure • Crown – exposed region;

root – below gumline; neck – separates crown and root

• Enamel covers crown – Calcium salts

• Cementum – calcified connective tissue – Attachment site of

periodontal ligament • Dentin – bulk of tooth

– Similar to bone composition – Central pulp cavity

• Extends through root canal • Vessels, nerve endings,

connective tissue

Teeth #/type • Baby/ deciduous teeth– 20 total – 2-1-0-2

2-1-0-2• Adult teeth – 32 total – 2-1-2-3 2-1-2-3

Pharynx and EsophagusStratified squamous epithelium

Esophageal/stomach junction

• Pharynx – Oropharynx and

laryngopharynx • Esophagus – Joins with stomach

and cardiac orifice • Cardiac or

gastroesophageal sphincter – Diaphragm helps

keep it closed

Stomach • Muscularis externa has a third layer –

oblique (deep to circular layer)• Rugae – inner lining folds

– Increases surface area – Allows stomach to expand

• Regions– Cardia/cardiac – Fundus – Body

• Lesser curvature – medial • Greater curvature – lateral

– Pylorus• Plyoric sphincter at junction with small

intestine

• Food and gastric juice mix to form thick acid chyme – Exits from stomach in spurts – Higher fat content stays in stomach

longer

Stomach histology

• Mucous cells • Parietal cells– Secrete hydrochloric

acid • Chief cells – Secrete pepsinogen

• In presence of HCl, converts to active pepsin – Chemical digests

protein

Small intestine • Site where most of chemical

digestion occurs, and absorption of nutrients

• 3 regions – Duodenum

• Smallest section; site of most digestion

• Entrance for hepatic and pancreatic secretions

– Jejunum – Ileum

• Contains Peyer’s patches– Large aggregates of lymphoid

material • Ileocecal valve – junction with

large intestine

Small intestine modifications for absorption

• Microvilli – plasma membrane extensions of cell • Villi – projections of mucosa – Both villi and microvilli increase surface area

• Pilcae circulares – Allows chyme to swirl through lumen

• Slows movement and increases contact with walls for absorption

Pancreas • Tail – next to spleen;

body – surrounded by duodenum

• Pancreatic secretions into duodenum – Bicarbonate ions

• Neutralized acidity of chyme

– Enzymes for digestion• Proteases, amylase,

lipase

Liver • Main digestive

function is to filter and process nutrient-rich blood from hepatic portal vein

• Produces bile salts – Emulsifies fat

• Mechanical breakdown of lipids into smaller droplets– Provides a greater

surface area for enzymes to work on

Gallbladder

• Concentrates and stores bile

• Bile exits through cystic duct to bile duct, then into duodenum

• Excess cholesterol can crystallize to form gallstones

Histology

Pancreas Liver

Large intestine • Absorbs water and

compacts waste • Contains bacteria

– Produces B12 and K vitamins, which are then absorbed

– Breakdown of molecules cause gaseous byproducts – flatus

• Longitudinal layer of muscularis is reduced to three bands – teniae coli– Causes walls to ‘pucker’ into

pouches called haustra • Epiploic appendages

Large intestine sections • Cecum “blind pouch”

– Ileocecal valve – Vermiform appendix – mass of

lymphoid tissue • Ascending colon – right side of

body – Turns at hepatic flexure

• Transverse colon– Turns at splenic flexure

• Descending colon – left side of body

• Sigmoid colon – “S” shape curves posteriorly

Large intestine sections cont

• Rectum • Anus – Internal sphincter

• Smooth muscle – External sphincter

• Skeletal muscle

Chemical digestion • Large macromolecules (polymers) are broken down into

monomer building blocks – Hydrolysis – addition of water molecule breaks one bond

• Carbohydrates– Enzymatic breakdown begins in mouth with amylase – Monomer = monosaccharide

• Glucose, fructose, galactose – Dissaccharide = 2 monosaccharides

• Sucrose, lactose– Polysaccharide = complex carbohydrate

• Glycogen, starch, cellulose (indigestible)

Chemical digestion cont • Proteins– Enzymatic breakdown begins in stomach by pepsin – monomer = amino acids

• Lipids – Enzymatic breakdown begins in duodenum by lipase in

adults • In children, begins in stomach

– Monomer = fatty acid (and glycerol) • Nucleic acids – Enzymatic breakdown begins in duodenum – Does not contribute to cellular fuel/creation of ATP– Monomer = nucleotide