chapter 22 – the digestive system
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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
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