Digestive System

Ingestion

The introduction of solid and liquid materials into the oral cavity

First step in the process of digesting and absorbing nutrients

Chemical digestion

Breaks down food into smaller molecules by using enzymes

Location: stomach, small intestines, mouth

Mechanical digestion

Physically breaks down food into smaller pieces

Location: mouth, stomach, small intestine

Peristalsis

The process of muscular contraction that forms ripples along part of the GI tract and forces material to move further along the tract

Location: esophagus, stomach, small intestines, and large intestines

Segmentation

Churning movements in the small intestines

Function: helps disperse the material being digested and combine it with digestive secretions

Location: small intestine

Secretion

Process of producing and releasing mucin or fluids

Absorption

Involves either active or passive transport of electrolytes, digestion products, vitamins, and water across the GI tract epithelium and into GI tract blood and lymph vessels

Location: small intestine and large intestine

Elimination

All undigestible materials and waste products secreted by the accessory organs into the GI tract are compacted into feces and then eliminated from the GI tract

Hard palate

Location: oral cavity

Structure: formed by the palatine process of the maxillae and horizontal plates of the palatine, covered in dense connective tissue, nonkeratinized stratified squamous epithelial tissue

Function: assist tongue to manipulate ingested food prior to swallowing

Soft palate

Location: oral cavity, extension of the hard palate

Structure: skeletal muscle, covered by nonkeratinized stratified squamous epithelial tissue

Function: elevates and tenses to close off nasopharynx when swallowing, breathing, speech

Uvula

Location: oral cavity, extending inferiority from the posterior soft palate

Structure: muscular tissue covered by a mucous membrane

Function: closed off the nasopharynx when swallowing, gag reflex, speech

Tongue

Accessory organ

Structure: skeletal muscle covered by stratified squamous epithelium tissue and contains papillae

Function: manipulates and mixes ingested materials during chewing and helps compress the materials against the palate to turn them into a bolus

Filiform papilla

Location: all over the tongue

Structure: roughness of tongue, keratinized stratified squamous epithelium tissue, irregular shape, no taste buds

Function: grip and manipulate food

Fungiform papillae

Location: dorsal end of the tongue

Structure: mushroom shape, taste buds, no deep pit

Function: grip and manipulate food, and sense of taste

Foliate papillae

Location: lateral edges of posterior tongue, not common in humans

Structure: symmetrical columnar shape, taste buds along proximal lateral surface, taste buds with pores

Function: grip and manipulate food, and sense of taste

Vallate papillae

Location: proximal end of the tongue

Structure: larger, deep pits lined with taste buds, salivary glands within skeletal muscle

Function: grip and manipulate food, and sense of taste

Gustatory cells

Location: tongue taste buds

Structure: specialized neuroepithelial cells

Function: detect taste

Supporting cells

Location: taste buds of the tongue

Structure: spindle shaped cells, elongated central nuclei

Function: maintain structure and function of taste buds

Salivary glands

Location: oral cavity

Structure: exocrine glands

Function: produce and secrete saliva

Saliva

Components: Ions (Na, K, chloride, bicarbonate), immunoglobulin A, lysosome, mucin, lingual lipase, salivary amylase

99.5% water 0.5% solutes

Slightly acidic

Function: assist in initial activities of digestion

Intrinsic salivary glands

Location: within the tongue

Function: secrete lingual lipase, an enzyme that is activated by the low pH levels in the stomach to break down lipids

Serous glands - intrinsic

Structure: pink looking, contain protein and ribosomes, granular appearance

Location: pits of vallate papillae

Function: secrete enzymes

Mucus glands - intrinsic

Location: tongue

Structure: purple in most slides, large frothy empty looking cells, nuclei pushed to the edge, smooth, vesicles filled with mucus

Function: secrete mucus rich material

Extrinsic salivary glands

Location: in the oral cavity not in the tongue

Parotid salivary gland

Extrinsic salivary gland

Structure: largest salivary gland, paired, only serous, prominent nucleus, granular appearance

Location: anterior and inferior to ear

Submandibular salivary gland

Extrinsic salivary gland

Location: inferior to the body of the mandible

Structure: mostly serous some mucous

Sublingual salivary gland

Extrinsic salivary gland

Location: inferior to the tongue

Structure: mostly mucous some serous

Mucous cells

Cell of the salivary glands

Function: secrete mucin which forms mucus upon hydration

Serous cells

Function: secrete a watery fluid containing ions, lysozyme, and salivary amylase

Acini

Structure: round structures formed by mucous and serous cells

Function: collect secretions and connect to a salivary duct

Pharynx

Structure: three parts nasopharynx, oropharynx, laryngopharynx. Nonkeratinized stratified squamous epithelial tissue (oropharynx and laryngopharynx) skeletal muscle in 2 layers (outer circular and inner longitudinal), superior middle and inferior pharyngeal constrictors

Function: facilitates controlled and safe transfer of food from oral cavity to esophagus

Palatine tonsils

Location: bilaterally in the oropharynx

Structure: oval shaped lymphoid tissue, surface stratified squamous epithelial tissue, secondary lymphoid organ

Function: immune surveillance, immune response initiation, antigen capture

Lingual tonsils

Location: posterior 1/3 base of the tongue

Structure: rounded mass of lymphoid tissue, lymphoid follicles covered by stratified squamous epithelial tissue, mucous glands, contain lymph nodules with immune cells

Function: immune protection

Epiglottis

Location: in the pharynx at the entrance

Structure: elastic cartilage covered by mucous membrane, leaf shape, contains glands

Function: protect airway during swallowing

Tunica Mucosa

Location: deepest tunic layer

Structure: 3 layers include epithelial tissue, loose connective tissue = lamina propria, and thin layer of muscle tissue = muscularis mucosae

Function: absorption, protection, secretion, immune defense

Tunica Submucosa

Location: layer superficial to epithelial layer

Structure: either areolar or dense irregular connective tissue, contains blood vessels, nerves, sensory receptors, and lymphatics. May contain small accessory salivary glands

Function: structural support, vascular supply, nerve supply, and glandular secretions

Tunica Muscularis Externa

Structure: 2 layers of smooth muscle tissue except esophagus which has skeletal and smooth and stomach which has 3 layers

Function: generate movement of food necessary for digestion and propulsion

Tunica Adventitia

Location: superficial outer layer

Structure: areolar connective tissue

Function: anchors organ to surrounding tissue and structural support

Tunica Serosa

Location: most superficial layer

Structure: mesothelium and loose connective tissue

Function: support, protection, lubrication

MALT

Mucosa Associated Lymphoid Tissue

Collection of lymphoid nodules located in the lamina propria

Function: 1st line of defense, initiation of immune responses, antibody production

GALT

Gut Associated Lymphoid Tissue

Location : GI tract

Structure: specific MALT for the GI tract

Esophagus

Location: between pharynx and stomach

Structure: thick wall, mucosa = nonkaratinized stratified squamous epithelial tissue, submucosa = thick with elastic fibers and mucous glands, muscularis = skeletal muscle superior 1/3 and skeletal and smooth muscle 2nd 1/3 and smooth muscle last 1/3

Function: transport food from the pharynx to the stomach

Superior esophageal sphincter

Location: where pharynx and esophagus meet

Structure: contracted ring of circular skeletal muscle

Function: prevents air entry, prevents aspiration, and regulates food passage

Inferior esophageal sphincter

Location: where esophagus and stomach meet

Structure: contracted ring of circular smooth muscle

Function: prevents back flow from stomach to esophagus

Esophageal glands

Location: lamina propria and submucosa

Structure: composed of mucus cells

Function: lubrication and protection

Stomach

Location: left upper quadrant of the abdomen, inferior to the diaphragm

Structure: muscular J shaped sac, 3 layers of muscle tissue

Function: mechanical and chemical digestion, produces chyme, and controls rate of food movement

Cardia

Small narrow superior entryway into the stomach from esophagus

Fundus

Dome shaped region lateral and superior to esophageal connection

Body of the stomach

Largest region inferior to cardiac and fundus

Pylorus

Narrow medially directed funnel shape pouch that forms the terminal region for the stomach

Rugae

Location : internally stomach walls

Structure: gastric folds

Function: expansion

Gastric pits

Indented depressions in the mucosa layer

Function: functional gateways

Gastric glands

Location: mucosa layer and extend the length of the mucosa

Structure:

Function: produce gastric juice to aid in digestion and protect the stomach

Chief cells

Location: distal part of the gastric glands in fundic stomach

Structure: dark purple, lots of granules, prominent nuclei

Function: synthesize and secrete enzymes, pepsinogen and lipase

Parietal cells

Location: proximal and middle of gastric glands in fundic stomach

Structure: small intercellular channels = canaliculi, lined with microvilli, pink, large spherical cell, central nuclei

Function: secrete hydrochloric acid and intrinsic factor

Small intestines

Location: between stomach and large intestines

Structure: thin walls coiled tube, innervated by superior metric plexus, venus supply from superior mesenteric artery, 3 segments, columnar epithelial tissue with microvilli and goblet cells, brush border

Function: digestion and absorption

Circular folds/plicae circulares

Location: small intestine

Structure: permanent folds in the wall of the small intestine

Function: increase surface area for absorption

Lacteals

Location: within the villi of the small intestine

Structure: lymphatic capillaries

Function: lipid absorption

Duodenum

Location: 1st segment of the small intestine, retroperitoneal

Structure: arched C shape, contains major duodenal papilla and minor duodenal papilla, Bruner glands in submucosa layer

Function: receives chyme, neutralizes stomach acid, mixes digestive juices, initiates nutrient absorption, peristalsis

Bruner’s glands

Location: proximal duodenum of the submucosa layer

Structure: mucus secreting cells

Function: secrete alkaline rich substance to neutralize stomach acid

Jejunum

Location: middle region of the small intestine, Intraperitoneal, suspended by mesentery proper

Structure: no defining features, columnar epithelial tissue with microvilli and goblet cells

Function: chemical digestion and nutrient absorption

Ileum

Location: last segment of the small intestine, Intraperitoneal, suspended in the abdomen by the mesentery proper

Structure: columnar epithelial tissue with microvilli and goblet cells, contain Peyer’s patches

Function: absorption of nutrients, digestion, immune surveillance

Peyer’s patches

Location: distal ileum in the lamina propria and extent to the submucosa layer

Structure: GALT, large aggregates of lymphatic follicles, B and T cells

Function: immune surveillance

Myenteric plexus

Location: between the circular and longitudinal musclaris externa layers of the small intestine

Structure: irregular patch of cells, synapse of parasympathetic nervous system, post ganglionic axons from sympathetic nervous system

Function: control involuntary contraction of the smooth muscle in the small intestines, peristalsis, mixing movements, and regulating sphincters

Submucosal plexus

Location: submucosa layer of small intestine

Structure: dense irregular CT, patch of cells, large nuclei, neuron cell bodies, parasympathetic synapse, post ganglionic neurons of sympathetic nervous system

Function: control secretion of glands

Paneth cells

Location: tunica mucosa at base of intestinal crypt/pit of small intestine

Structure: red cells, specialized columnar cells

Function: produce antimicrobial compounds

Duodenal papilla

Location: inner wall of the duodenum

Structure: one or two small raised projections, openings, main and minor, confluence of the common bile duct and main pancreatic duct, surface covered in duodenal mucosa

Function: gateway for release of bile and pancreatic juices

Ileocecal valve

Location: Lower right quadrant of the abdomen, point where the terminal ileum joins the cecum

Structure: valve with mucosal folds and a muscular sphincter,

Function: regulates flow of chyme from duodenum to cecum and prevents backflow

Cecum

Location: initial region of the large intestine, lower right abdominal quadrant, intraperitoneal

Structure: blind sac

Function: receives chyme from the small intestine

Large intestine

Location: connected to the small intestine and terminates at the anus, Intraperitoneal = cecum, transverse colon, descending colon, sigmoid colon Retroperitoneal = ascending colon

Structure: cecum at start, ascending colon, transverse colon, descending colon, sigmoid colon at end, simple columnar epithelium with goblet cells,

Function: reabsorption of H2O, packaging and elimination of waste

Teniae coli

Location: longitudinal smooth muscle of the muscularis externa layer of the large intestine

Structure: 3 thin distinct bundles of smooth muscle

Function: act like elastic bands to help bunch up the large intestine into many sacs

Haustra

Location: longitudinal layer of the muscularis externa of the large intestine

Structure: smooth muscle bunches of teniae coil

Function: absorption of water and electrolyte and slow propulsion of feces

Epiploic appendages/omental appendages

Location: along the colon

Structure: fat filled pouches of peritoneum

Function: energy storage, temperature regulation, cushioning

Vermiform appendix

Location: lower right quadrant of the abdomen attached to the cecum of the large intestine, intraperitoneal

Structure: small finger like projection, narrow blind ended tube, composed of all 4 layers, lined with lymphoid nodules

Function: immunity

Liver

Location: right upper quadrant of the abdomen, intraperitoneal

Structure: 4 lobes, supported by 2 ligaments

Histology: hepatic lobules

Function: detoxifies blood, produces bile, synthesize plasma proteins, regulates blood sugar

Portal triads

Location: at the periphery of each lobule in the liver

Structure: branches of: hepatic portal vein, common hepatic artery, and hepatic ductus

Function: facilitate the liver’s role as a central processing and detoxifying organ

Common bile duct

Location: extraheptic duct outside the liver

Structure: forms from the convergence of the common hepatic duct and the cystic duct

Function: transport bile from the liver and gallbladder to the duodenum

Common hepatic duct

Location: extraheptic duct outside the liver

Structure: formed by the junction of the right and left hepatic ducts and ends with its junction with the cystic duct to become the common bile duct

Function: transport bile produced by the liver

Common hepatic artery

Location: originates at the celiac trunk, enters the hepatosuodenal ligament and lies to the left of the common bile duct and anterior to the hepatic portal vein

Structure: tunica intima, tunica media, tunica adventitia

Function: deliver oxygenated blood to the liver, gallbladder, stomach, duodenum, pancreas

Hepatic portal vein

Location: upper right quadrant of the abdomen

Structure: formed by the confluence of the superior and inferior mesenteric vein and splenic vein, tunica intima, tunica media, tunica adventitia

Function: transports nutrient rich blood from digestive organs to the liver

Hepatocytes

Location: liver

Structure: large polygonal epithelial cells, large centrally located nuclei, pink when stained, cytoplasm appears granular

Function: maintain blood glucose levels, synthesize lipids, synthesize plasma proteins, detoxify blood, produce bile, store glycogen

Sinusoids

Location: between hepatocytes in the liver

Structure: highly specialized capillaries, discontinuous endothelium, lack of basement membrane, lined with endothelial cells and kupffer cells

Function: blood filtration and immune surveillance, regulation of blood flow and pressure, efficient exchange of substances

Epithelial cells of the sinusoid

Location: bile canaliculi of the liver

Structure: specialized endothelial cell

Function: highly permeable for substance exchange

Kupffer cells

Location: lumina of the hepatic sinusoid of the liver attached to liver sinusoidal endothelial cells

Structure: amoeboid shaped, irregular shape with projections, ovoid indented or lobular nuclei,

Function: macrophage cells, phagocytosis, blood filtration, immune surveillance

Bile canaliculi

Location: between adjacent heparocytes

Structure: blind ended lumen formed by the apposition of the y membranes of adjoining hepatocytes

Function: initiate formation and secretion of bile

Central vein

Location: the center of each lobule of the liver

Structure: thin walls, endothelial lining,

Function: drain blood from the liver

Pancreas

Location: Retroperitoneal, extends horizontally from the medial edge of the duodenum toward the left side of the abdominal cavity to touch the spleen

Structure: exhibits endocrine and exocrine functions, wide head, elongated body, tail

Histology: simple cuboidal epithelial cells called acinar cells organized into clusters

Function: secrete digestive enzymes to the duodenum and insulin and glycogen for blood sugar regulation

Serous secretory units

Location: pancreas

Structure: exocrine cells

Function: secrete digestive enzymes and bicarbonate to duodenum

Pancreatic islets / islets of Langerhans

Location: pancreas

Structure: endocrine cells

Function: secrete insulin and glycogen for blood sugar regulation

Pancreatic ducts

Location: pancreas

Structure: ducts

Function: transport pancreatic juices to the duodenum through the major and minor duodenal papilla

Gallbladder

Location: attached to the inferior surface of the liver

Structure: sac like accessory organ, 3 parts fundus, body, and neck

Function: stores bile

Cystic duct

Location: right upper quadrant of the abdomen connecting the neck of the gallbladder to the common hepatic duct

Structure: short narrow tube, mucosal lining, smooth muscle layer, fibrous connective tissue layer

Function: bile flow to the gallbladder for storage and concentration and bile flow from the gallbladder when fatty foods enter the duodenum