human anatomy lecture nineteen digestive system. general anatomy composed of a muscular tube - the...
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HUMAN ANATOMY
LECTURE NINETEEN
DIGESTIVE SYSTEM
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GENERAL ANATOMY• Composed of a muscular tube - the
digestive tract or gastrointestinal tract or alimentary canal
• Digestive organs- mouth or oral cavity with
salivary glands and tonsils- pharynx (throat) - esophagus- stomach- small intestine (duodenum,
ilium, jejunum)- liver, gall bladder and pancreas
(accessory organs)- large intestine (cecum,
colon, rectum and anal canal)- anus
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DIGESTIVE SYSTEM FUNCTIONS• Ingestion - introduction of food into stomach
• Digestion - physical digestion (chewing, mixing)
- chemical digestion (enzymes)
• Propulsion - deglutition (swallowing)
- peristalsis (movement of material of digestive tract by
waves of smooth muscle contraction and relaxation)
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FUNCTIONS cont…
• Secretion - mucus (throughout tract to lubricate food, coating and protecting
lining from chemicals)
- bile (emulsifies fats)
- enzymes (chemical digestion)
- water (liquefaction makes digestion and absorption easier)
• Absorption - movement of nutrients from digestive tract into circulation or
lymph
• Elimination - waste products (feces) removed from body by defecation
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DIGESTIVE TRACT TUNICSDigestive tract walls are composed of:
(1) Mucosa• Innermost layer of mucous
epithelium • Irregular connective tissue (lamina
propria) and a thin smooth muscle layer (muscularis mucosa)
• Thickened (stratified squamous epithelium) in mouth, esophagus, anus to resist abrasion
• Thinner in intestine for secretion and absorption
(2) Submucosa• Thick C.T. layer with nerves, blood vessels, small glands• Parasympathetic nerve plexus
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TUNICS cont…
(3) Muscularis• 2 or 3 layers of smooth muscle
(inner circular and outer longitudinal muscle)
• Muscle movements controlled by enteric nervous system - control movement and secretion
(4) Serosa or Adventita• Outermost layer
• Digestive tract covered by serosa - called visceral peritoneum (smooth epithelial layer)
• Rest of tract is surrounded by adventitia - C.T. layer continuous with surrounding C.T.
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PERITONEUM and MESENTARIESPeritoneum - lubricating serous membranes• Visceral - covers organs• Parietal - covers interior surface of body
wall• Retroperitoneal - covers organs that are
not within the peritoneal cavity eg: kidneys, pancreas, duodenum
Mesentaries - tissue holding organs in place• Two layers of serous membranes with
loose connective tissue in between• Lesser omentum - connects stomach to
liver and diaphragm• Greater omentum - connects stomach to
transverse colon and posterior body wall• Mesocolon - supports colon• Falciform ligament - stabilizes liver to
diaphragm and abdominal wall
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REGIONS OF THE DIGESTIVE SYSTEM
ORAL CAVITY• Surrounded anteriorly by lips and
posteriorly by fauces (opening into pharynx)
• Vestibule - space between lips/cheek and alveolar processes with teeth
• Lined with moist stratified squamous epithelium
LIPS (labia)• Orbicularis oris muscle within lips
• Keratinized stratified squamous exterior is thin so blood vessels give pink color
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CHEEKS• Lateral walls of oral cavity
• Contain buccinator muscle and buccal fat pad
PALATES• Roof of oral cavity
• Hard palate contains maxilla and palatine bone
• Soft palate contains skeletal muscle and connective tissue
TONSILS• Lateral posterior walls of oral cavity in nasopharynx and posterior surface of
tongue
TONGUE• Muscular with free anterior surface and attached posterior by lingual frenulum
• Covered with moist stratified squamous epithelium
• Intrinsic muscles - change its shape
• Extrinsic muscles - protrude or retract tongue, move side to side
• Terminal sulcus - groove dividing tongue into anterior 2/3 and posterior 1/3
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TONGUE cont…
• Anterior surface has papillae with taste buds
• Functions to move food in mouth, speech and swallowing
TEETH• Two sets:
- primary (milk teeth) children 20
- permanent (secondary) adults 32
• Types:
- incisors, canines, premolars, molars
• Function in mastication and speech
• Anatomical crown - enamel covered area of tooth
• Clinical crown - above gum line
• Neck - enameled part of tooth below gum line
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• Pulp cavity - center of tooth• Pulp - fills pulp cavity
- nerves, blood vessels, connective tissue• Dentin - living, cellular, calcified
tissue surrounding pulp cavity• Enamel - covers dentin in the crown - nonliving, acellular
- protects against abrasion• Cementum - covers surface of
dentin and anchors tooth in jaw• Periodontal ligaments - hold tooth
in socket• Teeth are rooted within alveoli
(sockets) along alveolar ridges within mandible and maxillae
• Gingiva - dense, fibrous connective tissue with moist squamous stratified epithelium that line the alveoli
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SALIVA• Compound alveolar salivary glands that
produce saliva
• Mixture of serous (watery) and mucous (contains enzymes) fluids
• Functions to:
- prevent bacterial infection
- lubrication
- contains salivary amylase that breaks
starch down into maltose
- helps form bolus for swallowing
• Parasympathetic input causes salivary production
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SALIVARY GLANDSThree pairs of salivary glands:(1) Parotid • largest, serous gland found just anterior to ear• enter oral cavity behind 2nd upper molar(2) Submandibular • mixed (mostly serous) glands found just under mandible• duct enters oral cavity on either side of lingual frenulum(3) Sublingual • mixed (mostly mucus) glands below tongue• each has 10-12 ducts
entering floor of oral cavity
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PHARYNX (throat)• Consists of nasopharynx, oropharynx, and laryngopharynx
• Posterior walls of oropharynx and laryngopharynx contain pharyngeal constrictor muscles that help with swallowing
ESOPHAGUS• Muscular tube that transports food to stomach
• Passes through esophageal hiatus (opening) of diaphragm
• Has upper and lower sphincters
- lower is often called cardiac sphincter
• Mucosa is moist stratified squamous epithelium that produces thick layer of mucus
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STOMACH• Openings
- gastropharyngeal (cardiac)
from esophagus
- pyloric to duodenum
• Regions
- cardiac where esophagus opens
- fundus is very top area
- body is main portion
- pyloric where duodenum opens
• Greater and lesser curvatures form the J-shape
• Sphincters - thick ring of smooth muscle
- cardiac sphincter from esophagus
- pyloric sphincter to duodenum
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• Layers
- serosa or visceral peritoneum is outermost layer
- muscularis (3 muscle layers)
- outer longitudinal
- middle circular
- inner oblique
- submucosa
- mucosa lined with gastric pits
• Rugae - folds of mucosa and submucosa that allow stomach to expand
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• Gastric pits are openings for gastric glands
• Cells of gastric pits:
- surface mucus cells secretes mucus
- parietal cells secrete HCl and intrinsic factor
- chief cells secrete pepsinogen
- endocrine cells secrete regulatory hormones
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STOMACH SECRETIONS
Gastric juice contains:
• Chyme - the ingested food plus stomach secretions
• Mucus - protects from acid and enzyme pepsin
- irritation of stomach mucosa causes greater mucus secretion
• Intrinsic factor - binds with vitamin B12 and helps it to be absorbed
• HCl - kills bacteria
- denatures proteins and inactivates enzymes in food
- breaks down plant cell walls and connective tissue in meat
- converts pepsinogen to pepsin
• Pepsinogen - converted into pepsin (functioning enzyme) that catalyzes breaking of covalent bonds in proteins
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SMALL INTESTINE• Consists of three sections:
duodenum, jejunum, ileum
DUODENUM• 1st 25 cm that curves to the left with
the pancreas head within the curve
• Common bile duct from liver and
pancreatic duct from pancreas enter
• Modifications to wall increase the surface area
- plicae circulares (circular folds)
- villi (folds of the mucosa)
containing capillaries and lacteals
- microvilli are folds of cell
membranes of the absorptive cells
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DUODENUM cont…Four kinds of cells within mucosa
(1) Absorptive cells
- cells with microvilli
- produce digestive enzymes
- absorb digested nutrients
(2) Goblet cells
- produce protective mucus
(3) Granular cells (Paneth’s cells)
- may help protect against bacteria
(4) Endocrine cells
- produce regulatory hormones
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Intestinal glands (crypts of Lieberkuhn)
- tubular glands in mucosa at base of
villi (granular and endocrine cells)
Duodenal glands (Brunner’s glands)
- tubular mucus glands in submucosa
opening into intestinal glands
Villi• covered with simple columnar epithelium
• ‘brush border’ of microvilli
• extensive capillary network
• each contains a lymphatic lacteal
• muscle contractions expose villi to digested material – improving absorption
• number decreases as move through small intestine
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JEJUNUM and ILEUM• Similar in structure to duodenum except for
decrease in:
- diameter
- thickness of wall
- number of circular folds
- number of villi as get further from stomach
• Main site of nutrient absorption
• Peyer’s patches - clusters of lymph nodules numerous in mucosa and submucosa of the ileum
• Ileocecal junction - where ileum meets large intestine with ileocecal sphincter and ileocecal valve allowing one-way movement of food
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LIVER• Found in right upper quadrant of
abdomen against inferior surface of diaphragm
• Divided into 2 major lobes (right and left lobe) separated by a septum (falciform ligament)
• 2 minor lobes, inferior, the caudate and quadrant
• Porta - located inferiorly - hepatic portal vein, hepatic artery,
hepatic nerve plexus, lymphatic vessels enter and exit • Right and left hepatic ducts
- transport bile- combine to form common hepatic duct joined by the cystic duct (from gall bladder) to form the common bile duct opening into the duodenum at the duodenal papilla
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• Septa divide liver into hexagonal lobules – porta follow branching
- each with a portal triad in corners
• Portal triads contain:
- hepatic artery
- hepatic portal vein
- hepatic duct
• Central vein runs through each
lobule and connects to form the hepatic vein that exits liver
• Hepatic cords radiate out from the central canal
- made up of functional liver cells (hepatocytes)
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• Bile canaliculi is a lumen (canal) between cells in the cord
- bile produced by hepatocytes runs
through canaliculus toward triad to
exit through hepatic ducts
• Hepatic sinusoids are blood channels that separate hepatic cords
- contain phagocytic cells that remove foreign particles from blood
- blood from hepatic portal vein and hepatic artery and mix within hepatic sinusoids
- mixed blood flows toward
center of the lobule into the
central vein
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BLOOD AND BILE FLOW THROUGH THE LIVER
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LIVER FUNCTIONS• Bile production
- bile salts emulsify fats
• Storage
- glycogen, fat, vitamins, Cu, Fe
• Nutrient conversion
- amino acids to energy producing compounds
- hydroxylation of vit D (vit D travels to kidney where it is activated and absorbed)
• Detoxification
- hepatocytes remove ammonia and convert to urea
- drug inactivation
- removal hormones and antibodies from bloodstream
- removal of toxins
• Synthesis
- albumins, fibrinogen, globulins, heparin, clotting factors
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GALLBLADDER• Sac lined with mucosa folded into rugae, inner muscularis, outer serosa
• Bile arrives constantly from liver is stored and concentrated
• Bile exits through cystic duct then into common bile duct emptying into duodenal ampulla (pouch) and then into duodenum through duodenal papilla
PANCREAS• Has head, body and tail regions
• Composed of both exocrine and endocrine tissue
• Endocrine - pancreatic islets that produce insulin, glucose, and somatostatin
• Exocrine - produces digestive enzymes and buffers
- groups of acini (exocrine cells) form lobules separated by septa
- secretions drain into pancreatic duct
- pancreatic duct joins the common bile duct, entering the duodenal
ampulla and then the duodenal papilla
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LARGE INTESTINE• Extends from ileocecal junction to
anus
• Consists of cecum, colon, rectum, anal canal
• Movement is sluggish while chyme is converted to feces
• Absorption of water, salts, secretion of mucus, extensive action of microorganisms (approx 90% volume is reabsorbed)
CECUM• Where small and large int. join
• Blind sac with veriform appendix attached
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COLON• Ascending, transverse, descending,
sigmoid regions
• Circular muscle layer is complete, longitudinal muscle layer is incomplete (teniae coli)
• Contractions of teniae form pouches called haustra
• Fat filled pouches called epiploic appendages on surface
• Mucosa has numerous straight tubular glands called crypts
- mainly goblet cells, but also
absorptive and granular cells
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RECTUM• Straight muscular tube with thick muscular tunic
• Anal canal - superior epithelium simple columnar, inferior epithelium stratified squamous (for protection)
- internal anal sphincter (smooth muscle)
- external anal sphincter (skeletal muscle)
SECRETIONS OF LARGE INTESTINE• Mucus provides protection - parasympathetic stimulation increases rate of
goblet cell secretion
• Bacterial actions produce gases (flatus) from particular kinds of carbohydrates found in legumes and artificial sugars
• Bacteria produce vit K which is absorbed
• Feces consists of water, undigested food (cellulose), microorganisms, sloughed-off epithelial cells, dead red blood cells
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