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HUMAN ANATOMY
LECTURE NINETEEN
DIGESTIVE SYSTEM
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
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)
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
DIGESTIVE TRACT TUNICS
Digestive 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
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.
PERITONEUM and MESENTARIES
Peritoneum - 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
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
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
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
• 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
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
SALIVARY GLANDS
Three 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
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
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
• 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
• 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
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
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
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
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
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
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
• 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)
• 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
BLOOD AND BILE FLOW THROUGH THE LIVER
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
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
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
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
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