0-1 Chapter 25 - digestive system Flashcards
gastroenterology
the study of the digestive tract and the diagnosis and treatment of its disorders
digestive system
the organ system that processes food, extracts nutrients from it, and eliminates the residue
five stages of digestion
ingestion digestion absorption- compaction defecation
ingestion
selective intake of food
digestion
mechanical and chemical breakdown of food into a form usable by the body
absorption-
uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph
compaction
absorbing water and consolidating the indigestible residue into feces
defecation
elimination of feces
mechanical digestion
the physical breakdown of food into smaller particles
–cutting and grinding action of the teeth
–churning action of stomach and small intestines
–exposes more food surface to the action of digestive enzymes
chemical digestion
aseries of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
–carried out by digestive enzymes produced by salivary glands, stomach, pancreas and small intestine
results:
- polysaccharides into monosaccharides
- proteins into amino acids
- fats into monoglycerides and fatty acids
- nucleic acids into nucleotides
some nutrients are present in a usable form in ingested food
–absorbed without being digested
–vitamins, free amino acids, minerals, cholesterol, and water
digestive system has two anatomical subdivisions
digestive tract
gastrointestinal
digestive tract
(alimentary canal)
–30 foot long muscular tube extending from mouth to anus
–mouth, pharynx, esophagus, stomach, small intestine, and large intestine
gastrointestinal (GI) tract
is the stomach and intestines
accessory organs
teeth, tongue, salivary glands, liver, gallbladder, and pancreas
General Anatomy
•digestive tract is open to the environment at both ends
•most material in it has not entered the body tissues
–is considered to be external to the body until it is absorbed by the epithelial cells of the alimentary canal
•in a strict sense, defecated food residue was never in the body
basis structural plan
most of the digestive tract follows the basis structural plan with digestive tract wall consisting of the following tissue layers, in order from inner to outer surface
basis structural plan
order
•mucosa –epithelium –lamina propria –muscularis mucosae •submucosa •muscularis externa –inner circular layer –outer longitudinal layer •serosa –areolar tissue –mesothelium
mucosa
(mucous membrane) –lines the lumen and consists of:
–inner epithelium
–lamina propria –
–muscularis mucosa
–mucosa-associated lymphatic tissue (MALT) –
inner epithelium
- simple columnar in most of digestive tract
* stratified squamous from mouth through esophagus, and in lower anal canal
lamina propria
loose connective tissue layer
muscularis mucosa
thin layer of smooth muscle
•tenses mucosa creating grooves and ridges that enhance surface area and contact with food
•improves efficiency of digestion and nutrient absorption
mucosa-associated lymphatic tissue (MALT)
the mucosa exhibits an abundance of lymphocytes and lymphatic nodules
submucosa
thicker layer of loose connective tissue
–contains blood vessels, lymphatic vessels, a nerve plexus, and in some places mucus secreting glands that dump lubricating mucus into the lumen
–MALT extends into the submucosa in some parts of the GI tract
muscularis externa
consists of usually two layers of muscle near the outer surface
inner circular layer
•in some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract
outer longitudinal layer
•responsible for the motility that propels food and residue through the tract
serosa
composed of a thin layer of areolar tissue topped by simple squamous mesothelium
–begins in the lower 3 to 4 cm of the esophagus
–ends just before the rectum
adventitia
a fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into the adjacent connective tissue of other organs
enteric nervous system
a nervous network in the esophagus, stomach, and intestines that regulated digestive tract motility, secretion, and blood flow
–thought to have over 100 million neurons
–more than the spinal cord
–functions completely independently of the central nervous system
•CNS exerts a significant influence on its action
composed of two networks of neurons
submucosal (Meissner) plexus
myenteric (Auerbach) plexus
submucosal (Meissner) plexus
in submucosa
•controls glandular secretion of mucosa
•controls movements of muscularis mucosae
myenteric (Auerbach) plexus
parasympathetic ganglia and nerve fibers between the two layers of the muscularis externa
•controls peristalsis and other contractions of muscularis externa
enteric nervous system contains sensory neurons that
monitor tension in gut wall and conditions in lumen
mesenteries
connective tissue sheets that loosely suspend the stomach and intestines from the abdominal wall
–allows stomach and intestines to undergo strenuous contractions
–allow freedom of movement in the abdominal cavity
–hold abdominal viscera in proper relationship to each other
–prevents the intestines from becoming twisted and tangled by changes in body position and by its own contractions
–provides passage of blood vessels and nerves that supply digestive tract
–contain many lymph nodes and lymphatic vessels
parietal peritoneum
a serous membrane that lines the wall of the abdominal cavity
–turns inward along posterior midline
–forms dorsal mesentery –a translucent two-layered membrane extending to the digestive tract
–the two layers of the mesentery separate and pass around opposite sides of the organ forming the serosa
–come together on the far side of the organ and continue as another sheet of tissue –the ventral mesentery
•may hang freely in the abdominal cavity
•may attach to the anterior abdominal wall or other organs
lesser omentum
a ventral mesentery that extends from the lesser curvature of the stomach to theliver
greater omentum
hangs from the greater curvature of the stomach
–covers the small intestines like an apron
–the inferior margin turns back on itself and passes upward
–forming a deep pouch between its deep and superficial layers
–inner superior margin forms serous membranes around the spleen and transverse colon
mesocolon
extension of the mesentery that anchors the colon to the posterior abdominal wall
intraperitoneal
when an organ is enclosed by mesentery on both sides
–considered within the peritoneal cavity
–stomach, liver, and other parts of small and large intestine
retroperitoneal
when an organ lies against the posterior body wall and is covered by peritoneum on its anterior side only
–considered to be outside the peritoneal cavity
–duodenum, pancreas, and parts of the large intestine
motility and secretion of the digestive tract are controlled by
neural, hormonal, and paracrine mechanisms
neural control
short (myenteric) reflexes
long (vagovagal) reflexes
short (myenteric) reflexes
stretch or chemical stimulation acts through myenteric plexus
•stimulates peristaltic contractions of swallowing
long (vagovagal) reflexes
parasympathetic stimulation of digestive motility and secretion
hormones
–chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract
–gastrin and secretin
paracrine secretions
–chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells
The Mouth
the mouth is known as the oral, or buccal cavity
functions include:
–ingestion (food intake)
–other sensory responses to food –chewing and chemical digestion
–swallowing, speech, and respiration
mouth enclosed by
cheeks, lips, palate, and tongue
oral fissure
anterior opening between lips
fauces
posterior opening to the throat
stratified squamous epithelium
lines mouth
–keratinized in areas subject to food abrasion –gums and hard palate
–nonkeratinizedin other areas –floor of mouth, soft palate, and inside of cheek and lips
cheeks and lips
–retain food and push it between the teeth for chewing
–essential for articulate speech
–essential for sucking and blowing actions, including suckling by infants
–fleshiness due to subcutaneous fat, buccinator muscle of the cheek, and the orbicularis oris of the lips
labial frenulum
median fold that attaches each lip to the gum between the anterior incisors
vestibule
the space between cheek or lips and the teeth
lips divided into three areas:
cutaneous area
cutaneous area
colored like the rest of the face
•has hair follicles and sebaceous glands
red (vermillion) area
hairless region where lips meet
•tall dermal papilla that allows blood vessels and nerves to come closer to epidermal surface
•redder and more sensitive than cutaneous area
labial mucosa
the inner surface of the lips facing the gums and teeth
tongue
muscular, bulky, but remarkably agile and sensitive organ
–manipulates food between teeth while it avoids being bitten
–can extract food particles from the teeth after a meal
–sensitive enough to feel a stray hair in a bite of food
nonkeratinized stratified squamous epithelium covers
its surface
lingual papillae
bumps and projections on the tongue that are the sites of the taste buds
body
anterior two-thirds of the tongue occupies oral cavity
root
posterior one-third of the tongue occupies the oropharynx
vallate papillae
a V-shaped row of papillae that mark the boundary between the body and root of the tongue
terminal sulcus
groove behind the V-shaped vallate papillae
lingual frenulum
median fold that attaches the body to the floor of the mouth
intrinsic muscles
are contained entirely within the tongue
•produce the subtle tongue movements of speech
extrinsic muscles
with origins elsewhere and insertions in the tongue
•produce stronger movements of food manipulation
•genioglossus, hyoglossus, palatoglossus, and styloglossus
lingual glands
serous and mucous glands amid the extrinsic muscles
•secrete a portion of the saliva
lingual tonsils
contained in the root
palate
separates the oral cavity from the nasal cavity
–makes it possible to breathe while chewing food
hard (bony) palate
anterior portion that is supported by the palatine processes of the maxillaeand the palatinebones
palatine rugae
transverse ridges that help the tongue hold and manipulate food
soft palate
posterior with a more spongy texture
–composed of skeletal muscle and glandular tissue
–no bone
uvula
conical medial projection visible at the rear of the mouth
–helps retain food in the mouth until one is ready to swallow
pair of muscular arches on each side of the oral cavity
–palatoglossal arch –anterior arch
–palatopharyngeal arch –posterior arch
–palatine tonsils are located on the wall between the arches
dentition
the teeth
masticate food into smaller pieces
–makes food easier to swallow
–exposes more surface area for action of digestive enzymes speeding chemical digestion
32 adult teeth –20 deciduous (baby) teeth
–16 in mandible
–16 in maxilla
from midline to the rear of each jaw
- 2 incisors–chisel-like cutting teeth used to bite off a piece of food
- 1 canine–pointed and act to puncture and shred food
- 2premolars –broad surface for crushing and grinding
- 3 molars–even broader surface for crushing and grinding
alveolus
tooth socket in bone
gomphosis joint
between tooth and bone
periodontal ligament
modified periosteum whose collagen fibers penetrate into the bone on one side and into the tooth on the other
–anchors tooth firmly in alveolus
–allows slight movement under pressure of chewing
gingiva (gum)
covers the alveolar bone
regions of a tooth
crown
root
neck
gingival sulcus
crown
portion above the gum
root
the portion below the gum, embedded in alveolar bone
neck
the point where crown, root, and gum meet
gingival sulcus
space between the tooth and the gum
•hygiene in the sulcus in important to dental health
dentin
hard yellowish tissue that makes up most of the tooth
enamel
covers crown and neck
cementum
covers root
•cementum and dentin are living tissue and can regenerate
•enamel is noncellular secretion formed during development
root canal
in the roots leading to pulp cavity in the crown
–nerves and blood vessels
apical foramen
pore at the basal end of each root canal
occlusion
meeting of the teeth with the mouth closed
Deciduous Teeth
20 deciduous teeth (milk teeth or baby teeth)
•teeth develop beneath the gums and erupt in a predictable order
–erupt from 6 to 30 months
–beginning with incisors
–between 6 and 32 years of age, are replaced by 32 permanent teeth
(wisdom teeth
third molars (wisdom teeth) erupt from 17 –25 –may be impacted–crowded against neighboring teeth and bone so they cannot erupt
Tooth and Gum Disease
the human mouth is home to more than 700 species of microorganisms, especially bacteria
plaque
sticky residue on the teeth made up of bacteria and sugars
–calculus –calcified plaque
–bacteria metabolize sugars and release acids that dissolve the minerals of enamel and dentin to form dental caries (cavities
root canal therapy
is necessary if cavity reaches pulp
calculus
in the gingival sulcus wedges the tooth and gum apart
–allows bacterial invasion of the sulcus
–gingivitis–inflammation of the gums
–periodontal disease –destruction of the supporting bone around the teeth which may result in tooth loss
mastication(chewing)
breaks food into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes
mastication
process
–first step in mechanical digestion
–food stimulates oral receptors that trigger an involuntary chewing reflex
–tongue, buccinator, and orbicularis oris manipulate food
–masseter and temporalis elevate the teeth to crush food
–medial and lateral pterygoids, and masseter swing teeth in side-to-side grinding action of molars
saliva
–moisten mouth
–begin starch and fat digestion
–cleanse teeth
–inhibit bacterial growth
–dissolves molecules so they can stimulate the taste buds
–moistens food and binds it together into bolus to aid in swallowing
Saliva
contents
hypotonic solution of 97.0% to 99.5% water and the following solutes:
–salivary amylase –enzyme that begins starch digestion in the mouth
–lingual lipase –enzyme that is activated by stomach acid and digests fat after the food is swallowed
–mucus–binds and lubricates the mass of food and aids in swallowing
–lysozyme–enzyme that kills bacteria
–immunoglobulin A (IgA) –an antibody that inhibits bacterial growth
–electrolytes-Na+, K+, Cl-, phosphate and bicarbonate
saliva
ph
pH of 6.8 to 7.0
intrinsic salivary glands
small glands dispersed amid other oral tissues
–lingual glands –in the tongue –produce lingual lipase
–labial glands –inside of the lips
–buccal glands –inside of the cheek
–all secrete saliva at a fairly constant rate
extrinsic salivary glands
three pairs connected to oral cavity by ducts
–parotid
–submandibular gland
–sublingual glands –
parotid
–located beneath the skin anterior to the earlobe
•mumps is an inflammation and swelling of the parotid gland caused by a virus
submandibular gland
–located halfway along the body of the mandible
•its duct empties at the side of the lingual frenulum, near the lower central incisors
sublingual glands
–located in the floor of the mouth
•has multiple ducts that empty posterior to the papilla of the submandibular duct
Histology of Salivary Glands
•compound tubuloacinar glands
–branched ducts ending in acini
•mucous cells secrete mucus
•serous cells secrete thin fluid rich in amylase and electrolytes
•mixed acinus has both mucous and serous cells
Salivation
extrinsic salivary glands secrete about of 1 to 1.5 L of saliva per day
cells of acini
filter water and electrolytes from blood and add amylase, mucin, and lysozyme
salivary nuclei
in the medulla oblongata and pons respond to signals generated by presence of food
–tactile, pressure, and taste receptors
–salivary nuclei receive input from higher brain centers as well
•odor, sight, thought of food stimulates salivation
–send signals by way of autonomic fibers in the facial and glossopharyngeal nerves to the glands
parasympathetics
stimulate the glands to produce an abundance of thin, enzyme-rich saliva
sympathetic stimulation
stimulates the glands to produce less, and thicker saliva with more mucus
bolus
mass swallowed as a result of saliva binding food particles into a soft, slippery, easily swallowed mass
pharynx
a muscular funnel that connects oral cavity to esophagus and allows entrance of air from nasal cavity to larynx
–digestive and respiratory tracts intersect
pharyngeal constrictors
(superior, middle, and inferior) -circular muscles that force food downward during swallowing
–when not swallowing, the inferior constrictor remains contracted to exclude air from the esophagus
–this constriction is considered to be the upper esophageal sphincter although it is not an anatomical feature
–disappears at the time of death when the muscles relax, so it is a physiological sphincter, not an anatomical structure
esophagus
a straight muscular tube 25-30 cm long
–begins at level between C6 and the cricoid cartilage
–extends from pharynx to cardiac orifice of stomach passing through esophageal hiatus in diaphragm
lower esophageal sphincter
food pauses at this point because of this constriction
•prevents stomach contents from regurgitating into the esophagus
•protects esophageal mucosa from erosive effect of the stomach acid
heartburn
burning sensation produced by acid reflux into the esophagus
esophagus
tissue type
–nonkeratinized stratified squamous epithelium
–esophageal glands in submucosa secrete mucus
–deeply folded into longitudinal ridges when empty
–skeletal muscle in upper one-third, mixture in middle one-third, and only smooth muscle in the bottom one-third
–meets stomach at level of T7
–covered with adventitia
swallowing
(deglutition) –a complex action involving over 22 muscles in the mouth, pharynx, and esophagus
swallowing center
pair of nuclei in medulla oblongata that coordinates swallowing
•communicates with muscles of the pharynx and esophagus by way of trigeminal, facial, glossopharyngeal, and hypoglossal nerves
swallowing occurs in two phases:
buccal phase
pharyngoesophageal phase
buccal phase
under voluntary control
•tongue collects food, presses it against the palate forming a bolus, and pushes it posteriorly
•food accumulates in oropharynx in front of the „blade‟ of the epiglottis
•epiglottis tips posteriorly and food bolus slides around it
•bolus enters laryngopharynx and stimulates tactile receptors and activates next phase
pharyngoesophageal phase
•three actions prevent food and drink from reentering the mouth or entering the nasal cavity or larynx
–the root of the tongue blocks the oral cavity
–the soft palate rises and blocks the nasopharynx
–the infrahyoid muscles pull the larynx up to meet the epiglottis while laryngeal folds close the airway
•food bolus is driven downward by constriction of the upper, then middle, and finally the lower pharyngeal constrictors
•bolus enters esophagus, stretches it, and stimulates peristalsi
peristalsis
wave of muscular contraction that pushes the bolus ahead of it
–an entirely involuntary reflex
stomach
a muscular sac in upper left abdominal cavity immediately inferior to the diaphragm
–primarily functions as a food storage organ
•internal volume of about 50 mL when empty
•1.0 –1.5 L after a typical meal
•up to 4 L when extremely full and extend nearly as far as the pelvis
stomach
function
mechanically breaks up food particles, liquefies the food, and begins chemical digestion of protein and fat
–chyme –soupy or pasty mixture of semi-digested food in the stomach
•most digestion occurs after the chyme passes on to the small intestine
chyme
soupy or pasty mixture of semi-digested food in the stomach
stomach
divided into four regions
cardiac region (cardia) fundic region (fundus body (corpus)
cardiac region (cardia)
small area within about 3 cm of the cardiac orifice
fundic region (fundus
dome-shaped portion superior to esophageal attachment
body (corpus)
makes up the greatest part of the stomach
pyloric region
narrower pouch at the inferior end
–subdivided into the funnel-like antrum
–and narrower pyloric canal that terminates at pylorus
–pylorus –narrow passage to duodenum
pyloric (gastroduodenal) sphincter
regulates the passage of chyme into the duodenum
stomach receives:
–parasympathetic fibers from
vagus
stomach receives:
sympathetic fibers from
celiac ganglia
supplied with blood by branches of the
celiac trunk
all blood drained from stomach and intestines enters
hepatic portal circulation and is filtered through liver before returning to heart
stomach
simple columnar epithelium covers
mucosa
–apical regions of its surface cells are filled with mucin
–swells with water and becomes mucusafter it is secreted
gastric rugae
mucosa and submucosa flat when stomach is full, but form longitudinal wrinkles called gastric rugae when empty
muscularis externa
has three layers instead of two
•outer longitudinal, middle circular and inner oblique layers
gastric pits
depressions in gastric mucosa
–lined with simple columnar epithelium
–two or three tubular glands open into the bottom of each gastric pit
cardiac glands
in cardiac region
pyloric glands
in pyloric regions
gastric glands
in the rest of the stomach
Cells of Gastric Glands
mucous cells
mucous cells
secrete mucus
–predominate in cardiac and pyloric glands
–in gastric glands, called mucous neck cells since they are concentrated at the neck of the gland
regenerative (stem) cells
found in the base of the pit and in the neck of the gland
–divide rapidly and produce a continual supply of new cells to replace cells that die
parietal cells
found mostly in the upper half of the gland
–secrete hydrochloric acid (HCl), intrinsic factor, and a hunger hormone called ghrelin
chief cells
most numerous
–secrete gastric lipase and pepsinogen
–dominate lower half of gastric glands
–absent in pyloric and cardiac glands
enteroendocrine cells
concentrated in lower end of gland
–secrete hormones and paracrine messengers that regulate digestion
gastric juice
2 –3 liters per day produced by the gastric glands
•mainly a mixture of water, hydrochloric acid, and pepsin
gastric juice has a high concentration of
hydrochloric acid
–pH as low as 0.8
parietal cells produce
HCl and contain carbonic anhydrase (CAH)
Functions of Hydrochloric Acid
•activates pepsin and lingual lipase
•breaks up connective tissues and plant cell walls
–helps liquefy food to form chyme
•converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)
–Fe2+ absorbed and used for hemoglobin synthesis
•contributes to nonspecific disease resistance by destroying most ingested pathogens
zymogens
digestive enzymes secreted as inactive proteins
–converted to active enzymes by removing some of their amino acids
pepsinogen
zymogen secreted by the chief cells
–hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins
–autocatalytic effect –as some pepsin is formed, it converts more pepsinogen into pepsin
pepsin digests dietary proteins into
shorter peptide chains
–protein digestion is completed in the small intestine
gastric lipase
produced by chief cells
•gastric lipase and lingual lipase play a minor role in digesting dietary fats
–digests 10% -15% of dietary fats in the stomach
–rest digested in the small intestine
intrinsic factor
a glycoprotein secreted by parietal cells
•essential to absorption of vitamin B12 by the small intestine
–binds vitamin B12 and intestinal cells absorb this complex by receptor-mediated endocytosis
vitamin B12 is needed to
synthesize hemoglobin
–prevents pernicious anemia
indispensable function of the stomach
secretion of intrinsic factor is the only indispensable function of the stomach
–digestion can continue if stomach is removed (gastrectomy), but B12supplements will be needed
Chemical Messengers
gastric and pyloric glands have various kinds of enteroendocrine cells that produce as many as 20 chemical messengers
–some are hormones enter blood and stimulate distant cells
–others are paracrine secretions that stimulate neighboring cells
–several are peptides produced in both the digestive tract and the central nervous system –gut-brain peptides
Gastric Motility
swallowing center of medulla oblongata signals stomach to relax
•food stretches stomach activating a receptive-relaxation response
–resists stretching briefly, but relaxes to hold more food
•soon stomach shows a rhythm of peristaltic contractions controlled by pacemaker cells in longitudinal layer of muscularis externa
allowing only a small amount into the duodenum enables the duodenum to:
- neutralize the stomach acid
* digest nutrients little by little
vomiting
the forceful ejection of stomach and intestinal contents (chyme) from the mouth
emetic center
in the medulla oblongata integrates multiple muscle actions
vomiting is usually preceded by
nausea and retching
retching
thoracic expansion and abdominal contraction creates a pressure difference that dilates the esophagus
vomiting –occurs when
abdominal contractions and rising thoracic pressure force the upper esophageal sphincter to open
–esophagus and body of the stomach relax
–chyme is driven out of the stomach and mouth by strong abdominal contractionscombined with reverse peristalsis of gastric antrum and duodenum
projectile vomiting
sudden vomiting with no prior nausea or retching
–common in infants after feeding
chronic vomiting causes:
dangerous fluid, electrolyte, and acid-base imbalances
bulimia–
eating disorder in which the tooth enamel becomes eroded by the hydrochloric acid in the chyme
most digestion and nearly all absorption occur after
the chyme has passed into the small intestine
•stomach does not absorb any significant amount of nutrients
alcoholis absorbed mainly by
small intestine
–intoxicating effects depends partly on how rapidly the stomach is emptied
living stomach is protected in three ways from the harsh acidic and enzymatic environment it creates
–mucous coat –
–tight junctions -
–epithelial cell replacement
•breakdown of these protective measures can result in inflammation and peptic ulcer
mucous coat
thick, highly alkaline mucus resists action of acid and enzymes
tight junctions
between epithelial cells prevent gastric juice from seeping between them and digesting the connective tissue of the lamina propria and beyond
epithelial cell replacement
stomach epithelial cells live only 3 to 6 days
•sloughed off into the chyme and digested with the food
•replaced rapidly by cell division in the gastric pits
gastritis
inflammation of the stomach can lead to a peptic ulceras pepsin and hydrochloric acid erode the stomach wall
•most ulcers are caused by acid-resistant bacteria, Helicobacter pylori that can be treated with antibiotics and Pepto-Bismol
Regulation of Gastric Function
nervous and endocrine systems collaborate
gastric activity is divided into three phases:
cephalic phase
gastric phase
intestinal phase
•phases overlap and can occur simultaneously
cephalic phase
stomach being controlled by brain
gastric phase
stomach controlling itself
intestinal phase
stomach being controlled by small
