digestive system Flashcards
digestive system
- also called gastrointestinal system
- composed of alimentary canal(gi tract)
- and accessory organs
alimentary canal
extends from the mouth to anus through ventral cavity
-9m or 30ft long
accessory organs
- teeth
- tongue
- salivary glands
- liver
- gallbladder
- pancreas
digestive system(functions)
- ingestion
- secretion
- digestion(chemical and mechanical)
- mixing and propulsion
- absorption
- defecation
metabolic processess
-gi tract and accessory organs like the liver and pancreas are responsible for facilitating the boys metabolic processes
mechanical digestion
- all movements that facilitate catabolic processes:
- mastication
- swallowing(deglutition)
- mixing:increase contact of food with digestive chemicals
- peristalsis: movement of muscles within the GI tract that facilitates movement of food.
chemical digestion
- mainly accomplished by using water to break chemical bonds(hydrolysis)
- fats are broken down into fatty acids and glycerol
- carbohydrates are broken down from polysaccharides into monosaccharides
- proteins are broken down into polypeptides and amino acids
walls of gi tract from lower esophagus to anal canal
- four basic layers
- deep - superficial
- mucosa, submucosa,muscularis,serosa/adventita
Mucosa
has epithelium, lamina propria, and muscularis mucosae
lamina propria
-mucous membrane made of various types of epithelium sitting on loose connective tissue
muscosa
-nonkertinized stratified squamous epithelium(for protection) lines the pharynx, esophagus, and anus.
mucosa
-simple columnar epithelium(for secretion/absorption) lines the stomach and intestines
mucosa
-located among the epithelial cells are various glandular cells that secrete muscus and fluid into the lumen of the tract(exocrine).
muscosa
- lamina propria contains a prominent lymphoid tissue (mucosa-associated lymphatic tissue or MALT) that protects against disease.
- underneath it is the muscularis
mucosae
-thin layer of smooth muscle that throws the lining of the stomach and small intestines into tiny folds(increase surface area to aid digestion/absorption)
submucosa
- composed of loose connective tissue the binds the mucosa to the muscular
- contains blood and lymph vessels(to receive absorbed substances)
- and an extensive network of neurons known as the submucosal plexus(plexus of messier)
- motor neurons of the submucosal plexus supply the secretory cells of the organs of GITq
muscularis
- of the mouth,pharynx,superior&middle parts of the esophagus,external anal sphincter contain skeletal muscle that allows for voluntary swallowing and control of defecation.
- rest of tract, it is smooth muscle
- arranged in inner circular and outer longitudinal sheets w/ myenteric nerve plexus continuing between them
- these plexus mostly control GI tract motility
- particulary frequency and strength of contraction of the muscular
- neurons of the ENS(enteric nervous system, brain of the gut are arranged in 2 plexuses explained above meissner and auerbach plexuses.
serosa/adventitia
- outermost layer
- if attached to surrounding tissue(around the esophagus) called adventitia
- if contained in the peritoneal cavity it is called series, which has a slippery mesotheliem layer
- serosa covers the intra abdominal organs as the visceral peritoneum
adventita
fibrous connective tissue arranged around the organ which it supports
peritoneum
- bodys largest serous membrane and it wraps around most abdominal pelvic organs
- the visceral peritoneum forms the series of the alimentary canal and covers other intra-abdominal organs
- it then continues around the abdominal wall as the parietal peritoneum
5 major peritoneal folds
- greater omentum
- falciform ligament
- lesser omentum
- mesentery
- mescolon
falciform ligament
-attaches the liver to the anterior abdominal wall and diaphragm
greater omentum
- largest peritoneal fold
- drapes over the transverse colon and the anterior coils of the small intestine like a fatty apron
- contains many lymph nodes that help combat and contain infections of the gi tract
- large amount of adipose tissue can greatly exapnd(as seen in people with beer bellies)
lesser omentum
- peritoneal fold that suspends the stomach and duodenum from the inferior edge of the liver
- forms a pathway for blood vessels(hepatic portal vein and common hepatic artery to enter the liver and it contains the common bile duct
mesentery(of the small intestine)
mesocolon(large intestine)
-attach the bowel to the posterior abdominal wall holding the intestine loosely in place as muscular contractions mix and move the luminal contents along the GI tract
organs in retroperitoneal space
- kidnerys-ureters
- pancreas
- adrenal gland
- aorta and inferior vena cava
- duodenum
- ascending and descending colons of the large intestine
Neural innervation of the gi tract
1-enteric nervous system
2-autonomic nervous system
3-gastrointestinal reflex pathways
enteric nervous system
1-submucosal plexus(plexus of meissner) is one network of neurons
a-it regulates movement of the mucosa, vasoconstriction of blood vessels, and innervates secretory cells of mucosal glands
2-the myenteric plexus(plexus of auerbach) consists of fibers from both divisions of the ANS
a-plexus mostly controls gi tract motility
autonomic nervous system
1-stimulation of the parasympathetic nerves that innervate the gi tract causes an increase in gi secretions and motility by increasing the activity of ENS neurons
2-sympathetic nerves that supply the gi tract cause a decrease in gi secretion and motility by inhibiting the neurons of the ENS
gastrointestinal reflex pathways
-made up of plexuses that regulate gi secretions and motility in response to stimuli within the gi tract
digestive activities of the gastrointestinal tract occur in three overlapping phases:
- cephalic phase
- gastric phase
- intestinal phase
cephalic phase
- smell,sight,thought,initial taste of food activates neural centers int he cerebral cortex, hypothalamus, and brain stem to prepare for digestion
- brain stem activates the facial(7) and glossopharyngeal(9) nerve to stimulate secretions of saliva, while vagus nerves stimulate secretions of gastric juice
gastric phase
- once food reaches the stomach gastric phase begins
- neural and hormonal mechanisms(HORMONE GASTRIN OF G-CELLS) promote secretions of gastric juice and increase gastric motility. stomach distention and high pH stimulate stretch receptors and chemoreceptors leading to peristalsis and gastric juice secretion.
intestinal phase
- begins once food enters small intestine
- neural(distention of duodenum due to chyme causes enterogastric reflex) and hormonal responses promote the continued digestion of food that have reached the small intestine.
2 major hormones secreted by the small intestine
1) cck(cholecystokinin) from cck cells of small intestine crypts of lieberkuhm and secretin from S cells of small intestinal crypts of lieberkuhn mediate intestinal phase of digestion.
2) secretin promotes secretion of bicarbonate ions into pancreatic juice and bile. it inhibits secretion of gastric juice and promotes normal growth and maintenance of the pancreas. enhances the effects of CCK. causes buffering of acid in chyme. CCK stimulates secretion of pancreatic juice rich in digestive enzymes and ejection of bile into duodenum. also shows gastric emptying
other hormones of small intestine
1) motilin,substance p,bombesin: stimulate intestinal movement
2) VIP(vasoactive intesintal peptide): stimulated secretion of ions and water by intestines and inhibits gastric secretions
3) Gastrin-releasing peptides-stimulates gastrin release
4) somatostatin: inhibits gastrin release
cephalic phase
- consist of reflexes initiated by sensory receptors in the head.
- stimulates gastric secretion and motility
gastric phase
a. can be regulated by neural and hormonal mechanisms
b. neural regulation begins when the stomach walls are distended or when pH increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated resulting in waves of peristalsis and continual flow of gastric juice
c. hormonal negative feedback also regulates gastric secretions during the gastric phase
- chemo receptors and stretch receptors stimulate the ANS to release acetylcholine which stimulates the release of gastrin by G-cells
- gastrin stimulates growth of the gastric glans and secretion of large amounts of gastric juice it also strengthens contraction of lower esophageal sphincter
- increases motility of the stomach, and realizes the pyloric and ileocecal sphincters
intenstinal phase
-intestinal phase begins when partially digested food enters the small intestine
neural regulation
-neural regulation is stimulated by distention of duodenum
-distention triggers the enterogastric reflex which reduces gastric emptyin
hormal regulation
- secretin: promotes secretion of bicarbonate ions into pancreatic juice and bile
- inhibits secretion of gastric juice and promotes normal growth and maintenance of the pancreas. enhances the effects of CCK. causes buffering of acid in chyme - cck stimulates secretion of pancreatic juice rich in digestive enzymes and ejection of bile into the duodenum.(slows gastric emptying)
d. other hormones secreted by and having effects on the Gi tract. they include motion,substance p, bombazine,vasoactive intestinal polypeptide, gastrin-releasing peptide, and somatostatin.
oral / buccual cavity
- formed by the cheeks,
- hard and soft palates and
- the tongue
mechanical digestion
-food through mastication(chewing) enables it to be mixed with saliva to form a soft flexible bolus that can be easily swallowed
saliva starts the process of chemical digestion of food
- saliva is 99.5% water
- tiny amounts of dissolved ions, gases, uric acid, lga,lysozyme, and salivary amylase(digestive enzyme that acts on starch)
mechanical and chemical digestion in mouth
- through mastication(chewing), food is mixed with saliva and shaped into a bolus that is easily swallowed
- the enzyme salivary amylase converts polysaccharides(starches) to disaccharides(maltose). this is the only chemical digestion that occurs in the mouth
3 pairs of salivary glands
- parotid(ducts open in vestibule)
- submandibular(ducts open near lingual frenulum)
- sublingual gland(ducts open in floor of mouth)
- smaller glands are found on the lips(labial)
- cheeks(buccal)
- palate(palatal)
- and tongue(lingual)
- daily salivary secretion average 1-1.5 liters
salivary regulation
- under control of the ANS
- parasympathetic stimulation promotes secretion of a moderate amount of saliva
- salivary centers are located in the brain stem and efferent nerve impulses are transmitted by the facial(7) and glossopharyngeal(9) nerve
- touch(pressoreceptors), smell, taste(taste buds), and psychological factors are also salivary stimulators
- sympathetic stimulation decreases saliva secretions
tongue
- composed of skeletal muscle under voluntary somatic motor control
- forces the moistened food bolus into position for swallowing(deglutition)
- places the bolus into contact with teeth for chewing
extrinsic muscle of tongue
-attach to bones in the area and move the tongue from side to side
intrinsic muscle of tongue
- orginate within tongue
- alter shape and size for speech and swallowing
mouth.oral/buccal cavitiy
- formed by cheek
- hard and soft palate
- lips
- tongue
vestibule of the oral cavity
-bounded externally by the cheeks and lips and internally by the gums and teeth
oral cavity
space that extends from the gums and teeth to the fauces
-opening between the oral cavity and the pharynx or throat
tongue
- together with associated structure form the floor of the oral cavity
- composed of skeletal muscle covered with muscus membrane
- extrinsic and intrinsic muscle permit the tongue to be moved to participate in food manipulation for chewing and swallowing in speech
- lingual frenulum is a fold of mucous membrane that attaches to the midline of the undersurface of the tongue
- the upper surface and sides of the tongue are covered with papillae. some papillae contain taste buds
- on the dorsum of the tongue are glands that secrete lingual lipase. which initiates digestion of triglycerides
teeth/dentes
- located in sockets of the alveolar processes of the mandible and maxillae. sockets are lined by the periodontal ligament.
- peridontal ligament: dense fibrous connective tissue that anchors the teeth to the socket walls and acts as a shock absorber during chewing
tooth(external regions)
- crown, root, neck
- neck of each tooth is covered by gingivae/gums which extend slightly into each socket
dentin
-calcified connective tissue that forms most of the tooth
dentin
- dentin of crown is covered by enamel
- harder than bone calcified material and encloses the pulp cavity
- space filled with pulp(connective tissue containing blood vessels, nerves, and lymph vessels
enamel
-calcium phospate and calcium carbonate and it protects against wear and tear
two dentitions
- humans have two sets of teeth
- 20 deceiduous or baby teeth
- begin to erupt around 6months of age
- 2 incisors, 1 canine, and 2 molars in each quadrant
- 32 permanent teeth numbered from right to left
- top(1-16) to bottom(17-32)
- thirds molars(teeth 1, 16,17, and 32) are the wisdom teeth
- 2 incisors for cutting food
- 1 canine(cuspid): for shredding and tearing food
- 2 premolars(bicuspids) and 3 molars used for crushing and grinding of the food
deglutition
act of swallowing food
- oropharynx and largynopharynx have digestive as well as respiratory functions
- swallowed food must transit them both on the way to the esophagus.
- must be done so while not going into the nasal cavity or the airway
- this complex process involves many muscles subject to a number of voluntary and involuntary controls
deglutition(3 stages)
- 3 stages
- voluntary
- pharyngeal
- esophageal
voluntary
-act of tongue forcing the bolus to the back of the oral cavity and into the oropharynx
involuntary stage(pharyngeal stage)
-begins as bolus passes the oropharnyx
-receptors send impulses to the deglutition center in the medulla and pons
-returning impulses cause the soft palate to move superiorly and posteriorly to close the nasopharynx
-the epiglottis moves slightly inferiorly to close the glottis
degultition
involuntary(esophageal stage)
- swallowing begins once the bolus enters the esophagus
- peristalsis: progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis
- push the bolus onward
esophagus
- only digestive function that occurs is propulsion(moving food to the stomach)
- beings inferior to the laryngopharynx and positioned posterior to the trachea
- leaving the neck it transverses the thoracic cavity down the posterior mediastinum before piercing the diaphargm through the esophageal hiatus
- mucosa of the esophagus is nonkertained stratified squamous epithelium
type of muscle in the muscular of the esophagus by region
- the superior 1/3 is skeletal muscle
- the intermediate 1/3 is skeletal muscle and smooth muscle
- the inferior 1/3 is smooth muscle
- the upper and lower esophageal sphincters are situated at each end of the tube
- the LES regulates the movement of food from the esophagus into the stomach
- incompetence of the LES results in gastro reflux disease(GERD) which manifest as heart burn.
stomach
- j shaped enlargement of the gi tract
- between the esophagus and the first part of the small intestine(duodenum)
- begins at the esophagus and ends at the pyloric sphincter
- position and size of the stomach varies continually
- the diaphragm pushes it inferiorly with each inhalation and pulls it superiorly with each exhalation.
- rugae are large folds in the mucosa of the empty stomach which enables gastric distention
- depending on the amount of stomach contents
- serves as a mixing and holding area for food
- begins the digestion of proteins
- continues the digestion of triglycerides
- converting a bolus to a liquid called chyme
- also absorb some substances
anatomy of stomach
- gross anatomical subdivision include
- cardia,fundus,body,pyloric part
- pyloric part consist of pyloric antrum,pyloric canal and pylorus
- when stomach is empty mucosa lies in folds called rugae
- the stomach is about the size of a large sausage
- distensible part of the GI tract and can expand to accommodate a large quantity of food
stomach
functions as a mixing chamber and holding resevoir
-as a functional adaptation the gastric muscular contains an additional 3rd inner oblique layer of muscle to facilitate the mixing action of mechanical digestion
stomach
- simple columnar epithelial cells(surface mucous cells) line the mucosal surface and secrete a protective mucous
- columns of secretory cells extend down into the lamina propria forming gastric glands
- several gastric glands open into the bottom of narrow channels called gastric pits
- variety of specialized exocrine and endocrine cell types populate the gastric glands pits
exocrine gland include:
- mucous neck cells which produce mucus
- partietal cells which produce intrinsic factor and HCl
- chief cells which secrete the protease pepsinogen and gastric lipase
stomach(enteroendocrine G cells)
- mainly in the pyloric antrum
- secrete the hormone gastrin into the bloodstream
stomach
- secretions of the mucous, partial, and chief cells from gastric juice which totals 2-3 L/day
- the stomach is protected from its own gastric juice by 1-3 mm thick layer of alkaline mucus secreted by surface mucous cells and mucous neck cells
stomach
-strongly acidic nature of gastric juice kills many microbes, partially denatures proteins in food, and converts pepsinogen into pepsin
pepsin
only proteolytic enzyme in the stomach
gastric lipase
-splits triglycerides to fatty acids and monoglycerides
intrinsic factor
- needed for absorption of vitB12 in the terminal ileum
- vit b12 is needed for rbc production
- disturbing the balance btw hydrochloric acid production, pepsin secretion, and mucosal defenses can lead to erosion of the stomach epithelial lining
mixing waves
-gentle, rippling, peristaltic movements pass over the stomach every 15-25seconds
chyme
these waves macerate food. mix it with secretions of the gastric glans and reduce it to a soupy liquid
stomach
- although digestion is a major function of the stomach
- epithelial cells are impermeable to most materials and very little absorptions takes place
- within 2-4hrs after eating a meal the stomach has emptied its contents into the duodenum
stomach
- foods rich in carbohydrates spend the least time in the stomach
- high protein foods remain somewhat longer
- emptying is slowest after a fat-laden meal containing large amounts of triglycerides
- the stomach allows a small amount of chyme to pass through the pyloric sphincter and enter the duodenum to begin the intestinal phase of digestion
- completion of digestion is a collective effort of pancreatic juice, bile, and intestinal juice in small intestine
histology of the stomach
- surface of the mucosa is a layer of simple columnar epithelial cells called mucous surface cells
- epithelial cells extend down into the lamina propria forming gastric pits and gastric glands
gastric glands
- mucous neck cells: secrete mucus
- chief/zymogenic cells: secrete pepsinogen and gastric lipase
- parietal/oxyntic cells:secrete HCL
gastic glands
- also contain enteroendocrine cells which are hormone producing cells
- G cells secrete the hormone gastrin into the bloodstream
submucosa
composed of areolar connective tissue
muscularis
- 3 layers
- longitudinal
- circular
- inner oblique layer
serosa
-part of the visceral peritoneum
at lesser curvature
-visceral peritoneum becomes the lesser omentum
at greater curvature
-visceral peritoneum becomes the greater omentum
mechanical digestion
-mechanical digestion consist of peristaltic movements called mixing waves
chemical digestion
-consist mostly of the conversion of proteins into peptides by pepsin
A. an enzyme that is most effective in the very acidic environment(ph2) of the stomach. the acid HCL is secreted by the stomachs parietal cells
B. gastric lipase splits certain molecules in butterfat of milk into fatty acids and monoglycerides and has a limited role in the adult stomach.
- stomach wall is impermeable to most substance however some water, electrolytes, certain drugs(especially aspirin), and alcohol can be absorbed through the stomach lining
- gastric emptying the periodic release of chyme from the stomach into the duodenum
- most food leaves the stomach 2-4hrs after ingestion.
- carbohydrates leave earliest, followed by proteins and then fats
pancreas
- digestion and absorption in the small intestine depend heavily on secretions from the pancreas and gallbladder(liver)
- pancreas is an oblong gland located posterior to the stomach in the retroperitoneal space
pancreas
-connected to the duodenum by the hepatopancreatic ampulla and accessory ducts
pancreas
- secretes enzymes which digest food in the small intestine
- sodium bicarbonate which buffers the acidic Ph of chyme
pancreas
-1-1.5 liters of alkaline pancreatic juice is secreted into the duodenum each day. it creates the proper pH for the following digestive enzymes in the small intestine
pancreas
starch digesting enzyme called pancreatic amylase
pancreas
serveral enzymes that cleave polypeptides into dipeptides and single amino acids: trypsin, chymotrypsin, carboxypeptidase, and elastase
pancreas
pancreatic lipase the major triglyceride(fat) digestive enzyme in adults
liver
- liver is divisible into left and right lobes serrated by the falciform ligament
- associated with the right lobe are the caudate and quadrate lobes
liver
-the gallbladder is sac located in depression on the posterior surface of the liver
liver
- liver receives a double supply of blood from the hepatic artery and the hepatic portal being
- all blood eventually leaves the liver via the hepatic vein
hepatic cells(hepatocytes)
-produce bile that is transported by a duct system to the gallbladder for concentration and temporary storage
liver
the fusion of individual crystals of cholesterol is the beginning of 95% of the all gallstones
- gallstones can cause obstruction to the outflow of bile in any portion of the duct system
- treat meant of gallstones consists of using gallstone-dissolving drugs,lithotripsy or surgery
hepatocytes
- major functional cells of the liver
- hepatocytes particpate in a number of digestive and non-digestive functions
digestive function of liver
- synthesis
- transformation and storage of proteins, carbohydrates and fats
- detoxification modification, and excretion of a variety of exogenous substances
non digestive function of liver
- phagocytosis of old worm out cells
- making heparin and other plasma protein(heparin, prothrombin, fibrinogen, albumin)
- modifying vitamin D to its active form(vitamin D3)
- venous blood(from the hepatic portal vein) and arterial blood (from the hepatic artery) feed the lobule from the triad on its outer margin
- the blood mixture percolates through endothelial lined spaces called sinusoids(a specialized capillary towards the central vein
liver
- venous blood(from the hepatic portal vein) and arterial blood(from the hepatic artery) feed the lobule from the triad on its outer margin
- the blood mixture percolates through endothelial-lined spaces called sinusoids(a specialized capillary) towards the central vein
path of the hepatic sinusoid
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