Digestive System Flashcards
anatomy of the digestive tract
- digestive system begins with oral cavity (mouth and pharynx) which serves as a receptacle for food
- swallowed food enters gastrointestinal tract (GI tract) consisting of esophagus, stomach, small intestine, and large intestine
- portion of the GI tract running from the stomach to the anus is also called the gut
digestion
chemical and mechanical breakdown of food
- takes place primarily in lumen of gut
accessory glandular organs
salivary glands, liver, gallbladder, and pancreas
digestion begins with chewing and secretion of saliva by three pairs of salivary glands:
(1) sublingual glands - under the tongue
(2) submandibular glands under the mandible (jawbone)
(3) parotid glands lying near the hinge of the jaw
esophagus
narrow tube that travels through the thorax to the abdomen
- walls are skeletal muscle but transition to smooth muscle
sections of stomach
- upper fundus
- central body
- lower antrum
pyloric valve
guards the pylorus or opening between stomach and small intestine
chyme
stomach continues digestion by mixing food with acid and enzymes
where does most of digestion take place?
small intestine
sections of small intestine
- duodenum
- jejunum
- ileum
how is digestion carried out?
intestinal enzymes, aided by exocrine secretions from two accessory glandular organs - pancreas and liver
sphincter of Oddi
keeps pancreatic fluid and bile from entering small intestine except during a meal
colon
proximal section of large intestine
what happens in the colon?
watery chyme becomes semisolid feces, as water and electrolytes are absorbed out of the chyme and into the extracellular fluid
rectum
terminal section of the large intestine
4 layers of GI tract
(1) inner mucosa facing lumen
(2) submucosa
(3) layers of smooth muscle known collectively as muscularis externa
(4) covering of connective tissue called serosa
Mucosa include:
(1) transporting epithelial cells into the small intestine
(2) endocrine and exocrine secretory cells
(3) stem cells
mucosal (apical) surface
cells secrete ions, enzymes, mucus, and paracrine molecules into lumen
serosal (basolateral) surface
substances being absorbed from lumen enter ECF
mucosa: mucosal epithelium
- the cell to cell junctions tie GI epithelial cells together
- in stomach+colon junctions have a tight barrier
- in small intestine junctions are leaky
- stem cells are rapidly dividing
mucosa: lamina propia
- subepithelial connective tissue: nerve fibers, blood vessels, and lymph vessels
- immune cells patrolling for invaders that enter through epithelium
- lymphoid tissue adjoining epithelium form small nodules and large Peyer’s patches
mucosa: muscularis mucosae
- thin layer of smooth muscle separates the lamina propria from the submucosa
- contraction of muscles in this layer moves the villi back and forth
submucosa
- middle layer of gut wall
- connective tissue with larger blood and lymph vessels running through it
- also contains submucosal or meissner’s plexus on of the major nerve networks of enteric nervous system
muscularis externa
- outer wall of Gi tract consists of 2 layers of smooth muscle: inner circular layer and outer longitudinal layer
contraction of circular layer
decreases diameter of lumen
contraction of longitudinal layer
shortens tube
myenteric plexus/Auerbach’s plexus
second nerve network
- lies between longitudinal and circular muscle layers
- controls and coordinates motor activity of muscularis externa
serosa
- outer covering of entire GI tract
- connective tissue membrane that is a continuum of the peritoneal membrane lining the abdominal cavity
four processes of digestive system
- digestion
- absorption
- secretion
- motility
secretion
movement of material from cells into lumen or ECF
digestion
chemical and mechanical breakdown of food into absorbable units
absorption
movement of material from GI lumen to ECF
motility
movement of material through the GI tract as a result of muscle contraction
challenges of digestive system
- autodigestion
- mass balance
- fluid loss
- defense
autodigestion
if protective mechanisms against autodigestion fail, raw patches known as peptic ulcers develop
mass balance
matching fluid input with output, dehydration
fluid loss
vomiting, diarrhea, loss of blood pressure
defense
GI tract is assisted by an array of physiological defense mechanisms, including mucus, digestive enzymes, acid, and the largest collection of lymphoid tissue in the body, the gut-associated lymphoid tissue (GALT)
mass balance in GI tract
- gastrointestinal epithelial cells are polarized, with distinct apical and basolateral membranes
- water follows osmotic gradient created by transfer of solutes from one side of epithelium to other
- water moves through the epithelial cells via channels or through leaky junctions between cells
digestive enzymes
- enzymes are secreted by exocrine glands (salivary glands and the pancreas) or epithelial cells in stomach and small intestine
- enzymes are proteins synthesized on rough ER packaged by Golgi complex into secretory vesicles, stored in cell until needed and released by exocytosis
- many intestinal enzymes remain bound to apical membranes of intestinal cells anchored by transmembrane protein “stalks” or lipid anchors
- some digestive enzymes are secreted in an inactive proenzyme form known collectively as zymogens
mucus
- viscous secretion composed primarily of glycoproteins collectively called mucins
- made in specialized exocrine cells called mucous cells in the stomach and salivary glands, and goblet cells in the intestine
- the signals for mucus release include parasympathetic innervation , variety of neuroproteins found in enteric nervous system and cytokines from immunocytes
- parasitic infections and inflammatory processes in the gut also cause substantial increases in mucus secretion
tonic contractions
sustained for minutes or hours, they occur in some smooth muscle sphincters and in the anterior portion of the stomach
