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Gut Tube and DigestionEmbryonic origin
Path of FoodEsophagus
Stomach
Small intestines
Large intestines
Rectum and anus
Blood supply to gut
Control of DigestionNeuronal
Hormonal
Path of FoodMouth--chewing
Pharynx--conscious swallowing
Esophagus--transport to stomach
Stomach--mechanical and chemical breakdown
Small Intestines--chemical digestion and absorption
Large Intestines--resorb water, form feces
Rectum---collect and expel feces
Esophagus
EsophagusPharynx to stomach
Smooth muscle (conscious swallowing is in pharynx)
Passes through esophageal hiatus in diaphragm, stomach against inferior diaphragm
Cardiac orifice, with esophageal hiatus guard opening to stomach, prevent regurgitation
GERD--gastroesophageal reflux diseaseSometimes due to hiatal hernia
Lower esophagus becomes ulcerous and precancerous
Treat with antacids and other acid-reducing drugs
Histological Layers of Digestive Tract Wall
Three layers generally present—details vary with each organInternal Layer = Mucosa
EpitheliumLamina propriaMuscularis mucosae
Middle Layer = SubmucosaCT w/ elastic fibers, nerves, vessels
Outer Layer = Muscularis Externa (two sub-layers) circular (“sphinchter” is more internal sub-layer) longitudinal (more external sub-layer)
Histology of esophagus
Stomach
Stomach
STRUCTUREJ-shaped but varies from “steerhorn” (high and horizontal) to vertically elongate (down to pelvis on tall, thin people)
From esophagus (cardiac orifice) to small intestine (pyloric sphincter)
Greater, lesser curvatures
FUNCTIONMechanical breakdown of food--smooth muscle in wall
Protein breakdown--pepsin secreted by epithelial lining
Acidic conditions--for pepsin to work and to kill bacteria
Absorption of water, ions and some drugs (e.g., aspirin, alcohol)
Histology of Stomach
MucosaRugae: mucosal folds allow expansion
Typical SubmucosaMuscularis externa
Oblique layerCircular layer
Pyloric sphincter
Longitudinal layer
Serosa
Histology of Stomach
Gastric GlandsGastric glands, in lamina propia, secrete digestive enzymes into fundus of stomach via gastric pits.1500 ml of gastric juice per day is pproducedParietal cells (proximal in gland) secrete
Intrinsic Factor that facilitates absorption of Vitamin BHCl components
Chief cells (distal in gland) secrete
Pepsinogen (pepsin precursor)Rennin in newborns (coagulates milk…used in cheese-making)Gastric lipase (begins digestion of milk fats)
Fig. 24-13Fig. 24-14Fig
When to keep food in stomach or send on: pyloric glands
In pyloric region, modified gastric glands secrete hormonesSecrete mucousGastrin—which stimulates other gastric glands and also contractions of smooth muscle (when neural/hormonal stimuli over-ride effect of somatostatin)Somatostatin—inhibits gastrin (continuously released)
No absorption of nutrients in stomach
Alcohol and other lipid-soluble drugs can be absorbed by penetrating epithelial bi-lipid cell membranes
Small Intestines
DuodenumC-shaped initial piece (5% of total)
Entries for pancreatic, bile ducts
Jejunum Fan-shaped coil (40% of total) at superior left abdomen
IleumInferior right part of coil
Ends at appendix in lower right quadrant
Location of Duodenum
Small Intestine: Modifications for absorptionLength
Increase surface area
Plicae circularisTransverse ridges of mucosa
Increase surface area
Slow movement of chyme
VilliMove chyme, increase contact
Contain lacteals: remove fat
Microvilli: Increase surface area
Modifications decrease distally
Secretion and absorption in small intestinesSECRETION
1800 ml of intestinal juice per dayMost is water that enters by osmosis across epithelial lining since chyme from stomach is very concentratedDuodenal glands also secrete mucous to protect liningSympathetic stimulation inhibits duodenal glands…thus duodenal ulcers are stress-relatedDigestive enzymes come from stomach (with chyme), pancreas and liver (more later)
ABSORPTIONIn each villus, nutrients diffuse into abundant capillariesFats and protein/fat packages are taken up by lacteals (too big to diffuse directly into circulationLacteals are modified lymph capillaries. Fats enter circulation by movement through lymph vessels, eventually to thoracic duct
Large Intestines
Large IntestinesFrame around rest of gut
Ascending, transverse, descendingStarts at cecum/appendixEnds at rectum, anal canal
Teniae coli“ribbons” or strips of muscle along length of colon (three around tube)Tension in teniae coli forms haustra or sacs
Little continuous movement, but mass peristaltic movement several times daily to force feces towards rectumAbsorption of water from food
Rectum + Anal Canal
Rectumdescends into pelvisno teniae colilongitudinal muscle layer completerectal valves
Anal Canal (more with pelvis)
passes through levator ani musclereleases mucus to lubricate feces Internal anal sphincter
involuntary, smooth m.
External anal sphinctervoluntary, skeletal m.
Blood supply--ventral branches off of aorta
Celiac a.--to stomach, liver, pancreas, spleen, duodenumSuperior (cranial) mesenteric a.--to small intestines and most of colonInferior (caudal) mesenteric a.--to descending colon, rectum
ParasympatheticWhat nerve?Where does it run?
SympatheticOnly thoracic output from spinal cordSplanchnic nerves from thorax lateral to vertebral bodies bring posteriorly to abdominal cavity and gutSynapse in celiac and superior mesenteric ganglia
Both Para- and Sympathetic follow aa. out to organsEnteric nervous system: High level of local control with network of synapses within ganglia and around gut
Innervation of gut
VAGUS
With aorta
Digestive Hormones also control secretion and absorption
Table 24-2
Overall breakdown and absorption of nutrients