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