Transcript Gut Tube and Digestion

Gut Tube and Digestion
Embryonic origin
Path of Food
Esophagus
Stomach
Small intestines
Large intestines
Rectum and anus
Blood supply to gut
Control of Digestion
Neuronal
Hormonal
Path of Food
Mouth--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
Esophagus
Pharynx 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 disease
Sometimes 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 organ
Internal Layer = Mucosa
Epithelium
Lamina propria
Muscularis mucosae
Middle Layer = Submucosa
CT 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
STRUCTURE
J-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
FUNCTION
Mechanical 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
Mucosa
Rugae: mucosal folds
allow expansion
Typical Submucosa
Muscularis externa
Oblique layer
Circular layer
Pyloric sphincter
Longitudinal layer
Serosa
Histology of Stomach
Gastric glands, in lamina
propia, secrete digestive
enzymes into fundus of
stomach via gastric pits.
1500 ml of gastric juice per
day is pproduced
Parietal cells (proximal in
gland) secrete
Gastric Glands
Intrinsic Factor that facilitates
absorption of Vitamin B
HCl 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. 24-13
Fig. 24-14
Fig
When to keep food in stomach or send on:
pyloric glands
In pyloric region, modified gastric glands
secrete hormones
Secrete mucous
Gastrin—which stimulates other gastric
glands and also contractions of smooth
muscle (when neural/hormonal stimuli override 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
Duodenum
C-shaped initial piece (5% of total)
Entries for pancreatic, bile ducts
Jejunum
Fan-shaped coil (40% of total) at superior left
abdomen
Ileum
Inferior right part of coil
Ends at appendix in lower right quadrant
Location of Duodenum
Small Intestine: Modifications for absorption
Length
Increase surface area
Plicae circularis
Transverse ridges of mucosa
Increase surface area
Slow movement of chyme
Villi
Move chyme, increase contact
Contain lacteals: remove fat
Microvilli:
Increase surface area
Modifications decrease distally
Secretion and absorption in small intestines
SECRETION
1800 ml of intestinal juice
per day
Most is water that enters by
osmosis across epithelial
lining since chyme from
stomach is very
concentrated
Duodenal glands also
secrete mucous to protect
lining
Sympathetic stimulation
inhibits duodenal
glands…thus duodenal
ulcers are stress-related
Digestive enzymes come
from stomach (with
chyme), pancreas and liver
(more later)
ABSORPTION
In each villus, nutrients
diffuse into abundant
capillaries
Fats and protein/fat
packages are taken up by
lacteals (too big to diffuse
directly into circulation
Lacteals are modified
lymph capillaries. Fats
enter circulation by
movement through lymph
vessels, eventually to
thoracic duct
Large Intestines
Large Intestines
Frame around rest of gut
Ascending, transverse, descending
Starts at cecum/appendix
Ends 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 rectum
Absorption of water from food
Rectum +
Anal Canal
Rectum
descends into pelvis
no teniae coli
longitudinal muscle
layer complete
rectal valves
Anal Canal (more with
pelvis)
passes through levator
ani muscle
releases mucus to
lubricate feces
Internal anal sphincter
involuntary, smooth m.
External anal sphincter
voluntary, skeletal m.
Blood supply--ventral branches off of aorta
Celiac a.--to
stomach, liver,
pancreas, spleen,
duodenum
Superior (cranial)
mesenteric a.--to
small intestines and
most of colon
Inferior (caudal)
mesenteric a.--to
descending colon,
rectum
Innervation of gut
Parasympathetic
What nerve? VAGUS
Where does it run?
With aorta
Sympathetic
Only thoracic output from spinal cord
Splanchnic nerves from thorax lateral to vertebral
bodies bring posteriorly to abdominal cavity and gut
Synapse in celiac and superior mesenteric ganglia
Both Para- and Sympathetic follow aa. out to
organs
Enteric nervous system: High level of local
control with network of synapses within ganglia
and around gut
Digestive Hormones also control
secretion and absorption
Table 24-2
Overall
breakdown
and
absorption
of nutrients