Download Digestive System

Digestion: breaking down of food into
components that can be absorbed
• Absorption: movement of nutrients, salts and water
across the GI epithelium into blood/lymph
• DS is designed to maximize digestion and absorption
• ANS, Chemical factors/hormones regulate
absorption/digestion and caloric intake by stimulation/
suppression of appetite
•
The lumen of
this tube is
continuous with
the external
environment.
•
The accessory
organs are the
salivary glands,
exocrine
glands, and
biliary system
(liver and
gallbladder).
1
Nasal passages
Hard palate
Soft palate
Uvula
Pharynx
Epiglottis
Bolus
Esophagus
Tongue
Trachea
Glottis at entrance
of larynx
Fig. 15-5a, p. 474
Swallowing center
inhabits respiratory
center in brain stem
Elevation of uvula
prevents food from
entering nasal passages
Position of tongue
prevents food from
re-entering mouth
Epiglottis is pressed
down over closed
glottis as auxiliary
mechanism to prevent
food from entering
airways
Tight apposition of vocal folds across
glottis prevents food from entering
respiratory airways (viewed from above)
Fig. 15-5b, p. 474
2
3 phases of control
• Cephalic phase: mediated by X N – prepare for food
• Gastric: food contents & volume in stomach initiate
reflexes for gastric secretions and motility
• Intestinal: food in the intestine initiates reflexes that
cause secretion of bicarbonate, digestive enzymes,
bile and begin segmenting contractions. Inhibitors
reflexes inhibit gastric emptying during intestinal
absorption
• PNS promotes digestion, SNS depresses/inhibits
3
•
Swallowing is initiated when a
food bolus contacts receptors in
the pharynx. Afferent impulses
are sent to the swallowing center
in the medulla.
•
Food is prevented from entering
the wrong passageways includes the closing of the glottis
and the tilting of the epiglottis,
preventing food from entering
the respiratory tract.
•
The pharyngoesophageal
sphincter prevents air from
entering the digestive tract
during breathing. The sphincter
keeps the esophagus closed,
preventing eructation.
•
The gastroesophageal sphincter
prevents reflux of gastric
contents.
The stomach stores food and begins protein digestion.
•
This J-shaped chamber
has a fundus, body, and
antrum. Its terminal
part has a pyloric
sphincter.
•
The most important
function of the stomach
is to store food.
•
It secretes HCl to begin
protein digestion.
•
The mixing movements
of the stomach/antrum
produces chyme.
•
The stomach
accommodate a twentyfold increase in volume.
4
Stomach
Esophagus
Gastroesophageal
sphincter
Duodenum
Pyloric sphincter
Movement
of chyme
Peristaltic
contraction
Direction of
movement
of peristaltic
contraction
Gastric emptying
Fig. 15-8a, p. 477
GI Hormones
• Gastrin: secreted in the stomach – stimulates HCL
• CCK: Intestinal hormone that stimulates GB contraction
and pancreatic juice release
• Secretin: Intestinal hormone that stimulates bicarbonate
secretion by liver and pancreas
5
Absorption
• Digestion occurs primarily in the small intestine.
• Absorption of salts, water and nutrients occurs in
the small intestine
• Most nutrients are absorbed by transepithelial
transport
• Monosaccharides and amino acids enter blood,
most fat enters the lymph
Table 15-3, p. 480
6
In oxyntic mucosa
In pyloric gland area
Surface epithelial cells
Gastric
pit
Mucosa cells
Gastric
gland
G cells
Chief cells
D cells
Parietal cells
Enterochomaffinlike (ECL) cells
Table 15-3 (2), p. 480
•
Mucous cells secrete a
thin, watery protective
mucus.
•
Parietal cells secrete HCl
and the intrinsic factoressential for the absorption
of vitamin B12.
•
Low pH in the stomach
converts pepsinogen into
the active form, pepsin.
•
Functions of HCl in the
stomach include pepsin
formation, the breakdown
of connective tissue and
muscle fibers, the
denaturation of proteins,
and the killing of most
microorganisms.
7
•
The stomach lining is protected from gastric secretions by the gastric mucosal barrier.
The mucosal membrane is almost impermeable to hydrogen ions.
•
Carbohydrate digestion continues in the body of the stomach. Proteins digestion begins
in the antrum. Salivary amylase continues to work in the internal mass of food.
The stomach absorbs alcohol and aspirin, but does not absorb food.
Large intestine -a drying and storage organ.
•
The colon consists of the
cecum, ascending colon,
transverse colon,
descending colon, and
sigmoid colon.
•
The colon receives
indigestible food,
unabsorbed biliary
components, and the
remaining fluid.
•
The colon extracts water
and salt and eliminates the
feces.
•
Contractions of the
haustrae slowly shuffle the
colonic contents back and
forth. Mass movements
propel colonic contents long
distances.
8
External
influence
Local changes in
digestive tract
Receptors in digestive tract
Extrinsic
automatic
nerves
Intrinsic
nerve plexuses
Smooth muscle
(contraction for motility)
Selfexcitable
Exocrine gland cells
(secretion of digestive juices)
= Short reflex
= Long reflex
= Hormonal pathway
Endocrine gland cells
(secretion of gastrointestinal
and pancreatic hormones)
Bile duct
from liver
Duodenum
Gastrointestinal
hormones
Fig. 15-3, p. 471
Stomach
Hormones
(insulin,
glucagon)
Blood
Endocrine portion
of pancreas
(Islets of Langerhans)
Duct cells
secrete aqueous
NaHCO3 solution
Acinar cells
secrete digestive
enzymes
Exocrine portion of panaceas
(Acinar and duct cells)
The glandular portions of
the pancreas are grossly
exaggerated
Fig. 15-11, p. 487
9
Acid in
duodenal
lumen
Fat and protein
products in
duodenal lumen
Secretion release
from duodenal
mucosa
CCK release
from duodenal
mucosa
(Secretin carried Neutralizes
by blood)
(CCK carried
by blood)
Digests
Pancreatic acinar
cells
Pancreatic duct
cells
Secretion of
pancreatic digestive
enzymes into
duodenal lumen
Secretion of aqueous
NaHCO3 solution into
duodenal lumen
Fig. 15-12, p. 488
Heart
Inferior
vena cava
Aorta
Hepatic vein
Hepatic
artery
Liver
sinusoids
Liver
Hepatic
portal vein
Digestive
capillaries
Digestive
tract
Fig. 15-13, p. 489
10
Branch of
hepatic
portal
vein
Central vein
Bile canaliculi
Cords of
hepatocytes
(liver cells)
Bile
duct
Branch
of
hepatic
artery
Hepatic
portal vein
To
hepatic
duct
Sinusoids
Hepatic
artery
Fig. 15-14a, p. 490
New
meal
Ascending
colon
Gastrin
Ileocecal valve
Ileocecal sphincter
Pushes valve open
and relaxes sphincter
Pushes
valve closed
and contracts
sphincter
Ileum
Cecum
Appendix
Fig. 15-19, p. 495
11
Table 15-6, p. 496
Lipid emulsion
Micelles
diffusion
Lumen
Lumen
Micelle
Micelles
Microvillus
Fatty acids,
monoglycerides
Aggregate and
coated with
lipoprotien
Epithelial
cell of villus
Short or
medium
chain
Passive absorption
Basement
membrane
(Exocytosis)
Central lacteal
Capillary
Fig. 15-23, p. 500
12
Table 15-7, p. 501
Transverse colon
Haustra
Taeniae coli
Descending colon
Ascending
colon
Ileocecal
valve
Appendix
Cecum
Sigmoid colon
Rectum
Internal anal sphincter
(smooth muscle)
External anal sphincter
(skeletal muscle)
Anal sphincter
Fig. 15-24, p. 502
13