Download The Digestive System

The Digestive System
Introduction
 The
digestive system consists of the
muscular digestive tract and various
accessory organs.
 Digestive functions include ingestion,
mechanical processing, digestion,
secretion, absorption, compaction, and
excretion.
Overview of the digestive
Tract
 The
digestive tract includes the oral
cavity, pharynx, esophagus, stomach,
small intestine, large intestine, rectum
and anus.
Histological Organization

The epithelium and underlying connective
tissue,the lamina propria, form the mucosa of
the digestive tract.
 Next, outward, are the submucosa, the
muscularis externa, and the adventitia, a
layer of loose connective tissue.
 In the peritoneal cavity, the muscularis
externa is covered by the serosa, a serous
membrane

Double sheets of
peritoneal
membrane called
mesenteries
suspend the
digestive tract.
Movement of Digestive
Materials

The neurons that innervate the smooth
muscle of the muscularis external are not
under voluntary control.
 The muscularis externa propels materials
through the digestive tract by means of the
contractions of peristalsis.
 Segmentation movements in areas of the
small intestine churn digestive materials.
Peristalsis
The Oral Cavity
 The
functions of the oral cavity are:
 Analysis
of potential foods
 Mechanical processing using the teeth,
tongue and palatal surfaces
 Lubrication by mixing with mucus and
salivary secretions
 Digestion by salivary enzymes
 The
oral cavity, or buccal cavity, is lined
by oral mucosa
 The hard palate and soft palate form its
roof and the tongue forms its floor.
The Tongue
 The
primary functions of the tongue
include:
 Mechanical
processing
 Manipulation to assist in chewing and
swallowing
 Sensory analysis.
The Salivary Glands
 The
salivary glands discharge their
secretions into the oral cavity
 Saliva lubricates the mouth, dissolves
chemicals, flushes the oral surfaces and
helps to control bacteria.
 Salivation is usually controlled by the
ANS.
Teeth
 Mastication
occurs through the contact
of the opposing surfaces of the teeth
 The periodontal ligament anchors the
tooth ins a bony socket.
 Dentin forms the basic structure of a
tooth
 The crown is coated with enamel, and
the root is covered with cementum
The 20 primary teeth or deciduous teeth are
replaced the the 32 teeth of the secondary
dentition during development.
The Pharynx and Esophagus
 The
pharynx is the passageway
between the oral cavity and the
esophagus
 The esophagus is a muscular tube
about 10 in. long and ¾ inches in diam.
 It
is lined with stratified squamous
epithelium that resists abrasion, hot or cold
temp. and chemical attack. Mucus glands
lubricate surface and keep material from
sticking.
Swallowing
Called deglutition
The mass of food
being swallowed
is called a bolus
The Stomach
 The
stomach has four major functions:
 Temporary
storage of ingested food
 The mechanical breakdown of food.
 The disruption of chemical bonds by acids
enzymes
 The production of intrinsic factor a
compound necessary for the absorption of
vitamin B12

Chyme forms in the stomach as gastric and
salivary secretions are mixed with food.
 The four regions of the stomach are the
cardia, fundus, body and pylorus
 The pyloric sphincter guards the exit from the
stomach.
 In a relaxed state the stomach lining contains
numerous rugae (ridges and folds)
The Gastric Wall
 Within
the gastric glands, parietal cells
secrete intrinsic factor and hydrochloric
acid.
 Chief cells secrete pepsinogen which
acids in the gastric lumen convert to the
enzyme pepsin (digests proteins)
 Gastric gland endocrine cells secrete
the hormone gastrin.
The Regulation of Gastric Activity
 Gastric
 The
secretion includes:
cephalic phase which prepares the
stomach to receive ingested materials
 The gastric phase which begins with the
arrival of food in the stomach
 The intestinal phase which controls the
rate of gastric emptying
The Small Intestine
 The
small intestine includes the
duodenum, the jejunum, and the ileum.
 The ileocecal valve, a sphincter, marks
the transition between the small and
large intestines.
The Intestinal Wall
 The
intestinal mucosa bears transverse
folds called plicae, or plicae circulares,
 and small projections called intestinal
villi.
 They increase the surface area for
absorption.
 Each villus contains a terminal
lymphatic called a lacteal which absorbs
lipids.
Intestinal Secretions

Intestinal glands secrete intestinal juice,
mucus,and hormones.
 Intestinal juice moistens the chyme, helps
buffer acids,and dissolves digestive enzymes
and the products of digestion.
 Intestinal hormones include secretin,
cholecystokinin(CCK) and gastric inhibitory
peptide.
Digestion and the Small intestine
 The
most important digestive and
absorptive functions occur in the small
intestine.
 Digestive enzymes and buffers are
provided by the pancreas, liver and
gallbladder.
The Pancreas
 The
pancreatic duct penetrates the wall
of the duodenum, where it delivers the
secretions of the pancreas.
 The
pancreas has two functions
 Endocrine
(secreting insulin and glucagon
into the blood)
 Exocrine (secreting water, ions, and
digestive enzymes into the small intestine)
 Pancreatic enzymes include lipases,
carbohydreases and proteases and
nucleases.
The Control of Pancreataic
Secretion
When chyme arrives in the small intestine
secretin triggers the pancreatic production of
a fluid containing buffers, primarily sodium
bicarbonate, what help bring the pH of the
chyme under control.
Also CCK is released which stimulates the
pancreas to secrete pancreatic amylase,
pancreatic lipase, nucleases and protease
enzymes.
The Liver
 Largest
visceral organ in the body, with
over 200 known functions.
Liver Functions
 Metabolic
regulation: controls the
composition of circulating blood. Filter
out toxins.
 Hematological regulation: liver is the
largest blood reservoir in the body.
Removes aged or damaged red blood
cells, debris, and pathogens.
 The gallbladder stores and concentrates
bile for release into the duodenum.
 Bile
salts emulsify lipids for digestion.
 The liver produces about 1 liter of
bile/day but it is only released into the
intestine when CCK is released.
The large Intestine
 The
main functions of the large intestine
are:
 Reabsorb
water and compact the feces
 Absorb vitamins liberated by bacteria
 Store fecal material prior to defecation.
Three Parts of Large Intestine
 Cecum:
collects and stores material
from the ileum. Vermiform appendix is
attached to the cecum.
 Colon: larger diameter and thinner wall
than sm. Intestine/
 Rectum: terminates in the anal canal
leading to the anus.
Physiology of the Large Intestine
 The
large intestine reabsorbs water and
other substances such as vitamins,
bilirubin, bile salts, and toxins;
 Bacteria residing in the large intestine
are responsible for intestinal gas or
flatus. They also produce vitamin K
(needed for clotting), Biotin (glucose
metabolism), and vitamin B5(steroid
hormones and neurotransmitters)
Digestion and Absorption
 The
digestive system breaks down the
physical structure of the ingested
material and then disassembles the
component molecules into smaller
fragments through hydrolysis.
 Amylase
breaks down complex
carbohydrates. These are broken down
into monosaccharides by other
enzymes and are absorbed by the
intestinal epithelium through facilitated
diffusion and co-transport.
 Triglycerides
are emulsified into large
lipid drops. The results interact with bile
salts to form micelles which diffuse
across the intestinal epitheliums.
 Protein digestion involves the gastric
enzyme pepsin and various pancreatic
proteases.
Water and Electrolyte Absorption
 About
2-2.5 liters of water are ingested
each day and digestive secretion
provide 6-7 liters
 Nearly all is reabsorbed by osmosis.
 Fat-soluble
vitamins are enclosed within
fat droplets are are absorbed with the
products of lipid digestion.
 The nine water-soluble vitamins are
important as cofactors and coenzymes
in enzymatic reactions..
Aging and the Digestive System
–related changes include
 a thinner and more fragile epithelium
 a reduction in epithelial stem cell
division and
 weaker peristaltic contraction as smooth
muscle tone decreases.
 Age