Digestive System General Functions of the Digestive System 1. Motility: a. Ingestion b. Peristalsis: rhythmic contractions moving food through the gastrointestinal tract. c. Defecation/excretion: removal the unabsorbed materials out of the body. 2. Secretion:
General Functions of the Digestive System
b. Peristalsis: rhythmic contractions moving food
through the gastrointestinal tract.
c. Defecation/excretion: removal the unabsorbed
materials out of the body.
Exocrine secretion: mucus, HCl, enzymes,
H2O... - digestive juice, 2-3 L/day (70 % is
Endocrine secretion: hormones that regulate
Mechanical and chemical decomposition of food.
4. Absorption: transport of digested products
into blood and lymph.
Lymphatic system: drain interstitial fluid and
return it into veins, absorb lipid molecules from
the digestive tract and send them to blood stream.
1). Primary organs – gastrointestinal tract:
Mouth pharynx esophagus stomach
small intestines large intestines rectum
(Food: Bolus chyme feces)
2). Accessory organs (2nd):
Teeth, tongue, salivary glands, pancreas, liver
and gall bladder.
Muscularis –inner circular and outer longitudinal
layers of smooth muscles, responsible for
segmental and peristaltic movement through the
Between the muscle layers: nerve fibers and
ganglia of sympathetic and parasympathetic
Serosa– connective tissue, forms the very outside
layer of the GI tract.
Functions of each organ:
A. Mouth:(mechanical digestion)
Saliva contains: H2O, mucus, and enzymes:
Amylase: breaks down starch into
Lysozyme: kills bacteria.
Immunoglobulin - IgA: an antibody
Functions of saliva: moistening food, dissolve
Tongue: The mucous membrane of tongue
contains taste buds, mucus glands, glands that
Inside of the tongue there are skeletal muscles
that makes voluntary movement.
Lipid digestion begins in the mouth by lingual
Functions of the tongue… participating swallowing.
Swallowing process is the result of corporation
of tongue, larynx and pharynx.
Tongue lifts up which causes the elevation of
larynx to close trachea and open esophagus.
So food enters the esophagus but not trachea.
This is a precisely controlled process.
The upper part contains skeletal muscle; the
lower part contains smooth muscle.
Skeletal muscle near pharynx perhaps
participates in swallowing action.
Lower esophageal sphinctercontracts after
food passes into the stomach, which prevents
the food from coming back to the esophagus.
Cardia - upper opening
Pylorus - lower opening
Pyloric antrum - is the area near pylorus.
Pyloric sphincter -
Smooth muscles in the stomach body are oriented
in different directions.
The epithelial cells of mucosa form gastric
glands and secrete different products to form
Goblet cells – secrete mucus;
Parietal cells – produce HCL
Chief cells – produce pepsinogen (a zymogen,
inactive form of pepsin);
ECL cells – secrete histamine and serotonin;
G cells (endocrine cells) – gastrin;
Intrinsic factor required for the absorption of
vitamin B12, is produced by parietal cells.
Gastric juice contains all the above substances
Food is mixed with gastric juice in stomach to
form paste like substance called chyme.
Several important components in the gastric
Pepsin, a protease that cleaves peptide bonds
and digests food proteins.
It’s optimal working pH is 1-2.
Pepsin is first synthesized as pepsinogen, which
is cleaved and activated after it gets into the lumen
of the stomach.
HCLis secreted by parietal cells.
H+ is transported into the stomach against its concentration gradient by H+/K+ exchange pump.
While H+ is pumped into the lumen of the stomach, K+ is taken up into the parietal cells.
Five functions of HCL:
a. Kills microorganisms;
b. Causes denature of proteins.
c. Breaks down cell walls of plants;
d. Activates pepsin, and provides an optimal
environment for pepsin to function.
Cooperative activities of pepsin and HCl
allows the partial digestion of ingested proteins
in the stomach.
e. The acidic environment is also good for the
absorption of Ca and Fe.
Mucus produced by Goblet cells, contains
HCO3- and forms an alkaline coating to prevent
the inner lining of stomach from being digested
by HCl and pepsin.
Digestion and Absorption in the Stomach
Proteins are partially digested in the stomach.
The polypeptide chains are cleaved by pepsin
into oligopeptides, tripeptides and dipeptides.
There is very little digestion for carbohydrates
and fats in the stomach.
The major digestion process is in the small
Absorption in the stomach is very limited.
The substances absorbed in stomach:
alcohol, water, small amount of minerals
(Ca, Fe, Na, Cl), and certain drugs, such as
aspirin and anti-inflammatory drugs.
The stomach empty time: for solid food is 4-6 hours;
water - about 30 min.
Pyloric sphincter controls the emptying speed
of stomach. It also prevents the intestinal content
from backing up into the stomach.
Regulation of The Stomach Function:
(Nervous and hormonal controls)
The movement and secretion of stomach are
both stimulated by parasympathetic nerves.
Cholinergic receptor blocker can decrease HCl
secretion and smooth muscle contraction …
Locally produced hormone, gastrin, can also
stimulate secretion and mobility of stomach.
Secretion of gastrin is triggered by food entering
The effect of ACh and gastrin on gastric secretion
Both ACh and gastrin can stimulate ECL cells
to produce histamine, which causes parietal
cells to produce HCL.
Gastritis and peptic ulcer:
Gastritis - inflammation in the stomach mucous
membrane, can be induced by certain drugs
(aspirin, steroids, anti-inflammatory drugs)
Infection of a bacterium, helicobacter has been
identified, as a direct cause of gastritis and
Emotional and physical stress also contribute
Peptic ulcer or digestive ulcer, is open wound
on the mucus membrane of the stomach or
duodenum, often developed on the basis of gastritis.
The mucous membrane of the stomach is digested
by HCl and pepsin.
General mechanism: combination of over
production of gastric acid and underproduction
of the alkaline mucus.
(Helicobector is also involved.)
More specifically, duodenal ulcer may be more
related with the over production of gastric acid.
Gastric ulcer is often due to insufficient production
of alkaline mucus.
Gastric ulcer is closely related with stomach cancer.
The traditional treatment for digestive ulcer is
acid control in combination with antacid drugs.
Cholinergic nerve blocker to inhibit ACh release
New generation: histamine receptors blocker:
Antibiotics are sometimes prescribed.
E. Small Intestines
The major site for digestion and absorption
Approximately 3 M long in a living person, but it
will measure 6 M in a dead body, because of the
relaxation of longitudinal muscles.
Small intestines are held by mesentery - a broad
sheet of connective tissue, which contains blood
vessels lymphatic tubes and nerves.
Three segments: duodenum, jejunum, and
Structure of Villus (Fig. 18.12)
A finger-like fold of mucosa that projects into the
Each villus is covered by columnar epithelial cells
with goblet cells in between.
In the center: connective tissue, capillary vessels,
and a lacteal.
The absorbed carbohydrates and amino acids enter
capillaries in the center of the villus, while lipids
enter the central lacteal.
Microvilli are brush like structure (brush border)
formed by epithelial cells on the surface of villus.
Functions of small intestines:
1), Receiving the digestive juice from pancreas
and liver (enzymes and bile salts).
Intestinal juice, 1.8 L/day. It moistens chyme
and neutralize it’s pH.
Before absorption, pH of the food content must
a. NaHCO3 - is secreted by pancreas and small
NaHCO3 Na+ + HCO3-
H+ + HCO3 H2CO3 CO2 + H2O
Duodenum is the major location of buffering action.
Duodenum (10 inches long), receives chyme
from stomach and neutralize it.
The pH of chyme here changes from 1 to 7.5.
pH of the rest of the small intestines is ~ 7.5,
which is optimal for most enzymes here.
Disaccharides, trisaccharides, dipeptides and
tripeptides are further broken down by enzymes
in the small intestines.
Brush border enzymes stick to the cell surface and
are exposed to the intestinal content.
Enterokinase is a brush border enzymes that
activates trypsin that is produced by pancreas.
Sucrase, maltase, and lactase are also produced
by intestinal epithelial cells.
(Lactase gene may stop expressing in adults.)
Intestinal epithelial cells also make dipeptidase
to cleave dipeptides into single amino acids which
can then be absorbed.
c. Small intestines also produce hormonethat
regulate secretion of stomach, pancreas and
The arriving of chyme to duodenum stimulates
the secretion of:
Chyme contianing lipids and proteins secretion
of CCK, which stimulates contraction of gall
bladder and secretion of pancreas.
Arrival of chyme into duodenum also causes
local production of secrenin, which stimulates
the secretion of alkaline buffer, Na2HCO3, to
neutralize the acidic chyme.
Secrenin also inhibits gastric secretion and
4). Digestion and absorption
The absorption of carbohydrate, lipids, Ca, Fe
mainly occurs in duodenum and jejunum.
Bile salts, water and electrolytes are absorbed
Movement of small intestines:
Segmentation is the major movement form in
Small, periodic, ringlike contractions cut chyme
into segments and move it back and forth.
Peristalsis is weak in the small intestines.
The smooth muscles in the small intestines are
controlled by vagus.
Smooth muscle cells have their own pace maker
to produce slow rhythmic contraction even when
there is no neural stimulation.
Stimulation from parasympathetic system can
accelerate both peristalsis and segmentation.
If small intestines are overdistended or irritated,
a strong peristaltic rush may pass through the
entire length, sweeping the contents of the small
intestines into the large intestines so quickly, that
water, electrolytes and other substances that
would normally be absorbed are excreted through
2). Colon: from right to left, it includes
ascending, transverse, and descending colons.
4). Anal canal.
The main function of large intestines is the
reabsorption of water and electrolytes.
There is 1500 ml of materials enters the large
intestines each day, but only 200 ml becomes
Other functions of large intestines:
Reabsorb and excrete bile salts.
E.coli that normally residing in the large intestines
produce vitamin K and some B vitamins, which
can be absorbed in the large intestines.
G. Liver, Gall bladder andPancreas
Liver located at the upper right corner of the
Gall balder is attached to the liver between its two
Liver is full of blood supply. Liver cells are
called hepatocytes, which form hepatic plates
that are one to two cells thick.
The spaces between hepatic plates are called
Sinusoids, whichmimic capillary vessels.
However, hepatic plates are more permeable.
They have large pores and lack basement membrane, which allow passage of proteins, fat
Within each hepatic plate, there are thin bile tubes
that collects bile juice produced by hepatocytes.
Kupffer cells ?
Hepatic portal system
All blood leaving the small intestines is transported
to liver through hepatic portal vein.
The blood is filtered in the liver and then enters
hepatic veins, which drains to inferior vena cava.
Hepatic portal system:
vein capillaries vein.
The purpose is to extract nutrients and to remove
toxins before they enter systemic circulation.
(The liver also receives blood from hepatic artery.)
Another example, alcohol is hydrolyzed and
broken down by alcohol dehydrogenase in the
Also, liver converts lactic acid into glucose;
and toxic purines into uric acid.
Disadvantage: drugs can be deactivated in the
3. Participating the metabolism of glucose,
protein and lipids
While being filtered by liver, blood glucose level
If blood glucose is high, liver absorbs some of it
for the synthesis of glycogen.
When blood glucose is low (fasting), glycogen
is broken down in to glucose, which is released
The liver monitors amino acids levels in the
It removes excessive amino acids and use them
for the synthesis of new plasma proteins or
convert them to glucose or lipids to be stored.
When blood amino acids are low, the liver will
break down glycogen and triglycerides to
synthesize and release amino acids to circulation.
Similarly, liver controls blood levels of triglyceride,
fatty acids and cholesterol.
When these molecules are low, liver breaks
down the fat storage to release them to blood.
When they are high, they are removed from
blood to be stored in the liver as glycogen or fat.
4. Storage:Besidesglycogen and fat, vitamins
(A, D, E, K and B12) and Fe can also be stored
in the liver.
Over ingestion of the lipid soluble vitamins can
cause liver damage.
Liver is a biochemical factory where many plasma
proteins are synthesized.
Albumin accounts for 70% of plasma proteins
and is important for maintaining blood osmotic
Thrombin and other clotting factors;
Transport proteins: LDL and HDL;
Bile is produced by hepatocytes. It contains bile
salts, billirubin, billiverdin, cholesterol and other
Bile is continuously produced by liver cells
but is not continuously released to small
After being produced, bile is stored in gallbladder.
It is only released into duodenum after meals.
When chyme reaches duodenum, CCK will be
produced which stimulates gallbladder to contract,
and bile will be ejected.
When the small intestine is empty, the sphincter
of ampulla at the end of common bile duct closes,
forcing bile into systic duct and gallbladder to
Pancreas duct and bile duct join into one tube that opens
to duodenum (duodenal papilla).
Bile juice produced by hepatocytes
hepatic duct cystic duct gallbladder
cystic duct common bile duct duodenum.
The function of bile salts is to assist the digestion
Pancreas - endocrine and exocrine gland
Located in the upper right abdomen behind
The endocrine function is carried out by
pancreatic islet cells.
Pancreas also secretes pancreas juice into
Components of pancreatic juice (Table 18.5)
contains water, bicarbonate and many digestive
a.Pancreatic amylase digest starch;
b,Pancreatic lipase that splits triglyceride
into fatty acids and glycerol.
c,Proteinases (trypsin, chymotrypsin, and
peptidase) cleave polypeptides into
dipeptides and tripeptides.
d, Pancreatic nuclease cleaves nucleotide
chains into single nucleotides.
Digestion and Absorption in Small Intestines
Bile is secreted in response to the arrival of lipids
After emulsification, pancreatic lipase hydrolyzes
triglycerides and phospholipids into glycerol, fatty
acids and lysolecithin.
These small subunits then associate with bile salts
and cholesterol to form mixed micelles.
These micelles are little particles that have
hydrophilic portion outside and hydrophobic
portion in the middle, which can be easily
absorbed by epithelial cells.
All digestive enzymes are listed in table 18.8.
Absorption of six categories of nutrients mainly
occurs in the small intestines
Monosaccharides (glucose) and amino acids are
absorbed into the intestinal epithelial cells and
secreted into capillaries through facilitated
diffusion and active transport.
Fatty acids, lysolecithin, cholesterol and glycerol
contained in micelles can diffuse into epithelial
Within the cytoplasm of epithelial cells,
triglyceride is resynthesized.
Triglycerides, together with cholesterol,
phospholipids are then coated with proteins to
form chylomicrons, which are secreted by
epithelial cells into lacteals through exocytosis.
Lymphatic fluid containing the absorbed lipids
will eventually join the blood circulation at
Water soluble vitamins are mostly absorbed in
small intestines via diffusion.
Fat soluble vitamins are mixed with triglycerides
and are absorbed together with them.
Absorption outside of the small intestines:
Stomach can absorb alcohol (diffusion), small
amount of water (by osmosis), vitamin B12
(active transport, bound with intrinsic factor),
Fe, and Ca (active transport).
Absorption in the large intestines:
Water (hand out), bile salts, Vitamins K and
Biotin (produced by E.coli).
Regulation of pancreas and gall bladder secretion