1. The Digestive System M 230A students Dr. Mukhallad Al-Janabi د

2. The digestive system consists of a tract plus the accessory organs.
The tract is a continuous tube that consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. Ingested contents pass through these organs in this order.
The accessory organs are the salivary glands, pancreas, and biliary system (liver and gallbladder).

3. Four main processes (functions)occur in digestive system
This system has four functions:
1.Motility is the muscular contractions that mix (mixing movements) and move (propulsive movements) the contents forward through the digestive tract.
2. Secretion is the transfer of digestive juices by exocrine glands into the digestive tract. The digestive secretion consists of water, electrolytes and organic substances like enzymes, bile salts and mucus. The secretions are controlled by neural or hormonal factors.

4. Functions of digestive system (continued)
3. Digestion is break down of food into smaller unites which can be absorbed. It involves chemical change (hydrolysis) of large molecules by digestive enzymes (e.g., carbohydrates, proteins, and fats) into their smaller subunits (e.g., starch into glucose, proteins into amino acids, etc).
An example of hydrolysis: the disaccharide maltose (the intermediate breakdown product of polysaccharides) is broken down into two glucose molecules by the addition of H2O at the bond site.
Glucose
Glucose
Maltose
4. Absorption is the passage of the products of digestion (e.g., monosaccharides like glucose, amino acids and fatty acids), along with water, vitamins, and electrolytes, into the blood and lymph.

5. Extra points about digestive processes
The smooth muscle in the walls of the digestive tract maintains a low level of contraction called tone. There are two types of digestive motility: propulsive movements and mixing movements. Propulsive movements push the contents forward through the tract. Mixing movements help in digestion and absorption.
Digestive secretions consist of water, electrolytes, and organic constituents such as enzymes, bile, salts, and mucus. They are released by hormonal or neural stimulation.
Polysaccharides (e.g., starch and glycogen) are chemically changed into disaccharides, which are changed into monosaccharides. Proteins are chemically changed into polypeptides, which are changed into amino acids. The end products of fat digestion are monoglycerides and free fatty acids.
Most absorption occurs in the small intestine.

6. Digestive motility and secretion are regulated by four factors.
Smooth muscle cells display rhythmic, spontaneous variations in membrane potentials. This is autonomous smooth muscle function.
The myenteric plexus and submucous plexus are the enteric nervous system (collection of neurons within the wall of GIT tract).
Extrinsic nerves of the autonomic nervous system innervate digestive structures from the outside. They modify digestive tract motility and secretion. Parasympathetic nerves signal maximum digestive activity.
Endocrine glands within the mucosa release hormones that signal digestive responses.
Within GIT there are chemoreceptors, mechanoreceptors, and osmoreceptors respond to changes in the digestive tract.

7. Receptors in digestive tract
External
influence
Local changes in
digestive tract
Receptors in digestive tract
Extrinsic
automatic
nerves
Intrinsic
nerve plexuses
Gastrointestinal
hormones
Smooth muscle
(contraction for motility)
Exocrine gland cells
(secretion of digestive juices)
Endocrine gland cells
(secretion of gastrointestinal
and pancreatic hormones)
Self-
excitable
= Short reflex
= Long reflex
= Hormonal pathway

8. The wall of the digestive tract consists of four layers.
1. The mucosa lines the luminal surface. The lamina propria is a middle layer of connective tissue. The muscularis mucosa is a sparse layer of smooth muscle.
2. The submucosa is under the mucosa This connective tissue has large blood and lymph vessels. It contains a submucous plexus.
3. The muscularis externa is the main smooth layer of the digestive tube. It has an inner circular layer and an outer longitudinal layer. Their contractions produce the propulsive and mixing movements. A myenteric plexus is between the two smooth muscle layers.
4. The serosa is the outer connective tissue layer. It is continuous with the mesentery along much of the tract.

9. The oral cavity is the entrance to the digestive tract.
Salivary secretion is continuous and can be reflexly increased.
Salivary Flow: L/day
Basal rate is about 0.5ml/minute
Maximum secretion 5 ml/minute
The amylase enzyme in salivary secretion digests starch into maltose. Lysozyme in the saliva lyses bacteria.
Saliva also keeps the mouth and teeth clean.
Both branches of the autonomic nervous system increase the rate of stimulation.

10. Function of Saliva
1. Moistens oral mucosa. It facilitates speaking , chewing and swallowing.
2. Moistens dry food, lubricates food to facilitate swallowing and cools hot food.
3. Provides a medium for dissolved foods to stimulate the taste buds.
4. Buffers oral cavity contents. Saliva has a high concentration of bicarbonate ions. It neutralizes acid in the food
5. Digestion. Amylase, contained in saliva, breaks polysaccharides into maltose.
6. Neutralizes any gastric acid that refluxes from stomach into the lower esophagus.
7. Controls bacterial flora of the oral cavity by:
Presence of enzyme called Lysozyme: lyses (destroys) certain bacteria by breaking their wall
Washing (rinsing) away remained food which acts as a food source for bacteria
** Absence of salivary secretion (xerostomia) leads to
Difficulties in swallowing, talking and chewing
Dental caries.

11. Innervation of salivary glands
Saliva production is controlled by salivary center in medulla through autonomic nervous system. Salivary secretion is the only digestive secretion that controlled only by neural factors
*Excitation of parasympathetic nerve fibers causes:
Increased watery secretion rich in electrolytes and enzymes.
*Excitation of sympathetic nerve fibers causes:
-slight increase in viscid saliva (containing mucus).

12. Swallowing
Swallowing center
inhabits respiratory
center in brain stem
Oropharyngeal stage: **moving the bolus from mouth through pharynx and then into esophagus.
**The following events occur during this stage
a. Uvula is elevated to prevent food to enter into nasal cavity.
b. Food is prevented from entering into trachea (respiratory system) by the following events
Elevation of larynx
Closure of glottis by closure approximation of vocal cords
Epiglottis tilted backward to cover the larynx
Respiration is stopped
Elevation of uvula
prevents food from
entering nasal passages
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

13. Swallowing (continued)
Involuntary swallowing is initiated when a food bolus contacts receptors in the pharynx. Afferent impulses are sent to the swallowing center in the medulla. This triggers various swallowing responses.
The pharyngoesophageal sphincter prevents air from entering the digestive tract during breathing. The sphincter keeps the esophagus closed.
Peristalsis waves push food through the esophagus.
A swallowing center initiates a primary peristaltic wave.
The gastroesophageal sphincter prevents reflux of gastric contents.
Esophagus secretes only mucus which protects the esophygeal lining against sharp edges of food and against any acid and enzyme of gastric juice if gastric reflux occurs.
Ringlike peristaltic contractions push bolus down

14. The stomach
Stomach has a fundus, body, and antrum. Its terminal part has a pyloric sphincter.
****One function of the stomach is to store food. Most of this occurs in the body of the stomach.
The stomach accommodates a twenty-fold increase in volume by receptive relaxation.
It also secretes HCl and pepsinogen enzyme to begin protein digestion.
****The mixing movements of the stomach mix food with gastric juice producing chyme.
Most of this mixing occurs in the antrum.

15. Gastric movements
**Mixing movements Occur mostly in antrum They mix the food with gastric content to produce chyme They are peristaltic contractions against closed pyloric sphincter
**Propulsive movements The peristaltic movements themselves push the chyme through pyloric region where chyme can leave stomach to duodenum
Pyloric sphincter
Peristaltic
contraction

16. Gastric emptying
After food is digested and mixed by peristalsis with gastric juice in the stomach. It is converted into chyme which is pushed by gastric motility into the small intestine.
***Gatric motility is increased by
1. Distension of the stomach
2. Signaling by the vagus nerve
3. the hormone gastrin
***Duodenal factors that decreases the gastric emptying.
a. Presence of undigested fat
b. Presence of acid
c. an increase in osmolarity of the chyme
d. Distension of the duodenum.
***Gastric emptying is inhibited by the following hormones
a. Secretin
b. CCK

17. Stomach Esophagus Gastroesophageal sphincter Pyloric sphincter
Duodenum
Direction of
movement
of peristaltic
contraction
FIGURE 15-8: Gastric emptying and mixing as a result of antral peristaltic contractions.
Peristaltic
contraction
Movement
of chyme
Gastric emptying
Fig. 15-8a, p. 477

18. VOMITING
***It is forceful expulsion of gastric contents out through the mouth.
The stomach does not actively participate in vomiting. The stomach, esophagus and upper and lower esophageal sphincters are relaxed
The force of vomiting arises from contraction of the diaphragm and abdominal muscles. These contractions leads to increased intra-abdominal pressure. The stomach is relaxed and squeezed to evacuate the food during vomiting. It is coordinated by the vomiting center in the medulla. During vomiting glottis is closed to prevent passage of vomited materials into respiratory passages. Respiration is also inhibited during vomiting.
****Causes of vomiting include tactile stimulation, irritation/distension of the stomach and duodenum, elevated intracranial pressure, chemical agents, and psychogenic factors.
***Severe vomiting can lead to dehydration, metabolic alkalosis and circulatory problems.

19. Table 15-2, p. 478

20. Gastric digestive juice
***Mucous cells secrete mucus.
***Parietal cells secrete HCl and the intrinsic factor. This factor is essential for the absorption of vitamin B12. The exocrine secretions of these cells are released into the gastric lumen.
Other secretory cells of the gastric mucosa release endocrine and paracrine regulatory factors.
The low pH in the stomach converts pepsinogen into the active form, pepsin.
Functions of hydrochloric acid in the stomach include pepsin formation, the breakdown of connective tissue and muscle fibers, the denaturation of proteins, and the killing of most microorganisms.
A mucus lining on the surface of the gastric mucosa is protective against acidity.

21. Types of gastric cells and their function
TABLE 15-3: The Stomach Mucosa and the Gastric Glands.

22. Parietal and chief cells
Acetylcholine from the intrinsic nerve plexuses stimulates these cells.
Histamine acts on parietal cells to increase hydrochloric acid secretion.
Gastrin (secreted by G cells) stimulates parietal and chief cells.
Somatostatin (secreted by D cells) inhibits secretions from parietal cells.
The control of gastric secretion involves three phases.
1. Cephalic phase: stimuli from the brain like thinking, tasting, smelling and chewing increase the secretion of hydrochloric acid and pepsinogen.
2. Gastric phase: stimuli in the stomach (e.g., proteins) distension initiate gastric secretions.
3. Intestinal phase: it is inhibitory, decreasing secretion of gastric juices.

23. Cephalic and gastric phases of gastric secretion

24. Other facts about stomach activity include:
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. Food is not mixed with gastric secretions in the body of the stomach.
The stomach absorbs alcohol and aspirin, but does not absorb food.

25. Gastric secretion is decreased by many factors:
1. Accumulation of acid in the stomach
2. As food leave the stomach, the protein concentration in the stomach is decreased and there is less distension of the stomach
3. When food reaches duodenum: presence of fat, acid, hypertonicity and distension of duodenum sends inhibitory signals to gastric mucosa to decrease the secretion

26. The pancreas has exocrine and endocrine cells
Endocrine cells of the islets of Langerhans secrete hormones.
The exocrine pancreas secretes digestive enzymes and an aqueous alkaline fluid. The alkaline fluid has sodium carbonate.
The enzymes are proteolytic enzymes, pancreatic amylase, and pancreatic lipase.
1. Trypsin, formed from trypsinogen, is a proteolytic enzyme. Chymotrypsinogen and procarboxypeptidase are other proteolytic enzymes.
2. Pancreatic amylase converts starch to disaccharides.
3. Pancreatic lipase hydrolyzes dietary lipids.
Pancreatic exocrine secretion is regulated by secretin and cholycycstokinin (CCK), enzymes secreted by the small intestine.
**Secretin causes secretion of sodium bicarbonate from the pancreas. **CCK regulates the secretion of pancreas digestive enzymes.

27. Exocrine portion of panaceas (Acinar and duct cells)
Bile duct
from liver
Duodenum
Stomach
Hormones
(insulin,
glucagon)
Blood
Endocrine portion
of pancreas
(Islets of Langerhans)
FIGURE 15-11: Schematic representation of the exocrine and endocrine portions of the pancreas. The exocrine pancreas secretes into the duodenal lumen a digestive juice composed of digestive enzymes secreted by the acinar cells and an aqueous NaHCO3 solution secreted by the duct cells. The endocrine pancreas secretes the hormones insulin and glucagon into the blood.
Duct cells
secrete aqueous
NaHCO3 solution
Acinar cells
secrete digestive
enzymes
The glandular portions of
the pancreas are grossly
exaggerated
Exocrine portion of panaceas
(Acinar and duct cells)

28. Acid in duodenal lumen Fat and protein products in duodenal lumen
Secretion release
from duodenal
mucosa
CCK release
from duodenal
mucosa
(Secretin carried
by blood)
Neutralizes
(CCK carried
by blood)
Digests
Pancreatic duct
cells
Pancreatic acinar
cells
FIGURE 15-12: Hormonal control of pancreatic exocrine secretion.
Secretion of
pancreatic digestive
enzymes into
duodenal lumen
Secretion of aqueous
NaHCO3 solution into
duodenal lumen

29. Pancreatic digestive enzymes
Pancreas secrets two types of secretion
1. Pancreatic enzymes from pancreatic acinar cells.
containing:
a. proteolytic enzymes (trypsinogen, chymotrypsin and procarboxypeptidase). All these enzymes are secreted in inactive forms.
In duodenal lumen trypsinogen is activated by entrokinase enzyme converting it into trypsin. Trypsin activates chymotrypsin and procarboxypeptidase.
b. Pancreatic amylase (secreted as active form) hydrolyzes starch into disaccharides
c. Pancreatic lipase (secreted as active form) acts on fat producing fatty acids and monoglycerides
2. Watery alkaline secretion from the pancreatic ducts for neutralizing the acid chyme and providing alkaline environment for optimal activity of pancreatic digestive enzymes.

30. Activation of pancreatic proteolytic enzymes

31. Liver functions It carries out the metabolic processing of nutrients.
It detoxifies or degrades body wastes.
It synthesizes plasma proteins.
It stores substances such as glycogen and fats.
It activates vitamin D.
It removes bacteria and worn-out old RBCs.
It excretes cholesterol and bilirubin.
Blood enters the liver from the digestive tract by the hepatic portal system. The portal vein of this system breaks into a capillary network, the liver sinusoids.
The liver lobules are delineated by vascular and bile channels. Hepatocytes continuously secrete bile into these channels.
Bile ducts from the lobules in the liver converge to form the common bile duct. This duct transports bile from the liver to the duodenum.
Bile is stored in the gallbladder between meals. After a meal the gallbladder secretes bile into the small intestine for fat digestion.
Hepatic artery
Portal vein

32. Bile salts aid fat digestion and absorption
The detergent action of bile emulsifies fats. Fat globules are broken into smaller droplets, increasing surface area to facilitate enzymatic attack (pancreatic lipase).
Pancreatic lipase is attached to a fat droplet by the polypeptide colipase.
Bile salts adsorb on the surface of small fat droplets, preventing the droplets for recoalescing. This also helps enzymatic attack.
Bile salts are the most potent stimulus for increased bile secretion. Its secretion occurs by a chemical mechanism, hormonal mechanism, and a neural mechanism.
Bilirubin is a waste product excreted in the bile.

33. Lipid-soluble portion
Negativity charged
H2O-soluble portion
(a carboxyl group at
the end of a glycine
or taurine chain)
Small lipid (fat)
droplet with bile
salt molecules
absorbed on
its surface
Lipid-soluble portion
(derived from cholesterol)
FIGURE 15-16: Schematic structure and function of bile salts. (a) Schematic representation of the structure of bile salts and their adsorption on the surface of a small fat droplet. A bile salt consists of a lipid-soluble part that dissolves in the fat droplet and a negatively charged, water-soluble part that projects from the surface of the droplet.

34. Large fat droplet Through action of bile salts Lipid emulsion
FIGURE 15-16: Schematic structure and function of bile salts. (b) Formation of a lipid emulsion through the action of bile salts. When a large fat droplet is broken up into smaller fat droplets by intestinal contractions, bile salts adsorb on the surface of the small droplets, creating shells of negatively charged, water-soluble bile salt components that cause the fat droplets to repel each other. This action holds the fat droplets apart and prevents them from recoalescing, increasing the surface area of exposed fat available for digestion by pancreatic lipase.

35. Micelle structure: it has two components
A. Outer is hydrophilic (water soluble)
B. Inner is lipophilic (lipid soluble )
Hydrophobic core
Hydrophilic shell
FIGURE 15-17: Schematic representation of a micelle. Bile constituents (bile salts, lecithin, and cholesterol) aggregate to form micelles that consist of a hydrophilic (water-soluble) shell and a hydrophobic (lipid-soluble) core. Because the outer shell of a micelle is water soluble, the products of fat digestion, which are not water soluble, can be carried through the watery luminal contents to the absorptive surface of the small intestine by dissolving in the micelle’s lipid-soluble core.
Water-soluble portion
Water-soluble portion
Lipid-soluble portion
Lipid-soluble portion
lipid-soluble products of fat
digestion like cholesterol,
Fatty acids and monoglycerides
Bile salt
Lecithin

37. The small intestine is where most digestion and absorption occur.
Its three segments are the duodenum, jejunum, and ileum.
The process of segmentation mixes
and slowly propels the food.
Segmentation mixes chyme
with secretions and slowly moves
the contents through the tract.
The circular smooth muscle responsiveness is influenced by the distension of the intestine, gastrin, and extrinsic nerve activity.
The ileocecal valve between
the small and large intestine.
It prevents contamination of the
small intestine by colonic bacteria.

38. Other facts about the small intestine
It does not secrete digestive enzymes. The pancreas secretes enzymes into small intestine.
The small intestine enzymes complete digestion intracellularly (inside intestinal epithelial cells). These include the disaccharidases and aminopeptidases.
The mucosal lining has a rapid turnover. The crypts of Lieberkuhn have stem cells for cell regeneration.

39. (Continue to next slide)
The small intestine has adaptations to maximize absorption. The mucosal lining has a large surface area due to its circular folds and fingerlike projections called villi. The epithelial cells also have microvilli. All these factors will increase the surface area of absorption 600 times
Circular
fold
Villus
FIGURE 15-20: Small-intestine absorptive surface. (a) Gross structure of the small intestine. (b) One of the circular folds of the small-intestine mucosa, which collectively increase the absorptive surface area threefold.
(Continue to next slide)

40. A villus has a cover of epithelial cells,
Capillaries
A villus has a cover of epithelial cells,
a capillary network, and the terminal
lymphatic vessel.
During absorption molecules
produced by digestion enter the
capillary or lymphatic vessel.
Mucous cell
Central lacteal
Crypt of Lieberkühn
Arteriole
Venule
Lymphatic vessel
FIGURE 15-20: Small-intestine absorptive surface. (c) Microscopic fingerlike projection known as a villus. Collectively, the villi increase the surface area another tenfold. (d) Electron microscope view of a villus epithelial cell, depicting the presence of microvilli on its luminal border; the microvilli increase the surface area another 20-fold. Altogether, these surface modifications increase the small intestine’s absorptive surface area 600-fold.
Microvilli

41. Absorption of carbohydrates and proteins
Glucose and galactose are absorbed from intestinal lumen into the interior of intestinal cells by secondary active transport with Na+. They are cotransported with sodium. Glucose and galactose leave the intestinal cell into extracellular fluid by facilitated diffusion.
Fructose is absorbed by
passive facilitated diffusion.
Amino and small peptides
are absorbed across intestinal
cells by secondary active
transport with Na+.

43. Absorption of water
Energy-dependent sodium transport absorption drives passive water absorption.
Sodium is pumped from the tract lumen into the interstitial fluid. From there it enters capillaries by diffusion.
The transport of sodium creates an osmotic pressure. Water follow the sodium as it is absorbed.

44. Fat absorption
Monoglycerides and free fatty acids are produced by hydrolysis of lipids.
These water-insoluble products are carried to the inside of water-soluble micelles.
On the mucosal surface these molecules leave the micelle and passively diffuse through the lipid bilayer of the luminal membrane. They are resynthesized into triglycerides inside the epithelial cells. There they form water-soluble chylomicrons which leave the cells by exocytosis.
They enter the central lacteals, lymphatic vessels.

45. Lipid emulsion
Micelles
diffusion
Lumen
Lumen
Micelle
Micelles
Microvillus
Fatty acids,
monoglycerides
FIGURE 15-23: Fat digestion and absorption.
Aggregate and
coated with
lipoprotien
Short or
medium
chain
Passive absorption
Basement
membrane
Epithelial
cell of villus
(Exocytosis)
Central lacteal
Capillary

46. Adsorption of vitamins, iron and calcium
Vitamin adsorption is mainly passive. Water-soluble vitamins are absorbed with water. Fat-soluble vitamins are absorbed in micelles.
Iron and calcium absorption is regulated. Only a part of ingested iron can be absorbed.
Some absorbed iron is immediately transported to the blood. Transferrin carries some iron to the bone marrow. Excess iron is stored in the ferritin pool. Unused iron is lost in the feces.
Most calcium is absorbed by active transport. About two-thirds of ingested calcium is absorbed. The remaining one-thirds is eliminated.
Most absorbed nutrients immediately pass through the liver for processing. The liver monitors nutrient molecules and controls their concentration in the blood or lymph leaving the liver.

48. Large intestine It consists of the colon, cecum, appendix, and rectum.
The colon consists of the ascending colon, transverse colon, descending colon, and sigmoid colon.
Most digestion and absorption has been accomplished in the small intestine. The colon receives indigestible food, unabsorbed biliary components and the remaining fluid.
The colon absorbs water and salt and eliminates the feces.

49. Large intestine (continued)
**Contractions of the haustrae slowly shuffle the colonic contents back and forth. They are initiated by the autonomous rhythmic contractions of the smooth muscle in the wall of the large intestine.
The large-intestine secretion is entirely protective. It consists of an alkaline mucus solution which protects the intestinal mucosa.

50. Large intestine (continued)
***The colon contains many beneficial bacteria. Their functions include
making vitamin K
2. Enhancing intestinal immunity
3. Increasing colonic motility
Mass movements propel colonic contents long distances. They occur three to four times a day and usually after meal. They drive the feces into the distal part of the large intestine and hence triggering defecation reflex.

51. Gastrointestinal hormones.
***Gastrin
increases the secretion of hydrochloric acid
enhances gastric motility.
***Secretin
inhibits gastric emptying and gastric secretion,
stimulates the pancreas to produce sodium bicarbonate
stimulates the liver to produce bile.
***CCK
inhibits gastric motility and secretion
stimulates pancreatic acinar cells to produce digestive enzymes
causes contraction of gallbladder