Download CHAPTER 17: DIGESTIVE SYSTEM

LECTURE 6: DIGESTIVE SYSTEM
INTRODUCTION
Well over half of the people who seek medical attention have gastrointestinal complaints. This striking fact is a
tribute to the complexity, diversity, and conspicuous impact of digestive system function and dysfunction. The
clinical problems highlight many of the key structures and processes found along the digestive tube. The oral
cavity may be affected, from tooth and gum problems to ulcers of the buccal and lingual mucosae. The pharynx
may be inflamed (“sore throat”). Swallowing may be disturbed (dysphagia), for example, after a stroke. The
lower esophageal sphincter may be too tight (achalasia) or too loose (gastric reflux). Erosive ulceration of the
gastric or duodenal mucosa may lead to fatal blood loss if the submucosa becomes involved. Gallbladder
disease may cause pain that mimics a myocardial infarction. Loss of the smooth muscle tone may paralyze
transport in the small bowel, and diabetic neuropathy may interfere with the large bowel. Varicosities of veins
that drain the pelvic floor may intrude into the anal canal as hemorrhoids. Constipation or diarrhea may follow
from osmotic and secretory disturbances as well as transport problems originating in the myenteric plexus and
muscularis externa. Digestion has two paramount functions: releasing nutrients from the food and absorbing
those nutrients. Beyond the challenge of finding suitable food, there are several problems that arise from these
roles. Digestive juices capable of breaking food down into its component nutrients must be quite powerful so
the digestive system must have protection from these chemicals to avoid auto digestion. The digestive juices
must be released at appropriate times and in adequate amounts to provide for proper food breakdown. Nutrient
absorption must also be efficient in order to supply adequate resources for the human body. Thus, large
absorptive surface areas are required. Finally, the indigestible remnants of the food must be eliminated.
Digestive system organs fall into two main groups: the alimentary canal and the accessory organs. The
alimentary canal, or gastrointestinal (GI) tract, is the continuous muscular digestive tube that winds through the
body digesting and absorbing foodstuff; its organs include: the mouth, pharynx, esophagus, stomach, small
intestine, and large intestine. Accessory digestive organs aid digestion physically and produce secretions that
break down foodstuff in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands,
liver, and pancreas. In this lecture we will discuss organs of digestive system and their role during the process
of: ingestion; propulsion; digestion; absorption; and defecation.
1. Ingestion - the simple act of putting food into the mouth;
2. Propulsion - moves food through the alimentary canal and includes both swallowing and peristalsis.
Peristalsis is waves of muscular contractions that propel the contents of the digestive tract from one point to
another.
3. Digestion is the process of breaking food into smaller particles so they could be absorbed
a. Mechanical digestion is the physical process of preparing the food for chemical digestion and
involves chewing, mixing, churning, and segmentation
b. Chemical digestion is a series of catabolic steps in which complex food molecules are broken
down to their chemical building blocks by enzymes.
4. Absorption is the passage of digested end products from the lumen of the GI tract through the mucosal
cells into the blood or lymph.
5. Defecation eliminates indigestible substances from the body via the anus as feces.
OVERVIEW
The digestive system creates an optimal internal environment for its functioning in the lumen of the GI tract, an
area that is technically outside of the body. Digestive activities within the GI tract are triggered by mechanical
and chemical stimuli. Controls of the digestive activity are both extrinsic and intrinsic (nervous and hormonal)
i.e. the activities of the digestive system are regulated by: the contents of the digestive tract (local);
parasympathetic and sympathetic neurons; and hormones.
1. Every cell require, oxygen and organic molecules for their function
2. Digestive system provide the organic molecules
3. Digestion - The process by which food substances are changed into forms that can be absorbed through
cell membranes.
4. Digestive Processes:
a.
b.
c.
d.
Ingestion = taking food into the mouth.
Movement of Food = the passage of food along the gastrointestinal (GI) tract.
Digestion = the breakdown of food by chemical and mechanical means.
Absorption = the passage of digested food from GI tract into bloodstream (and lymph) for
distribution to cells.
e. Defecation = the elimination of undigested material from GI tract.
5. Digestive Organs
a. Two categories:
i. Alimentary canal (GI Tract), which extends from mouth to anus: mouth → pharynx →
esophagus → stomach → small intestine → large intestine
ii. Accessory organs release secretions into the alimentary canal that help digest food.
Organs include: salivary glands; liver; gallbladder; and pancreas
GENERAL CHARACTERISTICS OF THE ALIMENTARY CANAL
1. Structure of the Wall
a. There are Four Distinct Layers of the wall.
i. Mucosa = innermost (surrounds lumen). Mucosa is the innermost, moist, epithelial
membrane that lines the entire digestive tract. It secretes mucus, digestive enzymes, and
hormones; absorbs digestive end products into the blood; and protects against infectious
disease.
1. Epithelium extends into lumen = villi (increases surface area).
2. Contains many glands that secrete mucus (lubrication & protection from harmful
action of digestive enzymes)
3. Function include: protection; secretion; and absorption (of nutrients)
ii. Submucosa = beneath mucosa. Submucosa is a moderately dense connective tissue layer
containing blood and lymphatic vessels, lymphoid follicles, nerve fibers and in some
region contains endocrine gland.
1. Function include: nourishment of mucosa; carrying absorbed nutrients away
iii. Muscularis = two layers of muscle. This layer is typically consists of smooth muscle
and is responsible for peristalsis and segmentation i.e. these layers play role in
mechanical processing and movement of material. Movement coordinated by neurons of
myenteric plexus.
1. The two layers of muscles are: circular muscle layer around submucosa; and
longitudinal layer around circular layer
2. Function: movements of food through canal (segmentation = mixing & peristalsis
= movement)
iv. Serosa = outermost layer. Serosa, the protective outer layer of the intraperitoneal organs,
is the visceral peritoneum
1. Functions include: lubrication and free movement of canal in abdominal cavity
2. Intestinal peritoneal extensions = mesentery.
a. suspend the length of the intestine within abdominal cavity
2. Movements of the Tube
a. Smooth Muscles show rhythmic cycles of activity. They undergo spontaneous depolarization
and their contraction trigger wave of contraction that spreads.
b. Mixing: (mechanical digestion)
i. Segmentation is the contraction that is produced in small intestine and some portion of
large intestine leads to mixing of food with digestive juices and mucus
c. Peristalsis:
i. Peristalsis is the contraction muscularis externa that propel food from one portion to
another portion of GI tract leads to movement of food
ii. Accomplished by movements of longitudinal muscle layer
d. Sphincter Muscles play an important role in movements throughout the GI tract also.
i. Sphincter = a strong circular muscle which prevents regurgitation of food.
ii. Locations: between (regions) organs of digestive tract.
1. Gastroesophageal sphincter (Between esophagus and stomach) – to prevent acid
reflex i.e. to prevent entry to chyme from stomach to esophagus (Heartburn is
usually caused when gastric juice from the stomach enters the esophagus).
2. Pyloric sphincter (between stomach and small intestine (duodenum) – to prevent
premature entry of chyme from the stomach to the first part of intestine called
duodenum
3. Ileocecal valve (between small and large intestine)
4. Internal and external anal sphincter (between large intestine to outside)
e. Innervation of the Tube
i. Autonomic Nervous System
1. Parasympathetic – activates digestion
2. Sympathetic – slows digestion
ii. Post-ganglionic networks
1. Submucosal plexus controls secretions.
2. Myenteric plexus (myenteric plexus is a network of neurons) controls peristalsis
MOUTH
The terms oral cavity and buccal cavity refer to the mouth. The mouth is a stratified squamous epithelial
mucosa-lined cavity with boundaries of the lips, cheeks, palate, and tongue. The mouth is adapted to receive
food and start mechanical and chemical digestion by chewing and mixing food with saliva. Mastication, or
chewing, begins the mechanical breakdown of food and mixes the food with saliva
1. Function: analysis of material, mechanical processing, lubrication by mixing with mucus and saliva,
limited digestion (e.g. carbohydrates digestion begins in oral cavity)
2. Anatomy: The mouth is surrounded by cheeks, lips, tongue and palate:
a. Cheeks and Lips
i. Cheeks form the lateral walls of mouth.
ii. Lips surround the opening of mouth; important in monitoring food temperature
b. Tongue
i. Muscular (skeletal) organ on floor of mouth
ii. Contains many bumps called papillae, which house taste buds i.e. the surface of tongue
is covered with forest of fine projections called lingual papillae
iii. Posterior root contains lymphatic tissue called the lingual tonsil
iv. its primary function is: mechanical processing (mixing food), manipulation to assist
chewing and swallowing, sensory analysis, secretion of mucus and enzyme (lingual
lipase)
c. Palate = roof of mouth
i. Anterior portion = hard palate.
ii. Posterior portion = soft palate.
iii. Median extension of soft palate = uvula.
3. Salivary Glands
a. Salivary glands produce saliva, which cleanses the mouth, dissolves food chemicals for taste,
moistens food, and contains chemicals that begin the breakdown of starches.
b. Saliva binds food together and begins chemical digestion of polysaccharides by the enzyme
salivary amylase.
c. There are three pair of salivary glands in oral cavity:
i. Parotid salivary gland- produces thick, serous secretion containing amylase (enzyme that
breakdown starch)produces 25% of saliva
ii. sublingual salivary glands- situated in the floor of mouth, produce watery mucous
secretion that act as buffer and lubricant, produces 5% of total saliva
iii. submandibular salivary glands –situated in floor of mouth along inner surface of
mandible, secret glycoprotein (responsible for lubrication) mixture of buffers, salivary
amylase, produces about 70% of total saliva
4. Saliva
a. Mastication, or chewing, begins the mechanical breakdown of food and mixes the food with
saliva
b. Saliva Composition:
i. 99.4 % water
ii. The remaining 0.6% is electrolytes, buffers, glycoproteins, Abs, lysozyme, enzymes, and
waste product
c. Functions:
i. Provide lubrication
ii. Help to control bacterial populations in the mouth
iii. Bind food together = moistening
iv. Begin chemical digestion of carbohydrates
v. Enzyme = salivary amylase breaks polysaccharides into disaccharides (starch
(polysaccharide)  disaccharides; glycogen (polysaccharide)  disaccharides)
PHARYNX
Pharynx (throat) serve as passageway for food and air thus it is shared by bot digestive and respiratory systems
i.e. passageway of food into esophagus and air into larynx/trachea. Foods pass through the oropharynx and
laryngopharynx to esophagus
1. Structure of the Pharynx:
a. Nasopharynx – superior to soft palate, posterior to nasal cavity
b. Oropharynx – posterior to mouth down to epiglottis
c. Laryngopharynx – inferior to oropharynx from epiglottis to cricoid cartilage
ESOPHAGUS
The esophagus provides a passageway for food and fluids from the laryngopharynx to the stomach where it
joins at the cardiac orifice
1. Anatomy: it is a hollow muscular tube with length of 25cm and diameter of 2cm. It is innervated by
sympathetic and parasympathetic fibers from esophageal plexus
2. Function: passageway for food from pharynx to stomach
3. Location: mediastinum behind trachea goes through diaphragm at esophageal hiatus
STOMACH
The stomach is a temporary storage tank where the chemical breakdown of proteins is initiated and food is
converted to chyme. It performs 4 major functions: storage, mechanical breakdown, disruption of chemical
bonds, and production of intrinsic factor.
1. Anatomy: The major regions of the stomach include the cardiac region, fundus, body, and pyloric
region:
a. The cardia – The portion of the stomach that connects to the esophagus. It is smallest part,
contain abundant mucous glands that secretes mucus which protect stomach from acid and
enzymes i.e. mucus prevents digestion of stomach wall by enzyme and stomach acid.
b. The fundus – gastric glands in in fundus and body secrete most of the acid and enzyme
c. The body –largest region, mixing food and secretin
d. The pylorus –it is the curve of the J-shaped, it is divided into pyloric antrum (connects to the
body) and pyloric canal (empties to duodenum), a pyloric sphincter regulate the release of chyme
to duodenum.
2. Microscopic Anatomy
a. The surface epithelium of the stomach mucosa is a simple columnar epithelium composed of
goblet cells, which produce a protective two-layer coat of alkaline mucus.
b. The gastric mucosa is pocked with depressions called gastric pits, that are lined with columnar
3.
4.
5.
6.
epithelium i.e. gastric pit are pockets in the lining of the stomach that contain secretory cells.
Two or three tubular glands open into the bottom of each pit. In the cardiac and pyloric regions,
they are called cardiac glands and pyloric glands. In the rest of the stomach, they are called
gastric glands.
c. The gastric glands of the stomach produce gastric juice, which may be composed of a
combination of mucus, hydrochloric acid, intrinsic factor, pepsinogen, and a variety of
hormones.
d. The mucosa and submucosa are flat and smooth when the stomach is full, but form longitudinal
gastric rugae when emptied i.e. there are folds called rugae that allows stomach to expand.
Functions of Stomach:
a. Mechanical digestion – churning
i. Mechanical breakdown is achieved by muscle contraction and mixing of food with
stomach juice that produce a viscous, highly acidic, soupy mixture called chyme
b. Chemical digestion of proteins – gastric juice
i. Chemical breakdown is achieved when food is mixed with gastric juice and chemical
bond of food components (e.g. denature and break peptide bonds of protein, break
connective tissue) are hydrolyzed (broken down) by enzymes and hydrochloric acid
produced by gastric glands.
c. Secretion
i. Secretes gastric juice (hydrochloric acid; pepsinogen; gastrin; intrinsic factor)
d. Storage
i. Serves as a storage - bulk storage of undigested food
Gastric Secretions (Gastric Juice):
a. Composed of:
i. Mucus –
1. Function: lubrication, protection of mucosa from digestion
ii. Digestive enzyme pepsin – Note: pepsinogen is produced by chief cells and then
pepsinogen (inactive form of enzyme) is converted to pepsin (active form of enzyme) by
hydrochloric acid.
1. Function: protein digestion (into peptides)
iii. Hydrochloric acid (HCl)
1. Functions: HCl kill most bacteria, denature proteins, inactivate food enzymes,
breakdown plant cells and connective tissue in meat, activate pepsinogen to
pepsin
iv. Intrinsic factor
1. Function: Aids absorption of Vitamin B12. Vitamin B12 is needed for
erythropoiesis (production of Red Blood Cells), thus problem with secretion of
intrinsic factors will lead to vitamin B12 deficiency and consequently lead to
anemia.
v. Gastrin
1. Function: Regulatory hormone. Gastrin stimulates parietal cells to produce HCl
and chief cells to secrete pepsinogen i.e. HCl and pepsinogen is produced in
response to gastrin
Four types of gastric cells in Gastric Glands:
a. Mucous cells secrete mucus.
b. Chief cells secrete pepsinogen and pepsinogen is converted to pepsin in acidic environment (by
hydrochloric acid).
c. Parietal cells secrete HCl & intrinsic factor.
d. G-cells secrete gastrin, and gastrin stimulates secretion of HCl and Pepsinogen
Regulation of Gastric Secretions:
a. Gastric secretion is controlled by both neural and hormonal mechanisms and acts in three distinct
phases: the cephalic phase, the gastric phase, and the intestinal phase.
i. Neural
1. Parasympathetic – increases gastric secretion
2. Sympathetic – decreases gastric secretion
ii. Hormonal
1. Gastrin – increases gastric secretion by stimulating parietal cells and chief cells to
produce HCl and pepsinogen
2. Somatostatin – decrease acid secretion from parietal cells
3. Cholecystokinin – decreases gastric motility
iii. Cephalic Phase
1. Increased parasympathetic due to sight, taste, smell, and thought of food
iv. Gastric Phase
1. Stretch of stomach wall increases gastrin secretion, which increases gastric
secretions.
2. pH changes alter gastrin release
v. Intestinal Phase
1. As duodenum fills, a sympathetic reflex inhibits gastric release.
2. Proteins and fats in duodenum stimulate cholecystokinin release which decreases
gastric motility.
7. Mixing and Emptying Actions
a. Mixing of bolus of food + gastric juice = chyme.
b. Peristaltic waves of stomach push chyme toward pyloric sphincter; It relaxes and food moves
into duodenum (small portions at a time i.e. all the content of stomach is not emptied into
duodenum at once)
c. The rate at which the stomach empties is determined by both the contents of the stomach and the
processing that is occurring in the small intestine.
d. Enterogastric reflex – ensures stomach slows down as duodenum fills.
PANCREAS
The pancreas is an accessory organ (gland) of digestive system that produces pancreatic juice. Pancreatic juice
consists mainly of water and contains enzymes that break down all categories of foodstuffs and electrolytes.
Secretion of pancreatic juice is regulated by local hormones and the parasympathetic nervous system.
1. Location: Inferior to stomach; retroperitoneal
2. Structure of the Pancreas: Shrimp–like with head in concave curve of duodenum; Tail extends to the
left to the spleen.
a. Endocrine cells produce hormones glucagon and insulin. Most cells of pancreas produce
pancreatic juice (exocrine cells).
b. Secretes pancreatic juice into a pancreatic duct; Pancreatic duct leads to duodenum (small
intestine).
c. Pancreatic duct joins bile duct before they enter duodenum.
3. Function of Pancreatic Juice:
a. Pancreatic juice contains four classes of enzymes that break down the four ingested organic
macromolecules.
i. Pancreatic amylase breaks Carbohydrates (Starch and Glycogen) → disaccharides
ii. Pancreatic lipase breaks Fats/Triglycerides → 2 fatty acids + monoglyceride
iii. Proteinases, trypsin, chymotrypsin, and carboxypeptidase breaks proteins → amino
acids
1. These enzymes are stored and released in an inactive form.
iv. Pancreatic nucleases breaks Nucleic Acids
nucleotides
v. Note bicarbonate ions are also released to neutralize acidic chyme entering from the
stomach.
4. Regulation of Pancreatic Secretions:
a. Hormones secretin and cholecystokinin regulate pancreatic secretions.
i. Secretin Activation:
1. When chyme from the stomach enters duodenum, the acidic nature of chyme will
stimulate duodenum to release secretin from the intestinal wall.
2. Secretin stimulates the release of bicarbonate rich pancreatic juice into
duodenum to neutralize the chyme and prevent intestinal wall injury by acid.
ii. Cholecystokinin (CCK) Activation
1. Fats content of chyme in the duodenum stimulates (signal) release of
cholecystokinin from the intestinal wall.
c.
CCK leads to constriction of gallbladder and stimulates the release of enzyme
rich pancreatic juice into the duodenum. Constriction of gallbladder leads to
ejection of bile into duodenum
LIVER
The liver and gallbladder are accessory organs associated with the small intestine. The liver is the largest gland
in the body and has four lobes. Each lobe of the liver is composed of lobules, which are made of plates of liver
cells (liver cells called hepatocytes). Liver has many functions, but the digestive function of the liver is to
produce bile, which is a fat emulsifier. Bile is a yellow-green, alkaline solution containing bile salts, bile
pigments (primarily bilirubin), cholesterol, neutral fats, phospholipids, and a variety of electrolytes. Bile that is
produced by the liver is delivered to the gallbladder, where it is stored and concentrated. Bile does not usually
enter the small intestine until the gallbladder contracts when stimulated by cholecystokinin i.e. the fat content of
chyme that enters duodenum will stimulate release of CCK by the intestine wall, and then CCK will stimulate
gallbladder to contract and eject the bile into the duodenum.
1. The term hepatic refers to the liver. The liver has many functions; however its digestive function is
production of bile
2. Function of bile is emulsification of fats.
3. Location/Description:
a. below diaphragm
b. right side
4. Liver Structure:
a. 4 lobes
b. Each lobe is made up of Hepatic lobules = functional unit of the liver.
i. Lobule structure us like hexagon shaped around a central vein
ii. Contains hepatocytes (liver cells) which provide most functions
5. Blood Supply to Liver:
a. Hepatic artery supplies oxygenated blood
b. Hepatic portal vein supplies deoxygenated blood filled with newly absorbed nutrients from
small intestine i.e. nutrient absorbed by the intestine is delivered to the liver, so liver could store
them or distribute them to other parts of the body based on their needs.
6. Composition of Bile
a. Bile salts (digestive function)
b. Bile pigments (biliverdin and bilirubin)
c. Cholesterol
d. Electrolytes
7. Gall Bladder
a. Stores bile between meals
b. Location: underside of liver,
c. Abnormality: Blockage due to gallstones. Gallstones are usually composed of cholesterol.
8. Regulation of Bile Release
a. Hormone = CHOLECYSTOKININ (CCK)
b. When small intestine fills with fatty chyme:
c. CCK is released into blood.
d. CCK stimulates walls of gallbladder to contract.
e. Bile passes down the common bile duct and bile is deposited into duodenum.
9. Functions of Bile Salts: Emulsification of fat molecules!
a. Emulsification = breaking up of fat globules (large fat droplets) into small droplets to increase
the surface area and increases effectiveness of lipases.
b. Bile is released into duodenum to emulsify fat.
c. Bile also aids in absorption of fats and fat-soluble vitamins.
SMALL INTESTINE
Food takes 3 to 6 hours to complete its digestive path through the small intestine, the site of virtually all nutrient
absorption. Most substances (enzymes) required for chemical digestion within the small intestine are imported
(released) from the pancreas and the liver to the small intestine. Optimal digestive activity in the small intestine
depends on a slow, measured delivery of chyme from the stomach. Segmentation is the most common motion
of the small intestine, which continuously mixes chyme with enzyme to ensure complete digestion.
1. Parts of Small Intestine:
a. Duodenum – first part of the small intestine, the part that is attached to the stomach (to the
pylorus of the stomach), the part that receives chyme from the stomach; the part that receive bile
and pancreatic juice.
b. Jejunum - mid-region, the part where mostly digestion is complete and absorption begins.
c. Ileum - near large intestine; the part that is attached to the large intestine
i. The distal end of the ileum narrows to form the ileocecal valve (sphincter muscle
between small & large intestine).
2. Functions of Small Intestine
a. Major site of chemical digestion (duodenum). The followings are important in facilitating
chemical digestion:
i. Bile released by the gallbladder to the duodenum
ii. Pancreatic juice released by the pancreas to the duodenum
iii. Small intestinal digestive enzymes produced by the rush border of the small intestine
3. Secretions
a. Secretes mucus
b. Secretes digestive enzymes. The brush border of small intestine produces digestive enzymes:
i. Peptidases breaks down peptides
amino acids
ii. Sucrase, maltase, lactase breaks down disaccharides
monosaccharide
iii. Intestinal lipases beaks down triglycerides
3 fatty acids + glycerol
4. Major site of absorption of Nutrients
a. About 90% of all nutrients are absorbed in small intestine. NOTE: absorption mainly begins in
the second part of small intestine (Jejunum).
b. Simplest food (amino acids, fatty acids, glycerol, monosaccharide) forms of ingested molecules
are absorbed into the intestinal mucosa
c. Absorbed nutrients are carried away by blood & lacteals.
i. Amino acids and monosaccharide cross from the intestine to the blood
ii. Fatty acid and glycerol cross from the intestine via lacteals to the lymph
d. Monosaccharide are absorbed by facilitated diffusion or active transport and then delivered by
blood to the liver
e. Amino acids are absorbed by active transport and then delivered to the liver by blood
f. Fatty acids are absorbed by simple diffusion into intestinal mucosa. Once in mucosal cells’ ER,
the fats collect in clusters encased in protein to form chylomicrons (lipoproteins). Chylomicrons
are absorbed by lacteals (lymphatic capillaries) into the lymphatic system.
5. Structure of the Small Intestine Wall
a. Intestinal villi project into lumen (increasing surface area).
b. Each villus is composed of microvilli which further increases the surface area.
6. Movements of the Small Intestine
a. Mixing of chyme + intestinal juices by segmentation (mechanical digestion)
b. Peristaltic waves push residual chyme toward ileocecal sphincter; it relaxes, moving food into
large intestine.
LARGE INTESTINE
The large intestine absorbs water from indigestible food residues and eliminates the latter as feces. The large
intestine has the following subdivisions: cecum, appendix, colon, rectum, and anal canal. The mucosa of the
large intestine is thick and has crypts with a large number of mucus-producing goblet cells. Bacteria entering
the colon via the small intestine and anus colonize the colon and ferment some of the indigestible
carbohydrates. The movements seen in the large intestine include haustral contractions and mass movements.
Feces forced into the rectum by mass movements stretch the rectal wall and initiate the defecation reflex.
1. Parts of Large Intestine:
a. Cecum - nearest ileum of small intestine i.e. portion of the large intestine that is attached to the
small intestine. Appendix is a blind pouch in this region.
b. Colon - majority of length of the large intestine is colon. Colon is subdivided into 4 regions:
ascending; transverse; descending; and sigmoid.
c. Rectum - distal region of colon
d. Anal canal - narrowing of rectum and opening to outside
2. Structure of the Large Intestinal Wall
a.
Lacks villi
3. Functions of Large Intestine:
a. Secretion = mucus.
b. Absorption = water & electrolytes.
c. Storage = feces.
d. Minimal digestion by intestinal flora (bacteria) which digest substances humans cannot digest.
4. Movements of the Large Intestine:
a. Mass movements only 2-3 times a day
b. Peristaltic waves of large intestine move residual chyme toward anal sphincter muscles.
5. Control of Anal Sphincter Muscles:
a. Both voluntary & involuntary nervous control
6. Feces:
a. Undigested & unabsorbed material
b. Color due to bile pigments
c. Odor due to intestinal bacteria and bacterial products formed in digestion
7. Defecation = emptying of rectum.
LIFE SPAN CHANGES
1. Mouth changes (Enamel thins increasing sensitivity to hot and cold.
2. Muscle contraction reduces: Slowing peristalsis, and slow peristalsis causes heartburn and constipation
3. Absorption of nutrients decreases with age.
SUMMARY DIGESTIVE SYSTEM
Site of Enzyme/Hormone/other products Origin
Parotid Gland
Product
Enzyme/other/H
ormones
Salivary
Amylase
Effect
Result of Action
Breakdown Carbohydrate to
oligosaccharide and
disaccharide
Sublingual Gland
Submandibula Gland
Mucous (this is
not enzyme)
Glands in the pylorus
produce primarily a mucous
Protect the cells from
acid
Mucous Cells
Gastric Pit
Parietal Cells
Chief Cells
Gastric Gland
G Cells
In the fundus and body of the stomach, each gastric pit
communicates with several gastric glands. Gastric
glands are dominated by two types of secretary cells:
parietal cells and chief cells . Together, they secrete
about 1500 ml of gastric juice each day.
Absorption do not take place in the stomach because:
1)The epithelial cells are covered by a blanket of
alkaline mucus and are not directly exposed to chyme,
2) the epithelial cells lack the specialized transport
mechanisms
3) the gastric lining is relatively impermeable to
water,
4) digestion has not been completed.
Some drugs e.g. Aspirin can be absorbed in the
stomach
The arrival of lipids and carbohydrates in the
duodenum stimulates the secretion of the hormones
CCK, and GIP.
A drop in pH below 4.5 stimulates the secretion of the
hormone secretin, so the secretin level in the blood
increases.
Vasoactive intestinal polypeptide (VIP), a polypeptide
hormone, is secreted by cells throughout the intestinal
tract
Prior to absorption, disaccharides and trisaccharides
are fragmented into monosaccharides (simple sugars)
by brush border enzymes of the intestinal microvilli.
The brushborder of intestine produces the following
enzymes: maltase, Sucrase and Lactase.
The brushborder of intestine also produces enzymes
such as dipeptidase to break dipeptides to amino acids
HCl
Parietal cells also
secrete a
glycoprotein
called intrinsic
factor
Pepsinogen is an
inactive
proenzyme
Gastrin
secretion, rather than
enzymes or acid.
intrinsic factor enables
vitamin B-12 absorption.
HCl converts pepsinogen to
pepsin, denatures proteins,
kills microorganisms, breaks
connective tissue
pepsinogen is converted by
the acid in the gastric lumen
to pepsin.
Pepsin is a digestive protease
that degrades food protein
into peptide. Pepsin attacks
only specific types of peptide
bonds, not all of them, so
digestion of protein is not
completed in the stomach.
Gastrin is a hormone that:
stimulates secretion by both
parietal and chief cells.
Stimulate contraction of
stomach wall
Vitamin B-12 is
absorbed, Protein is
denatured, Pepsin is
activated
Connective tissue is
broken.
pepsin breaks down
complex proteins into
smaller peptide and
polypeptide before the
chyme enters
duodenum.
Production of HCl and
pepsinogen begins
GIP (gastric
inhibitory
peptide)
inhibits gastric secretion.
Stimulates release of insulin
by the pancreas
Gastric secretion
stops.
Insulin released
Secretin
inhibits parietal cell and chief
cell activity in the stomach.
stimulates the production of
buffers (biocarbonate) by the
pancreas to protect the
duodenum by neutralizing the
acid
stimulates the secretion of
bile
inhibits gastric secretion of
acids and enzymes
Stimulates gall bladder to
release bile
Dilation of capillaries
Production of acid and
pepsinogen is stopped.
Biocaronates is ejected
by pancreas.
Bile is ejected by
gallbladder.
CCK
(cholecystokinin)
VIP
maltase
Sucrase
Lactase
maltase splits bonds between
the two glucose molecules of
the disaccharide called
maltose .
Sucrase breaks the
disaccharide sucrose into
glucose and fructose.
Lactase hydrolyzes the
disaccharide lactose into a
molecule of glucose and one
of galactose .
Bile is ejected into
duodenum
stimulates the
secretion of
electrolytes and water
by the intestinal
mucosa
Glucose a
monosaccharide
Glucose and fructose
both are
monosaccharide
Glucose and galactose
both are
monosaccharides.
Amylase
Peptidase
Chymotrypsin
Trypsin
Lipase
And other
enzymes
The pancreas is primarily an exocrine organ,
producing digestive enzymes and buffers. Pancreatic
juice (about 1000 ml per day) of pancreas is an
alkaline mixture of digestive enzymes, water, and
ions.
Gallbladder
Most dietary lipids are not water soluble. Mechanical
processing in the stomach creates large drops
containing a variety of lipids. Pancreatic lipase is not
lipid soluble, so the enzymes can interact with lipids
only at the surface of a lipid droplet.
Store and modify
bile
These enzymes do most of
the digestive work in the
small intestine. Secretion of
pancreatic enzymes are
controlled by hormones of
small intestine such as CCK
and Secretin
Amylase an enzyme that
breaks down certain starches.
Lipase , which breaks down
certain complex lipids,
releasing products (such as
fatty acids).
Protease (e.g. trypsin) break
certain proteins.
bile is produced by the liver
and delivered for storage to
the gallbladder. Bile salts
break the fat droplets apart in
a process called
emulsification.
Disaccharide and
trisaccharide
Monogluceride and
fatty acid
Short peptides
increases the surface
area of fat droplet
accessible to
enzymatic attack