DIGESTIVE SYSTEM
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identify the main structures including organs that make up the digestive system.
Identify the two groups of digestive system.
describe the functions of the structures and organs of digestive system.
describe the structures of the lower gastrointestinal organs.
explain the actions and functions of the lower gastrointestinal organs.
describe the structures of the accessory organs of the Digestive system.
Explain the functions of the accessory organs of the digestive system.
explain the six processes of food breakdown in the digestive system
describe the phases of the digestive system.
explain the implications occurring due to problems of the digestive system.
INTRODUCTION
 The digestive system takes in food (ingests food) breaks it down physically and chemically into nutrient
molecules (digests it) and absorbs the nutrients into the bloodstream. Then it rids the body of the
indigestible remains (defecates).
 The organs of the digestive system can be separated into 2 main groups:
1. alimentary canal
2. accessory digestive organs
 The alimentary canal performs the whole menu of digestive functions; ingests, digests, absorbs and
defecates.
Organs of the Alimentary Canal
 Also called the gastrointestinal (GI) tract.
 A continuous coiled, hollow muscular tube that winds through the ventral body cavity and is open at
both ends.
 Its organs are the
 Mouth
 Pharynx
 Oesophagus
 Stomach
 Small intestine
 Large intestine
 anus
 In a cadaver, the alimentary canal is
approximately 9 meters (30 feet) long, but
in a living person, it is considerably
shorter because of its relatively constant
muscle tone.
The Accessory Organs
 The accessory organs are the:
 Teeth
 Salivary glands
 Pancreas
 Liver and Gallbladder
 The accessory organs assist the process
of digestive breakdown in various ways.
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THE ALIMENTARY CANAL
MOUTH
 Food enters the digestive tract through the mouth, or oral
cavity, a mucous-membrane lined cavity.
 The lips (labia) protect its anterior opening.
 The cheeks form its lateral walls.
 The hard palate forms its anterior roof and the soft palate
forms its posterior roof.
 The uvula is a fleshy fingerlike projection of the soft palate
which extends inferiorly from the posterior edge of the soft
palate.
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The space between the lips and cheeks externally, and the teeth and gums internally is the vestibule.
The muscular tongue occupies the floor of the mouth. The tongue has several bony attachments
– two of these are to the hyoid bone and the styloid processes of the skull.
The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and
limits its posterior movements.
At the posterior end of the oral cavity are paired masses of lymphatic tissue, known as the palatine
tonsils.
The lingual tonsil covers the base of the tongue just beyond.
Having tonsils at the oral entrance to the pharynx provide a protective value because:
 The mouth is a favoured site of body entry by bacteria and the presence of tonsils, lymphocyte-and
-macrophage filled organs is very effective in preventing many pathogens from getting further into
the digestive tract.
As food enters the mouth, it is mixed with saliva and masticated
The cheeks and the closed lips hold the food between the teeth during chewing.
The nimble tongue continuously mixes food with saliva during chewing and initiates swallowing.
Papillae containing taste buds, or taste receptors, are found on the tongue surface. Besides it food
manipulating function, the tongue allows us to enjoy and appreciate the food as we eat.
PHARYNX
 The pharynx is subdivided into:
 Nasopharynx –part of the respiratory passageway
 Oropharynx–posterior to the oral cavity
 Laryngopharynx–continuous with the oesophagus below.
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From the mouth, food passes
posteriorly into the oropharynx and
laryngopharynx, both of which are
common passageways for food,
fluids and air.
Walls of pharynx contain two
skeletal muscle layers:
 The cells of the inner layer run
longitudinally
 the outer layer (the constrictor
muscles) run around the wall in
a circular fashion.
Alternating contraction of these two
muscle layers propel food through the
pharynx into the oesophagus below.
OESOPHAGUS
 The oesophagus or gullet, runs from the pharynx through the diaphragm to the stomach.
 About 25cm (10 inches) long, it is essentially a passageway that conducts food to the stomach.
 The walls of the alimentary canal organs from the oesophagus to the large intestine are made up of 4
basic layers, or tunics:
1. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen of the organ.
It consists primarily of a surface epithelium, plus a small amount of connective tissue (lamina
propria) and a scanty smooth muscle layer.
2. The submucosa is found just beneath the mucosa. It is a soft connective tissue layer containing
blood vessels, nerve endings, mucosa-associated lymphoid tissue, and lymphatic vessels.
3. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer
longitudinal layer of smooth muscle cells.
4. The serosa is the outermost layer of the wall. It consists of a single layer of flat, serous fluidproducing cells, the visceral peritoneum.
The visceral peritoneum is continuous with the slick, slippery parietal peritoneum, which lines the
abdominopelvic cavity by way of a membrane extension, the mesentery.
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When the peritoneum is infected, a condition called peritonitis, the peritoneal membranes tend to stick
together around the infection site. This helps to seal off and localize many intraperitoneal infections (at
least initially) providing time for macrophages in the lymphatic tissue to mount an attack.
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The alimentary canal wall contains 2 important
intrinsic nerve plexuses:
1. Submucosal nerve plexus
2. Myenteric nerve plexus
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These networks of nerve fibres are actually part
of the autonomic nervous system. They help
regulate the mobility and secretory activity of GI
tract organs.
STOMACH
 The J-shaped stomach is on the left side of the abdominal
cavity, nearly hidden by the liver and diaphragm.
 Different regions of the
stomach have been named:
 the cardiac region (named
for its position near the
heart) surrounds the
cardioesophageal
sphincter through which
food enters the stomach
from the oesophagus.
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The fundus is the expanded part of the stomach lateral to the cardiac region.
The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum,
and then the funnel shaped pylorus, the terminal part of the stomach.
The pylorus is continuous with the small intestine through the pyloric sphincter or valve.
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Stomach varies from 15 to 25cm (6-10”) in length. But diameter and volume depend on how much food
it contains, can hold about 4 litres (1 gallon) of food.
When empty it collapses inward on itself,
and its mucosa is thrown into large folds
called rugae.
The stomach act as a temporary storage
tank for food as well as a site for food
breakdown.
The stomach has longitudinal, circular and
oblique muscle layers which not only move
food along the tract but also to churn, mix,
pummel the food and physically breaking
it down to smaller fragments.
The mucosa of the stomach is composed many cells:
1. mucus cells - produce bicarbonate rich alkaline to protect stomach wall from being damaged by
acid and digested by enzymes.
2. Chief cells – produce protein digesting enzymes called pepsinogen.
3. Parietal cells –produce hydrochloric acid, making stomach contents acidic and activates enzyme
for conversion of pepsinogen to pepsin by HCL.
The smooth lining of stomach has million deep gastric pits leading into gastric glands secreting gastric
juice.
After food has been processed in the stomach, it resembles heavy cream called chyme.
The convex lateral surface of stomach is the greater curvature and concave medial surface is the lesser
curvature.
A double layer of peritoneum extends from the liver to the lesser curvature known as the lesser
omentum.
Another extension of peritoneum drapes downward and covers the abdominal organs like a lacy apron
before attaching to the posterior body wall called greater omentum.
The greater omentum is riddled with fat, which helps to insulate, cushion and protect the abdominal
organs and has large collections of lymphoid follicles containing macrophages and defensive cells of
the immune system.
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SMALL INTESTINE
 The small intestine is a muscular tube extending from pyloric sphincter
to the large intestine.
 The longest section of alimentary tube, an average length of 2 to 4m
(7 to 13 feet).
 The small intestine has three subdivisions:
1. Duodenum
2. Jejunum
3. Ileum
 Ileum meets large intestine at ileocecal valve which joins them.
 The small intestine is able to process only small amount of food.
 The pyloric sphincter controls food movement into the small intestine from the stomach from being
overwhelmed.
 The duodenum joins with the pancreatic and bile ducts to form hepatopancreatic ampulla.
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The wall of the small intestine has 3 structures that allow food absorption:
1. Microvilli
2. Villi
3. Circular folds
Microvilli –tiny projections of plasma membrane giving cell surface fuzzy appearance referred as brush
border, producing enzymes that complete protein and carbohydrates digestion in the small intestine.
Villi –finger like projections giving velvety appearance. Within each villus, is a rich capillary bed and
lymphatic capillary called lacteal, where foods are absorbed.
LARGE INTESTINE
 The large intestine is much longer in diameter than the small intestine, but shorter in length.
 About 1.5m (5 feet) long, extending from ileocecal valve to the anus.
 It absorbs water and dry out digestible food and eliminate the residues from body as feces.
 It frames small intestine on 3 sides and has subdivisions:
1. Cecum –saclike, 1st part of large intestine.
2. Appendix –hanging from cecum, wormlike.
3. Colon –ascending, transverse, descending
and sigmoid colon.
4. Rectum –Sigmoid colon join the rectum and
anal canal.
5. Anal canal –ends at anus that open to
exterior.
It
has
internal
anal
sphincter(involuntary) external anal sphincter
(voluntary). Ordinarily the sphincters closed
except during defecation. Anal mucosa
produces alkaline mucus that act as lubricant
to ease passage of feces.
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THE ACCESSORY ORGANS
TEETH
 The teeth tear and grind the food, breaking it down into smaller fragments by mastication or chewing.
 By age 21 two sets of teeth have been formed:
1. Deciduous teeth or baby/milk teeth –begin to erupt at 6
months. Full set (20 teeth) complete by the age of 2 years.
1st teeth to appear, lower central incisors.
2. Permanent Teeth as it enlarged and develop, roots of milk
teeth are reabsorbed. Between ages 6 –12 years the teeth
loosen and fall out. All permanent teeth erupt by end of
adolescence, except 3rdmolars, the wisdom teeth that
emerge between ages 17 and 25, but sometimes fail to
erupt at all.
 Complete full set of permanent teeth are 32.
 Teeth are classified according to the shape and function:
 Incisors –chisel shaped, adapted for cutting
 Canines –fanglike for tearing and piercing
 Premolar and Molars –broad crowns with rounded
cusps (tips) best suited for grinding.
 Teeth consist of 2 major regions the crown and root. Both
connected by the neck.
 The exposed part of the tooth above gingiva or gum is the
enamel.
 Enamel is the hardest substance in the body, is fairly brittle
because its heavily mineralized with calcium salt.
 The outer surface of the root is covered by a substance called
cement, which attaches the tooth to the periodontal
membrane(ligament). The ligament holds the tooth in place in
the bony jaw.
 Dentine, a bonelike material underlies the enamel and forms
the bulk of the tooth.
 The dentine surrounds the pulp cavity, containing number of
structures, collectively called pulp.
 Pulp supplies nutrients to the tooth tissues and provides for
tooth sensations.
SALIVARY GLANDS
 There are 3 pairs of salivary glands:
 Parotid glands –lie anterior to the ears.
Inflammation of parotid glands can cause
Mumps, a common childhood disease.
 Submandibular glands and Sublingual
glands –empty their secretions into the
floor of the mouth, through tiny ducts.
 The product of salivary glands, saliva is a mixture
of mucus and serous fluids which helps to moisten
and binds food together into a mass called bolus,
so that chewing and swallowing will be easier.
 Saliva contains the enzyme salivary amylase for
digestion of starch in the mouth.
 Saliva also contains lysozyme and antibodies that inhibit bacteria.
 Saliva dissolves food chemicals so they can be tasted.
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PANCREAS
 Pancreas is a soft pink, triangular gland that extends across the abdomen from the spleen to the
duodenum.
 The function of Pancreas is to produce pancreatic enzymes, secreted into the duodenum in an alkaline
fluid that neutralizes the acidic chyme. Pancreas produce the hormones insulin and glucagon.
LIVER & GALL BLADDER
 Liver is the largest gland in the body, suspended from the diaphragm and abdominal wall by a delicate
mesentery cord, falciform ligament.
 Liver produces bile which leaves the liver through the common hepatic duct and enters duodenum
through the bile duct.
 Bile is a yellowish to green, watery solution containing bile salts, bile pigments (bilirubin, a breakdown
product of hemoglobin), cholesterol, phospholipids and a variety of electrolytes.
 Bile does not contain enzymes, but its bile salts
emulsify fats by breaking large fat globules into
smaller ones, for fat-digesting enzymes to work
on.
 Gall bladder is a small, thin walled green sac that
snuggles in a shallow fossa in the inferior surface
of the liver.
 When food digestion is not occurring, bile backs
up the cystic duct and enters the gall bladder.
 Function of gallbladder; Gallbladder stores bile
and causes bile to be concentrated by removing
the water. When fatty food enters the duodenum,
a hormonal stimulus prompts the gallbladder to
contract and spurt out stored bile making it
available to the duodenum.
FUNCTIONS OF THE DIGESTIVE SYSTEM
The essential activities of the Gastrointestinal (GI) tract include the following 6 processes:
1. Ingestion
2. Food breakdown: mechanical digestion
3. Food breakdown: chemical digestion
4. Propulsion
5. Absorption
6. Defecation
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 What is INGESTION
 Food must be placed into the mouth before it can be acted on. This is an active, voluntary process
called ingestion.
 MECHANICAL & CHEMICAL DIGESTION
 Once food is in the mouth, both mechanical and chemical digestion begin to break down the food
through chewing and mixing with saliva.
 What is Mechanical digestion:
 Mechanical digestion prepares food for further degradation through chewing.
 The food is physically fragment into smaller particles.
 Degradation is carried out by the enzymes.
 What is chemical digestion?
 The sequence of steps in which large food molecules are broken down to their building blocks by
enzymes which are protein molecules that act as catalyst. These reactions are called hydrolysis
reactions because a water molecule is added to each bond to be broken. Water is also necessary
as a dissolving medium and a softening agent for food digestion.
 The salivary amylase begins chemical digestion of starch breaking it down into maltose. (Note:
Chewing bread longer before swallowing will make it taste sweeter as sugars are released).
 Saliva is secreted continuously to keep the mouth moist, but as food enters the mouth, larger
amounts of saliva pour out.
 Some stimuli can make the mouth watery due to parasympathetic fibres in cranial nerves.
 What is PROPULSION
 The action of pushing forward to the next part.
 Because food must be processed by more than one digestive organs, hence it must be propelled
from one organ to the next.
 In order to move the food from the mouth to the organs internally, swallowing (deglutition) has to
take place.
 Swallowing is a complex process involving the coordinated activity of several structures
 Tongue
 soft palate
 pharynx and
 oesophagus.
 Deglutition or swallowing depend largely on propulsive process called peristalsis for food
movement.
 Peristalsis is involuntary and involves alternating waves of contraction and relaxation of muscles in
the organ wall. The effect is to squeeze the food along the tract.
 Involve 2 phases of swallowing:
1. The voluntary buccal phase.
2. The involuntary pharyngeal oesophageal phase.
What is buccal phase?
 The voluntary phase that occurs in the mouth. Once the food has been chewed and well mixed with
saliva it becomes a bolus or food mass and forced into the pharynx by the tongue. As food enters the
pharynx it passes out of our control and into the realm of reflex activity.
What is pharyngeal oesopharyngeal phase?
 The involuntary phase that transports food through the pharynx and oesophagus. The Vagus nerves
controls this phase and promotes the mobility of digestive organs from this point on. Food is moved
through the pharynx and then into the oesophagus below by wavelike peristaltic contractions of their
muscular wall, first the longitudinal muscles contract and then the circular muscles contract.
 As food enters, the tongue blocks off the mouth, soft palate closes off the nasal passages, larynx rises
and covered with epiglottis.
 If we try to talk while swallowing, food may enter the respiratory passages which can trigger a protective
reflex-coughing-during which air rushes upward from the lungs in an attempt to expel the food.
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Food moved through the pharynx by peristalsis, an involuntary alternating
wave of contraction and relaxation of the muscles in the organ wall. The net
effect is to squeeze the food along the tract.
The pharynx and oesophagus have no digestive function as they only
provide passageways to carry food to the next processing site, the stomach.
Once food reaches distal end oesophagus, it presses against the
cardioesophageal sphincter, causing it to open and the food enters the
stomach.
Sometimes cardioesophageal sphincter fails to close tightly and gastric juice
from the stomach backs up into the oesophagus resulting in a characteristic
pain known as heartburn.
Activities of the stomach
 As food enters and fills the stomach, its wall begins to stretch, at the same time gastric juices are
secreted.
 The muscles of stomach walls compress and pummel the food, breaking it apart physically and
continuously mixing the food with enzyme-containing gastric juice so that semifluid chyme is formed.
 Little chemical digestion occurs in the stomach.
 Chemical digestion involves sequence of steps, where large food molecules are broken down to their
building blocks by enzymes.
 Protein are digested to their building blocks, which are amino acids. The products of protein digestion
are polypeptides and peptides.
 When food has been well mixed, a rippling peristalsis begins in the upper half of the stomach and the
contractions increase in force as the food approaches the pyloric valve.
 The pylorus of the stomach holds about 30mls of chyme.
 Because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3mls or
less of chyme into the small intestine.
 The contraction also closes the valve, so the rest (about 27mls) is propelled backward into the stomach
for more mixing, a process called retropulsion.
 Generally, it takes about 4 hours for the stomach to empty completely after the person eats and about
6hrs or more if the meal has a high fat content.
How vomiting may occur
 Local irritation of stomach from bacterial food poisoning may activate emetic center in the brain causing
vomiting. Vomiting is due to contraction of abdominal muscles and diaphragm and increase pressure
in the abdominal organs.
 The emetic centre may also be activated through disturbance of the equilibrium apparatus of the inner
ear when a person spins for a long period going on a boat ride on the rough water.
 What is ABSORPTION
 The digestive end products are transported from the lumen of the gastrointestinal tract to the blood
or lymph. For absorption to occur, the digested foods must first enter the mucosal cells by active or
passive transport processes. The small intestine is the major absorptive site.
Absorption -activities of the small intestine
 Chemical food digestion is accelerated in the small intestine as the food takes a rather wild 3-6 hours
journey through the looping coils and twists.
 The small intestine is the major absorption site of water and end products of digestion.
 When chyme enters small intestine, secretin and cholecystokinin hormones influence the release of
pancreatic juice from pancreas and bile from liver.
 If either bile or pancreatic juice are absent, no fat digestion or absorption goes on, causing fatty bulky
stools.
 Most substances are absorbed through the intestinal cell plasma membranes by the process of active
transport.
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They enter the capillary beds in the villi to be transported in the blood to the liver via hepatic portal vein
except lipids or fats that are absorbed by the process diffusion.
Lipid breakdown products enter capillary beds and lacteals in villi and are carried to liver by blood and
lymphatic fluids.
At the end of ileum, all that remains is some water, indigestible food materials (plant fibres such as
cellulose) and large amounts of bacteria. Cellulose is important because it provides fibre which is
important for moving faeces along the colon and ease the action of defecation.
This debris enters large intestine through ileocecal valve by peristalsis with rhythmic segmental
movements produce by local constrictions of the intestine.
 What is DEFECATION
 This is the elimination of indigestible residues from the gastrointestinal tract via the anus in the form
of faeces. Mass movements are long, slow moving but powerful contractile waves force the contents
toward the rectum.
Activities of the large intestine
 Large intestine does not produce digestive enzymes. It aids with defecation.
 What is finally delivered to the large intestine contains few nutrients, but that residue still has 12 to 24
hours more to spend there.
 The resident bacteria that live in the lumen of the colon, metabolize some of the remaining nutrients,
releasing gases (methane and hydrogen sulphide) that contribute to odor of feces.
 About 500mls of gas (flatus) is produced each day and much more when certain carbohydrate rich
foods (such as beans) are eaten.
 Feces, the more or less solid product delivered to the rectum, contain undigested food residues, mucus,
millions of bacteria and just enough water to allow their smooth passage.
 When there is food residue, the colon begins contractions which are quite slow, known as the haustral
contractions. This is a segmenting movement lasting about 1 min, that occur every 30mins or so.
 Bulk or fiber in the diet increases the strength of colon contractions and soften stools allowing colon to
act as a well-oiled machine.
 As feces are forced through anal canal, messages reach the brain giving us time to decide whether
external voluntary sphincter should open or constricted to stop passage of feces.
 Defecation can be delayed temporarily and with the next mass movement, defecation reflex is initiated
again.
 Watery stools or diarrhoea occur as a result from irritation of the colon by bacteria or any condition that
rushes food residue through large intestine before the organ has sufficient time to absorb the water.
 Because fluids and ions are lost from the body, prolonged diarrhoea may result in dehydration and
electrolyte imbalance and if severe can be fatal.
 However, constipation may occur if there is a lack of fiber in the diet, poor bowel habits and laxative
abuse, where feces remain in large intestine for extended period, water is absorbed, stool becomes
hard and difficult to pass.
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