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1. PHYSIOLOGY OF THE DIGESTIVE
SYSTEM
Dr. Muhammad Usman Hashmi
PGT Physiology
Rawalpindi Medical University

2. DIVISIONS OF DIGESTIVE SYSTEM
• The 2 division of digestive system are alimentary tube and the
accessory organs.
• The alimentary tube extends from mouth to the anus. Its consists of
the oral cavity, pharynx, esophagus, stomach, small intestine, and
large intestine.
• The accessory organs of digestion are the teeth, tongue, salivary
glands, gallbladder, liver and pancreas. Digestion doesn’t take place
within this organs, but its contributes something in digestion process.

3. TYPES OF DIGESTION
• There are 2 complementary processes: mechanical and chemical
digestion.
• Mechanical digestion are the physical breaking down the food into
smaller pieces. Example is chewing.
• Chemical digestion (enzymes), broken of food particle which is in
complex chemical molecules changed into much simpler chemicals
that can be utilize in our body.

4. ORAL CAVITY
• Food enters the oral cavity by mouth.
• Boundaries of the oral cavity are the hard and soft palates superiorly;
• The cheeks laterally;
• The floor of mouth inferiorly.
• In oral cavity, there’re teeth and tongue and opening of the salivary
ducts

5. TEETH:
 Function; chewing (mechanical digestion)
 Deciduous teeth (gigi sulung) begins erupt at about 6 months of age
and the set of 20 teeth usually complete by the age of 2 years.
 The permanent teeth consists of 32 teeth; the types of teeth are
incisors, canines, premolars, molars, wisdom teeth/third molar.

6. • The crown is visible above the gum (gingiva).
The outermost layer of the crown are enamel, made
by ameloblasts.
 Within enamel is dentin, similarly with bone and
produced by odontoblast. Dentin also forms the
roots of a tooth.
 The innermost is the pulp cavity, containing blood
vessels and nerve endings of 5th cranial nerve.
(trigeminal)
 The periodontal membranes lines the socket and
produces a bone like-cement that anchors the
tooths.

7. TONGUE:
• Made of skeletal muscle innervated by hypoglossal nerves (12th
cranial).
• On the upper surface of tongue there’re small projections called
papillae containing taste buds.
• Sensory nerves for taste: facial (7th cranial) and glossopharyngeal
(9th).
• Function of tongue also keeping the food between the teeth and
tongue mixing it with saliva

8. SALIVARY GLANDS: (CONSISTS 3 PAIRS)
• The parotid glands, submandibular,sublingual glands.
• Secretion of saliva is continuous, but the amount is varies.
• The parasympathetic response mediated by the facial
and glossopharyngeal nerve. The smell of food also
increase secretion of saliva.
• Sympathetic stimulation making the mouth dry
and swallowing difficult.
Saliva made from blood plasma and contains many
of the chemical.

9. PHARYNX
• The oropharynx and laryngopharynx are passageways connecting the
oral cavity to the esophagus. There’re no digestion takes place.
• Related functional with swallowing (mechanical movement).
• The reflex center for swallowing is in medulla, coordinates many
actions; constriction of pharynx, cessation of breathing, elevation of
soft palates to nasopharynx, elevation larynx and close of epiglottis
and peristalsis of esophagus.

10. ESOPHAGUS
• A muscular tube takes food from esophagus to stomach.
• Peristalsis of esophagus propels food in one direction and ensures that
food gets to the stomach even if the body is horizontal or upside down.
• At the junction of stomach and the lumen of esophagus, there’s lower
esophageal sphincter (LES), a circular smooth muscle.
• LES relaxes when food goes down to the stomach and contracts to prevent
the backup of stomach contents.
• GERD, due to the gastric juice splash into the esophagus because of LES
doesn’t close completely. This is painful condition and we also called
heartburn.

15. GASTRIC PITS:
• The glands lining in the stomach and consists several
types of cells.
• Mucous cells: secrete mucus helps prevent the
erosion by gastric juice.
• Chief cells: secrete pepsinogen – pepsin.
• Parietal cells: produce HCL and its helps pepsinogen
convert into pepsin and also gives gastric juice an
acidic PH. Also secrete intrinsic factor necessary for
vit b12 absorption.
• G cells: secrete hormone gastrin/gastric juice –
induce by sight of smell of food.

16. The three phases of secretion of gastric juice.

19. SMALL INTESTINE
• Diameter: 1inch (2.5cm) and approximately 20 feet (6m)
• Digestion completed in small intestine
• The end products absorbed into lymph and blood.
• The mucosa has simple columnar epithelium; consists microvilli and goblet cells to
secrete mucus.
• Lymph nodules (Peyer’s Patches) abundant in ileum to destroy absorbed pathogens.
• The waves of peristalsis can takes place w/o CNS. The enteric nervous system can
function independently and promote normal peristalsis.
DUODENUM JEJUNUM ILEUM
• The first10 inches (25cm)
• There’s ampulla of vater : the common
bile ducts enter the duodenum
• Is about 8 feet long • Is about 11 feet in length.

20. COMPLETION OF DIGESTION AND
ABSORPTION
• The intestinal glands (crypts or Liberkuhn) stimulated by the presence
of food in the duodenum.
• The intestinal enzymes are; peptidase, sucrase, maltase and lactase.

21. ABSORPTION
• Most absorption of the end products of digestion takes place.
• Absorption takes place in large surface area – modified with plica ciculares; folds of
mucosa.
• The mucosa folded into projections called villi – the inner surface of the intestine.
• Each columnar cell (except the goblet cells) of the vlli also has microvilli on its free surface.
• The absorption takes place from the lumen of intestine into vessels within the villi.
• Within each villi, there’re capillary vessels ( absorbed water-soluble nutrients) and a lacteal
(fat-soluble nutrients)
• Once absorbed, fatty acid recombined with glycerol – TG; form globules include cholesterol
and protein – chylomicrons; most absorbed fat transported by lymph enters the blood in
left subclavian vein.
• Blood from the capillary networks in the villi doesn’t retrun directly to the heart, but first
travels through the portal vein to liver.
• Thus, liver enables to regulate the blood levels of ( glucose, amino acids, store certain
vitamin) and also remove potential poisons from blood.

24. LIVER
• Consists of 2 large lobes; right and left
• The structural unit of liver is the liver nodule (hepatocytes)
• The hepatic artery brings oxygenated blood, the portal vein brings
blood from digestive organs and spleen.
• The capillaries of lobules are sinusoid; permeable vessels and receive
blood from hepatic artery and portal vein.
• Their function is production of bile. Bile enters from bile canaliculi –
hepatic duct (takes bile out from liver) – unites with cystic duct formed
– common bile duct (takes bile to duodenum)

25. • Bile mostly water and excretory function carries bilirubin and excess
cholesterol to the intestine for elimination in feces.
• Bile accomplished by bile salts, emulsifying fats into small globules.
(mechanical digestion)
• Production of bile stimulated by hormone secretin – produced by
duodenum when food enters small intestine.

26. • The structural unit; liver lobule a
hexagonal column of liver cells.
• Between adjacent lobules are branches
of the hepatic artery and portal vein.
• The central veins of all the lobules
unites to form the hepatic veins, which
take blood out of the liver to inferior
vena cava.

27. OTHER FUNCTIONS OF THE LIVER
CARBOHYDRATE
METABOLISM:
regulates level blood
glucose – excess glucose
will be convert into
glycogen (glycogenesis).
On hypoglycemia,
glycogen convert back to
glucose (glycogenolysis) –
increase glucose level.
AMINO ACID
METABOLISM
The non essential amino
acids are synthesizes by
transamination; excess
amino acids are changed
to carbohydrates or fats
by deamination; the
amino groups are
converted to urea and
excreted by the kidneys.
LIPIDMETABOLISM:
liver forms lipoproteins;
transport of the fats in
blood to other tissues. Its
also synthesizes and
excrete excess cholesterol
into bile and eliminated in
feces.
SYNTHESIS OF PLASMA
PROTEIN:
liver synthesizes many
proteins to circulate in
blood also the clotting
factors. Its also synthesis
ἀ and B globulins
(proteins serve as carriers
for other molecules such
as; fats in blood).

28. FORMATION OF
BILIRUBIN:
liver phagocyte the old
RBC’s and bilirubin
formed from heme portion
of Hb. Liver removes it
from blood, the bilirubin
formed in the spleen and
red bone marrow and
excretes it into bile and
eliminated by feces.
PHAGOCYTOSIS BY
KUPFFER CELLS:
macrophages in liver
called kupffer cells –
destroying the old RBC
and phagocytize the
pathogens/ foreign
materials that circulatein
liver.
STORAGE:
liver stores the fat-soluble
vitamins A.D.E.K and
water-soluble vitamins
B12. Its also stores
minerals iron and copper -
(needed for myoglobin
and hemoglobin) and
enzyme needed for
hemoglobin synthesis.
DETOXIFICATION:
enables synthesizes
enzyme that will detoxify
harmful substances.
(alcohol and others
medications) – those
substances will be
excreted by kidneys.

31. GALLBLADDER
• Is a sac about 3-4 inches located on the undersurface of right lobe of
the liver.
• Bile in the hepatic duct of the liver flows through the cystic duct into
gallbladder.
• When fatty enters duodenum, duodenal mucosa secrete the
hormone of cholecystokinin – stimulates contraction of smooth
muscle in the wall of gallbladder – forces bile out to cystic duct –
common bile duct – duodenum.

33. PANCREAS
• Located in the upper left abdominal and between quadrant of duodenum and spleen.
• The exocrine glands of pancreas called acini (singular: acinus)
• They produce 3 types of enzymes; amylase, trypsin, lipase.
• The pancreatic enzymes carried by small ducts unites to form larger ducts finally main pancreatic duct.
• Its also produces bicarbonate juice (containing sodium bicarbonate) to neutralize the gastric juice that enters
duodenum.
• Secretion of pancreatic juice stimulated by hormone secretin and cholecystokinin, produced by duodenal
mucosa when chyme enters small intestine.

35. LARGE INTESTINE – COLON
• Approximately 2.5 inches (6.3 cm) in diameter and 5 feet (1.5 m)
• It extends from ileum of the small intestine to the anus, the terminal
opening.
• The cecum is the first portion, junction with the ileum is the ilececal
valve – prevents the backflow of fecal material
• Attached to the cecum is the appendix – a small dead end tube with
abundant of lymphatic tissue (vestigial organ)
• The rectum is about 6 inches long – the anal canal is the last inch
colon that surrounds the anus.
• Clinically, the terminal end of colon usually referred as the rectum.

36. • No digestion takes place in colon – colonic mucosa, lubricates the passage
of the fecal material.
• The longitudinal smooth muscle – taenia coli.
• The haustra – puckers or pockets which provide for more surface area
within colon.
• Functions of colon – absorption of water, minerals, vitamins and eliminate
un-digestible material.
• About 80% of water absorbed (400-800mL)/day
• The vitamins absorbed are those produced by the normal flora, the trillions
of bacteria living in the colon.
• Vitamin K is produced and absorbed in amounts usually sufficient to meet a
person;s daily need.
• Function of normal flora; inhibit the growth of pathogens.

39. ELIMINATION OF FECES:
 Feces consists of cellulose and other un-digestable materials, living
bacteria and water.
 Its accomplished by defecation reflex – spinal cord reflex that may be
controlled voluntarily.
 The rectum usually empty until food enters duodenum – colon push the
feces –
wall of rectum stretched by the entry of feces (stimulus of defecation reflex).
 Stretch receptors in the rectum generated by sensory impulse that travel
from sacral spinal cord.
 In the anus – the internal anal sphincter – made by smooth muscle –
permitting defecation (relax the sphincter)
 The external anal sphincter (made from skeletal muscle and surrounds
the internal anal sphincter – delayed the defecation – voluntarily
contracted to close the anus.
 These receptors will be stimulated when the next wave of peristalsis
reaches the rectum.

41. AGING AND THE DIGESTIVE SYSTEM
• Sense of taste become less acute, less saliva produced.
• The effectiveness of peristalsis diminishes – indigestion frequently occurs especially if
the LES loses its tone and there’s a greater chance of esophageal damage.
• In the colon, diverticula may form; these are bubble-like outpouchings of the the
weakened wall of the colon – may be asymptomatic or become infected.
• Intestinal obstruction occurs with greater frequency among elderly.
• Sluggish peristalsis contributes to constipation, which may contribute to
hemorrhoids.
• The risk of oral cancer or colon cancer also increases with age.
• The liver usually function adequately – unless there’s damage by pathogens such as
hepatitis viruses.
• The pancreas usually functions well – although acute pancreatitis of unknown cause
more likely in elderly.