Digestive System
 Introduction
 Organs
 General Structure
 Structures and functions
 Digestion
 Absorption
Digestive system
 Nutrients required by body cells have to be in simple
and soluble form.
 Ingested food is in complex form which has to be
broken into simpler form.
 The conversion of complex form of food into simple
soluble form brought by digestive system.
 Different organs that makes up digestive system are
involved in digestion.
Main Organs (in order)
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Mouth and Oral Cavity
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Accessory Organs
 Teeth and tongue
 Salivary glands
 Liver
 Gallbladder
 Pancreas
General Structure
 Walls of the digestive tract consist of four layers
throughout its length.
 From lumen to outward:
 Mucosa/mucous membrane – epithelial layer coated
with mucous
 Submucosa – loose connective tissue: contain blood
vessels, lymphoid, nerves and glands
 Muscularis/musculosa–smooth muscle that produces
wave-like contractions (peristalsis) to move food,
consists of two layers, outer-longitudinal, inner-circular
 Serosa/adventitia – connective tissue; outermost
covering.
Baisc structure of Alimentary canal
Structures and functions
Oral cavity
 Hollow chamber with a roof, floor and walls. Food enters
through the mouth.
 Hard palate – bony structure in the anterior or front
portion of the mouth.
 Soft Palate – located above the posterior or rear portion
of the mouth; muscle tissue.
 Uvula – hangs down from the soft palate; helps prevent
food or liquid from entering the nasal cavities above
the mouth.
 Tongue – skeletal muscle covered with mucous
membrane. Involved in mixing, swallowing and
tasting.
Structures and functions
 Teeth – 4 major types
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Incisors – cutting function during mastication.
Canines (cuspids) – pierce or tear food.
Premolars (bicuspids) – large flat surfaces with 2 or 3
grinding or crushing cusps on their surface. Grind food
into a bolus so it can be swallowed.
Molars (tricuspids) – same as premolars.
Deciduous teeth = baby teeth, 20 in number; adults
have 32 teeth
Structures and functions
 Teeth cont’d
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Parts of a tooth
 Crown – portion that is exposed and visible in the
mouth; covered with enamel.
 Neck – narrow portion surrounded by the pink gingiva
or gum tissue.
 Root – fits into the socket (joint) of the upper or lower
jaw.
Structures and functions
Structures and functions
Salivary Glands(3 pairs)
 Parotids – largest, lie just below and in front of each ear
at the angle of the jaw (mumps).
 Submandibular – open into the mouth on either side of
the lingual frenulum.
 Sublingual – open into the floor of the mouth.
 Saliva – contains mucus and a digestive enzyme called
salivary amylase which begins the process of chemical
digestion of carbohydrates.
Location of salivary glands
Structures and functions
Esophagus
 Muscular, mucus lined tube that connects the pharynx
with the stomach (about 25 cm or 10 inches long).
 Peristalsis moves food from the mouth to the stomach.
Structures and functions
Stomach
 Lies in the upper part of the abdominal cavity just under the
diaphragm.
 Serves as a pouch that food enters after it has been chewed,
swallowed and passed through the esophagus.
 At the junction of the esophagus and the stomach is the
cardiac sphincter (muscle) that keeps food from reentering
the esophagus
 Divided into three regions:
 Fundus – enlarged portion to the left of and below the
cardiac sphincter.
 Body – central part of the stomach
 Pylorus – lower, narrow section which joins the first
part of the small intestine.
Structure of stomach
Structures and functions
 The lining of the stomach has numerous folds when empty
called rugae; they allow for expansion when we eat.
 Has numerous gastric glands that secrete “gastric juice”
into the stomach.
 Most common cells found in the glands are:
 Parietal cells - produce HCl acid that denatures
proteins.
 Chief cells - produce pepsin that begins protein
digestion.
 Mucus cells - produce mucus the protects from HCl
action.
Gastric pits and gland
Structures and functions
 The stomach wall has 3 layers of muscle that allow mixing
of the food with gastric juice, breaking it down into a
semisolid mixture called chyme.
 Pyloric sphincter (muscle) holds food in the stomach or
allows for emptying of chyme into the small intestine.
Controlled by the small intestine.
LS of stomach
Structures and functions
Small intestine
 Roughly 7 meters in length.
 Smaller in diameter compared to the large
intestine.
 Divided into three sections:
 Duodenum
 Jejunum
 Ileum
Structures and functions
Small intestine
 Mucous lining, contains thousands of microscopic
glands which secrete intestinal digestive juices.
 Intestinal lining has multiple circular folds, plicae,
that increase the surface area available for
absorption of nutrients.
 The surface is covered with tiny finger-like
projections called villi.
LS of small intestine
Structures and functions
Liver
 Fills the entire upper right section of the abdominal cavity
and extends part way into the left side.
 The largest gland in the body secretes bile into hepatic
ducts.
 Functions:
 Produces bile
 Stores vitamins and minerals
 Plays a part in maintaining blood sugar (glycogen) and
blood pressure (blood proteins).
Structures and functions
 Liver cont’d
 Bile breaks up (emulsifies) large fat globules into
smaller particles to increase the surface area for
digestion.
 Bile gives the feces its color (no bile - stools become
clay colored). Bile absorbed into the blood gives the
skin a yellowish color = jaundice or icterus.
Liver – anterior view
Structures and functions
 Gall Bladder
 Stores bile
 Cystic duct arising from it joins with hepatic duct.
 When fat enters the duodenum, the mucosal cells
release a hormone called cholecystokinin (CCK).
 CCK stimulates the contraction of the gallbladder
and forces bile into the duodenum via the common
bile duct.
Gall bladder
Structures and functions
Pancreas
 Lies behind the stomach in the C-shaped curve of the
duodenum.
 Endocrine portion produces insulin and glucagon.
 Exocrine portion (ascini) secretes pancreatic juice into
ducts that connect to the duodenum at the same area
as the bile duct.
 Pancreatic juice contains enzymes such amylase,
trypsin, chymotrypsin and lipase and also bicarbonate
that neutralizes the HCl in the chyme.
Pancreas
Structures and functions
 Large Intestine
 About 5 feet in length, forms the lower portion of the
digestive tract.
 Undigested and unabsorbed food material enter the
large intestine after passing through a sphincter-like
structure called the ileocecal valve.
L/S of Large intestine
Structures and functions
 Large Intestine
 One primary function of the large intestine is to remove
water from the feces.
 A few vitamins and minerals are also absorbed.
 Bacteria living in the large intestine break down some of
the remaining material.
 Bacteria also produce some of the B-complex vitamins
and vitamin K.
Structures and functions
 Large Intestine
 Inner surface has no folds or villi – not well suited for
absorption.
 Large Intestine
 Cecum – ileocecal valve opens into this pouchlike area.
 Ascending colon – right side of body
 Transverse colon – extends across the front of the
abdomen
 Descending colon – left side of the body
 Sigmoid colon – leads to the rectum
Large intestine
Digestion
 Complex process that occurs in the alimentary canal,
consists of mechanical and chemical changes that prepare
food for absorption.
 Mechanical digestion
 Breaks food into tiny particles, mixes them with digestive
juices, moves them along the alimentary canal and finally
eliminates the digestive wastes from the body. Includes
mastication, deglutition, peristalsis and defecation.
Digestion
 Chemical digestion
 Breaks down large, non-absorbable food molecules into
smaller, absorbable molecules that can easily pass through
intestine into blood and lymph.
 Process includes numerous chemical reactions catalyzed by
specific enzymes in saliva, gastric, pancreatic, and
intestinal juices.
Digestion
Carbohydrates
 Digestion begins in mouth (salivary amylase)
 Salivary and pancreatic amylase converts starches into
disaccharides (double sugars).
 Intestinal enzymes (sucrase, maltase, lactase) break down
disaccharides into monosaccharides.
Polysaccharide salivary amylase maltose &small polysaccharide
Undigested polysaccharides pancreatic amylase maltose &
diaaccharides maltse,sucrase, lactase monosaccharides
Digestion
Proteins
 Starts in the stomach and ends in the small intestine.
 Pepsin in stomach,trypsin and chymotrypsin in pancreatic
juice digest protein.
 Aminopeptidase present in intestinal secretion finally digest
protein into amino acids.
Protein pepsin short polypeptides
Short polypeptides trypepsin, chymotrypsin small polypeptides &
peptides carboxypeptidase, peptidases,dipeptidases amino acids
Digestion
Lipids (fat)
 Bile produced by liver is poured into the duodenum.
 It brings about emulsification.
 Pancreatic lipase splits the lipid molecules into fatty acids
and glycerol.
Fat bile salts emulsified fat droplets pancreatic lipase fatty acids &
glycerol
Absorption
 Stomach absorbs only a few substances
 Main absorption occurs in small intestine
 Absorption occurs by a combination of simple diffusion,
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facilitated diffusion and active transport
Amino acids and glucose are directly transported to liver by
hepatic portal vein
Fatty acids and glycerol are absorbed by intestinal cells
Resembled as triglycerides in ER
Packed into protein covered fat droplets-chylomicrons
Chylomicrons enter the lacteals and enter into blood circulation
Hormonal control
Gastrin(stomach) causes gastric glands in stomach to
secrete pepsinogen.
Stimulated by distention of stomach by food, partially
digested proteins and caffine
Secretin(duodenum) causes the pancreas and liver to
secrete sodium bicarbonate and bile respectively.
Stimulated by the presence of acidic chyme in the
Duodenum.
CCK/cholecytokinin(duodenum)
Stimulates pancreas to release digestive enzymes and
gall bladder to empty bile
Stimulated by the presence of fatty acids and partially
digested protein in duodenum.
Gastric inhibitory peptide/GIP(duodenum)
Decrease stomach churning, thus slowing emptying.
Stimulated by the presence of fatty acids or glucose in
duodenum