The document summarizes the key stages and processes of digestion. It describes the functions of the main parts of the digestive system including the mouth, stomach, small intestine, liver, gallbladder and pancreas. It explains the mechanical and chemical breakdown of food as well as the roles of enzymes and hormones in digesting carbohydrates, proteins and fats. Absorption and motility in the small intestine is also summarized.

1. Digestive System
Dr Kamelia

4. Digestive Functions & Processes
• Functions
– ingestion = intake of food
– digestion = breakdown of molecules
– absorption = uptake of nutrients into blood or lymph
– defecation = elimination of undigested material
• Processes
– motility = muscular contractions that break up food,
mix it with enzymes & move it along
– secretion = digestive enzymes & hormones
– membrane transport = absorption of nutrients

5. Stages of Digestion
• Mechanical digestion is physical breakdown of food into smaller
particles
– teeth & churning action of stomach & intestines
• Chemical digestion is series of hydrolysis reactions that break
macromolecules into their monomers
– enzymes from saliva, stomach, pancreas & intestines
– results
• polysaccharides into monosaccharides
• proteins into amino acids
• fats into glycerol and fatty acids

6. Saliva
• Functions of saliva
– moisten, begin starch & fat digestion, cleanse teeth, inhibit
bacteria, bind food together into bolus
• Hypotonic solutions of 99.5% water and solutes:
– amylase = begins starch digestion
– lingual lipase = digests fat after reaches the stomach
– mucus = aids in swallowing
– lysozyme = enzyme that kills bacteria
– immunoglobulin A = inhibits bacterial growth
– electrolytes = Na+, K+, Cl-, phosphate & bicarbonate
• pH of 6.8 to 7.0

8. Swallowing

9. The esophagus
Saliva contains salivary amylase. It hydrolyzes the glucose polymers starch and glycogen. The
epiglottis blocks the top of the windpipe when we swallow. Peristalsis moves the bolus along the
epiglottis.

10. The stomach
The stomach can hold 2L of food and water. The lining of the stomach secretes gastric acid.
Gastric acid contains HCl and pepsin. Pepsin is an enzyme that hydrolyzes proteins. But it breaks
proteins into smaller polypeptides.

11. Functions of Hydrochloric Acid
• Activates enzymes pepsin & lingual lipase
• Breaks up connective tissues & plant cell
walls
– liquifying food to form chyme
• Converts ingested ferric ions (Fe+3) to
ferrous ions (Fe+2) that can be absorbed &
utilized for hemoglobin synthesis
• Destroys ingested bacteria & pathogens

12. Small Intestine
• Nearly all chemical digestion and nutrient absorption
occurs in the small intestine

13. Oral Cavity Stomach
Small Intestine
Lumen Brush border
Carbohy
drates
Salivary Amylase
Polysaccharides ==>
smaller Polysaccharides
(starch,glycogen)
(maltose)
Pancreatic Amylase
Polysaccharides ==> d
isaccharides (maltose)
Disaccharidases
==> monosaccharides
Proteins
Pepsin
Proteins ==> s
mall polypeptide
Trypin, Chymotrypin
Polypeptides ==> smal
ler polypeptides ==>
aminopeptidase,
carboxypeptidase
==> Amino Acids
Dipeptidases
small
polypeptides ==>
Amino Acids
Nucleic
Acids
Nucleases
DNA,
RNA ==> Nucl
eotides ==>
Nucelotidases
==> nuclesides ==>
Nucelosidases
==> Nitrogenous
bases, sugars,
phosphates

14. Carbohydrates
Proteins
Fats

15. The digestive enzymes in the table below are summarized
according to type of food that they digest.
FOOD TYPE
ENZYME SOURCE PRODUCTS
CARBOHYDRATES Salivary amylase
Pancreatic amylase
Maltase
Salivary glands
Pancreas
Small intestine
Maltose
Maltose
Glucose
PROTEINS Pepsin
Trypsin
Peptidases
Stomach mucosa
Pancreas
Intestinal mucosa
Peptides
Peptides
Amino acids
FATS Lipase Pancreas Fatty acids
and glycerol

16. The table below shows digestive enzymes grouped by source of the enzyme.
SOURCE
ENZYME FOOD PRODUCT
MOUTH (salivary glands) Salivary amylase Polysaccharides Maltose
STOMACH Pepsin Proteins Peptides
PANCREAS Pancreatic amylase
Trypsin
Lipase
Polysaccharides
Proteins
Fats
Maltose
Peptides
Fatty acids
and glycerol
SMALL INTESTINE Maltase
Peptidases
Maltose
Peptides
Glucose
Amino acids

17. • Digestive function is affected by hormones produced in many
endocrine glands, but the most profound control is exerted by
hormones produced within the gastrointestinal tract. The
gastrointestinal tract is the largest endocrine organ in the
body and the endocrine cells within it are referred to
collectively as the enteric endocrine system. Three of the best-
studied enteric hormones are:
• Gastrin: Secreted from the stomach and plays an important
role in control of gastric acid secretion.
• Cholecystokinin: A small intestinal hormone that stimulates
secretion of pancreatic enzymes and bile.
• Secretin: Another hormone secreted from small intestinal
epithelial cells; stimulates secretion of a bicarbonate-rich
fluids from the pancreas and liver.

18. HORMON
ES
Major Activities Stimuli for Release
Gastrin
Stimulates gastric acid secretion and proliferation of gastric
epithelium
Presence of peptides and amino acids in gastric lumen
Cholecyst
okin
in
Stimulates secretion of pancreatic enzymes, and contraction and
emptying of the gall bladder
Presence of fatty acids and amino acids in the small
intestine
Secretin
Stimulates secretion of water and bicarbonate from the pancreas
and bile ducts
Acidic pH in the lumen of the small intestine
Ghrelin
Appears to be a strong stimulant for appetite and feeding; also a
potent stimulator of growth hormone secretion.
Not clear, but secretion peaks prior to feeding and
diminishes with gastric filling
Motilin
Apparently involved in stimulating housekeeping patterns of motility
in the stomach and small intestine
Not clear, but secretion is associated with fasting
Gastric
inhi
bitor
y
poly
pept
ide
Inhibits gastric secretion and motility and potentiates release of
insulin from beta cells in response to elevated blood glucose
concentration
Presence of fat and glucose in the small intestine

19. Hormones and Digestion:Feedback loops.The hormone Gastrin stimulates the production of
gastric acid.The pyloric sphincter regulates the release of chyme (nutrient broth) into the small
intestine.Bile contains bile salts that aid digestion and absorption of fats.

20. • Secretin hormone production is stimulated by acid
chyme entering the duodenum. This hormone
stimulates the pancreas to release bicarbonate to
neutralize the acid.
• CCK stimulates the gall bladder to release bile and
the panceas to release pancreatic juices.
• If the chyme is rich in fats causes the release of
enterogastrone. This hormone inhibits peristalsis
and the release of secretion of acid.

21. The small intestine

23. Essential Amino Acids
• The human liver can synthesize 9 of the 20 amino acids used
in proteins. Those that cannot be synthesized are called the
essential amino acids. They must be supplied by diet.

24. Bile Ducts and Gallbladder
Definition:
Bile is produced by the liver cells and collects in the small bile ducts between the
microscopic liver lobules which lead to the larger ducts. Outside the liver, two
outflowing hepatic ducts unite to form the common hepatic duct. From here the bile
enters the cystic duct and the common bile duct . The common bile duct joins the
pancreatic duct to enter the duodenum. The opening is controlled by a valve which
regulates the flow of gastric juices (bile, pancreatic enzymes) into the duodenum.

25. • Function:
Bile is produced in the liver and aids the digestion of fatty
food substances. Between 250 and 1000 millilitres of bile are
produced each day. The most important constituents of bile
are: water, bile salts, cholesterol , phospholipids . Between
meals, the bile is stored in the gallbladder, which contracts
once or twice as food is taken. This causes the bile to enter
the duodenum via the common bile duct. 80 to 90% of the
bile acids are later reabsorbed by the small intestine and re-
enter the liver via the bloodstream (portal system).

26. Fat Digestion & Absorption

27. Absorption and Motility
• Transit time is 12 to 24 hours
– reabsorbs water and electrolytes
• Feces consist of water & solids (bacteria, mucus, undigested fiber, fat &
sloughed epithelial cells
• Haustral contractions occur every 30 minutes
– distension of a haustrum stimulates it to contract
• Mass movements occur 1 to 3 times a day
– triggered by gastrocolic and duodenocolic reflexes
• filling of the stomach & duodenum stimulates motility
• moves residue for several centimeters with each contraction

28. Defecation
• Stretching of the rectum stimulates defecation
– intrinsic defecation reflex via the myenteric plexus
• causes muscularis to contract & internal sphincter to relax
– relatively weak contractions
• defecation occurs only if external anal sphincter is voluntarily relaxed
– parasympathetic defecation reflex involves spinal cord
• stretching of rectum sends sensory signals to spinal cord
• splanchnic nerves return signals intensifying peristalsis
• Abdominal contractions increase abdominal pressure as levator ani lifts anal
canal upwards
– feces will fall away