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Physiol 2130 Module 15—Digestive System
Introduction to Human Physiology (The University of Western Ontario)
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Module 15: Digestive System
The Digestive System
15.4: Anatomy and Basic Function
Main function of digestive system ⇒ Break down organic nutrients so they
can be reabsorbed
● Carbs, proteins, fats, water, vitamins, and minerals
Basic Function:
1. In the mouth, food is broken up by chewing (mastication) and is mixed with saliva
2. Salivary glands produce saliva to moisten and begin digesting some food particles
3. Esophagus is a straight muscular tube that connects the mouth and the pharynx to the
stomach
4. Stomach stores, mixes, and digests more food and delivers food to the small intestine
5. The liver has many functions; for the digestive system it produces and secretes bile
6. Gallbladder stores and concentrates bile
7. Pancreas secretes digestive enzymes, hormones, and bicarbonate
8. Small intestine allows digestion and absorption of most food particles
9. Large intestine stores and concentrates undigested material and absorbs salt and
water
10. Rectum is the site where the defecation reflex is triggered
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15.6: Basic Processes
● Secretion involves the release of digestive fluids into the lumen of the digestive tract.
This fluid may include water, mucus, acid, electrolytes, enzymess, bile salts, and
digestive enzymes
● Digestion is the process whereby food is broken down into smaller molecules by the
digestive enzymes so they can be taken up by the body
● Absorption is the process whereby these small molecules are taken up by the
circulatory system and are distributed throughout the body
● Motility is the movement of the “food” through the digestive system by the contraction
of smooth muscle that lines the walls of the tract
● Excretion is the removal of the unwanted waste products from the body after almost
all the wanted material is absorbed
15.7: The Mouth
Digestion begins in the mouth
● Bolus: Saliva mixed with food is broken down by chewing (mastication)
● Saliva mostly contains water (99.5%) with ions and proteins (0.5%)
○ Saliva is secreted by 3 glands: Parotid, submandibular, sublingual
○ Helps lubricate the bolus of food and begins digesting carbohydrates
■ Enzyme amyla in saliva helps digesting carbohydrates
■ Under control of autonomic nervous system
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15.8: Swallowing
● The bolus gets pushed to the back of the mouth by the tongue and the swallowing
reflex will be initiated
○ Uvula of the soft palate closes over the nasopharynx
○ The larynx is lifted by the muscles in the neck and the epiglottis bends back
over the glottis to cover the larynx
○ Bolus moves down the esophagus through the cardiac orifice (aka lower
esophageal sphincter) and into the stomach via smooth muscle contractions
(peristalsis)
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The Stomach
15.9: Structure
Divided into 3 general areas
● Upper dome-shaped fundus
● Middle body (largest part of the stomach)
● Lower antrum (aka pyloric region)
Pyloric sphincter is located at the distal portion and regulated the
emptying of the stomach into the first part of the small intestine (duodenum)
When the stomach is empty, there are folds (rugae) which increases the surface area and
allows for expansion of the stomach as it fills with food
15.10: Function
The stomach liquefies, mixes and stores each bolus of food from the meal.
● This mixture is called the chyme and is slowly released into the small intestine
● Most of the digestion and absorption takes place in the small intestine
● The slow release regulates the amount of food entering the small intestine
Only substances like alcohol and aspirin can cross the lining of the stomach to be absorbed
into the bloodstream
● Mixing of chyme is achieved by peristaltic contraction of the stomach walls that begins
at the fundus
○ The contractions help move the chyme through the pyloric sphincter into the
small intestine where most of the digestion and absorption takes place
15.11: Function of Gastric Secretion
Stomach secretes a variety of substances
● Mucus, hydrochloric acid (HCL), pepsinogen, gastrin, and intrinsic factor
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The Pancreas
15.12: Structure
Pancreas has digestive (exocrine) and endocrine functions
● Exocrine products of the pancreas are secreted into a long pancreatic duct
● Duct merges with the common bile duct (from liver & gallbladder) just before entering
the duodenum
15.13: Function
Pancreas produces and secretes:
● Carbohydrate-digesting enzyme amylase
● Protein-digesting enzymes trypsin, chymotrypsin, and proteases
● Fat-digesting enzyme lipase
● Sodium bicarbonate into the duodenum to neutralize the acid from the stomach
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Small Intestine
15.14: Structure
Stomach contents empty through the pyloric sphincter into the small intestine
● Longest section of the digestive tract
● Ends at the ileocecal sphincter and empties into the ascending colon of the large
intestine
Divided into 3 segments:
● Duodenum (shortest segment)
● Jejunum
● Ileum (longest segment)
Inner wall of the small intestine has folds and
contain fingerlike projections (villi) to result in
large surface area
The villi contain a capillary network and the lymphatic lacteal to absorb digested material
● Layer of epithelial cells secrete digestive enzymes which covers the villi
● Microvilli that face out into the lumen of the intestine form a brush border
15.16: Function
Almost all digestion and absorption takes place in the first 2 segments of the small intestine
(duodenum and jejunum)
● Digestion of the chyme occurs by digestive enzymes from the pancreas and epithelial
cells
● Bile (from the liver) helps digest fat → Bile is NOT a digestive
enzyme
**There are very specific digestive enzymes for each part of our meal. Some enzymes digest he
protein of the meat, some digest the carbohydrates of the fries, and some digest the fat in both
the meat and fries**
Small intestine is the site where most of the digested material gets absorbed into the
circulatory and lymphatic system
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15.17: Carbohydrate Digestion
Carbohydrates are made up of a single building block ⇒ Monosaccharides
● E.g: Glucose, fructose, galactose..
Disaccharides consists of two monosaccharides
● E.g: Maltose, lactose, sucrose
Polysaccharides consists of 2+ monosaccharides
In order to absorb larger carbohydrates, polysaccharides must be broken down into
monosaccharides
● Once in this form ⇒ they can be absorbed by transport system in the
walls of the small intestine
15.18: Carbohydrate Digestion (cont.)
Digestion begins in the mouth (@ the salivary enzyme, amylase)
● This enzyme breaks down large polysaccharides into smaller polysaccharides and
maltose
Once food reaches the stomach ⇒ digestion of the carbohydrates stop because
of the acidic environment which denatures the salivary amylase
15.19: Carbohydrate Digestion (cont.)
Food reaches the small intestine
● Digestion of the carbohydrates begins again because of the pancreas secretion:
○ Amylase into the duodenum ⇒ continue digestion
○ Bicarbonate ions ⇒ neutralize the acid from the stomach
15.20: Carbohydrate digestion (cont.)
Each disaccharide requires a specific enzyme to break it down to monosaccharides
● The enzymes are located in the brush border of the microvilli
● The brush border is located on the epithelial cells
Lactase breaks down lactose to galactose and glucose
Matase breaks down maltose to 2 x glucose
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Sucrase breaks down sucrose to glucose and fructose
15.21: Carbohydrate Digestion—Lactose Intolerance
Lactose is a carbohydrate found in milk
● People that are lactose intolerant lack the
ability to produce lactase
● If lactose is not digested and cannot be
absorbed, it will stay in the digestive tract lead
to diarrhea
People who suffer from lactose intolerance can purchase synthetic lactase which is generally
taken before a meal that may contain lactose
15.22: Carbohydrate Absorption
Carbohydrate absorption process is almost identical to glucose reabsorption in the kidneys
Intestinal epithelial cells contain Na+/K+ pumps on their basal side which establish a
concentration gradient for Na+
● High conc. Na+ on the outside
Concentration gradient powers the Na+/glucose cotransported (secondary active transport)
● The transporter moves glucose into the cell as Na+ move in
● Once glucose is inside the cell it will then diffuse out through the basal side by
facilitated diffusion
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15.23: Protein Digestion
20 different AA
● 11 nonessential AA that can be produced by the body
● 9 essential AA that must come from the diet
Different groups of AA require different enzymes to break them apart
● Proteins must be broken down into the AA building blocks before they can be absorbed
by transport systems in the small intestine
15.24: Protein Digestion (cont.)
Hydrochloric acid (HCl) converts the inactive pepsinogen to the active enzyme pepsin
● HCl also helps uncoil the long twisted strands of proteins
● The unfolding of proteins give the pepsin access to the long protein chains in order to
● digest them into smaller chains (polypeptides)
● Polypeptides are pass through the pyloric sphincter into the small intestine where they
continue to be digested and absorbed into the body
15.25: Protein Digestion (cont.)
Pancreatic enzymes continue protein digestion in neutral pH to work optimally
● Chyme from the acidic stomach is neutralized with bicarbonate secreted from the
pancreas
In neutral pH, pepsin is inactivated. Trypsin and chymotrypsin secreted from the pancreas
continue digestion in the small intestine.
15.26: Protein Digestion (cont.)
Proteases continue digesting proteins into AA
● Produced in the pancreas and secreted into the small intestine
● Found along the brush border of the intestinal epithelial cells
Endopeptidases break the bonds between AA in the inner part of the protein
Exopeptidases break the bonds between AA at the ends of the protein
15.27: Protein Absorption (cont.)
Absorption of AA is through secondary active transport requiring the presence of Na+
concentration gradient
● As Na+ moves move into the intestinal epithelial cells down their concentration
gradient, they power a co-transport that also moves AA into the cell
● The remaining small peptides are absorbed by endocytosis
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15.28: Short Summary
● Carbohydrates begin to be digested in the mouth by the enzyme amylase that is found
in the saliva
● Little carbohydrate digestion and some protein digestion takes place in the stomach.
● HCl converts pepsinogen to an active enzyme (pepsin) which breaks the long strands
of protein into smaller polypeptides
In the small intestine;
● Carbohydrates are broken down by the pancreas amylase to sucrose, maltose and
lactose which are broken down into monosaccharides by sucrase, maltase and lactase
○ Monosaccharides are absorbed into the body by a secondary active transport
system using Na+/glucose cotransporter
● Pancreatic enzymes (endopeptidase and exopeptidase) break the polypeptides into
AA.
○ AA are also absorbed into the body by Na+/AA cotransporters relying on the
Na+ concentration gradient
15.29: Fat and Lipids
Lipids = fats
● E.g: Triglycerides, phospholipids, cholesterol, long-chain fatty acids
● Not water soluble
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15.30: Fat and Lipids (cont.)
Emulsification: Churning action of the stomach helps break large drops of fat into smaller ones
● Small drops of fat are easier to digest than large fat droplets
○ It is important to keep the lipids emulsified through the whole digestion process
15.31: Fat Digestion—Bile
Bile is produced in the liver and transported to the gallbladder to where it is stored and
concentrated
● Gallbladder releases bile into the duodenum of the small intestine during meals
● The salts from the bile keep the lipid droplets emulsified ⇒ preventing them to
form back into large droplets
15.32: Fat Digestion—Co
Lipid droplets are coated in bile salts
● Enzyme lipase cannot reach and digest the lipids
Colipase is secreted from the pancreas ⇒ not a digestive enzyme
● This protein allows for the lipase to get access to the lipid interior of the fat droplet
● Pancreatic lipase can digest the lipids
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15.33: Fat Absorption
Fatty acids and monoglycerides (lipid soluble) can diffuse through the membrane of the
epithelial cells that line the small intestine
● Cholesterol molecules are transported into the cells by a specific active transport
system
After absorption of lipids; bile salts are left behind
● Bile salts are reabsorbed by a cell transport system in the ileum (end of small intestine)
● Bile salts are returned to the liver where they are reused
15.35: Fat Absorption (cont.)
Absorbed lipids are still inside the intestinal epithelial cells
● Monoglycerides and fatty acids will enter the smooth endoplasmic reticulum to
combine with cholesterol and proteins to form chylomicrons
● Chylomicrons are packed up into secretory vesicles by the golgi apparatus and leave
the cell to enter the lacteals of the lymphatic system which eventually drains into the
circulatory system
Organ:
Substance secreted:
Mouth
Amylase
Pancreas
Amylase, Endopeptidase, Exopeptidase
Small Intestine
Sucrase, Maltase, Lactase
Stomach
Pepsinogen, Pepsin, Hydrochloric Acid
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15.36: Absorption of Vitamins
Vitamins and minerals are absorbed by the small intestine
Lipid soluble substances can diffuse directly through the cell membrane
● Lipid soluble vitamins can be absorb in the same way as fats
● E.g: Vitamin A, D, E and K
Water soluble substances are absorbed via carrier-mediated mechanisms
● Absorption of these vitamins are similar to absorption of sugars and AA
○ Require Na+ concentration gradient and Na+ cotransporter
● E.g: Vitamin B, C, folic acid, and niacin
15.37: Absorption Of Water
~80% of water is reabsorbed from water contained in:
● Saliva, digestive enzymes from the stomach and pancreas, bile and the secretions from
the intestine
First half of the small intestine (duodenum & Jejunum)
⇒ absorbs ~44% of
total water
Last half of the small intestine (Ileum)
⇒ absorbs ~38% of
total water
Large intestine
⇒ absorbs ~1.5% of total
water
Remaining water (100mL) is excreted in the feces
15.38: Absorption Of Water (cont.)
Water is reabsorbed in the small intestine much in the same way as it is in the kidneys
● A conc. gradient is built up as molecules of glucose, AA, and lipids are absorbed in the
intestinal epithelial cells
These cells develop a higher solute concentration inside
● This causes water to flow into the cells by osmosis and down its concentration gradient
● Water ends up flowing into the cells via osmosis and down the concentration gradient
○ Solutes move out of the cell into circulation, water continues to follow by
osmosis into the blood
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15.39: Absorption of Ions
Absorption of Na+ is similar to the kidney’s mechanism
● Occurs via Na+/K+ pump located on the basal lateral surface of the intestinal cells
○ Pumps establishes a conc. gradient for Na+ (low on the inside)
■ Na+ will move into the cell and down their conc. gradient from the
lumen of the intestine
■ Na+ will be absorbed along with carbohydrates and AA
● This requires the presence of Na+ for their absorption
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Large Intestine
15.40: Structure
Large intestine (aka colon) begins at the
ileocecal valve and is ~1m
● Ascending colon
● Longitudinal/transverse Colon
● Descending colon
● Curved sigmoid section
● Rectum
● Anus
Diameter of the large intestine is much larger
than the small intestine but lacks the many folds and villi in the smaller counterpart
Bolus of food would pass through:
Mouth > Pharynx > Esophagus > Stomach > Duodenum > Jejunum > Ileum > Cecum >
Ascending Colon > Transverse Colon > Descending Colon > Rectum > Anus
15.41: Function
Main function of the large intestine is to absorb water and electrolytes and to store and
concentrate the feces
● Sodium is absorbed by active transport mechanism, and water follows by osmosis
● Similar to small intestine most of this absorption takes place in the first half of the
colon
The large intestine also secretes mucus to protect its lining from chemical and mechanical
damage and to lubricate the forming feces
● Some K+ and bicarbonate and secreted into the colon
Very small amount of digestion occurs when undigested polysaccharides are metabolized by
bacteria to free fatty acids that are absorbed by passive diffusion
● Bacteria also produce gas (glatus) and vitamin K for blood coagulation
**All carbohydrates and amino acids require secondary active transport/co-transporters from
Na+ conc. From the Na+/K+ pump of the basal side of the intestinal cells**
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Regulation
15.42: Regulation
Process of digestion and absorption are unregulated
Process of motility and enzyme secretion and carefully regulated
● Muscular contractions of the digestive tract
● Involves both neural and hormonal mechanisms
Hormonal Mechanisms: Release of chemicals from intestinal cells in response to direct
mechanical stimulation from food in the stomach or intestine
● Hormones can also be released in response to neural activation
Neural Mechanisms: Involves enteric nervous system (division of the autonomic nervous
system)
15.43: The Enteric Nervous System
Autonomic nervous system (ANS) can influence the activity of the digestive tract
● Sympathetic and Parasympathetic nervous divisions
ANS alters the activity of the nerves in the enteric nervous
system which affects:
● smooth muscle in the walls of the digestive tract
● enzyme-secreting cells
● endocrine cells (enteroendocrine cells)
● blood vessels of the tract
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15.44: The Enteric Nervous System (cont.)
2 types of reflect loops
● Long loops: Travel through the central nervous system
● Short loops: Travel locally within the digestive system
Short Loops:
Begin by a mechanical distension (stretching of the digestive tract) or chemical
changes such as pH or osmolarity within the tract.
● These changes are detected by sensors of the enteric nervous system
that initiate a reflex to activate the effector organs and secretory cells
(smooth muscles in the walls of the tract) causing a release of enzymes
to alter gastric motility.
Long Loops:
Include input from the higher brain centres. The stimulus may be
sight or smell (of food).
● Signals are then sent through the parasympathetic nervous
system to the enteric nervous system to alter the digestive
function—enzyme release and gastric motility
14.45: The Autonomic Nervous System
PSYN increases gastric motility, secretions and cause the
production of water saliva
● PSYN associated with rest and relaxation
SYN decreases gastric motility, secretions and produces a dry sensation in the mouth
● SYN associated with fight or flight response
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14.46: Gastrointestinal Motility
Motility is the muscular contractions of the stomach and intestine that mix the food and propel
it through the digestive tract
● Important to regulate this process for digestion and absorption to be completed
It is important that the chyme be released by the stomach in discrete amounts in order to
maximize the digestive and absorptive processes
● Digestion of fats is a long complex process
○ If there is too much fatty foods released from the stomach → The
intestine may not be able to completely digest it
14.47: Gastrointestinal Motility (cont.)
Interstitial cells (smooth muscle cells) are similar to pacemaker cells of the heart
● These surround the digestive tract
● Can spontaneously alter their membrane potentials to produce a basal electrical
rhythm or BER (aka slow waves) that travel down the digestive tract
○ The membrane potentials do not cause muscles to contract b/c it does not reach
threshold
14.48: Gastrointestinal Motility (cont.)
Reaching threshold requires external stimuli (mechanical, nervous or hormonal input)
● If threshold is reached → Action potential fires and causes smooth muscle
contraction
○ Contractions travel down the intestine in a wave-like fashion
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Hormones of the Intestine
15.49: Hormones of the Intestine
Secretin is released in response to the presence of acid in the intestine
● Inhibits emptying from the stomach and causes release of pancreatic bicarbonate and
bile from the liver
○ Bicarbonate release from the pancreas neutralizes the acid from the stomach
and the intestinal digestive enzymes can function optimally
Cholecystokinin (CCK) is released in the presence of fats
● Slows emptying from the stomach
● Stimulates pancreas to release digestive enzymes
● Causes gallbladder to contract → releasing bile
Glucose-dependent insulinotropic peptide is secreted in response to glucose and AA
● Stimulates the release of insulin from the pancreas and may inhibit emptying of the
stomach
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15.50: Hormones of the Intestine (cont.)
1. Food in the intestine causes release of hormones
2. Hormones cause the release of a particular digestive enzyme to deal with food type
3. Hormones slow the output from the stomach → allowing food to be
properly
digested and
absorbed
15.51: Gastric Hormones
Gastrin is secreted from the G cells of the antrum of the stomach in response to the presence
of:
● Proteins
● Mechanical distension from food
● Stimulation from the parasympathetic nervous system
Gastrin’s primary purpose ⇒ secretion of hydrochloric acid
(HCL)
● HCL converts pepsinogen to the active enzyme pepsin
● Pepsin digests proteins
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Phases of Gastric Acid Secretion
15.52: Three Phases of Gastric Acid Secretion
HCL secreted by parietal cells in the stomach
● Amount of HCL secretion varies through the eating-digestion process
● **HCL can be secreted before you start eating
Secretion of gastric acid is divided into 3 phases:
● Cephalic Phase (refers to the brain)
● Gastric Phase (refers to the stomach)
● Intestinal Phase (refers to the intestine)
15.53: Cephalic Phase
**Refers to the brain**
Cephalic phase is initiated in response to sight, smell, taste and chewing
of food
● This triggers the parasympathetic nervous system
○ Causes parietal cells to release HCl and the G cells to
release gastrin
○ Increases gastric motility
Cephalic phase is an anticipatory response to the act of eating and
involves the activation of the enteric nervous system ⇒ long loop reflex
15.54: The Gastric Phase
**Refers to the stomach**
Gastric phase of acid secretion starts once the food is swallowed by the stimulus:
● Mechanical Stimulus: Presence of food in the stomach that distends the wall
● Chemical Stimulus: Presence of AA from the breakdown of
proteins
● Increases gastric motility
The stimuli involves the activation of the enteric nervous system to
release HCL, gastrin and pepsinogen ⇒ short loop reflex
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15.55: The Intestinal Phase
**Refers to the intestine**
Intestinal phase starts when the food has left the stomach and entered the intestine
● Intestine = Most of the digestive and absorptive processes take place here
○ Carefully regulated
● Stimulus for intestinal phase:
○ Presence of glucose, fat and acidic chyme in the intestine
● Enteric nervous system, Secretin, CCK, Glucose-dependent insulinotropic peptide
work together to decrease gastric motility
○ Inhibits the secretions in the stomach
**During intestinal phase of digestion; CCK, insulin and secretin are released**
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