Functional Human Physiology
for the Exercise and Sport Sciences
The Gastrointestinal System
Jennifer L. Doherty, MS, ATC
Department of Health, Physical Education, and
Recreation
Florida International University
Overview of Gastrointestinal System
Function




Digestion
Absorption
Secretion
Motility
Organs of the Gastrointestinal
System
The alimentary canal

Continuous hollow tube extending from the
mouth to the anus


Called the gastrointestinal (GI) tract as is passes
inferior to diaphragm
Functions:


Digestion
Absorption of digested fragments into blood
Organs of the Gastrointestinal
System
Accessory organs
 Assist in the
chemical process of
digestion by
secreting saliva,
enzymes, and bile




Salivary glands
Pancreas
Liver
Gallbladder

Assist in the
mechanical process
of digestion


Teeth
Tongue
Digestion


The process mechanically breaking
down food into particles small enough
to be absorbed through cell
membranes
Two methods of food breakdown


Chemical
Mechanical
Digestion: 5 Integrated Steps

(1) Ingestion


(2) Propulsion


Foodstuff enters the GI tract via the mouth
The process that moves foodstuff through the GI
tract via coordinated reflexive contraction activity
(3) Digestion

The process of breaking down large food particles
into smaller particles via chemical and mechanical
action
Digestion: 5 Integrated Steps

(4) Absorption



The movement of digested end products through
the intestinal wall and into the blood or lymph
End products include small organic molecules,
electrolytes, & H2O
(5) Defecation

The process of discharging undigested and
unabsorbed foodstuff
The GI Tract
The Gastrointestinal Wall
 4 layers
 (1) Muscosa





Lamina propria
Muscularis mucosa
(2) Submucosa
(3) Muscularis Externa
(4) Serosa
Mucosa




Inner most mucous membrane
Lines lumen
Composed of simple columnar
epithelium
Contains many mucus secreting goblet
cells
Mucosa

Lamina propria



Underlying loose CT
Contains blood vessels, sensory nerve
endings, lymph vessels, and scattered
lymph tissue
Muscularis mucosa


Layer of smooth muscle
Produces local movements that change
the shape of the lumen
Mucosa
Functions:
 Protect underlying tissue
 Absorb digested material
 Secrete mucous or digestive juices
 Increase surface area


Folds in the mucosa
Villi present in the small intestine
Submucosa


Composed of loose CT
Contains blood vessels and lymphatics


Function to circulate absorbed nutrients
Contains nerves from the ANS


Form the submucosal plexus
A component of the intrinsic nervous system
of GI tract
Muscularis Externa

Composed of two layers of smooth
muscle


Inner layer - circular fiber arrangement
Outer layer - longitudinal fiber arrangement
Muscularis Externa
Inner layer - circular fiber arrangement


When fibers contract = ↓ lumen size
Forms sphincter muscles

Prevent backflow of materials
Outer layer - longitudinal fiber arrangement

When fibers contract = mix and propel food
along the alimentary canal
Muscularis Externa

Myenteric plexus
Extensive nerve network between the
smooth muscle layers

Regulate motility



Movement/contraction of the GI tract walls
Regulate glandular secretions

Secretions into the lumen of the GI tract
Serosa


Outermost layer of the GI wall
Inner layer



Outer layer - Mesothelium


Consists of fibrous CT
Provides structural support
Consists of epitelium
Secretes a water lubricating fluid allowing
organs to slide past one another
Peritoneal Cavity


The space between the visceral
peritoneum and parietal peritoneum
Both visceral and parietal peritoneum
secrete serous fluid into the peritoneal
cavity

Lubricates and protects abdominal tissues
as they slide past one another
GI Motility Patterns


Contractions of the muscularis externa
Two methods


Peristalsis
Segmentation
GI Motility Patterns
Peristalsis
 Propelling motion produced by alternate
waves of contraction and relaxation of
muscularis externa layer
 Occurs due to contraction of one part of
wall with simultaneous relaxation of the
wall ahead
 Propels food along tube
GI Motility Patterns
Segmentation
 Occurs due to rhythmic, local
contractions of the smooth muscle in the
muscularis externa layer
 Mechanically grinds foodstuff in the
stomach and intestines mixing it with
digestive juices
Gastrointestinal Regulation

Intrinsic Control



Submucosal Plexus
Myenteric Plexus
Extrinsic Control


Parasympathetic nerve fibers
Sympathetic nerve fibers
Gastrointestinal Regulation: Intrinsic
Control

Provided through the




Submucosal Plexus
Myenteric Plexus
Local stimulus = distension of submucosa or
muscularis externa walls
Local response = activation of stretch
receptors


↑ glandular secretions
↑ smooth muscle contractions in the immediate area
Gastrointestinal Regulation: Extrinsic
Control


Includes Parasympathetic and
Sympathetic input from the ANS
Fibers within the muscularis externa
layer specifically assist in controlling the
rate and strength of contractions
Gastrointestinal Regulation: Extrinsic
Control
Parasympathetic
activity
 Impulses carried by
the vagus nerve


↑ motility
↑ glandular
secretions
Gastrointestinal Regulation: Extrinsic
Control
Sympathetic activity
 Opposes parasympathetic activity



↓ motility
↓ glandular secretions
Causes sphincters to contract thus
slowing the movement of foodstuff
through the GI tract
Digestion: 5 Integrated Steps





(1) Ingestion
(2) Propulsion
(3) Digestion
(4) Absorption
(5) Defecation
Digestive Function: Ingestion


The digestive process begins in the
mouth
Includes…


Mechanical fragmentation of foodstuff
Foodstuff mixes with saliva
Digestive Function: Ingestion
Organs and associated structures
Tongue
 Mixes foodstuff with saliva during chewing
 Initiates swallowing
 Contains taste buds


Sensitive to chemical differences among food
molecules
Differentiate sweet, sour, salty, or bitter tastes
Digestive Function: Ingestion
Organs and associated structures
Teeth
 Tear and grind food


Mastication
Mechanical breakdown of foodstuff into
smaller fragments
Digestive Function: Ingestion
Organs and associated structures
Lips and Cheeks
 Keep food in mouth
 Involved in speech
Digestive Function: Ingestion
Organs and associated structures
Palate
 Forms roof of mouth
 2 distinct parts


Hard Palate
Soft Palate
Digestive Function: Ingestion
Organs and associated structures

Hard palate



Anterior region composed of bone
Forms a hard surface against which foodstuff is pushed
during chewing
Soft palate


Posterior region composed of skeletal muscle
Rises reflexively to close off nasopharynx during
swallowing
Digestive Function: Ingestion
Organs and associated structures
Salivary glands - 3 pairs
 1). Parotid glands



2). Submandibular glands


Largest salivary glands
Anterior to the ears
Inferior to the jaw
3). Sublingual glands

Inferior to the tongue
Digestive Function: Ingestion
Organs and associated structures
Saliva
 Produce 1 - 1.5 L/day
 Basic composition





Water (98 - 99%)
Salivary Amylase: a digestive enzyme
Mucins: mucous that lubricates the mouth and food
Ions, buffers, metabolites, antibodies, etc
Dissolves foodstuff
Digestion: 5 Integrated Steps





(1) Ingestion
(2) Propulsion
(3) Digestion
(4) Absorption
(5) Defecation
Digestive Function: Propulsion
Swallowing
 Reflex response which moves foodstuff
through the pharynx and down the
esophagus
 The swallowing reflex is triggered when
material moves into the pharynx
Digestive Function: Propulsion
Components of the swallowing reflex:


The soft palate rises
The epiglottis covers the opening of the larynx



Prevents foodstuff from entering the air passageways
Peristaltic contractions along the pharyngeal and
esophageal walls propel foodstuff through the GI
tract
Relaxation of lower esophageal sphincters allows
foodstuff to enter the stomach
Digestive Function: Propulsion
Esophagus
 A muscular tube which collapses when it is
not in use
 Contains 2 sphincters



Upper esophageal sphincter
Lower esophageal sphincter
The sphincters contract to prevent the
backflow of stomach acids into the
esophagus
Digestive Function: Propulsion

Both the pharynx and the esophagus are
only passageways for foodstuff


Peristalsis
The pharynx and the esophagus are not
directly involved in digestive activities
Digestion: 5 Integrated Steps





(1) Ingestion
(2) Propulsion
(3) Digestion
(4) Absorption
(5) Defecation
Digestive Function: Digestion
Saliva

Dissolves foodstuff via chemical processes
 Contains salivary amylase
Chemical breakdown of starches
Allows food to be tasted



Moistens foodstuff and converts it into bolus

Mass of moistened food that can be easily swallowed
Digestive Function: Digestion
Control of Salivation
 Regulated by both the PNS and SNS


Receptors in mouth send signals to brain
Controlled primarily by the PNS


Causes ↑ salivation
Rate of secretion and composition of saliva
changes in response to…

sight, sounds, smells, pressure of food in mouth
Digestive Function: Digestion
Stomach
 Stores foodstuff



Gradual delivery to the small intestine
Mixes foodstuff with gastric juices
Initiated the digestion of proteins
Digestive Function: Digestion
Stomach
 Protein digestion occurs via pepsin


Enzyme that breaks down proteins into
smaller polypeptide and amino acid
fragments
Protein digestion is the only type of
chemical digestion occurring in stomach
Digestive Function: Digestion
Stomach
 Foodstuff mixed with gastric juices
forming a creamy paste called Chyme


Chyme is passed out of the stomach into the
small intestine
Limited absorption occurs in the stomach

Absorb small amounts of H2O, glucose,
salts, alcohol, and lipid-soluble drugs
The Stomach


Distends to accommodate foodstuff
Empty stomach


Full stomach


Walls collapse forming folds (rugae) in the
inner lining
Rugae smooth out
Able to hold 1 - 1.5 L of foodstuff
The Stomach
Regions of the stomach
 Body - large, main portion of stomach
 Cardiac region - surrounds opening
where esophagus enters the stomach
 Fundus - domed-shaped region that
projects above the cardiac region

Serves as a storage area
The Stomach
Regions of the stomach
 Pylorus - exit
 Antrum (pyloric antrum) - funnel-shaped
region near the pylorus
 Pyloric canal - narrow region
terminating at the pylorus
 Pyloric sphincter - controls stomach
emptying
The Stomach
Modifications of the Stomach Wall
 The muscularis layer is modified according to the
functions of the stomach



Greater churning/mixing ability
Mechanical breakdown of foodstuff into smaller
pieces
Composed of 3 layers
 Circular
 Longitudinal
 Oblique layer - additional innermost layer
The Stomach
Modifications of the Stomach Wall
 The mucosa layer contains gastric pits


Gastric pits lead into gastric glands


Millions of pockets in the epithelium
Gastric glands are located deep in the lamina
propria
Gastric glands produce gastric juices

Stomach secretions
Gastric Glands
4 types of gland cells:
 1) Chief cells


Zymogenic cells
2) Parietal cells

Oxyntic cells

3) Mucous neck cells

4) Enteroendocrine cell
Gland Cells: Chief cells (Zymogenic cells)

Secrete pepsinogen



Inactive form of pepsin
 Protein-digesting enzyme
Activated by stomach acids
Also secrete gastric lipase



Fat-digesting enzyme
Not very active in stomach due to low pH
Main action is on butter fat
Gland Cells: Parietal cells (Oxyntic cells)

Secretes HCl



HCl is a strong acid that ↑ the acidity of the
stomach (pH = 1.5 - 3.5)
Activates pepsin
Also secretes Intrinsic factor


Required for vitamin B12 absorption in the
small intestine
Only stomach function essential for life
Gland Cells: Mucous Neck Cells


Secretes alkaline mucous
Protects stomach lining from…


Damaging acidity in the stomach
Damaging action of protein-digesting
enzymes
Gland Cells: Enteroendocrine Cells



Secrete a variety of compounds
directly into the lamina propria
These compounds then diffuse into
blood capillaries
Function as local hormones to regulate
the functions of various digestive
organs

i.e.) Gastrin – regulates the stomach
Gland Cells

Gastric juices do not digest the walls of the
stomach due to…



The mucous barrier is alkaline


Mucous barrier
Epithelial cells
Neutralizes acid on the stomach lining
Tight junctions between epithelial cells
prevent acid from leaking to underlying tissue


Rapid turnover of epithelial cells
Replaced every 3 days
Regulation of Gastric Secretion
Neural control

Parasympathetic input


Conducted via the vagus nerve
Increases secretion from all gastric gland
cells



↑ Pepsin concentration in stomach
↑ HCl concentration in stomach
↑ Gastrin concentration in stomach
Regulation of Gastric Secretion
Neural control

Sympathetic input



Overrides Parasympathetic input
Inhibits gastric gland secretion

↓ Pepsin concentration in stomach

↓ HCl concentration in stomach

↓ Gastrin concentration in stomach
Occurs during times of…


Physical activity
Emotional stress
Regulation of Gastric Secretion
Hormonal control
 Provided primarily by Gastrin
 Increases secretion from all gastric gland cells


↑ Pepsin concentration in stomach

↑ HCl concentration in stomach

↑ Gastrin concentration in stomach
Three phases of gastric secretion

Occur almost simultaneously
Phases of Gastric Secretion
1). Cephalic phase (Reflex phase)


Triggered by sight, smell, thought, or taste of
food
Sensory input is relayed to hypothalamus



Vagus nerve sends impulses to gastric glands


Sensory input is integrated in hypothalamus
Vagus nerve is stimulated
Increase secretions from all gastric gland cells
Prepares the stomach in advance for the arrival
of foodstuff
Phases of Gastric Secretion
2). Gastric phase


Begins when foodstuff reaches the stomach
Stimulation of the vagus nerve occurs due to 2
stimuli
 a). Distension of stomach walls


b). Chemical stimuli


Stimulation of stretch receptors
Provided by partially ingested foodstuff
Increase secretions from all gastric gland cells
Phases of Gastric Secretion
3). Intestinal Phase

Purpose = control the rate of gastric emptying


Provides the small intestine with enough time for
digestion and absorption
Phase begins when chyme enters the
duodenum

First portion of the small intestine
Phases of Gastric Secretion
3). Intestinal Phase cont.

Presence of foodstuff in the stomach stimulates
the release of intestinal gastrin


Hormone similar to gastrin
Increase secretions from all gastric gland cells


Helps speed digestion in the stomach before
stomach-emptying is complete
Released only when stomach begins to empty
Inhibition of Gastric Secretions

Can be accomplished via…



Emotional upset
Sympathetic innervation
The rate of gastric secretion may also
be inhibited by the…


Stomach
Small intestine
Inhibition of Gastric Secretions


Gastric secretion may be inhibited
during the gastric phase
Gastric secretion decreases when the
mucosa becomes too acidic (pH < 2)


Slows the rate of further gastric secretion
Occurs as the stomach empties and
buffering capacity of food material
decreases
Inhibition of Gastric Secretions



Gastric secretion may be inhibited
during the intestinal phase
Most regulatory controls during the
intestinal phase are inhibitory
Duodenum responds to various stimuli
through the enterogastric reflex
Inhibition of Gastric Secretions

Enterogastric reflex


This is an inhibitory reflex in the small intestine
that slows gastric secretions
Stimuli



Distension of the duodenum
Presence of acidic (H+) and/or hypertonic chyme
Presence of fats, partially digested proteins,
and/or irritants
Inhibition of Gastric Secretions
Inhibitory effects: enterogastric reflex


 PNS input to the stomach mucosa
 SNS input to the pyloric sphincter



Prevents additional foodstuff from entering the
small intestine
 gastric motility
 hormone release from the small intestine,
which inhibits gastric secretions
Digestive Function: Digestion
Mechanical Digestion and Propulsion
 Mechanical digestion is achieved through
contractile activity of the stomach


Propulsion is achieved through peristalsis
and segmentation


Mechanically mix and breakdown foodstuff
Propel chyme from the stomach to small intestine
These mechanisms are triggered via
distension of the stomach

~ 1 L of food enters
Digestive Function: Digestion
Mechanical Digestion and Propulsion cont.



Peristaltic waves sweep across the
stomach toward the pylorus
Foodstuff is pushed against the pyloric
sphincter, which is normally closed
Large food materials are churned and
mixed in the stomach until the food
particles are small enough to pass
through the pyloric sphincter
Regulation of Gastric Emptying


The rate of gastric emptying depends on the
type of food ingested
Fluids



Pass through quickly
90 minutes
Solids



Remain in the stomach until reduced to very small
particles
Remain in the stomach until dissolved in gastric juices
3 - 4 hours
Regulation of Gastric Emptying

Gastric emptying of nutrients



Carbohydrates are emptied first
Followed by proteins
Fats take the longest to leave the stomach
Regulation of Gastric Emptying



Dependent upon the duodenum
Gastric emptying is regulated along with gastric
secretions
Stimuli



Responses




Distention of the wall in the small intestine
Presence of acid (H+), fat, or hypertonic solution in the small
intestine
Trigger the enterogastric reflex
↓ gastric motility
Slows rate of gastric emptying
Purpose

Provides time for the small intestine to digest and absorb foodstuff
Small Intestine (SI)


Chyme leaves the stomach ~3-4 hours
after ingestion
Carbohydrates and proteins are partially
digested upon entering the SI


Too large to be absorbed through SI wall
Fats are undigested upon entering the SI
Small Intestine (SI)
In the duodenum, bile from liver is
added to chyme
 Also in the duodenum, enzymes from
pancreas are added to chyme
 Enteroendocrine Cells in the duodenum
also secrete enzymes
***All nutrient absorption occurs in
small intestine***

Small Intestine (SI)


The SI extends from the pyloric
sphincter of the stomach to the iliocecal
valve in the large intestine
Subdivisions



Duodenum
Jejunum
Ileum
Small Intestine (SI)


The SI contains ducts that carry bile
and pancreatic juices to the duodenum
Hepatopancreatic Ampulla


A single duct that empties into duodenum
Sphincter of Oddi


Also called hepatopancreatic sphincter
Duct that controls entry of bile and
pancreatic juices into duodenum
Digestion: 5 Integrated Steps





(1) Ingestion
(2) Propulsion
(3) Digestion
(4) Absorption
(5) Defecation
Digestive Function: Absorption

Anatomy of the SI



Plicae


Permanent transverse folds in mucosa & submucosa
layers
Villi


Function to increase the surface area
Increased surface area = Increased absorption
Finger-like projections in mucosa layer
Microvilli

Finger-like projections on the lumenal surfaces of
mucosa cells
Digestive Function: Absorption
SI Secretions: Intestinal juice
 A watery secretion with neutral pH (7.0)
 Serves as a medium for digestion and
absorption of nutrients
 Glands normally secrete ~1-2 L/day
 Stimuli for secretion


Distension of SI walls due to the presence of
chyme
Irritation of SI mucosa caused by hypertonic or
acidic chyme
Digestive Function: Absorption

SI Secretions: Digestive enzymes


Disaccharidases


Carbohydrate digestion: breakdown disaccharides into
monosaccharides
Peptidases


Secreted by epithelial cells in the SI
Protein digestion: breakdown polypeptides into amino
acids
Lipases

Fat digestion: breakdown fats into glycerol and fatty acids
Digestive Function: Absorption

Gastric Secreting Gland Cells


Goblet cells


Located in the mucosal epithelium of the SI
Secrete alkaline mucous
Enteroendocrine cells

Secrete local hormones to regulate activity
of the SI
Intestinal Glands




Location
 At base of villi in intestinal crypts
Secrete intestinal juices
 A watery mixture with neutral pH (7.0)
 Glands normally secrete 1 - 2 L/day
Function
 Serves as medium for digestion and absorption of
nutrients
Stimuli
 Distension of SI walls due to the presence of chyme
 Irritation of SI mucosa caused by hypertonic or acidic
chyme
Intestinal Glands


Specialized structures located in submucosa
Peyer's patches



Collections of lymph tissue
Preventing bacteria in undigested food from
entering the systemic circulation
Brunner's glands



Only located in the initial part of duodenum
Secrete alkaline mucous
Protects duodenal walls by neutralizing the acidic
chyme entering the SI from the stomach
Regulation of Intestinal Secretions


Stimuli to ↑ intestinal secretions

Presence of chyme in duodenum

Distension of SI walls
Stimuli elicits a response from the
Parasympathetic nervous system


Impulses carried via vagus nerve
Results in the secretion of…
 Alkaline mucous
 Digestive enzymes
The Pancreas


Both endocrine and exocrine function
Endocrine Function



Alpha cells release glucagon
Beta cells release insulin
Exocrine Function

Acinar cells



Secrete pancreatic juice into the pancreatic duct
Watery alkaline fluid (pH = 8)
Contains HCO3- and various digestive enzymes
The Pancreas

Pancreatic Enzymes




All are proteolytic or protease enzymes


Trypsin,
Chymotrypsin, and
Carboxypeptidase
Protein-digesting
Require alkaline environment

Provided by HCO3-
Pancreatic Enzymes


Secreted in inactive form
Activated in the duodenum


Pancreatic amylase


Breaks down almost all carbohydrates
Pancreatic lipase


Prevents self-digestion of pancreas
Breaks down fats
Pancreatic nuclease

Breaks down nucleic acids
Regulation of Pancreatic Secretion

Begins during the cephalic and gastric
phases of gastric secretion


via PNS activation
Major stimulus for secretion is…


Presence of chyme in the duodenum
Occurs during the intestinal phase
Regulation of Pancreatic Secretion

Chyme in duodenum stimulates the secretion
of…

Secretin


Cholecystokinin (CCK)



Targets duct cells to secrete watery alkaline fluid
Targets acinar cells to secrete pancreatic juice
containing digestive enzymes
Pancreatic secretions trigger the enterogastric
reflex
The enterogastric reflex slows gastric emptying
The Liver


Liver is the largest visceral organ
Blood circulation in the liver



O2 blood delivered via hepatic artery
Nutrient-rich blood delivered via hepatic portal
vein from the intestines
Blood from these 2 sources mixes as it
flows through the liver sinusoids

Blood leaves the liver via central veins -->
hepatic veins --> heart
The Liver
Functions:

Contributes to blood maintenance

Phagocytosis



Pathogens and old RBCs
Synthesize plasma proteins
Detoxify drugs and poisons

Metabolic regulation

Aids in digestion

Synthesis and secretion of bile
Bile


Yellow-green alkaline solution
Composition


Water
Bile salts


Synthesized in the liver from cholesterol
Function: Emulsification of fat globules


Mechanical break down of fat globules into small
droplets
Small droplets have larger surface area thus
allowing lipases to act more effectively
Bile Pigments

Biliverdin



Metabolized by bacteria in
SI
Exit body in feces (gives
brown color)




Metabolized by bacteria in
SI
Exit body in feces (gives
brown color)




Aid in digestion
Phospholipids

Bilirubin

Bile salts
Aid in digestion
Lecithin
Cholesterol
Neutral fats
Electrolytes
Bile Pathway

Bile exits the liver via canaliculi


Tiny bile canals
Bile ducts from the liver join together to
form the common hepatic duct

Extends downward toward the duodenum
Bile Pathway

Cystic duct from the gall bladder joins
common hepatic duct as well


Common bile duct


Forms the common bile duct
Joins with the pancreatic duct via the
hepatopancreatic ampulla
Pancreatic Duct


Empties bile and pancreatic juices into the initial
portion of the duodenum
Sphincter of Oddi guards this entry into duodenum
Gallbladder


Small green sac located on the inferior
surface of the liver
Concentrates and stores bile


Does not synthesize bile
Sphincter of Oddi


When closed, bile cannot enter duodenum
Bile is then stored in the gallbladder
Regulation of Bile Release
Bile release from the gall bladder into the
duodenum…
 The presence of fat and protein in the
duodenum stimulates the secretion of
Cholecystokinin (CCK)


Targets acinar cells to secrete pancreatic juice
containing digestive enzymes
CCK stimulates the gallbladder to contract and
sphincter of Oddi to relax

Results in bile release into the duodenum
Digestive Function: Digestion and Absorption


The SI is the major site of digestion and
absorption
Requires that chyme be mixed with…



Bile, and
Digestive enzymes
Necessary to expose foodstuff to the SI
mucosa for absorption
Digestive Function: Digestion and Absorption

Segmentation



Begins as soon as chyme enters the SI
Do not propel chyme onward
Peristalsis


Propels chyme onward
Requires time to move chyme

Allows more time for digestion and absorption
Digestion and Absorption of Major
Nutrients
Carbohydrates (CHO)
 Amylase

Salivary amylase


Began CHO digestion in mouth
Pancreatic amylase

Continues digestion of CHO in the SI
Digestion and Absorption of Major
Nutrients
Carbohydrates (CHO)


Glucose is the major end-product of CHO
digestion
Cellulose and other indigestible CHO provides
dietary fiber



Cannot be digested by humans
Passed to the large intestine
Simple sugars are absorbed across intestinal
mucosa of the SI
Digestion and Absorption of Major
Nutrients
Proteins (PRO)
 In Stomach


Dietary PRO is initially broken into smaller
polypeptide fragments by pepsin
In Small Intestine

Trypsin and chymotrypsin are enzymes from
pancreas that further break down
polypeptides into smaller fragments
Digestion and Absorption of Major
Nutrients
Proteins (PRO)

Complete PRO digestion occurs in the SI via
the following enzymes…



Carboxypeptidase (from pancreas)
Peptidases (from SI)
Final breakdown products are amino acids

Absorbed in the SI and travel to the liver via the
hepatic portal vein
Digestion and Absorption of Major
Nutrients
Fats (Lipids)
 The SI is basically the only site of fat digestion
 Triglycerides (TG) are the most abundant
dietary fat

As TGs enter the SI they are emulsified by bile
salts


Emulsification droplets
Pancreatic lipase

Enzyme that breaks down TGs into…


2 free fatty acids (FFA)
1 monoglyceride (glycerol)
Digestion and Absorption of Major
Nutrients
Fats (Lipids)
 FFAs and Glycerides interact with bile
salts to form micelles



Fatty elements clustered around bile salts for
the purpose of increasing solubility
Micelles enhance fat absorption in the SI
Once absorbed,

2 FFAs and 1 Glycerol form a TG
Digestion and Absorption of Major
Nutrients
Fats (Lipids)

TGs then combine with other lipid material to
form chylomicrons


Water-soluble lipoprotein droplets that are
processed in the cell
Chylomicrons are secreted into interstitial fluid


Carried by lacteals through the lymphatic
system
Ultimately enter the systemic circulation
Digestion and Absorption of Major
Nutrients
Fats (Lipids)


Chylomicrons are present in the systemic
circulation
Systemic capillaries contain lipoprotein lipase

Enzyme that breaks down TGs into…



2 FFA
1 Glycerol
FFAs and Glycerol may then diffuse out of
the blood and into cells


Used for energy, or
Stored as fat
Digestion and Absorption of Major
Nutrients
Vitamins

Fat-soluble vitamins
(A, D, E, K)


Bind to ingested lipids
Absorbed with lipids
via lacteals

Water-soluble vitamins


Mostly absorbed with
water
Exception - vitamin B12
 Must bind with
intrinsic factor, which
is produced by
parietal cells located
in the stomach
 Absorbed via
endocytosis
Digestion and Absorption of Major
Nutrients
Electrolytes

Na+ and Cl

K+


Absorbed via active transport
Absorbed via passive diffusion
H 2O


Absorbed via osmosis
Moves freely across intestinal mucosa
Digestion: 5 Integrated Steps





(1) Ingestion
(2) Propulsion
(3) Digestion
(4) Absorption
(5) Defecation
Digestive Function: Defecation
The Large Intestine
 2 major regions


Colon
Rectum
Digestive Function: Defecation
The Colon
 The largest portion of the large intestine
 Extends from the ileocecal valve to the
rectum
 Subdivisions:




Ascending colon,
Transverse colon,
Descending colon, and
Sigmoid (S-shaped) colon
Digestive Function: Defecation
The Rectum
 Distal portion of GI tract
 Includes last 6" of large intestine
 Extends to anal canal


Opens to the exterior of body at the anus
Sphincters


Internal sphincter
External sphincter
The Large Intestine (LI)
Anatomical Modifications
 Mucosa contains many goblet cells

No villi or microvilli

Alkaline mucosa

Holds feces together
 Protects the lining of the large intestine
Longitudinal muscle layer arranged in 3 bands



Produces out-pocketings of the LI wall
Called haustra
The Large Intestine (LI)
Motility Patterns

(1) Haustral churning





Similar to segmentation in the SI
Local contractile activity mixes chyme
Places chyme close to the haustra
Facilitates absorption of water
(2) Peristalsis


Very sluggish
Contributes little to movement of chyme
The Large Intestine (LI)
Motility Patterns cont.
 (3) Mass movements



Powerful peristaltic waves
Pass over large areas (8" segments) of the
colon ~ 3 times/day
Responsible for…


Moving foodstuff to the rectum
Producing the urge to defecate
The Large Intestine (LI)
The LI has limited digestive and absorptive
function…

Digestion: the lumen contains millions of
bacteria


Metabolize remaining nutrients
Produce gases


Hydrogen, methane, CO2, hydrogen sulfide
Absorption: limited

Some vitamins, electrolytes, & H2O
Digestive Function: Defecation

Feces are forced into the rectum via
mass movements


This causes distension of the LI wall
Triggers the defecation reflex
Digestive Function: Defecation


Regulation of the defecation reflex is
provided by centers in the sacral cord
Stimulates…



Contractions in rectum and distal LI
Relaxation of internal anal sphincter
Input to the cerebral cortex allows decision
regarding appropriate behavior

Relaxation of both the external and internal
sphincter if appropriate
Digestion: 5 Integrated Steps





(1) Ingestion
(2) Propulsion
(3) Digestion
(4) Absorption
(5) Defecation