Chapter 24

Copyright © John Wiley & Sons, Inc. All rights reserved.

Chapter 24

The Digestive System


The Digestive SystemGastroenterology is the study of the

gastrointestinal system.

The digestive system

functions are ingestion,

secretion, digestion

(chemical and mechanical),

mixing and propulsion,

absorption and

defecation.


The Digestive SystemThe digestive system, also called the

gastrointestinal system, is composed of the

alimentary canal (GI tract), and the accessory

organs.

The alimentary canal extends from the

mouth to the anus through the ventral body

cavity (approximately 9 m, or 30 ft.).

The accessory organs include the teeth,

tongue, salivary glands, liver, gallbladder, and

pancreas.


The Digestive SystemThe GI tract, and accessory organs like the liver

and pancreas, are responsible for facilitating the

body’s metabolic processes.

Catabolism: Larger molecules are broken

into smaller molecules (mouth, stomach,

duodenum).

◦ In the GI tract, this is called digestion and

can occur by either mechanical or chemical

means.

Anabolism: Smaller molecules are used as

building blocks for larger molecules (liver).


The Digestive SystemMechanical digestion includes all movements

that facilitate catabolic processes:

Mastication

Swallowing

Mixing

◦ Increase contact of food with digestive

chemicals

Peristalsis

◦ Movement of muscles within the GI tract

that facilitates movement of food


The Digestive SystemChemical digestion is mainly accomplished by

using water to break chemical bonds

(hydrolysis).

Fats are broken down into fatty acids and

glycerol.

Carbohydrates are broken down from

polysaccharides into monosaccharides.

Proteins are broken down into polypeptides

and amino acids.


The Digestive System(Interactions Animation)

Enzyme mediated hydrolysis

You must be connected to the internet to run this animation

http://www.wiley.com/college/jenkins/0470227583/animations/index_23_02_04.html


Anatomy OverviewThe wall of the GI tract from the lower

esophagus to the anal canal has the same

basic, four-layered arrangement of tissues.

The four layers of the tract, from deep to

superficial, are the mucosa, submucosa,

muscularis, and serosa/adventitia.

The lumen is

the inside of

the tube.


Anatomy OverviewThe mucosa is a mucous membrane made of

various types of epithelium sitting on a loose

connective tissue called the lamina propria.

Nonkeratinized stratified squamous epithelium

(for protection) lines the pharynx, esophagus,

and anus.

Simple columnar epithelium (for

secretion/absorption) lines the stomach and

intestines.

◦ Located among the epithelial cells are

various glandular cells that secrete mucus

and fluid into the lumen of the tract

(exocrine).


The lamina propria contains a prominent

lymphoid tissue (mucosa-associated lymphatic

tissue or “MALT”) that protect against disease.

Underneath it is the muscularis

mucosae, a thin layer of smooth muscle that

throws

the lining of the stomach

and small intestines into

tiny folds (increases

surface area to aid

digestion/absorption).

Anatomy Overview


Anatomy OverviewThe submucosa is composed of loose

connective tissue that binds the mucosa to the

muscularis.

It contains blood and lymphatic vessels

(to receive absorbed substances)

and an extensive network

of neurons known as

the submucosal

plexus.


The muscularis of the mouth, pharynx,

superior and middle parts of the esophagus,

and anal sphincter contains skeletal muscle that

allows for voluntary swallowing and control of

defecation. Throughout the rest of the

tract, it is smooth muscle, arranged in

inner circular and outer longitudinal

sheets, with the myenteric

nerve plexus continuing

between them.

Anatomy Overview


Anatomy OverviewThe serosa/adventitia is the outermost layer.

If attached to surrounding tissues (e.g. around

the esophagus), it is called adventitia - a

fibrous connective tissue arranged around the

organ which it supports.

If contained in the peritoneal cavity, it is

called serosa , which has a slippery

mesothelium surface layer.

◦ Serosa covers the intra-abdominal organs as

the visceral peritoneum.


Anatomy Overview


Anatomy OverviewThe peritoneum is the body’s largest serous

membrane, and it wraps around most

abdominopelvic organs.

The visceral peritoneum forms the “serosa”

of the alimentary canal and

covers other intra-abdominal

organs.

It then continues around

the abdominal wall as the

parietal peritoneum.


Anatomy OverviewUnlike the pericardium and pleurae, which

smoothly cover the heart and lungs, the

peritoneum contains large folds that bind the

organs to one another and to the cavity walls.

There are five

major peritoneal folds:

◦ greater omentum

◦ falciform ligament

◦ lesser omentum

◦ mesentery

◦ mesocolon


Anatomy Overview

The greater omentum is the largest peritoneal

fold.

It drapes over the transverse colon and the

anterior coils of the small intestine like a “fatty

apron”.

◦ It contains many lymph nodes that help

combat

and contain infections

of the GI tract.

◦ The large amount of adipose tissue can

greatly

expand (as seen in people with “beer

bellies”).


Anatomy OverviewThe falciform ligament attaches the liver to

the anterior abdominal wall

and diaphragm.


The lesser omentum is a peritoneal fold that

suspends the stomach and duodenum from the

inferior edge of the liver.

It forms a pathway for blood

vessels (hepatic portal

vein and common

hepatic artery)

to enter the liver,

and it contains the

common bile duct.

Anatomy Overview


Together, the mesentery (of the small

intestine) and mesocolon (of the large

intestine) attach the bowel to the posterior

abdominal wall,

holding the intestines

loosely in place as

muscular contractions

mix and move the

luminal contents

along the GI tract.

Anatomy Overview


Anatomy OverviewSome abdominopelvic organs are covered by

visceral peritoneum only on their anterior

surfaces. The portion of the organ that lies

behind the peritoneum is said to be

“retroperitoneal”. Organs in the

retroperitoneal space include:

The kidneys and ureters

Most of the pancreas

The adrenal glands

The aorta and inferior vena cava


Physiology OverviewDigestive activities of the gastrointestinal tract

occur in three overlapping phases:

1. The cephalic phase

2. The gastric phase

3. The intestinal phase

As we study the glands of the

mouth, stomach, and small

intestine – as well as the

secretions of the accessory organs the liver and

pancreas – we will learn about the specifics of

all 3 phases.


Physiology OverviewDuring the cephalic phase of digestion, the

smell, sight, thought, or initial taste of food

activates neural centers in the cerebral

cortex, hypothalamus, and brain stem to

prepare for digestion.

The brain stem activates the facial (CN VII)

and glossopharyngeal (CN IX) nerves to

stimulate secretion of saliva, while the

vagus nerves (CN X) stimulate secretion of

gastric juice.


Physiology OverviewOnce food reaches the stomach, the gastric

phase of digestion begins.

Neural and hormonal mechanisms (the

hormone gastrin is a key player) promote

secretion of gastric juice and increase gastric

motility.

The intestinal phase of digestion begins

once food enters the small intestine.

Neural and hormonal responses promote the

continued digestion of foods that have

reached the small intestine.


The MouthThe oral or buccal cavity, is formed by the

cheeks, hard and soft palates, and the tongue.

Mechanical digestion of food through

mastication (chewing) enables it to be mixed

with saliva to form a soft flexible bolus that

can be easily swallowed.

Saliva starts the process of chemical

digestion of food

◦ Saliva is 99.5% water, with tiny amounts of

dissolved ions, IgA, lysozyme (a bacteriolytic

enzyme), and salivary amylase (a digestive

enzyme that acts on starch).


Three large salivary glands secrete most of

the saliva: the parotid, submandibular, and

sublingual glands. The smaller glands are

found on the lips (labial), cheeks (buccal),

palate

(palatal), and tongue

(lingual).

Daily salivary

secretions average

1–1½ liters.

The Mouth


The MouthSalivary regulation is under the control of the

ANS

Parasympathetic stimulation promotes

secretion of a moderate amount of saliva.

Salivary centers are located in the brain stem

and efferent nerve impulses are transmitted by

the facial (VII) and glossopharyngeal (IX)

nerves.

◦ Touch (pressoreceptors), smell, taste (taste

buds), and psychological factors are also

salivary stimulators.

Sympathetic stimulation decreases saliva

secretions.


The Mouth(Interactions Animation)

Carbohydrate Digestion in the Mouth





Lipid Digestion in the Mouth




The MouthThe tongue is composed of skeletal muscle

under voluntary somatic motor control - it

forces the moistened food bolus into position for

swallowing (deglutition) and places the

bolus into contact with the teeth for chewing.

The extrinsic muscles of the tongue attach

to bones in the area and move the tongue

from side to side.

Intrinsic muscles originate within the

tongue and alter its shape and size for speech

and swallowing


The Mouth


The MouthThe teeth or dentes are located in sockets of

the alveolar processes of the mandible and

maxillae. The sockets are lined by the

periodontal ligament - a dense fibrous

connective tissue that anchors the teeth to the

socket walls and acts as a shock absorber

during chewing.

A typical tooth has three major external

regions: the crown, root, and neck.

◦ The neck of each tooth is covered by the

gingivae , or gums, which extend slightly

into each socket.


The MouthDentin is a calcified connective tissue that

forms most of the tooth. The dentin of the crown is

covered by enamel, a

harder-than-bone

calcified material and

encloses the pulp cavity,

a space filled with pulp

(a connective tissue

containing blood vessels,

nerves, and lymphatic vessels).


The MouthHumans have two dentitions or sets of teeth.

There are 20 deciduous or “baby teeth”

which begin to erupt around 6 months of age.

◦ All are lost between 6–12 yrs. of ages

There are 32 permanent teeth

numbered from right to left, top

(1–16) to bottom (17–32).

◦ The third molars (teeth 1,

16, 17, and 32) are the

wisdom teeth.


Mastication





DeglutitionDeglutition is the act of swallowing food. The

oropharynx and laryngopharynx have digestive

as well as respiratory functions, and swallowed

food must transit them both on the way to the

esophagus – and it must do so while not going

into the nasal cavity or the airway.

This complex process involves many muscles

subject to a number of voluntary and

involuntary controls.

◦ Deglutition has 3 stages: voluntary,

pharyngeal, and esophageal.


DeglutitionThe first stage is the voluntary act of the tongue

forcing the bolus to the back of the oral cavity and

into the oropharynx.

The involuntary pharyngeal stage begins as the

bolus passes into the oropharynx - receptors send

impulses to the deglutition center in the medulla

and pons.

Returning impulses cause the soft palate to move

superiorly and posteriorly to close the

nasopharynx.

The epiglottis moves slightly inferiorly to close the

glottis.


DeglutitionThe esophageal stage of swallowing begins once the bolus enters the esophagus. Peristalsis, a progression of coordinated

contractions and relaxations of the circular and longitudinal layers of the muscularis, push the bolus onward.


Deglutition(Interactions Animation)

Deglutition




The EsophagusThe only digestive system function that occurs

in the esophagus is propulsion (moving food

into the stomach).

The esophagus is a muscular tube that

begins inferior to the laryngopharynx,

and positioned posterior to the trachea.

Leaving the neck, it traverses the

thoracic cavity down the posterior

mediastinum before piercing the

diaphragm through the esophageal

hiatus.


The mucosa of the esophagus is nonkeratinzed

stratified squamous epithelium The type of muscle in the

muscularis of the esophagus

varies by region

◦ the superior 1/3 is

skeletal muscle

◦ the intermediate 1/3 is

skeletal and smooth muscle

◦ the inferior 1/3 is

smooth muscle

The Esophagus


The EsophagusUpper and lower esophageal sphincters (UES

and LES) are situated at each end of the tube.

The LES regulates the movement of food

from the esophagus into the stomach.

◦ Incompetence of the LES

results in Gastroesophageal

Reflux Disease (GERD),

which manifests as

“heart burn”.


The stomach is a J-shaped enlargement of the GI tract situated

between the esophagus and the first part of the small intestine (the

duodenum).

The position and size of the stomach varies continually - the

diaphragm pushes it inferiorly with each inhalation and pulls it

superiorly with each exhalation.

Rugae are large folds in the

mucosa of the empty stomach

which enable gastric

distension, depending

on the amount of stomach

contents.

The Stomach


The StomachEmpty, the stomach is about the size of a large

sausage, but it is the most distensible part of

the GI tract and can expand to accommodate a

large quantity of food.

Because a meal can be eaten much more

quickly than the intestines can digest and

absorb it, one of the functions of the stomach

is to serve as a mixing chamber and holding

reservoir.

As a functional adaptation, the gastric

muscularis contains an additional 3rd inner

oblique layer of muscle to facilitate the

mixing action of mechanical digestion.


Note the additional oblique layer of smooth muscle in the gastric

muscularis, which is limited primarily to the body of the

stomach

The Stomach

The stomach has four main regions:

the cardia

the fundus

the body

the pylorus


The StomachSimple columnar epithelial cells (surface

mucous cells) line the mucosal surface and

secrete a protective mucous.

Columns of secretory

cells extend down into

the lamina propria

forming gastric glands.

Several gastric glands open

into the bottom of narrow

channels called

gastric pits.


The StomachA variety of specialized exocrine and

endocrine cell types populate the gastric

glands and pits. Exocrine gland cells include:

◦ mucous neck cells which produce mucus

◦ parietal cells which produce intrinsic

factor and HCl

◦ chief cells which secrete the protease

pepsinogen and gastric lipase Enteroendocrine G cells, located mainly in

the pyloric antrum, secrete the hormone

gastrin into the bloodstream.


The StomachThe secretions of the mucous, parietal, and

chief cells form gastric juice, which totals 2–3

l/d.

The stomach is protected

from its own gastric

juice by a 1–3 mm

thick layer of alkaline

mucus secreted by

surface mucous cells

and mucous neck cells.


The StomachThe strongly acidic nature of gastric juice kills

many microbes, partially denatures proteins in

food, and converts pepsinogen into pepsin.

Pepsin is the only proteolytic enzyme in the

stomach.

Gastric lipase splits triglycerides.

Intrinsic factor (IF) is needed for absorption

of vitamin B12 in the terminal ileum.

◦ Vitamin B12 is needed for RBC production.


The StomachDisturbing the balance between hydrochloric

acid production, pepsin secretion, and mucosal

defenses can lead to erosion of the stomach's

epithelial lining.

This graphic shows an

endoscopic view of

a gastric erosion, possibly

caused by consumption of

too much alcohol or use

of an NSAID drug such as

aspirin or ibuprofen.


The StomachGentle, rippling, peristaltic movements called mixing

waves pass over the stomach every 15 to 25 seconds.

These waves macerate food, mix it with secretions of

the gastric glands, and reduce it to a soupy liquid

called chyme.


The Stomach(Interactions Animation)

Stomach Peristalsis




Chemical Digestion - Gastric Acid





Protein Digestion in the Stomach





Lipid Digestion in the Stomach





The Stomach


The StomachAlthough digestion is a major function of the

stomach, its epithelial cells are impermeable to

most materials, and very little absorption takes

place.

Within 2 to 4 hours after eating a meal, the

stomach has emptied its contents into the

duodenum.

Foods rich in carbohydrate spend the least

time.

High-protein foods remain somewhat longer.

Emptying is slowest after a fat-laden meal

containing large amounts of triglycerides.


The StomachAt appropriate intervals, the stomach allows a

small amount of chyme to pass through the

pyloric sphincter

and enter the duodenum to begin the

intestinal phase

of digestion.

Completion of digestion

is a collective effort of

pancreatic juice,

bile, and intestinal juice

in the small intestine.


The PancreasDigestion and absorption in the small intestine

depend heavily on secretions from the pancreas

and gallbladder (liver).

The pancreas is an oblong gland located

posterior to the stomach in the

retroperitoneal space.

◦ It is connected to the duodenum by the

hepatopancreatic ampulla and accessory

ducts.

◦ It secretes enzymes, which digest food in

the small intestine, and sodium bicarbonate,

which buffers the acidic pH of chyme.


The Pancreas


The PancreasAbout 99% of pancreatic acini (glandular

clusters) participate in exocrine secretion – only

1% of the clusters, called pancreatic islets,

form the endocrine

portion of the gland

(secreting the hormones

glucagon, insulin, and

somatostatin and

pancreatic polypeptide).


The PancreasAbout 1-1.5 liters of alkaline pancreatic juice

is secreted into the duodenum each day. It

creates the proper pH for the following digestive

enzymes in the small intestine:

A starch digesting enzyme called pancreatic

amylase

Several enzymes that cleave polypeptides into

dipeptides and single amino acids: trypsin,

chymotrypsin, carboxypeptidase, and

elastase

Pancreatic lipase, the major triglyceride (fat)

digesting enzyme in adults


Carbohydrate Digestion – The Pancreas

The Pancreas(Interactions Animation)




Lipid Digestion - Bile Salts and Pancreatic Lipa

se

The Pancreas(Interactions Animation)





The liver is the body’s largest gland and second

largest organ. It has 2 main lobes

(right and left –

divided by the falciform

ligament) and is covered

by visceral peritoneum.

The liver is made up of

repeating functional units

called liver lobules.

The Liver and Gallbladder


The Liver and GallbladderHepatocytes are the major functional cells of

the liver. As the body’s “chemical factories”,

their metabolic versatility is truly remarkable.

Hepatocytes participate in a number of

digestive and non-digestive functions.

Important digestive functions include:

◦ the synthesis, transformation, and

storage of proteins, carbohydrates,

and fats

◦ detoxification, modification, and excretion

of a variety of exogenous and endogenous

substances


The heparin molecule

The Liver and GallbladderNon-digestive liver functions include:

Phagocytosis of old or worn-out cells

Making heparin (anticoagulant) and other

plasma proteins (prothrombin, fibrinogen, and

albumin)

Modifying vitamin D to its active form

Human albumin Vitamin D3, the

active form of the molecule

http://upload.wikimedia.org/wikipedia/commons/a/aa/Heparin-2D-skeletal.png

http://upload.wikimedia.org/wikipedia/commons/a/aa/Heparin-2D-skeletal.png

http://upload.wikimedia.org/wikipedia/commons/5/5d/Albumin_pyrimidin_ring.png

http://upload.wikimedia.org/wikipedia/commons/5/5d/Albumin_pyrimidin_ring.png


Venous blood (from the hepatic portal vein) and

arterial blood (from the hepatic artery) feed the

lobule from the triad on its outer margin. The blood mixture percolates through

endothelial-lined

spaces called

sinusoids

(a specialized

capillary)

towards the

central vein.




Path of blood in hepatic sinusoid

Microstructure of the liver lobule


The Liver and GallbladderFixed macrophages within the sinusoids called

Kupffer cells destroy red cells, white cells, and

bacteria in blood draining

from the GI tract.

An important function of lobule

hepatocytes is to secrete bile, an

excretory product that helps emulsify fats for

the watery environment of small intestine

digestive juices.

Hepatocytes secrete about 1 liter of bile per

day.


The Liver and GallbladderBile is an alkaline solution consisting of water,

bile salts, cholesterol, and bile pigments. It is

both an excretory product and a digestive

secretion.

Bile salts are used in the small intestine for

the emulsification and absorption of lipids.

◦ Without bile salts, most of the lipids in food

would be passed out in feces, undigested.

The dark pigment in bile is called bilirubin and

comes from the catabolism of old red blood

cells.


The Liver and GallbladderBile secreted into the canaliculi (located

between the hepatocytes) exits the liver in the

common hepatic duct.

This duct joins the

cystic duct from the

gallbladder to form

the common bile

duct (CBD).


The CBD works its way towards the duodenum

and joins with the pancreatic duct to form

the hepatopancreatic

ampulla just proximal

to the second part of the

duodenum.

The duodenal papilla

(“nipple”) pierces the

intestinal mucosa to

deliver its contents.



The Liver and GallbladderBetween meals, the

sphincter of the

hepatopancreatic

ampulla is closed – bile

“backs-up” into the gall

bladder where it is

stored and

concentrated up to ten-

fold through the

absorption of water

and ions.


The Liver and GallbladderUnder the influence of the hormone

cholecystokinin (CCK), the gallbladder

contracts and ejects stored bile.

Although not necessary for life, normal gall

bladder function is highly desirable.

After surgical

removal of the

gall bladder (called a

cholecystectomy), a person

would experience severe indigestion

if they ate a large meal high in fat content.


The Liver and Gallbladder(Interactions Animation)

Chemical Digestion – Bile




The Small IntestineThe small intestine is divided into 3 regions:

The duodenum (10 in)

The jejunum (8 ft)

The ileum (12 ft)

◦ If measured in a cadaver, the intestines are

longer than if measured in a live person due

to the loss of smooth muscle contraction.

In the small intestine, digestion continues, even

while the process of absorption begins.


The Small IntestineMechanical digestion in the small intestine is a

localized mixing contraction called

segmentations.

Segmentations is a type of peristalsis used to

mix chyme and bring it in contact with the

mucosa for absorption.

It begins in the lower portion of the stomach

and pushes food forward along a small stretch

of small intestine.

◦ It is governed by the myenteric plexus.


The Small Intestine(Interactions Animation)

Segmentation Animation




The Small IntestineCircular folds called the plicae circulares are

permanent ridges of the mucosa and

submucosa that encourage

turbulent flow of chyme.


The Small IntestineVilli are multicellular structures that can barely

be seen by the naked eye. They form finger-like

projections that are covered with a simple

columnar epithelium.


Microvilli are microscopic folds in the apical

surface of the plasma membrane on each

simple columnar cell (about 200 million/mm2).

The plicae circulares,

villi, and microvilli all

contribute to increase

the surface area of the

small intestine, allowing

for maximum reabsorption of nutrients.

The Small Intestine


The small intestinal mucosa contains many

deep crevices lined with glandular epithelium

(intestinal glands) that secrete intestinal

juice. Its function is to complete the digestive

process begun by

pancreatic juice.

Trypsin exists in pancreatic

juice in the inactive form

trypsinogen - it and other

enzymes are activated by

intestinal juice.

The Small Intestine


The Small IntestineMost of the enzymatic digestion in the small

intestine occurs inside the epithelial cells or

on their surfaces (rather than in

the lumen of the tube) as

intestinal juice comes in

contact with the brush

border of the villi.



Digestion on the Brush Border





Before discussing the absorption of nutrients, the events of gastric and intestinal digestion are reviewed in this animation. Hormonal Control of Digestive Activities




The Small IntestineIntestinal absorption is the passage of

digested nutrients into the blood or lymph: 90%

of all intestinal absorption occurs in the small

intestine.

Proteins (amino acids), nucleic acids, and

sugars (monosaccharides) are absorbed

into blood capillaries

by facilitated diffusion or active transport.

Triglycerides (fats) aggregate into globules

along with phospholipids and cholesterol and

become coated with proteins. These large

spherical masses are called chylomicrons.


The Small IntestineChylomicrons, too large to enter blood

capillaries, enter specialized lymphatic vessels

called lacteals and

eventually drain

into the superior

vena cava and

mix with blood.

All dietary

lipids are absorbed

by simple diffusion.



Carbohydrate Absorption in the Small Intestine




Protein Absorption in the Small Intestine





Nucleic Acid Absorption in the Small Intestine





Lipid Absorption in the Small Intestine





The large intestine is about 5 feet in length.

Starting at the ileocecal valve, the large

intestine has 4 parts:

The cecum

The colon

◦ ascending

◦ transverse

◦ descending

◦ sigmoid

The rectum

The anal canal

The Large Intestine


The Large IntestineThere are no circular folds or villi in the large

intestine.

The mucosa is mostly an absorptive

epithelium (mainly for water), and microvilli

are plentiful.

Interspersed goblet

cells produce mucous,

but no digestive

enzymes are secreted.


The Large IntestineThe large intestine is attached to the posterior

abdominal wall by its mesocolon peritoneal

membrane.

Teniae coli are 3 separate longitudinal ribbons of

smooth muscle that run the length of the colon.

Because the teniae coli is shorter than the intestine,

the colon becomes sacculated into small pouches

called haustra (giving it a segmented appearance).

◦ As one haustrum distends, it stimulates muscles

to contract, pushing the contents to the next

haustrum.


The Large IntestineHanging inferior to the ileocecal valve is the

cecum, a small pouch about 2.5 in long.

Attached to the cecum is a 3 in coiled tube

called the appendix.

The open end of the cecum merges with a long

tube called the colon, with its various parts.

Both the ascending and descending colon

are retroperitoneal; the transverse and

sigmoid colon

are not.


The Large IntestineThe rectum is the last 8 in of the GI tract and

lies anterior to the sacrum and coccyx.

The terminal 1 in of the rectum is

called the anal canal . The mucous

membrane of the anal canal is

arranged in longitudinal folds

called anal columns that contain

a network of arteries and veins.

◦The opening of the anal canal

to the exterior is called the anus.


The Large Intestine


The Large IntestineIncluding the 2 liters we

drink, about 9 liters of fluid

enter the small intestine

each day. The small intestine

absorbs about 8 liters;

the remainder passes

into the large

intestine, where most

of the rest of it is also

absorbed.

◦ Only 100 mL/d of water

is excreted in the feces.


The Large IntestineFeces are the waste leftover after digesting

and absorbing all the nutrients we can from

eaten material. Though it is lower in energy

than the food it came from, feces may still

contain a large amount of energy, often 50% of

that of the original food.

The characteristic brown coloration comes

from a combination of bile and bilirubin.

The distinctive odor is due to bacterial action

- both aerobic and anaerobic bacteria

participate.


The Large IntestineThough the human body consists of about 100

trillion cells, we carry about ten times as many

microorganisms in the intestines. Bacteria

make up most of the flora in the colon and

about 60% of the dry mass of feces.

As these bacteria digest/ferment left-over food,

they secrete beneficial chemicals such as

vitamin K, biotin (a B vitamin), and some amino

acids (they are our main source of some of

these nutrients.)


The Large IntestineThe mechanical events

associated with defecation

include localized haustral

churning and peristalsis.

Two autonomic nervous system

reflexes that initiate strong

bouts of mass peristalsis are the

gastroileal reflex and the

gastrocolic reflex.

◦ Both reflexes occur with

distension of the stomach.Gastric distension

initiates mass peristalsis by the ANS


The Large IntestineThe gastroileal reflex causes relaxation of the

ileocecal valve, intensifies peristalsis in the

ileum, and forces any chyme into the cecum.

The gastrocolic reflex intensifies strong

peristaltic waves that begin at about the middle

of the transverse colon and quickly drive the

contents of the colon into the rectum.

This mass peristalsis takes place three or

four times a day during or immediately after a

meal, and may lead to defecation.


The Large IntestineThe defecation reflex is activated by stretch

receptors stimulated by filling of the rectum.

The events leading to defecation include:

◦ Food in the stomach stimulates mass

peristalsis.

◦ Food moves through the intestine into the

rectum.

◦ Rectal pressoreceptors respond to distention

and longitudinal muscles shorten the rectum.

◦ ANS releases the internal anal sphincter and

gives a conscious awareness of distention.

◦ Release of external sphincter is under

conscious control.


Mechanical Digestion in the Large Intestine




Chapter 24

Science

Transcript of Chapter 24