Large Intestine在线视频

Hello, everyone.

The topic for this session

is the large intestine,

the last segment

in the gastrointestinal (GI) tract.

Firstly,

let's look at the case of a young patient.

3-year-old Rose

developed a fever and cough.

Her mother thought it

was a respiratory infection

and gave Rose antibiotics for one week.

When the cough almost disappeared,

Rose started to have diarrhea.

Her mother gave Rose another kind of

antibiotic.

Five days later,

the diarrhea became more severe.

Rose was taken to the hospital.

The doctor said Rose

hadan intestinal dysbacteriosis.

Rose'mother wondered

what was dysbacteriosis?

Why didn't the antibiotics work?

After we finish the discussion

on the large intestine (bowel),

you should be able to describe

the general features

and main functions

of the large intestine,

and also explain some clinical

manifestations,

for example

providing clues to explain Rose'problem

to her mother.

The large intestine consists of

the cecum, colon, rectum, and appendix.

It is about 1.5 meters long

and has an average diameter

of about 6 cm.

Since our food is digested

and nutrients are absorbed

in the small intestine,

what are the roles of the large intestine?

To understand the functions

of the large intestine,

let us first get to know

its microstructure.

Under the light microscope,

the wall of large intestine

is composed of 4 layers.

Just as the general structures

of the GI tract,

from inner side to outer side,

they are mucosa,

submucosa, muscularis and adventitia.

The mucosa of the large bowel

is penetrated throughout its length

by tubular intestinal glands.

The intestinal lumen and glands

are lined by goblet cells

and absorptive cells.

The columnar absorptive cells

have irregular microvilli

and dilated intercellular spaces

suggesting active fluid absorption.

Goblet cells producing lubricating mucus

become more numerous

along the length of the colon

and in the rectum.

The main functions of the large intestine

are absorption of water

and electrolytes

from the undigested dietary components,

mainly fibers and formation of feces.

Some of the indigestible food matters

are also fermented

by the intestinal bacteria.

This process

generates short-chain fatty acids

and vitamin K which are absorbed.

In the mammalian large intestine,

dietary substrates that

are hard to digest are broken down

by microbial fermentation

to form short-chain fatty acids.

From animal experiments,

scientists found that

small amount of antibiotic

did not reduce the overall bacterial counts

in the large bowel.

But the antibiotic treatment

changed the microbial composition towards

species responsible for fermentation,

resulting in

more efficient energy retrieval from

ingested food,

and more short-chain fatty acids absorption

by the host.

The results indicated that

the bacteria in the large intestine

are involved

in additional nutrient absorption.

Besides, the normal intestinal bacteria

will inhibit the

proliferation of harmful bacteria.

Colonies of these harmless bacteria

form our microbiota.

If this microbiota is disrupted by factors

such as

prolonged broad-spectrum antibiotic use,

harmful bacteria will overgrow.

This is dysbacteriosis,

which means

an imbalance between beneficial

and pathogenic microorganisms.

The most common symptom

of intestinal dysbacteriosis is diarrhea.

The intestinal microbiota

also has an effect on feces or stool.

There are over

400 species of bacteria in the colon.

Bacteria make up

30%~50% of the total dry matter in the feces,

or even 75% according to another calculation.

This will help you understand

the stools of some special populations.

For example,

breast milk has zero fiber,

yet healthy babies

pass stools several times daily,

and people who fast for days at a time

also have stools,

even though they consume nothing but water.

Among different members

of the large intestine,

the appendix is a special one.

It is attached to the cecum,

has a very small lumen, fewer mucosal glands,

while lymphoid tissue fills its mucosa

and submucosa.

This slide

is the appendix stained with hematoxylin

and eosin under light microscope.

It is hard to identify the mucosal

and submucosal layers

due to the abundant lymphoid tissues.

The function of the appendix is not clear.

One theory is that the lymphoid tissue

is part of the mucosa-associated

lymphoid tissue,

which gives the appendix an important role

in immunity.

Other experts believe the appendix

is just a useless

remnant in the process of human evolution.

In fact,

surgical removal of the appendix causes

no obvious health problems.

This slide

shows the end of the large intestine,

the lower part of the anal canal.

The epithelium changes from single columnar

to stratified squamous.

Single columnar epithelium

facilitates absorption of food

and other substances

from the intestinal lumen.

It is derived from the endoderm

during embryonic development.

Stratified squamous epithelium

is continuous with the epidermis of skin,

originated from the embryonic ectoderm.

Compared to single columnar epithelium,

stratified squamous epithelium provides

more protection

and more resistance against friction.

The epithelial transformation site

is the pectinate line or dentate line,

which also separates internal (above dentate line)

hemorrhoids from external ones clinically.

Above we have described the structural features

and associated functions

of the large intestine.

Please compare these with other sections

of the GI tract.

Can you now explain what is dysbacteriosis

and why the antibiotics did not work

for Rose'diarrhea?

Here are the references.

Thank you!