Campbell's Physiology Notes For Nurses by John Campbell

Author:John Campbell
Language: eng
Format: mobi, pdf
Published: 0101-01-01T00:00:00+00:00

Layers

The walls of all the sections of the

GI tract from the oesophagus to the

rectum have the same basic layered

structure. The lumen is surrounded

by mucosa, submucosa, a muscular

layer and an outer serosa.

Mucosa

The

inner

layer,

immediately

surrounding the lumen, is called the

mucosa. This is composed of a

mucous

membrane

lined

with

mucus.

A mucous membrane is

simply a layer of tissue that secretes

m u c u s . Mucus

protects

the

underlying cells and also acts as a

lubricant to ease the passage of

material along the lumen. The

thickness of the mucosal epithelium

varies along the length of the tract.

In

areas

such

as

the mouth,

oesophagus and anus, which are

subject to a lot of mechanical forces

and abrasion, the mucosa is

composed of a stratified epithelium.

In

sections

associated

with

absorption it is important for the

lining to be thin to facilitate

diffusion of nutrients. In these areas

the mucosa is composed of a simple

columnar epithelium.

Underneath the epithelium but still

part of the mucosa is a layer of

connective tissue containing small

blood vessels. These vessels supply

nutrients

and

oxygen

to

the

epithelial cells. In some areas of the

GI tract they may also be involved

in the process of absorption. Under

the connective tissue is a thin layer

of smooth muscle. This supports the

overlying connective and epithelial

tissues and in some areas of the gut

pulls the lining up into a series of

folds. These folds have the effect of

enlarging the internal surface area

of the lumen thus increasing the rate

of digestion and absorption.

Submucosa

The layer underneath the mucosa is

called the submucosa. This is

composed of loose connective

tissue and also carries numerous

blood vessels that perfuse the gut

wall with blood. In certain areas of

the 01 tract, the submucosa contains

exocrine glands that produce some

o f the digestive enzymes. Areas of

lymphatic tissue are found in the

submucosa; these are referred to as

lymphatic nodules and provide an

immune function. This layer also

contains a network of intrinsic

nerve fibres collectively called the

submucosal plexus. ('Plexus' means

à network of blood vessels or

nerve fibres'.)

Muscular layer

Under the submucosa is the

muscular layer or muscularis. There

a r e two

layers

of

smooth,

involuntary muscle: the outer layer

is composed of longitudinal muscle

fibres and the inner layer of circular

muscles. The wall of the stomach

has an additional inner layer of

muscle fibres, which run obliquely.

Activity of the muscular layer is

coordinated

by a network of

intrinsic, autonomic nerve fibres

called the myenteric plexus.

Contraction and relaxation of

these muscle layers allows material

to be propelled along the GI tract.

This muscular contraction also

helps with the physical breakdown

of food and aids the mixing in of

digestive enzymes. In addition to the

involuntary muscles found in the

wall of the GI tract, the mouth,

pharynx, oesophagus and anus also

contain voluntary muscles.

Serosa

The outer layer of the gut wall is

called the serosa. This is composed

o f loose connective tissue with a

serous membrane on the external

surface of the bowel. Like all

serous membranes it secretes a

small volume of serous fluid (fluid

derived from serum, i.e. plasma)

that acts as an external lubricant.

Below the diaphragm, the serosal

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