The Digestive System The alimentary tract provides the body with a - PowerPoint Presentation

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The Digestive System

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The alimentary tract provides the body with a

continual supply

of water, electrolytes, and

nutrients.To

achieve

this requires

(

1) movement of food through

the alimentary tract; (2) secretion of digestive juices

and digestion of the food; (3) absorption of water,

various electrolytes, and digestive products; (4)

circulation of blood through the gastrointestinal

organs to carry away the absorbed substances; and

(5) control of all these functions by local, nervous, and hormonal systems.

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Introduction to the gastrointestinal system

The

gastrointestinal tract

(GIT) consists of a hollow muscular tube starting from the oral cavity, where food enters the mouth, continuing through the pharynx,

oesophagus

, stomach and intestines to the rectum and anus, where food is expelled. There are various

accessory organs

that assist the tract by secreting enzymes to help break down food into its component nutrients. Thus the salivary glands, liver, pancreas and gall bladder have important functions in the

digestive system

. Food is propelled along the length of the GIT by peristaltic movements of the muscular walls.

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functions of the digestive system include:

1

. Motility

. This refers to the movement of food through

the digestive tract through the processes of

a.

Ingestion: Taking food into the mouth.

b.

Mastication: Chewing the food and mixing it with saliva.

c.

Deglutition: Swallowing food.

d.

Peristalsis: Rhythmic, wavelike contractions that

move food through the gastrointestinal tract.

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2

. Secretion.

This includes both exocrine and

endocrine secretions

.

a.

Exocrine

secretions:Water

, hydrochloric acid,

bicarbonate, and many digestive enzymes are secreted

into the lumen of the gastrointestinal tract. The

stomach alone, for example, secretes 2 to 3

liters

of

gastric juice a day.

b.

Endocrine secretions:

The stomach and small

intestine secrete a number of hormones that help to

regulate the digestive system.

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3

.Digestion

.

This refers to the breakdown of food

molecules into

their smaller subunits, which can be absorbed

.

4

. Absorption.

This refers to the passage of digested

end products

into the blood or lymph.

5

. Storage and elimination.

This refers to the

temporary storage

and subsequent elimination of indigestible

food molecules

.

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Layers of the Gastrointestinal Tract

The GI tract from the

esophagus

to the anal canal is composed of four layers, or

tunics.

Each tunic contains a dominant tissue type that performs specific functions in the digestive process. The four tunics of the GI tract, from the inside out, are the

mucosa,

submucosa

,

muscularis

, and

serosa

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Mucosa

The mucosa, which lines the lumen of the GI tract, is the absorptive and major

secretory

layer. It consists of a simple columnar epithelium supported by the

lamina

propria

, a thin

layer of

areolar

connective tissue containing numerous lymph nodules, which are important in protecting against disease

External to the lamina

propria

is a thin layer of smooth muscle

called the

muscularis

mucosae

.

This is the muscle layer responsible for the numerous small folds in certain portions of the GI tract. These folds greatly increase the absorptive surface area. Specialized goblet cells in the mucosa secrete mucus throughout most of the GI tract.

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Submucosa

The relatively thick , submucosa is a highly vascular layer of connective tissue that serves the mucosa. Absorbed molecules that pass through the columnar epithelial cells of the mucosa enter into blood and lymphatic vessels of the submucosa. In addition to blood vessels, the submucosa contains glands and nerve plexuses. The

submucosal

plexus (

Meissner’s

plexus)

provides an autonomic nerve supply to the

muscularis

mucosae.

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Muscularis

The

muscularis

(also called the

muscularis

externa

)

is responsible for segmental contractions and peristaltic movement through the GI tract. The

muscularis

has an inner circular and an outer longitudinal layer of smooth muscle. Contractions of these layers move the food through the tract and physically pulverize and mix the food with digestive enzymes. The

myenteric

plexus

(

Auerbach’s

plexus), located

between the two muscle layers

,

provides the major nerve supply to the GI tract. It includes

fibers

and ganglia from both the sympathetic and parasympathetic divisions of the autonomic nervous system

Serosa

The outer

serosa

completes the wall of the GI tract. It is a binding and protective layer consisting of

areolar

connective tissue covered with a layer of simple

squamous

epithelium

.

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The effects of the

sympathetic nerves

reduce peristalsis and

secretory

activity and stimulate the contraction of sphincter muscles along the GI tract; therefore, they are antagonistic to the effects of parasympathetic nerve stimulation.

The gut also contains the

enteric nervous system

, a discrete web of 100 million neurons that can control gut function without extrinsic control. It consists of the

myenteric

plexus, located between longitudinal and circular muscle layers, and the

submucosal

plexus. Reflexes mediate

motility

,

secretions

,

hormone release

, and vasculature.

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components of the gastrointestinal system

Oral cavity

The oral cavity or mouth is responsible for the intake of food.

Mastication

refers to the mechanical breakdown of food by chewing and chopping actions of the teeth. The tongue, a strong muscular organ, manipulates the food bolus to come in contact with the teeth. It is also the sensing organ of the mouth for touch, temperature and taste using its

specialised

sensors known as papillae.

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Insalivation

refers to the mixing of the oral cavity contents with salivary gland secretions. The

mucin

(a glycoprotein) in saliva acts as a lubricant. The oral cavity also plays a limited role in the digestion of carbohydrates. The enzyme serum amylase, a component of saliva, starts the process of digestion of complex carbohydrates. The final function of the oral cavity is absorption of small molecules such as glucose and water, across the mucosa. From the mouth, food passes through the pharynx and

oesophagus

via the action of swallowing.

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Salivary glands

Three pairs of salivary glands communicate with the oral cavity. Each is a complex gland with numerous

acini

lined by

secretory

epithelium. The

acini

secrete their contents into

specialised

ducts. Each gland is divided into smaller segments called lobes. Salivation occurs in response to the taste, smell or even appearance of food. This occurs due to nerve signals that tell the salivary glands to secrete saliva to prepare and moisten the mouth. Each pair of salivary glands secretes saliva with slightly different compositions

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Parotids

:

The parotids produce a watery secretion which is also rich in proteins.

Immunoglobins

are secreted help to fight microorganisms and amylase proteins start to break down complex carbohydrates.

Sublingual

:

They produce approximately 5% of the saliva and their secretions are very sticky due to the large concentration of

mucin

. The main functions are to provide buffers and lubrication.

Submandibular

:These glands produce a more viscid (thick) secretion, rich in

mucin

and with a smaller amount of protein.

Mucin

is a glycoprotein that acts as a lubricant

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Esophagus

The

esophagus

is that portion of the GI tract which connects the pharynx to the stomach. It is a muscular tube approximately 25 cm (10 in.) long, located posterior to the trachea within the

mediastinum

of the thorax. the

esophagus

passes through the diaphragm by means of an opening called the

esophageal

hiatus. The

esophagus

is lined

with a

nonkeratinized

stratified

squamous

epithelium; its walls contain either skeletal or smooth muscle, depending on the location.

the middle third contains a mixture of skeletal and smooth muscle, and the terminal portion contains only smooth muscle

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Swallowed food is pushed from the oral to the end of the

esophagus

by a wavelike muscular contraction called

peristalsis

the bolus along the digestive tract occurs because the circular smooth muscle contracts behind, and relaxes in front of, the bolus. This is followed by shortening of the tube by longitudinal muscle contraction. These contractions progress from the superior end of the

esophagus

to the

astroesophageal

junction at a

rate of 2 to 4 cm per second as they empty the contents of the

esophagus

into the cardiac region of the stomach.

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The stomach:

the stomach

is a J shaped expanded bag, located just left of the midline between the

oesophagus

and small intestine. The stomach stores up to 2 liters of food

The functions of the stomach include

:

The short-term storage of ingested food.

Mechanical breakdown of food by churning and mixing motions.

Chemical digestion of proteins by acids and enzymes.

Stomach acid kills germs.

Some absorption of substances such as alcohol.

Most of these functions are achieved by the secretion of stomach juices by gastric glands in the body and

fundus

. Some cells are responsible for secreting acid and others secrete enzymes to break down proteins.

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gastric juice:

thin, strongly acidic (

p

H varying from 1 to 3), almost

colorless

liquid secreted by the glands in the lining of the stomach. Its essential constituents are the digestive enzymes

pepsin

, hydrochloric acid, and mucus. Pepsin converts proteins into simpler, more easily absorbed substances; it is aided in this by hydrochloric acid, which provides the acid environment in which pepsin is most effective. Gastric secretion is stimulated by a number of hormones and chemical substances, by the presence of food in the stomach, and by a number of psychological factors, such as the smell of a

favorite

food. Certain cells of the stomach lining secrete a substance known as intrinsic factor, which is necessary for the absorption of vitamin B

12

; absence of this substance results in pernicious

anemia

, or B

12

deficiency

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Cell types in the stomach that help with digestion

There are four main types of cells for stomach secretions spread all over the inner surface of the stomach:

Mucous cells

secrete the alkaline mucous for shielding the epithelium from hydrochloric acid. These are found in the

fundic

, cardiac, and pyloric region.

Parietal cells

, located in the

fundic

, cardiac, and pyloric region, secrete hydrochloric acid; the acid activates release of pepsin for protein digestion. The acid also kills micro-organisms swallowed with the food.

Chief cells

secrete pepsin. These cells are located in the

fundic

region.

G cells

are found in the

fundic

, pyloric, and gastric region. These secrete

gastrin

which stimulates the secretion of hydrochloric acid.

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Motor Functions of the

Stomach

The motor functions of the stomach are threefold: (1)

storage of large quantities of food until the food can

be processed in the stomach, duodenum, and lower

intestinal tract; (2) mixing of this food with gastric

secretions until it forms a semifluid mixture called

chyme

; and (3) slow emptying of the

chyme

from the

stomach into the small intestine at a rate suitable for

proper digestion and absorption by the small intestine.

Figure 63–2 shows the basic anatomy of the

stomach. Anatomically, the stomach is usually divided

into two major parts: (1) the

body

and (2) the

antrum

.

Physiologically, it is more appropriately divided into

(1) the “

orad

” portion, comprising about the first two

thirds of the body, and (2) the “

caudad

” portion, comprising

the remainder of the body plus the antrum.

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Storage Function of the Stomach

As food enters the stomach, it forms concentric circles

of the food in the

orad

portion of the stomach, the

newest food lying closest to the esophageal

opening

and the oldest food lying nearest the outer wall of the

stomach. Normally, when food stretches the stomach,

a “

vagovagal

reflex” from the stomach to the brain

stem and then back to the stomach reduces the tone

in the muscular wall of the body of the stomach so that

the wall bulges progressively outward, accommodating

greater and greater quantities of food up to a limit in

the completely relaxed stomach of 0.8 to 1.5

liters.The

pressure in the stomach remains low until this limit is

approached.

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