CP Intestinal Obstruction chap5

7/31/2019 CP Intestinal Obstruction chap5

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Chapter V

ANATOMY AND PHYSIOLOGY

I. Introduction to Gastrointestinal System Processes

Nutrition permits us to take in and use food substances that the

body converts to energy and body structure. The digestive system

includes all the organs and glands involved in this process of eating

and digesting. Starting in the mouth, a long muscular tube provides

continual fluid and vital nutrients. The coiled intestines alone are about

24 feet long. After we consume food, the body mechanically and

chemically breaks it down, and then transports it for absorption and

defecation (final waste removal). The digestive glands (salivary glands,

pancreas, liver, and gallbladder) produce or store secretions that the

body carries to the digestive tract in ducts and breaks down chemically.


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A. Ingestion

Food processing begins with ingestion (eating). The teeth aid in

mechanical digestion by masticating (chewing) food. Mastication

permits easier deglutition (swallowing) and faster chemical breakdown

in the digestive tract. During mastication, salivary glands secrete saliva

to soften the food into a bolus (semi-solid lump). Saliva contains the

salivary amylase enzyme, which digests carbohydrates (starches), and

mucus (a thick liquid), which softens food into a bolus. Ingestion starts

both chemical and mechanical digestion.

In deglutition, the tongue pushes the bolus toward the pharynx

(throat) and into the esophagus, a muscular tube that leads from the

throat to the stomach. To prevent food or liquid from entering the

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trachea (windpipe), the epiglottis (a small flap of tissue) closes over the

opening of the larynx (voice box) during deglutition.

Upon entering the esophagus, peristalsis (wave-like

contractions) of smooth muscle carries the bolus toward the stomach.

Two layers of smooth muscle, the outer longitudinal (lengthwise) and

inner circular, contract rhythmically to squeeze food through the

esophagus. Throughout the digestive tract, smooth muscle peristalsis

aids in transporting food.

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From the esophagus, the bolus passes through a sphincter

(muscular ring) into the stomach. All sphincters located in the digestive

tract help move the digested material in one direction. When the

stomach is empty, the walls are folded into rugae (stomach folds),

which allow the stomach to expand as more food fills it.

B. Digestion: stomach

In the stomach, food undergoes chemical and mechanical

digestion. Here, peristaltic contractions (mechanical digestion) churn

the bolus, which mixes with strong digestive juices that the stomach

lining cells secrete (chemical digestion). The stomach walls contain

three layers of smooth muscle arranged in longitudinal, circular, and

oblique (diagonal) rows. These muscles allow the stomach to squeeze

and churn the food during mechanical digestion.

Powerful hydrochloric acid in the stomach helps break down the

bolus into a liquid called chyme. A thick mucus layer that lines the

stomach walls prevents the stomach from digesting itself.

Food is digested in the stomach for several hours. During this

time, a stomach enzyme called pepsin breaks down most of the protein

in the food. Next, the chyme is slowly transported from the pylorus

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(end portion of the stomach) through a sphincter and into the small

intestine where further digestion and nutrient absorption occurs.

C. Digestion and Absorption: Small Intestine

The small intestine is about 20 feet (6 meters) long and has

three parts: the duodenum, jejunum, and ileum. The duodenum is

where most chemical digestion takes place. Here, bile from the

gallbladder and enzymes from the pancreas and intestinal walls

combine with the chyme to begin the final part of digestion.

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Bile liquid is created in the liver and stored in the gallbladder.

Bile emulsifies (breaks into small particles) lipids (fats), which aids in

the mechanical digestion of fats. The pancreas and gland cells of the

small intestine secrete digestive enzymes that chemically break down

complex food molecules into simpler ones. These enzymes include

trypsin (for protein digestion), amylase (for carbohydrate digestion),

and lipase (for lipid digestion). When food passes through the

duodenum, digestion is complete.

From the duodenum, chyme passes to the jejunum and ileum.

Here, tiny villi (finger-like projections) cover the walls of the small

intestine. The cells that line the villi are covered with small projections

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called microvilli (brush border). These projections increase the surface

area of the small intestine, allowing the chyme to contact more of the

small intestine wall. The increased contact causes more efficient food

absorption.

During food absorption, food molecules enter the bloodstream

through the intestinal walls. Capillaries (microscopic blood vessels)

within the villi absorb products of protein and carbohydrate digestion.

Lymph vessels (lacteals) within the villi absorb products of fat digestion

and eventually lead to the bloodstream.

From the small intestine, digested products travel to the liver,

one of the body's most versatile organs. Hepatocytes (liver cells)

detoxify (filter) blood of harmful substances such as alcohol and

ammonia. And, hepatocytes store fat-soluble vitamins and excess

substances such as glucose (sugar) for release when the body

requires extra energy.

D. Absorption: Large Intestine

Once food has passed through the small intestine, it is mostly

undigestible material and water. It enters the colon (large intestine),

named for its wide diameter. The large intestine has six parts: the

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cecum, ascending colon, transverse colon, descending colon, sigmoid

colon, and

rectum.

The large pouch-shaped cecum marks the beginning of the

colon. Attached near the cecum bottom is the vermiform (worm-like)

appendix. The appendix contains lymphoid tissue and intercepts

pathogenic microorganisms that enter the digestive tract. Sometimes,

fecal matter may become trapped in the appendix, resulting in

appendicitis (infection and inflammation).

The other parts of the colon absorb water and minerals from the

undigested food and compact the remaining material into feces.

E. Elimination

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Defecation is the digestive process final stage: feces (consists

of undigested food, inorganic materials, water, and bacteria) are

carried to the rectum through peristalsis and eliminated through the

anus.

Elimination begins with distention of the rectum, which initiates

reflex contractions of the rectal musculature and relaxes the normally

closed internal anal sphincter. The internal sphincter is controlled by

the autonomic nervous system; the external sphincter is under the

conscious control of the cerebral cortex.

During defecation, the external anal sphincter voluntarily relaxes

to allow colonic contents to be expelled. Normally, the external anal

sphincter is maintained in a state of tonic contraction. Thus, defecation

is seen to be a spinal reflex that can be inhibited voluntarily by keeping

the external anal sphincter closed. Contracting the abdominal muscles

facilitates emptying the colon. The average frequency of defecation is

once daily, but this varies among people.

II. Anatomy and Physiology

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

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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.

The primary purpose of the gastrointestinal tract is to break food

down into nutrients, which can be absorbed into the body to provide

energy. First food must be ingested into the mouth to be mechanically

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processed and moistened. Secondly, digestion occurs mainly in the

stomach and small intestine where proteins, fats and carbohydrates

are chemically broken down into their basic building blocks. Smaller

molecules are then absorbed across the epithelium of the small

intestine and subsequently enter the circulation. The large intestine

plays a key role in reabsorbing excess water. Finally, undigested

material and secreted waste products are excreted from the body via

defecation (passing of feces).

In the case of gastrointestinal disease or disorders, these

functions of the gastrointestinal tract are not achieved successfully.

Patients may develop symptoms of nausea, vomiting, diarrhea,

malabsorption, constipation or obstruction. Gastrointestinal problems

are very common and most people will have experienced some of the

above symptoms several times throughout their lives.

A. Four Layers of the Wall

Mucosa

The innermost layer of the digestive tract has specialized

epithelial cells supported by an underlying connective tissue layer

called the lamina propria. The lamina propria contains blood vessels,

nerves, lymphoid tissue and glands that support the mucosa.

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Depending on its function, the epithelium may be simple (a single

layer) or stratified (multiple layers).

Areas such as the mouth and esophagus are covered by a

stratified squamous (flat) epithelium so they can survive the wear and

tear of passing food. Simple columnar (tall) or glandular epithelium

lines the stomach and intestines to aid secretion and absorption. The

inner lining is constantly shed and replaced, making it one of the most

rapidly dividing areas of the body! Beneath the lamina propria is the

muscularis mucosa. This comprises layers of smooth muscle which

can contract to change the shape of the lumen.

Submucosa

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The submucosa surrounds the muscularis mucosa and consists

of fat, fibrous connective tissue and larger vessels and nerves. At its

outer margin there is a specialized nerve plexus called the submucosal

plexus or Meissner plexus. This supplies the mucosa and submucosa.

Muscularis externa

This smooth muscle layer has inner circular and outer

longitudinal layers of muscle fibers separated by the myenteric plexus

or Auerbach plexus. Neural innervations control the contraction of

these muscles and hence the mechanical breakdown and peristalsis of

the food within the lumen.

Serosa/ Mesentery

The outer layer of the GIT is formed by fat and another layer of

epithelial cells called mesothelium.

B. Individual Components of the Gastrointestinal System

Oral cavity

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

is lined by a stratified squamous oral mucosa with keratin covering

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those areas subject to significant abrasion, such as the tongue, hard

palate and roof of the mouth. 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 specialized sensors known as

papillae.

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 esophagus via the action of swallowing.

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 specialized 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

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saliva to prepare and moisten the mouth. Each pair of salivary glands

secretes saliva with slightly different compositions.

Parotids

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The parotid glands are large, irregular shaped glands

located under the skin on the side of the face. They secrete 25% of

saliva. They are situated below the zygomatic arch (cheekbone)

and cover part of the mandible (lower jaw bone). An enlarged

parotid gland can be easier felt when one clenches their teeth. The

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

Immunoglobins are secreted help to fight microorganisms and a-

amylase proteins start to break down complex carbohydrates.

Submandibular

The submandibular glands secrete 70% of the saliva in the

mouth. They are found in the floor of the mouth, in a groove along

the inner surface of the mandible. 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.

Sublingual

The sublinguals are the smallest salivary glands, covered by

a thin layer of tissue at the floor of the mouth. 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.

Esophagus

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The esophagus is a muscular tube of approximately 25cm in

length and 2cm in diameter. It extends from the pharynx to the

stomach after passing through an opening in the diaphragm. The wall

of the esophagus is made up of inner circular and outer longitudinal

layers of muscle that are supplied by the esophageal nerve plexus.

This nerve plexus surrounds the lower portion of the esophagus. The

esophagus functions primarily as a transport medium between

compartments.

Stomach

The stomach is a J shaped expanded bag, located just left of

the midline between the esophagus and small intestine. It is divided

into four main regions and has two borders called the greater and

lesser curvatures. The first section is the cardia which surrounds the

cardial orifice where the esophagus enters the stomach. The fundus is

the superior, dilated portion of the stomach that has contact with the

left dome of the diaphragm. The body is the largest section between

the fundus and the curved portion of the J.

This is where most gastric glands are located and where most

mixing of the food occurs. Finally the pylorus is the curved base of the

stomach. Gastric contents are expelled into the proximal duodenum via

the pyloric sphincter. The inner surface of the stomach is contracted

into numerous longitudinal folds called rugae. These allow the stomach

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to stretch and expand when food enters. The stomach can hold up to

1.5 Liters of materials. The functions of the stomach include:

1. The short-term storage of ingested food.

2. Mechanical breakdown of food by churning and mixing motions.

3. Chemical digestion of proteins by acids and enzymes.

4. Stomach acid kills bugs and germs.

5. 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.

Small Intestine

The small intestine is composed of the duodenum, jejunum, and

ileum. It averages approximately 6m in length, extending from the

pyloric sphincter of the stomach to the ileo-caecal valve separating the

ileum from the cecum. The small intestine is compressed into

numerous folds and occupies a large proportion of the abdominal

cavity.

The duodenum is the proximal C-shaped section that curves

around the head of the pancreas. The duodenum serves a mixing

function as it combines digestive secretions from the pancreas and

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liver with the contents expelled from the stomach. The start of the

jejunum is marked by a sharp bend, the duodenojejunal flexure. It is in

the jejunum where the majority of digestion and absorption occurs. The

final portion, the ileum, is the longest segment and empties into the

cecum at the ileocaecal junction.

The

small intestine performs the majority of digestion and absorption of

nutrients. Partly digested food from the stomach is further broken down

by enzymes from the pancreas and bile salts from the liver and

gallbladder. These secretions enter the duodenum at the Ampulla of

Vater. After further digestion, food constituents such as proteins, fats,

and carbohydrates are broken down to small building blocks and

absorbed into the body's blood stream.

The lining of the small intestine is made up of numerous

permanent folds called plicae circulares. Each plica has numerous villi

(folds of mucosa) and each villus is covered by epithelium with

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projecting microvilli (brush border). This increases the surface area for

absorption by a factor of several hundred. The mucosa of the small

intestine contains several specialized cells. Some are responsible for

absorption, whilst others secrete digestive enzymes and mucous to

protect the intestinal lining from digestive actions.

Large Intestine

The large intestine is horse-shoe shaped and extends around

the small intestine like a frame. It consists of the appendix, cecum,

ascending, transverse, descending and sigmoid colon, and the rectum.

It has a length of approximately 1.5m and a width of 7.5cm.

The cecum is the expanded pouch that receives material from

the ileum and starts to compress food products into fecal material.

Food then travels along the colon. The wall of the colon is made up of

several pouches (haustra) that are held under tension by three thick

bands of muscle (taenia coli).

The rectum is the final 15cm of the large intestine. It expands to

hold faecal matter before it passes through the anorectal canal to the

anus. Thick bands of muscle, known as sphincters, control the

passage of feces.

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The mucosa of the large intestine lacks villi seen in the small

intestine. The mucosal surface is flat with several deep intestinal

glands. Numerous goblet cells line the glands that secrete mucous to

lubricate faecal matter as it solidifies.

The functions of the large intestine can be summarized as:

1. The accumulation of unabsorbed material to form feces.

2. Some digestion by bacteria. The bacteria are responsible for the

formation of intestinal gas.

3. Reabsorption of water, salts, sugar and vitamins.

Liver

The liver is a large, reddish-brown organ situated in the right

upper quadrant of the abdomen. It is surrounded by a strong capsule

and divided into four lobes namely the right, left, caudate and quadrate

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lobes. The liver has several important functions. It acts as a

mechanical filter by filtering blood that travels from the intestinal

system. It detoxifies several metabolites including the breakdown of

bilirubin and estrogen. In addition, the liver has synthetic functions,

producing albumin and blood clotting factors. However, its main roles

in digestion are in the production of bile and metabolism of nutrients.

All nutrients absorbed by the intestines pass through the liver and are

processed before traveling to the rest of the body. The bile produced

by cells of the liver, enters the intestines at the duodenum. Here, bile

salts break down lipids into smaller particles so there is a greater

surface area for digestive enzymes to act.

Gall bladder

The gallbladder is a hollow, pear shaped organ that sits in a

depression on the posterior surface of the liver's right lobe. It consists

of a fundus, body and neck. It empties via the cystic duct into the

biliary duct system. The main functions of the gall bladder are storage

and concentration of bile. Bile is a thick fluid that contains enzymes to

help dissolve fat in the intestines. Bile is produced by the liver but

stored in the gallbladder until it is needed. Bile is released from the gall

bladder by contraction of its muscular walls in response to hormone

signals from the duodenum in the presence of food.

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Pancreas

Finally, the pancreas is a lobular, pinkish-grey organ that lies

behind the stomach. Its head communicates with the duodenum and

its tail extends to the spleen. The organ is approximately 15cm in

length with a long, slender body connecting the head and tail

segments. The pancreas has both exocrine and endocrine functions.

Endocrine refers to production of hormones which occurs in the Islets

of Langerhans. The Islets produce insulin, glucagon and other

substances and these are the areas damaged in diabetes mellitus. The

exocrine (secretrory) portion makes up 80-85% of the pancreas and is

the area relevant to the gastrointestinal tract.

It is made up of numerous acini (small glands) that secrete contents into

ducts which eventually lead to the duodenum. The pancreas secretes fluid

rich in carbohydrates and inactive enzymes. Secretion is triggered by the

hormones released by the duodenum in the presence of food. Pancreatic

enzymes include carbohydrases, lipases, nucleases and proteolytic

enzymes that can break down different components of food. These are

secreted in an inactive form to prevent digestion of the pancreas itself.

The enzymes become active once they reach the duodenum.

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CP Intestinal Obstruction chap5

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