11321614 all about git physiology

7/30/2019 11321614 All About GIT Physiology

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Physiology of GIT

Hawler Medical University

College of Medicine

Department of Medical Physiology

2008-2009


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Terms and Activities

M: Motility

S: Secretion

D: Digestion A: Absorption

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Oral Cavity and Esophagus

M: Swallowing and chewing.

S: Saliva from salivary gland and

lipase.

D: Carbohydrates and minimal for

fats.

A: None

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Stomach

M: Peristaltic mixing and propulsion.

S: HCl (Parietal Cells); pepsinogen and

gastric lipase (Chief cells); mucus and

HCO-3 (Surface mucus cells); gastrin (G

cells) histamine (ECL cells).

D : proteins and fats.

A: Lipid soluble substances such as

alcohol and aspirin.

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Small Intestine

M: Mixing and Propulsion primarily by segmentation;

enzymes; HCO-3 and enzymes (pancreas); bile (liver)

S: Mucus (goblet cells); hormones; CCK, Secretin, GIP,

and other hormones.

D: Carbohydrates, fats, polypeptides and nucleic acids.

A: Peptides by active transport; amino acids, glucose and

fructose by secondary active transport; fats by simple

diffusion; water by osmosis; ions, minerals and vitamines

by active transport.

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Large Intestine

M: Segmental mixing; mass movement for

propulsion

S: mucus by goblet cells.

D: None (Except by bacteria).

A: Ions, water, minerals, vitamines

produced by bacteria.

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Organs of GIT

The alimentary canal (also called thegastrointestinal tract) consists of themouth, pharynx, esophagus, stomach,

small intestine and large intestine.

The accessory digestive organs are theteeth, tongue, salivary glands, liver,

gallbladder, and pancreas.


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Digestive Processes

INGESTION

PROPULSION (peristalsis)

MECHANICAL DIGESTION (physicallypreparing food for chemical digestion)

CHEMICAL DIGESTION (enzymaticbreakdown of food)

ABSORPTION (into blood or lymph fromsmall intestine)

DEFECATION (elimination of waste)


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Questions?

What provokes mechanical and chemical

activity of the GIT?

What Kind of receptors do we have in

GIT?

What is the role of receptors?


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Digestive Activity

A range of mechanical and chemicalstimuli provokes digestive activity

Various receptors located in the GI tract respondto several stimuli such as stretching of the wall by

food, osmolarity and pH, and presence ofsubstrates.

When receptors are stimulated, they:

Initiate reflexes that either activate or inhibit

glands that secrete digestive juices and/orhormones

Mix lumen contents and moves them along bystimulating smooth muscle of GI tract walls


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Questions?

What are the reflexes that control GIT

functions?

What is the role of hormone producing cells

in these functions?


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Controls of digestive activity areboth extrinsic and intrinsic

The entire GI tract is lined with nerve plexuses thatresult in two reflexes:

A. The short reflexes are mediated by the local(enteric) plexuses in response to GI tract

stimuli.B. Long reflexes are initiated by stimuli arising

within or outside the GI tract. They involve theCNS centers and extrinsic autonomic nerves.

C. The stomach and small intestine also containhormone-producing cells that reach theirtarget cells through the blood. Whenstimulated, their contents contribute tosecretory or contractile activity.


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Digestive Processes occurring in the mouth, pharynxand esophagus

Within the mouth, food is chewed, mixed, and

moistened with saliva which contains enzymes

that aid in the process of chemical digestion

Up to 99.5% of SALIVA is composed of water. It is slightly acidic and its solutes include

electrolytes (such as sodium and potassium),

salivary amylase a digestive enzyme, mucin

which hydrates the food, and wastes such as

urea.


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Saliva

The average output of saliva is 1000 1500 mlper day!

Saliva also contains lysozyme, defensins, and

IgA antibodies for protection againstmicroorganisms.

The role of saliva is to cleanse the mouth,dissolve food so it can be tasted, moisten food

and aid in propulsion of food through thedigestive tract, and to begin the chemicalbreakdown of starchy foods.


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Control of Salivation

Salivation is controlled by the parasympatheticdivision (PS) of the nervous system.

Humans contain receptors in their mouths whichwhen stimulated, send signals to the pons.

The PS activates cranial nerves VII(facial) and IX (glossopharyngeal) toincrease saliva output.

In contrast, the sympathetic division

causes release of thick, mucin richsaliva..


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Control of Salivation

When strongly stimulated, the blood vessels

serving the salivary glands constrict, which

ceases release of saliva and causes

drymouth.

Dehydration also inhibits salivation because

low blood volume results in reduced

filtration pressure at capillaries


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Mechanical Processes

Mastication, or chewing, is the role of the teeth.Mastication begins voluntarily, but stretchreflexes take over when receptors in the cheeks,gums, and tongue are stimulated.

Deglutition or swallowing involves thecoordinated activities of over 22 muscle groups.

The buccal phase, the first phase in the mouth,is voluntary.


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Once food is forced into the pharynx by the voluntaryelevation of the tongue, receptors in the pharynx sendmessages to the pons and medulla to reflexively cause:

The tongue to block the mouth The soft palate to rise

The epiglottis to close off the glottis

Peristalsis to move the food down the esophagus

Relaxation of the cardiac sphincter so that food can

enter the stomach. Within the stomach, food is mechanically churned and

proteins are chemically broken down by salivaryamylase.

Further chemical breakdown of carbohydrates and lipidsdoes not begin until food reaches the small intestine.

Very little is absorbed in the stomach.


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Some Questions

to you


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Questions

I. Define:1. Gastric pits

2. Gastric glands

3. Gastric juice

4. Intrinsic factorII. Determine the role of:

1. Goblet cells

2. Mucous neck cells

3. Parietal cells4. Chief cells

5. Enteroendocrine cells


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Chemical Processes

The lining of the stomach is dotted with millionsofgoblet cells that secrete mucus.

The lining also has gastric pits, which lead intogastric glands throughout the stomach.

These glands contain a variety of secretory cellsthat collectively produce gastric juice:Mucous neck cells found in the upper regions of the

glands produce a different kind of mucus than that ofthe goblet cells. Its function is not yet fullyunderstood.

Parietal cells are found in the middle region of theglands and secrete hydrochloric acid and intrinsicfactor.


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Chemical Processes

HCl makes the stomach extremely acidicwhich is necessary to activate pepsin and

kill any bacteria ingested.

Intrinsic factor is a glycoprotein requiredfor absorption of Vitamin B12 in the small

intestine.


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Chemical Processes

Chief cells produce pepsinogen, the inactive formof the protein-digesting enzyme pepsin. Theyare located in the basal regions of gastricglands.

When chief cells are stimulated, HClencountered in the gland activates the firstpepsinogen molecules they release.

Once pepsin is present, it also catalyzed the

conversion of pepsinogen to pepsin.This positive feedback process is limited only bythe amount of pepsinogen present.

Chief cells also secrete small amounts of lipase.


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Chemical Processes

Enteroendocrine cells secrete avariety of hormones such as gastrin,histamine, endorphins, serotonin,cholescystokinin, and somatostatin.These all influence several digestivesystem target organs.


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How the stomach is not digesting itself?


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The Stomach is Protected

The stomach is exposed to an extremely harshacidic environment.

It protects itself by creating a mucosal barrier. A

thick coating of mucus is built up on the stomachwall

The cells of the mucosal layer are joinedtogether by tight junctions that prevent the

gastric juices from leaking into underlying tissue.Damaged epithelial cells are quickly shed and

replaced


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Regulation of Gastric Secretion

Gastric secretion is controlled by both neuraland hormonal mechanisms acting through bothintrinsic and extrinsic reflexes.

The cephalic phase prepares the stomach to

receive food. The sights, smell, taste, or thoughtof food triggers it. The receptors involved inthese triggers activate parasympathetic outputfrom the medulla, which increases secretoryactivity of the gastric glands.

Sympathetic innervations decreases theseevents, and may occur as a result of emotionssuch as anger, fear or anxiety.


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The gastric phase ensures that gastricsecretion and motility continue. Foodentering the stomach stimulates

mechanoreceptors and chemoreceptors.Mechanoreceptors respond to stomach

distension and trigger reflexes, which leadto Ach release.

This of course promotes gastric secretionand smooth muscle activity..


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Chemoreceptors respond to stomach pHand the presence of peptides and/orcaffeine.

Stimulation leads to the secretion ofgastrin from the enteroendocrine cells

Food entering the stomach increases the

pH of the stomach because the food andsaliva have a higher pH than that ofnormal stomach contents


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When gastrin is released, it enters thebloodstream; it stimulates the gastricglands to release more gastric juice.

Although it increases the release ofpepsinogen, it particularly increases therelease of HCl.

Since the release of gastrin is triggered bya rise in pH, this is a negative feedbacksystem.


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Physiologic Anatomy of the

Stomach


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Phases of Gastric Secretions


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Start here,


As the pH begins to decrease due to therelease of more HCl, the release ofgastrin willdecrease and hence the level of HCl willdecrease.

GASTRIN also causes:constriction of the cardiac sphincter, increases the motility of the stomach and

relaxes the pyloric sphincter and ileocecal sphincter.

When food is present in the stomach, histamineand Ach are released and work with gastrin toincrease secretions from gastric glands to mixand churn the food.


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Gastrin Hormone Functions

1) Stimulates the gastric glands to release

more gastric juice.

2) It increases the release of pepsinogen, it

particularly increases the release of HCl.

3) Constriction of the cardiac sphincter,

4) Increases the motility of the stomach and

5) Relaxes the pyloric sphincter and

ileocecal sphincter.


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Through the actions ofpepsin and acid,the food is now broken down, particularlythe proteins.

There are THREEsets of reflexes set inmotion by chyme in the intestinal phase.

These reflexes are to inhibit gastric

secretion and reduce intestinal motilityso that the small intestine is notdamaged by acid.


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Reg. Gast. Secr.//first reflex

The first reflex is a short excitatory reflexthat releases intestinal gastrin.

Its release is stimulated by the presence of

chyme in the duodenum and it further

promotes digestive activity of the stomach.

What is the aim of the first reflex?


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Reg. Gast. Secr.//Secod reflex

The second reflex is the enterogastric reflex that isstimulated by the distension of the duodenum and thepresence of hydrogen, fats, partially digested proteinsand irritating substances via mechano and

chemoreceptors. This in turn:

a) Acts via the medulla to inhibit parasympathetic outflow to thestomach.

b) Inhibit intrinsic reflexes in the stomach

c) Activates sympathetic fibers that cause the pyloric sphincterto tighten and prevent further release of chyme into the smallintestine.

What do a to c means?


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Reg. Gast. Secr.//Third reflex

The third reflex triggers the release of aseries of intestinal hormones including:

Secretin

Gastric inhibitory peptidCholescystokinin (CCK).

These three inhibit gastric secretion and

reduce gastric motility.Compare the functions of Gastrin with the

above hormones.


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Gastric Stretching and Emptying

The initial response of the stomach to filling is toRELAX. This is due to:Receptive relaxation :the stomach relaxes when

food is in the esophagus.

Adaptive relaxation Stretch receptors on thestomach wall respond, and cause the stomach todilate.

Plasticityof the stomach allows it to respond tostretch without greatly increasing its tension andcontracting expulsively.


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Gastric Contractile Activity

In the wall of the stomach are pacemaker cellswhich slowly depolarize and repolarize at a rateof about 3 per minute (called the basic electricalrhythm or BER).

The opening of the valve is very small, so onlyliquid and small particles will pass through.

As the stomach distends, gastrin is released.Gastrin further stimulates contraction of thesmooth muscle.

The larger the meal, the greater the activity ofthe gastric smooth muscle.


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Gastric Emptying

Only a small amount of chyme is propelled out ofthe stomach with each peristaltic wave.

Reflexes arising from the duodenum control theactual volume that exits.

Food will only enter the small intestine at therate that it can be processed.

The less acid, fat, and protein in the chyme, thefaster the chyme will be allowed to enter the

small intestine.A liquid meal or a meal rich in carbohydrates

will pass through the stomach faster than ameal rich in fats and protein.


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Digestive processes occurring in the smallintestine

The food reaching the small intestine hasbeen altered significantly and now appearsas a creamy fluid.

Carbohydrates and proteins are onlypartially digested, and no fat digestion hasoccurred at this point.

The rest of digestion and nutrientabsorption takes place in the smallintestine.


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Requirements for optimalintestinal digestive activity

The CHYME entering the small intestine ishighly acidic, hypertonic, and still containspepsin, the powerful protease.

The walls of the small intestine cannot beprotected in the same way as the stomach, asabsorption needs to take place here.

The chyme must be neutralizedto protectagainst the acid and at the same time,deactivate the pepsin.

R i f i l


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Requirements for optimalintestinal digestive activity

Because chyme is hypertonic, it must also

be diluted otherwise it would draw water

out of the blood across the intestinal wall.

This is why chyme must be released

slowly form the stomach.

It must be released at a rate that facilitates

dilution and neutralization in the smallintestine.

I i l J i C i i


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Intestinal Juice; Compositionand control of production

This is produced by the mucosa of the smallintestine.

The majorstimulation for its production isdistension of the intestinal mucosa and irritationby acidic or hypertonic chyme.

It consists mainly ofwater and mucus secretedby the duodenal glands and goblet cells.

It has very few enzymes

Its job is to (1) dilute the hypertonic chyme and(2) act as a transport medium.


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Daily Secretion of Intestinal Juices


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Liver and Gall Bladder

The liver has many functions, one of which is toproduce bile to aid in the digestive process.

Bile acts to help neutralize the chyme as it

enters the small intestine and is a fat emulsifier. It breaks up fats into tiny particles so that they

are more accessible to digestive enzymes.

The gallbladder is chiefly a storage organ for

bile.Secretin stimulates the liver to increase its rate

of producing the watery, bicarbonate rich bile.


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Composition of bile

Bile is an alkaline solution containing a numberof components.

The important ones for digestion are bile salts,cholesterol and phospholipids.

Bile salts act as a fat emulsifier they breakglobules of fat entering the small intestine into

millions of fatty droplets.This provides a large surface area for the fat

digesting enzymes to work on.


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Composition of bile

Bile salts also facilitate fat and cholesterol

absorption.

These salts are conserved:

They reabsorbed during digestion in the

ileum, returned to the liver by the hepatic

portal vein and reused.


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Regulation of bile release

The liver makes bile continuously.

When there is no food in the small

intestine, the hepatopancreatic sphincter

(the entrance of the common bile duct and

pancreatic duct into the small intestine) is

closed and the bile backs up into the

gallbladder.


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When food enters the small intestine,activation of mechano andchemoreceptors leads to parasympathetic

stimulation.This mildly stimulates gallbladder

contraction

This also stimulates the release ofcholecystokinin and secretin from theduodenal and enteroendocrine cells.

Liver secretion and gallbladder emptying


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Liver secretion and gallbladder emptying


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Besides their action on the stomach, these

hormones also have effects on the

gallbladder and liver.

Cholecystokinin stimulates the gallbladderto contract and release its contents.

It also allows the hepatopancreatic

sphincter to relax and allow the bile toenter the small intestine.


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Composition of Bile


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The Pancreas

The pancreas is an accessory organ

which lies right below the stomach.

It produces many of the digestiveenzymes and also secretes and

alkaline

fluid that helps neutralize the acid inchyme.


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Composition of pancreatic juice

Pancreatic juice consists mainly ofwater, enzymes, and bicarbonate

ions.This high pH enables pancreatic fluid

to neutralize the acid chyme enteringthe duodenum and provides andoptimal environment for the activity ofenzymes


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The Pancreas

Pancreatic proteases are secreted in an inactive

form and activated in the duodenum. This

prevents the pancreas from self-digestion.

For example, within the duodenum, trypsiongenis activated to trypsin by enterokinase, and

intestinal brush border enzyme.

Trypsin, a proteolytic enzyme, then activates

procarboxypeptidase and chymotrypsinogen.


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The Pancreas

Just like the secretion of bile,

parasympathetic nerve stimulation leads to

the release of pancreatic juice.

Also, secretin leads to the release of thewatery, bicarbonate rich component and

the cholecystokinin leads to the release of

the enzyme rich component of thepancreatic juice.

Regulation of Pancreatic Secretions


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Regulation of Pancreatic Secretions



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Mechanical ProcessesMotility of the Small Intestine

Segmentation is the most common motion ofthe small intestine, which is the contracting and

relaxing of smooth muscle.Pacemaker cells in the smooth muscle initiate

segmentation, although the duodenum

depolarizes more frequently (1214

contractions per minute) then the ileum (8 9contractions per minute).



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Mechanical ProcessesMotility of the Small Intestine

This allows ample time for complete

digestion and absorption as contents

move towards the ileocecal valve.

Long and short reflexes and hormonesalter the intensity of segmentation.

Parasympathetic activity enhances and

sympathetic activity decreasessegmentation.


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Peristalsis

Peristalsis occurs after most nutrients have beenabsorbed and is regulated on the basis of which neuronsare stimulated.

Peristaltic waves sweep slowly along the duodenum to

sweep out debris, bacteria, and meal remnants. This is called the migrating mobility complexand its

function is to keep bacteria from settling into the smallintestine.

The enteric neurons of the GI tract coordinate the

mobility patterns. Impulses sent proximally by the cholinergic neurons

cause contraction and shortening of the muscle layer.


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Peristalsis

Impulses sent distally to certain interneuronscause shortening of the longitudinal musclelayer and distension of the intestine, in responseto Ach releasing neurons.

Other impulses sent distally by activated VIP orNO releasing enteric neurons that causerelaxation of the circular muscle.

As a result, as the proximal area constricts andforces chyme along the tract, the lumen of theintestine enlarges to receive it, where it movestoward the ileocecal sphincter.


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Peristalsis

Most of the time, the ileosphincter valve is constrictedand closed.

Two mechanisms; one neural, the other hormonal causeit relax and allow food residues to enter the cecum.

Enhanced activity in the stomach initiates the gastroilealreflex that enhances force of segmentation in the ileum.

Gastrin released by the stomach increases motility of theileum and relaxes the ileocecal sphincter.

Once the chyme has passed through, it exertsbackpressure that closes the valves flaps and preventsregurgitation.


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Segmentation movements of the small intestine


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Peristalsis

Digestive Processes Occurring


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Digestive Processes Occurringin the Large Intestine

The major function of the large intestine isto absorb water from indigestible food

residues and eliminate the residue assemisolid feces.

Chemical Processes

The only chemical processing whichoccurs in the large intestine is a result ofthe bacterial flora that resides there.

Digestive Processes Occurring


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Digestive Processes Occurringin the Large Intestine

These bacteria colonize the colon and ferment

some of the remaining carbohydrates that are

indigestible by the body.

They produce irritating acids and a mixture ofgases, some of which are quite odorous.

They also synthesize B complex vitamins and

most of the vitamin K which the liver requires to

synthesize some blood clotting proteins.


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Haustra ContractionsThe longitudinal muscles of the large intestine

are tonically active and pull the wall of the large

intestines into pockets called haustra.The smooth muscles within the walls of

individual haustra are activated by distensionand they move food along to the next haustrum.

This is a local reflex that moves the food residueand mixes it which aids in water absorption.Peristalsis occurs at a very slow rate.

Mass Movements


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Mass Movements(mass peristalsis)

Strong, slow moving waves of peristalsis movealong the colon three or four times a day.

They are triggered by distension of the stomach,therefore they happen right after a meal.

Stomach distension also gives rise to the twofollowing gastric reflexes:

The gastroileal reflex which opens the ileocecal

sphincterThe gastrocolic reflex, which forces the contents

of the large intestine towards the rectum.


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Defecation

When feces are forced into the rectum by

mass peristalsis, stretching of the rectal

wall initiates the defecation reflex.

This is a spinal cord mediatedparasympathetic reflex that causes the

walls of the final segment of the colon and

the rectum to contract and the analsphincter to relax.


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Defecation

As feces enter the anal canal, messages reach thebrain indicating that defecation is imminent.

At this point voluntary control can override the

reflex and stop passage of the feces temporarily.If defecation is delayed, the reflex contractions

stop and the rectal walls relax until the next

wave of mass peristalsis initiates the reflex

again.


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Daily Fluid Balance

Over the course of an average day an

individual ingests 2L of fluid, 1L of saliva,

2L of gastric juice, 1L of bile, 2L of

pancreatic juice, 1L intestinal juiceTotal fluid ingested 9L daily

0.1L is excreted in feces

8L are reabsorbed by the small intestine

0.9L are reabsorbed by the large intestine


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Review Questions

Describe the structure and function of the entericnervous system.

Define the basic electric rhythm (BER) and the

migrating motor complex (MMC), and describethe function of each in the regulation of

gastrointestinal motility.

List the principal gastrointestinal hormones, the

sites where each is secreted, and the mainphysiologic function of each of these hormones.


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Review Questions

Summarize the functions of the mouth, thesalivary glands, and the esophagus.

Outline the functional anatomy and histology ofthe stomach.

Describe how acid is secreted by cells in thegastric mucosa.

Describe the mechanisms that regulate the

secretion and motility of the stomach.Describe the relationship between

cyanocobalamin and intrinsic factor.


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Review Questions

List the main components of pancreatic juice, and outlinethe mechanisms that regulate its secretion.

Describe the functional anatomy of the liver, and discussthe formation of bile.

Discuss the function of the gallbladder and theprocesses that regulate the passage of bile to theintestinal lumen.

List the types of movement seen in the small intestineand the function of each.

Describe the functional consequences of having bacteriain the gastrointestinal tract.

Outline the physiologic changes that lead to defacation.

Q


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Review Questions

General Questions1. What factors regulate the secretion of the

exocrine portion of the pancreas?

2. Discuss the abnormalities of gastrin secretion

seen in disease states.3. What abnormalities would you expect to be

produced by resection of the terminal portion ofthe ileum with connections of the remainingportion of the small intestine to the colon? Why?

4. What is heartburn? Discuss its pathophysiologyand ways it might be treated.

R i Q i


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Review Questions

5. What causes gallstones? How would you treat them?6. Several surgical procedures have been recommended

for the treatment of severe obesity that fails to respondto other forms of treatment. What are these procedures?How do they cause weight loss? What are their long-term complications?

7. The bacteria in the colon exist in a symbioticrelationship with the host. How does the host benefitfrom this relationship, and what harmful or potentially

harmful effects may occur to the host?

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