gastrointestinal anatomy and physiology - part 2

7/30/2019 Gastrointestinal Anatomy and Physiology - Part 2

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Gastrointestinal Anatomy and PhysiologyPart 2: The stomach: initial digestion and absorption

Lumen of the GI tract is outside the body

If you fed an object into your mouth, it would eventually come out theother side

The lumen of the GI tract is continuous with the outside of the body

Perforation of the lumen is equivalent to having a perforation (i.e. knifestab wound) in the abdominal wall from the outside

A knife wound to the abdomen is no different and no less serious than aperforation of the lumen of the intestine into the peritoneal cavity

Objectives

Know how the peritoneum supports the contents of the abdominal cavity,as well as the blood and neural supply.

o Sac supports contents and provides route for the blood and neuralsupply

Understand the roles of the layers within the intestinal wall. Understand the digestion processes that occur in the stomach, the cells

that regulate them.

Understand the process of gastric emptying

Key Concepts

Digestion depends on both mechanical (ie stretch) and chemical (such aspH) stimuli.

o Response to mechanical change in the stomach food arrivesand stretches the stomach and stimulates digestion

o When food arrives in the stomach it dilutes the acid (causes thepH to rise) and this stimulates digestion as well

Digestion is controlled by both extrinsic and intrinsic nervous andhormonal input

o Digestive system has its own intrinsic nervous input, but it isalso effected by extrinsic nervous and hormonal inputs

We all wear an apron


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The Omentum is a large sheet of the peritoneum

The peritoneal sac is entirely analogous to the pericardial sac

o Two layer serousal saco parietal layer adjacent to the wall of the abdominal cavityo visceral layer on the surface of the viscera

large flap of the peritoneum covering the entire front area of the

abdominal cavity (left) the Omentum

it has been flipped up in the right picture

it hangs off the transverse colon of the large colon

Sagittal view of the abdomen


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The omentum hangs down at the front

It is a 4 layered saco Do not bother to try and track the layers since this an artistic

renditiono (cannot see it as clearly as the pericardial sac where you could see

the parietal portion against the inside of the fibrous pericardium

and then you could see how it looped around the top and becamethe visceral layer of the heart)

The sac wraps around and supports the stomach, the colon (immediatelybelow the stomach)

There is a portion of it, called the mesentery, which supports the smallintestine (loops that come out around the small intestine)

There are different names for portions of the peritoneal sac greateromentum, lesser omentum, mesentery

o Called the lesser omentum where it narrows near the liver

Parietal peritoneum along the abdominal wall; visceral peritoneum againstthe organs

Not all of the GI tract and the organs within the abdomen are within theperitoneal saco A number are plastered against the rear wall of the abdominal

cavity pancreas and duodenum (below the pancreas)

o (between the parietal layer and the wall retroperitoneal behindthe peritoneal)

Q. What holds the abdominal contents in place?

The peritoneumQ. What structure supports the intestinal vascular supply?


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The peritoneum

The nerves and the blood vessels follow this tract down to the variousparts of the small intestine and other parts of the intestine as well

How does everything fit in there?

This shows the location of things

There is the large colon with the ascending, transverse, and descendingportion which ends up at the rectum

Theres the small intestine in the middle

The stomach has part or the lesser omentum wrapping over it (as well asthe liver and gull bladder)

In one of the movies you can see a lesion in the stomach that has gone

right through to the gallbladdero The gallbladder lies right against the stomach

Abdomen -tranverse section at the level of pancreas


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Inferior view of a transverse section

Shows a number of the contents of the abdominal cavity

The pylorus is the structure out of which the contents of the stomachcome to go to the small intestine

The two top corners of the large intestine can be seen

The top end of the kidney can be seen and a little bit of the small intestine

The top end of the dark structure that wraps around the body of thevertebral column is the bottom edge of the muscular of the diaphragm

This section is cut fairly high at the level of the pancreas and just caught

the bottom end of the diaphragm

The intestine has specialized layers


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The layers are essentially the same the whole way througho There is an extra layer in the stomach

Mucosao Made up of the epithelium (stratified squamous layer that provides

the closure and ability withstand some abrasion for the lining of theGI tract)

o Lamina propria connective tissue (underlies the same was as theepidermis has dermis underneath)

o Muscular muscosa does not have role in peristalsis; has a role tomove the endothelium around to help clear it of any material thatmay come to rest

Submucosao Connective tissue that carries the blood vessels (like layer

underlying the skin)o Carries the first of the nerve plexuses that look after the operation

of the GI tract submucosal plexus

Nerves the run from one end of the GI tract to the other

Provides communication in terms of sensory input (changesin stretch or pH)

o two muscular layers are immediately above that

inner layer is circular muscle (wraps around)

outer layer is longitudinal

between these two layers is the second plexus of nerves myenteric plexus

muscle plexus myo = muscle; teric = GI tract

communicates with the submusucosal plexus

major role in controlling the peristaltic waves stimulates the muscle to contract

Adventia is the connective tissue on the outside of an organ called theSerosa hereo The adventia is also the visceral layer of the peritoneumo Mesentery that supports part of the GI tract with vessels and nerves

arising and coming along it, so it is referred to as the Serosa (but isone in the same as the adventia)

The intestines have a special nervous system


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Picture intended to emphasize the nervous system at the wall of theintestine

Brain of the GI tract (component that can function on its own in terms of

regulating the process that can go on) Pacemaker cells Interstitialcells of Cajal

o Controls the frequency of contractions of the muscleo Major research going on at mac about this

Its internal nervous system is impinged upon by the Autonomic NervousSystem

Sympathetic innervation

Q. What is the role of the sympathetic innervation of the intestinal tract?A. Reduce activity for Fight, Fright or Flight

In a stress situation, the liver does not do nothing!

The liver is where a lot of the readily available carbohydrate energy isstored (glucose stored as glycogen)


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Under sympathetic innervation, breakdown of the glycogen occurs to yieldglucose to be distributed into circulation to maintain activity in fight,flight, fright situation

It isnt completely a case of having everything slowed down

Energy is diverted from process of digestion though

Cant automatically say the entire GI tract shuts down under sympatheticcontrol

The vagus nerve -parasympathetic

Q. What is the role of the parasympathetic innervation of the intestine?

A. SLUDD Salivation, Lacrimation, Urination, Digestion, Defecation (Rest

and Rumination) Liver will be storing sugar that you are getting from diet as glycogen

Blood supply -1

Descending (abdominal) aorta

Celiac Trunk provides circulation to a number of the organs in the GI tract


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Superior Mesenteric artery and Inferior mesenteric artery provide bloodsupply to the intestines

Blood supply -2

The stomach has been lifted in this image (normally sitting in front of thepancreas)

The celiac trunk gives rise to the hepatic artery (supplies blood to theliver) and gives rise to the right gastric artery

There are anastomoses everywhere to limit the risk of tissues becoming

ischemic Left gastric artery comes off the celiac trunk ; right gastric artery comes

off the hepatic artery

Do not need to know all the names:o Know

Gastric runs around the small curvature

Gastroepiploic runs around the large curvature

Splenic

Pancreaticoduodenal supplies the pancreas and duodenum

Recognize the fact that they almost all have anastomoses

Superior mesenteric supplies all of the small intestine and the ascending

and transverse portion of the large intestine Inferior mesenteric artery supplies the descending portion of the large

intestine

The intestinal venous drainage


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There are matching veins

The Veins do not go back to the inferior vena cava

They go to the Hepatic Portal You can see the superior mesenteric vein coming up and there is a

contact between it, the inferior mesenteric and the splenic vein in behindo They combine and take the nutrient rich blood directly to the liver

Q. Why do all intestinal veins drain through the liver?A. Processing Centre

It is the place where all the nutrients that have been collected from thesmall intestine are either transported for storage (sugars) or processing(amino acids)

The stomach is a blender


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Organs do not have air spaces in them this picture makes it look likethere is a space in the stomach (makes you think food falls to the bottom)

In reality, when the stomach is empty the two walls are against each otherwith a little bit of fluid in between them

Parts of the stomacho Lower esophageal sphinctero Cardia enterance way

o Fundus large area that has a lot of folds along its inner surface

Where the food arrives to

Capable of stretching maximally to hold food

Can get 1L of food into our stomach (large part of thataccommodated in the fundus)

o Body main area where digestion occurs

o Pyloric area

Pyloric antrum

Pyloric acal

Pyloric sphinctero Duodenum is beyond the pyloric area

There is a difference between the wall structure of the stomach and therest of the GI tract has a third layer of muscle

o It is inside the other twoo Therefore, longitudinal, then circular, and obliqueo Rather than just squeezing to mechanically break down the food, it

can twist


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The rugae of the stomach

Coronal section

You can see the rugae (the folds) of the stomach, primarily in the area ofthe fundus

o Allows the expansion of the stomach to accommodate largequantities of food

o Gets much smoother as you move further down

Endoscopy of normal stomach

In order to do endoscopy, air must be pumped into the stomach once theendoscope is beyond the esophagus

o This is what makes endoscopy uncomfortable This stomach has been inflated not what the stomach usually looks like

You can see the folds (rugae) in the fundus part and it is smoother as yougo further down

The Stomach

Is normally a very, very acid environment (pH 1.5-3.5)o Usually more down towards 1.5


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Digests proteins.

Breaks down, mixes and puts chyme into duodenum.o Mechanicallyo (ex. Hamburger is mashed up and the meat itself is broken down

but fats will undergo very little breakdown)

Produces one essential protein: intrinsic factor used in Vitamin B12absorption

o Important in the iluim to take up Vitamin B12o Necessary for synthesis of red cells

o Without Vitamin B12 you get Pernicious anemia

Absorbs some drugs like alcohol and ASA (some drugs).o Well vascularized so you would expect some absorption

The gastric wall

The wall of the stomach is specialized to do its role

Everything in the muscularis and submucosa is the same as the rest of theGI tract (but has an additional oblique muscle layer)

The Mucosa varies

There are pits with glands in the muscoa

In the pit


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There is an upper portion where there are columnar cellso Unlike the esophagus which was just a transfer tube and had

stratified squamous cells

It is a simple layer of columnar cellso Columnar cells are classically cells that secrete something

In the neck of the pit, they secrete a mucous which is rich in bicarbonate

o The mucous protects the cells that line the stomach from the acido It is a thick layero The presence of the bicarbonate prevents the acid from affecting

the cells below

o These cells undergo rapid turnover constant flow of movement ofcells up cells on the surface are constantly replenished to replacecells damaged by the acid

The working part of the pit is in the area identified as the gastric glando Contains three types of cells

Parietal cell produced intrinsic factor and HCl

Chief cells produce pepsinogen (inactive form of pepsin)

Pepsinogen converted to pepsin in the presence of HCl

You dont want an active protease inside the cell or itwould break down the cell

It is exposed to acid in the lumen and converted to

pepsin so it can become active and break down theproteins

Enteroendocrine Cells Major one is gastrin

Produced in situations where the pH has risen

Gastrin goes into the blood and stimulates the parietalcells to secrete more acid

Gastrin Hormone used by the stomach to control theproduction of acid


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Gastrin goes into the blood because in the area of thestomach, there is primarily parietal cells in the fundus

In the area of the stomach towards the pylorus theproportion of enteroendocrine cells increases

o Sends a message based on what its seeing interms of the contents of the stomach andsending a message back up to the top of thestomach

How does the parietal cell make acid?

Gastrin produced by enteroendocrine cells stimulates the parietal cells toproduce acid

The receptors are on the basal side (adjacent to blood vessels)

ACh arrives transmitter for parasympathetic nervous system in casesfor rumination/digestion is required, parasympathetic system can turn onproduction of acid

Internal second messenger transduction signaling gets an acid pumpgoing

Carbon dioxide and water forms carbonic acid which dissociates to aproton and bicarbonate (like it respiratory lecture)


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The easiest way to get acid is to produce carbonic acid which willdisassociate uses carbonic anhydrase which catalyzes the condensationof water and carbon dioxide to yield carbonic acid

o Parietal cells are rich in carbonic anhydrase and produce largequantities of protons and bicarbonate

o Protons are pumped by a specific pump requiring ATP energy into

the lumen of the stomach in exchange for potassium which comesin

o Potassium builds up in the cell and leaves through channels in thecell wall

There is a lot of bicarbonate in the cell, it will follow the concentrationgradient and move into blood and extracellular fluid through facilitatedtransport in exchange for chloride coming into the cell

o The concentration of chloride builds up in the cell and there arechannels in the parietal cells which allow the chloride to move outinto the lumen

Hydrogen and Chloride are both in the lumen so they form hydrochloric

acid The apical surface (surface facing the stomach lumen) of the cells is very

invaginated to maximize the surface area for these processes to occur

The bicarbonate ends up in the blood and alcholonizes the blood (makes itmore basic)

o This doesnt matter because pancreatic fluid ends up puttingprotons into the blood and it balances out


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Q. What is the role of the stomach in digestion?

Three processes in terms of what goes on in the stomacho Cephalic Phase

When we taste, smell, think about food a message is sent viathe sympathetic nervous system to the stomach to tell it toget ready

Starts the production of acid and gets things ready for foodto arrive

o Gastic Phase

Stretch and rise in pH (dilution of acid) is detected Stimulates the production of gastin which stimulates the

production of acid

Based on the food arrivingo Intestinal Phase

Intestines only want to deal with so much at a given time


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Feedback from the intestines as food moves from thestomach into the intestines which limits the amount of foodthat enters the intestine at any given time

Handled by feedback with cholecystokinin

Why doesnt the stomach autodigest?

The bicarbonate rich mucous secreted by the cells at the neck of thegland that prevent the acid from interacting with the cells at the surface

If this mucous is damaged or effected you will get damage to the surfaceof the stomach

Diseases of the stomach related to acid

Acute gastritis (alcohol, NSAIDS non-steroidal anti-inflammatories [likecheap asprin]): loss of mucus layer/ bicarbonate

Helicobacter pylori infection: loss of gastric function. Treatment is 85-95%effective to prevent recurrent ulcers

o Buries itself in the mucous layer and effects the mucous

o Treated with antibiotics and in many cases relieves ulcers The risk of ulcer formation is worsened by smoking, alcohol and stress.

Acute Gastritis

The red dots are small bleeds in the wall due to the mucous layer notbeing continuous and acid getting through

Gastric ulcer

Probably caused by aspirin sitting on stomach wall

The stomach wall has been eroded

The green at the bottom is bile leaking in


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Erosion has gone through stomach wall and through the wall of thegallbladder

This is an emergency situation

Bleeding gastric ulcer

Blood spurting due to the erosion of the stomach wall down to thesubmucosa and blood vessels

Gastric Cancer

Gastric Emptying

Once the contents (ex. Of a hamburger) of have been turned around tophysically break up the structure and the pepsin and acid havesubstantially broken down the protein it is time for the chyme that resultsto move to the small intestine and into the duodenum for absorption ofthe nutrients


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Initially a new set of peristaltic waves occurs in the stomach to push thechyme towards the pylorus

If the small intestine is ready to receive chyme the pyloric valve will openand a small amount of chyme will go into the first part of the duodenum

The pyloric valve will close again and feedback from the duodenum willreduce the motility in the stomach until that batch of chyme has beenprocessed in the duodenum at which point the process will occur again

Regulation of gastric emptying

Gives outline of process of initiation of the peristaltic wave that moves thematerial into the duodenum (combination of food being almost digestedand parasympathetic input)

The right side shows feedback from the duodenum that slows the activityand reduces motility of the stomach until the chyme has been movedthrough and processed

Control of gastric emptying

Neural: the presence of neuronal NO allows the stomach to relax toaccommodate up to 1 litre of food

o Muscularature of pyloric valve is controlled by NO not nerveso Plays a central role in opening pyloric valve and relaxing the

stomach Hormonal: gastrin and serotonin stimulate, VIP, GIP and somatostatin

inhibito Dont worry about VIP

o GIP (Gastric Inhibitory Peptide) is a source of confusion used to beconsidered a hormone produced by the duodenum that fed back to

inhibit gastric activity; turns out that that isnt what it is renamedit to glucose-dependent insulinotropic peptide (a hormone that is


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produced by the duodenum when it has carbohydrates in it tostimulate insulin production by the pancreas switch that turns oninsulin production in anticipation of blood glucose level rising)

o Somatostatin does inhibit gastric acivity

The duodenum controls the emptying based on content:o Carbohydrates, water easy

(i.e. pasta, bread)

easily processed by the small intestine material movesrapidly through the stomach into the intestine

o Small amounts of acid

Moves quickly

If amount of acid is larger it will move more slowlyo Fats enter very slowly

Leaves stomach slowly

This is what the fast food industry is based on

The fattier the food is, the longer it will take the stomach to

enter You need one relatively small-fatty hamburger and your

stomach will be full for a long time (and therefore satisfied)

Chinese food which is low in fat moves on quickly so youneed to eat a much larger amount to remain satisfied for thesame amount of time

Q. What common disease is associated with problems in gastric emptying?

Vomiting goes the other way if it cant empty!

Why do we vomit?

Extreme stretch, or irritants (bacterial toxins, excessive EtOH, certainfoods, drugs)

Under the above conditions, there is efferent stimulation of medulla.

Afferent signals (from the medulla) to stimulate contraction of diaphragmand abdominal muscles, relaxation of Lower Esophageal Sphincter, closingof soft palate and epiglottis

Pyloric stenosis

Unless the inhibitory fibers at the pylorus develop and produce theinhibitory neurotransmitter NO (nitric oxide) the sphincter does not relax.

o If the NO production is slow in developing the sphincter will notopen and food will come flying out

If it doesnt then the only place for gastric contents to go is up theesophagus and out (Stand back the projectile vomiting will cross theroom).

It is most common in first male babies (young boys)o Gives rise to projectile vomiting

If mild wait; if severe treated by slitting the pyloric sphincter

gastrointestinal anatomy and physiology - part 2

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