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Structure of the Small Intestine: list

1.8 L of intestinal juice is produced each day: Osmosis (chyme is more concentrated than the cytoplasm of the epithelial cells, so water flows out of the mucosa), and secretion by intestinal glands.

Structure of the Small Intestine: explain

The small intestine is structurally modified to maximize absorption.
About 1.8 L of intestinal juice is produced via osmosis and by the secretions from intestinal glands.
This fluid is important for digestive and absorptive processes.
The major stimulus for production of intestinal juice is distension of the intestinal mucosa by hypertonic or acidic chyme.
The normal pH of intestinal juice is slightly alkaline (7.4-7.8)
and is made up mainly of water but also contains some mucus and electrolytes (isotonic to the blood).

Small Intestine: Main site of Absorption: list structures/layers

There are three levels of structural modifications that facilitate maximal absorption.
Plicae circulares
Villi
Microvilli

plicae circulares

Permanent circular folds of the mucosa and submucosa,
about 1 cm tall and visible to the naked eye, called the plicae circulares
force the chyme to spiral around the lumen.
The plicae circulares function to slow down the movement of chyme which allows more time for processing and absorption.
In addition, the plicae circulares increase surface area for absorption by about 4x.

Villi

The villi are fingerlike projections of the mucosa.
They give the lining of the intestine a velvety appearance to the naked eye,
and increase surface area for absorption by about 3x more.

microvilli

The microvilli are specializations of the apical plasma membrane of all the columnar absorbing cells of the small intestine. Altogether, they increase surface area for absorption by about 50x. Collectively, these three levels of structural modifications increase surface area of the small intestine for absorption by 600 times which is about equal to the floor space of an average two-story house. Because most of all nutrient absorption occurs in the more proximal part of the small intestine, these features get smaller and fewer from proximal to distal

Small Intestine: Main site of Absorption: list

90% of all absorption of nutrients takes place in the small intestine
Mechanisms for increasing surface area for absorption:
1. Plicae circulares
Folds of mucosa and submucosa
Increases surface area by 4 x
2. Villi
Folds of mucosa
Increase surface area by 3x
3. Microvilli
Increases surface area by 50x

an enlargement of plica (upper diagram) and of a section of intestinal wall

This slide shows an enlargement of plica (upper diagram) and of a section of intestinal wall showing the fingerlike villi.
Also, notice that in addition to the outward projections of the mucosa to form villi, there are downward pits as well.
These are called crypts or intestinal glands.

Intestinal Villus: list epithelium type of cells

Intestinal Villus: explain
Looking at the microscopic anatomy of a villus, we can see that the epithelium consists mainly of two types of
cells—simple columnar epithelium (= columnar absorbing cells) and
mucous secreting goblet cells (not shown in this diagram).

Intestinal Villus: explain

The glands contain a variety of cell types including Paneth cells which secrete lysozyme for destruction of bacteria,
and a number of endocrine cells that secrete a variety of hormones that function in regulating digestive processes.
The lamina propria of the villus contains an extensive capillary bed to carry away absorbed nutrients,
and a lymphatic vessel called a lacteal which collects absorbed fats.
There is also some smooth muscle in the core of the villi.
Contraction and relaxation of this smooth muscle causes the villi to alternate between shortening and lengthening.
These pulsations increases contact between the contents of the intestinal lumen and the villi which increases the efficiency of absorption.
They also help to move the lymph along through the lacteals.

Intestinal Villus: list

Epithelium:
- Columnar absorbing cells
- Mucous secreting goblet cells
Glands:
Paneth cells: Lysozyme
Endocrine cells:
- Gastrin, secretin, CCK, somatostatin, gastric inhibitory polypeptide, Glucagon-like substance

Columnar Abosorbing Cells: Microvilli

Each columnar absorbing cell is studded on its apical plasma membrane with microvilli.
Collectively, the microvilli form a fuzzy appearance when viewed under a light microscope which is referred to as the brush border.
In addition to increasing surface area for absorption of nutrients, the plasma membranes of the microvilli contain enzymes (brush border enzymes), which complete digestion of carbohydrates and proteins.

Regional Specializations: list

Although all three regions of the small intestine share similar features,
there are some regional specializations that facilitate regional functions.
Duodenum
Jejunum
Ileum

Duodenum

Secrete mucous which protects the epithelium and raises pH of chyme; releases Secretin into the blood.
The duodenum is the first part of the small intestine.
It receives acidic chyme from the stomach.
Therefore, the duodenum has extensive mucus-secreting duodenal glands
(AKA Brunner's glands) in its submucosa in addition to epithelial goblet cells.
The mucus from the duodenal glands is very high in bicarbonate content which helps to neutralize the acidic chyme.
The duodenum also receives pancreatic juice and bile.

Jejunum

Maximal absorption
The jejunum is where most of the absorption of nutrients takes place so it has the most extensive plicae and villi.

Ileum

Lymphoid nodules protect the blood from bacteria in this region of the small intestine.
Although the mucosa of all the regions of the small intestine contain lymphoid cells,
the ileum has large aggregates of lymphoid follicles called Peyer's patches.
This is because as we approach the distal end of the small intestine, there is an increasing number of bacteria.
These must be prevented from entering the bloodstream and causing septicemia (infection).
There are also a lot of bacteria in the colon.
These bacteria aid in the final breakdown of plant materials and production of certain vitamins.

The Large Intestine: list fn

Completion of digestion
- Bacteria ferment the remaining carbohydrates
- Release hydrogen, carbon dioxide and methane gas
- Bacteria convert remaining proteins into amino acids
Completion of absorption
Manufacture of certain vitamins
- Bacteria manufacture some B vitamins and vitamin K
Absorption of water
Formation of feces
Expulsion of feces from the body

The Large Intestine: explain fn

The large intestine is greater in diameter than the small intestine, but it is much shorter.
It functions to complete the final stages of digestion.
The "good" bacteria housed in the large intestine breakdown any remaining carbohydrates via fermentation.
In so doing, they release hydrogen, carbon dioxide and methane gases (look out global warming)
which is released from the anus as flatulence.
The bacteria also convert any remaining pieces of protein into amino acids,
and help to make some vitamins that our bodies need but cannot make themselves such as some B vitamins and vitamin K.
The large intestine doesn't absorb much in the way of nutrients,
but it is very important in retrieving the water remaining in indigestible foods.
In removing water, it helps the body reduce water loss and forms the semisolid feces so they can be voided from the body.

large intestine anatomy

The large intestine is subdivided into the cecum
(a blind-ended pouch which lies below the junction with the ileum & ileocecal valve).
Attached to the posterior surface of the cecum is the vermiform appendix which is part of MALT.
The colon has several regions:
(1)the ascending colon travels up the right side of the abdominal cavity, makes sharp turn at the liver (right colic or hepatic flexure);
(2) the transverse colon goes across the abdomen and makes another sharp bend at the spleen (splenic or left colic flexure); it descends down the left side of the abdominal cavity as the
(3) descending colon, enters the pelvis as the S-shaped
(4) sigmoid colon.
(5)The large intestine then joins the rectum and finally ends as the anal canal.

specialized features of the large intestine. : list

ileocecal valve
Cecum
anal canal
an involuntary internal anal sphincter
voluntary external anal sphincter
hemorrhoidal veins
anal columns

specialized features of the large intestine. explain

There are several specialized features of the large intestine.
The ileocecal valve is visible to the naked eye.
The cecum can be a site where fecal material collects.
This may block off the appendix which becomes infiltrated with bacteria resulting in inflammation called appendicitis.
The appendix swells cutting off blood supply which results in tissue death or necrosis.
The appendix may burst spewing bacteria and feces through out the adominopelvic cavity resulting in peritonitis.
The anal canal is the last segment of the large intestine.
It has two sphincters,
an involuntary internal anal sphincter composed of smooth muscle and controlled by the autonomic nervous system,
and a voluntary external anal sphincter composed of skeletal muscle.
There are some large hemorrhoidal veins that sometimes become distended with blood and can get itchy or painful,
especially if you sit on your ___too long.
There are also some folds in the wall called anal columns which release mucus when they are squeezed by feces.
This helps to slick things up so to speak.
The anal canal is lined with non-keratinized stratified squamous epithelium to protect it from the abrasiveness of the fecal material.

Special Features of the Colon: list only

teniae coli
haustra.
Epiploic appendages

Special Features of the Colon: explain

There are some other structural adaptations that pertain only to the colon.
First of all, the longitudinal layer of the muscularis externa does not form a complete layer around the colon.
Instead it consists of three thick bands of smooth muscle called the teniae coli (=ribbons of the colon).
The muscle tone of these bands cause the colon to form pouches called haustra.
In addition, there are expansions of the visceral peritoneum that are filled with fat. These are called epiploic appendages.

Special Features of the Colon: list

1. The smooth muscle comprising the outer longitudinal layer are in 3 thick bands called the teniae coli.
2. The tenia coli bands cause the colon to form pouches called haustra.
3. Epiploic appendages are expansions of the serous membrane containing adipose.

Mucosa and Glands of the Colon

The mucosa of the colon is simple columnar and
contains predominately mucus secreting goblet cells that help lubricate the colon for the passage of feces.
There are no plicae circulares or villi because virtually all absorption of nutrients has already taken place.
There are no enzyme-secreting cells either.
There are numerous crypts but they contain goblet cells.

Defecation Reflex: list

Powerful peristaltic waves push feces out of sigmoid colon into rectum.
Distension of rectal wall triggers stretch receptors.
This causes additional local peristaltic waves to move feces toward anus. Parasympathetic division of ANS (sacral plexus) stimulates mass movement.
Internal sphincter is under autonomic (involuntary control); external sphincter is voluntarily controlled.

Defecation Reflex: explain

Now for everyone's favorite conversation piece: the defecation reflex; a real party thriller.
This reflex is every bit as complex as deglutition,
and similarly requires coordination between the voluntary and involuntary divisions of the nervous system.
The defecation reflex is mediated by the parasympathetic division at the level of the sacral region of the spinal cord.
It can be subdivided into long and short reflexes.
Mass movement of fecal material from the sigmoid colon into the rectum occurs
when the parasympathetic nerve fibers cause powerful peristaltic contractions in the muscularis externa.
The walls of the rectum distend with feces which triggers stretch receptors.
In the short reflex loop, the myenteric plexus causes increased local peristalsis which further distends the rectum.
The stretch stimulus is also carried to the parasympathetic motor neurons in the spinal cord which stimulates increased peristalsis,
but also allows somatic sensory afferent fibers to carry information to our brains alerting us of the situation.
If we choose to ignore the "urge" the reflex contractions end in a few seconds and the rectal walls relax.
There comes a point, however, when we can no longer ignore the message.
At this point we voluntarily relax the external anal sphincter and viola, mission accomplished.

Sphincter Contraction—one way flow : list only

Upper esophageal sphincter
Lower esophageal (cardiac) sphincter
Pyloric sphincter
Ileocecal valve
Internal and External Anal Sphincter

Sphincter Contraction—one way flow : list

1. Upper esophageal sphincter
Top of the esophagus
Opens in response to swallowing
2. Lower esophageal (cardiac) sphincter
Opens to let esophageal contents into stomach
Prevents reflux of stomach contents into esophagus
3. Pyloric sphincter
Between stomach and duodenum
Stays closed during stomach mixing
Insures particles in liquid are the correct size for absorption
Prevents intestinal contents from entering back into the stomach
4. Ileocecal valve
Between small intestine and large intestine
Controls emptying of material from small intestine into colon
5. Internal and External Anal Sphincter
Under involuntary and voluntary control, respectively

Sphincter Contraction—one way flow : explain

Well that brings us to the end of the GI tract.
As a reminder, there are a series of sphincters
(modifications of the circular layer of the muscularis externa)
that insure one way flow of food, chyme and feces.
Be sure you know where the valves are located and their role.
Vomiting or emesis is an exception to the one-way flow rule,
however, this only happens under physiologically atypical conditions—
you really overate, you drank too much alcohol, you have bacterial toxins from "food poisoning", took certain drugs, etc.
The emetic (vomiting) center is located in the medulla oblongata of the brainstem.

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