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Digestive System General Functions of the Digestive System 1. Motility: a. Ingestion b. Peristalsis: rhythmic contractio - PowerPoint PPT Presentation

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Digestive System General Functions of the Digestive System 1. Motility: a. Ingestion b. Peristalsis: rhythmic contractions moving food through the gastrointestinal tract. c. Defecation/excretion: removal the unabsorbed materials out of the body. 2. Secretion:

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Digestive System

General Functions of the Digestive System

1. Motility:

a. Ingestion

b. Peristalsis: rhythmic contractions moving food

through the gastrointestinal tract.

c. Defecation/excretion: removal the unabsorbed

materials out of the body.

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2. Secretion:

Exocrine secretion: mucus, HCl, enzymes,

H2O... - digestive juice, 2-3 L/day (70 % is


Endocrine secretion: hormones that regulate

digestion activities.

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3. Digestion:

Mechanical and chemical decomposition of food.

4. Absorption: transport of digested products

into blood and lymph.

Lymphatic system: drain interstitial fluid and

return it into veins, absorb lipid molecules from

the digestive tract and send them to blood stream.

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1). Primary organs – gastrointestinal tract:

Mouth  pharynx  esophagus  stomach

 small intestines  large intestines rectum

 anus

(Food: Bolus  chyme  feces)

2). Accessory organs (2nd):

Teeth, tongue, salivary glands, pancreas, liver

and gall bladder.

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  • Layers of GI tract:

  • Mucosa – lines the lumen of the tract and form

  • folds. It contains epithelial cells, connective

  • tissue, lymph nodules, capillary vessels and a

  • thin layer of smooth muscle.

  • Submucosa:capillary vessels, lymphatic tube,

  • glands and nerves (submucosal plexus).

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Muscularis –inner circular and outer longitudinal

layers of smooth muscles, responsible for

segmental and peristaltic movement through the

GI tract.

Between the muscle layers: nerve fibers and

ganglia of sympathetic and parasympathetic


Serosa– connective tissue, forms the very outside

layer of the GI tract.

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Functions of each organ:

A. Mouth:(mechanical digestion)

Saliva contains: H2O, mucus, and enzymes:

Amylase: breaks down starch into


Lysozyme: kills bacteria.

Immunoglobulin - IgA: an antibody

Functions of saliva: moistening food, dissolve


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Tongue: The mucous membrane of tongue

contains taste buds, mucus glands, glands that

secrete lipase.

Inside of the tongue there are skeletal muscles

that makes voluntary movement.

Lipid digestion begins in the mouth by lingual


Functions of the tongue… participating swallowing.

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B. Pharynx:

Swallowing process is the result of corporation

of tongue, larynx and pharynx.

Tongue lifts up which causes the elevation of

larynx to close trachea and open esophagus.

So food enters the esophagus but not trachea.

This is a precisely controlled process.

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C. Esophagus

The upper part contains skeletal muscle; the

lower part contains smooth muscle.

Skeletal muscle near pharynx perhaps

participates in swallowing action.

Lower esophageal sphinctercontracts after

food passes into the stomach, which prevents

the food from coming back to the esophagus.

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D. Stomach


Cardia - upper opening

Stomach body

Pylorus - lower opening

Pyloric antrum - is the area near pylorus.

Pyloric sphincter -

Smooth muscles in the stomach body are oriented

in different directions.

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The epithelial cells of mucosa form gastric

glands and secrete different products to form

gastric juice.

Goblet cells – secrete mucus;

Parietal cells – produce HCL

Chief cells – produce pepsinogen (a zymogen,

inactive form of pepsin);

ECL cells – secrete histamine and serotonin;

G cells (endocrine cells) – gastrin;

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Intrinsic factor required for the absorption of

vitamin B12, is produced by parietal cells.

Table 18.1

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Gastric juice contains all the above substances

and water.

Food is mixed with gastric juice in stomach to

form paste like substance called chyme.

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Several important components in the gastric


Pepsin, a protease that cleaves peptide bonds

and digests food proteins.

It’s optimal working pH is 1-2.

Pepsin is first synthesized as pepsinogen, which

is cleaved and activated after it gets into the lumen

of the stomach.

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HCLis secreted by parietal cells.

H+ is transported into the stomach against its concentration gradient by H+/K+ exchange pump.

While H+ is pumped into the lumen of the stomach, K+ is taken up into the parietal cells.

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Five functions of HCL:

a. Kills microorganisms;

b. Causes denature of proteins.

c. Breaks down cell walls of plants;

d. Activates pepsin, and provides an optimal

environment for pepsin to function.

Cooperative activities of pepsin and HCl

allows the partial digestion of ingested proteins

in the stomach.

e. The acidic environment is also good for the

absorption of Ca and Fe.

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Mucus produced by Goblet cells, contains

HCO3- and forms an alkaline coating to prevent

the inner lining of stomach from being digested

by HCl and pepsin.

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Digestion and Absorption in the Stomach

Proteins are partially digested in the stomach.

The polypeptide chains are cleaved by pepsin

into oligopeptides, tripeptides and dipeptides.

There is very little digestion for carbohydrates

and fats in the stomach.

The major digestion process is in the small


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Absorption in the stomach is very limited.

The substances absorbed in stomach:

alcohol, water, small amount of minerals

(Ca, Fe, Na, Cl), and certain drugs, such as

aspirin and anti-inflammatory drugs.

The stomach empty time: for solid food is 4-6 hours;

water - about 30 min.

Pyloric sphincter controls the emptying speed

of stomach. It also prevents the intestinal content

from backing up into the stomach.

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Regulation of The Stomach Function:

(Nervous and hormonal controls)

The movement and secretion of stomach are

both stimulated by parasympathetic nerves.

Cholinergic receptor blocker can decrease HCl

secretion and smooth muscle contraction …

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Locally produced hormone, gastrin, can also

stimulate secretion and mobility of stomach.

Secretion of gastrin is triggered by food entering

the stomach.

The effect of ACh and gastrin on gastric secretion

is indirect.

Both ACh and gastrin can stimulate ECL cells

to produce histamine, which causes parietal

cells to produce HCL.

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Gastritis and peptic ulcer:

Gastritis - inflammation in the stomach mucous

membrane, can be induced by certain drugs

(aspirin, steroids, anti-inflammatory drugs)

and alcohol.

Infection of a bacterium, helicobacter has been

identified, as a direct cause of gastritis and

gastric ulcer.

Emotional and physical stress also contribute

to gastritis.

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Peptic ulcer or digestive ulcer, is open wound

on the mucus membrane of the stomach or

duodenum, often developed on the basis of gastritis.

The mucous membrane of the stomach is digested

by HCl and pepsin.

General mechanism: combination of over

production of gastric acid and underproduction

of the alkaline mucus.

(Helicobector is also involved.)

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More specifically, duodenal ulcer may be more

related with the over production of gastric acid.

Gastric ulcer is often due to insufficient production

of alkaline mucus.

Gastric ulcer is closely related with stomach cancer.

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The traditional treatment for digestive ulcer is

acid control in combination with antacid drugs.

Cholinergic nerve blocker to inhibit ACh release


New generation: histamine receptors blocker:


Antibiotics are sometimes prescribed.

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E. Small Intestines

The major site for digestion and absorption

The anatomy:

Approximately 3 M long in a living person, but it

will measure 6 M in a dead body, because of the

relaxation of longitudinal muscles.

Small intestines are held by mesentery - a broad

sheet of connective tissue, which contains blood

vessels lymphatic tubes and nerves.

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Three segments: duodenum, jejunum, and


  • The walls of small intestines fold into a structure

  • called plica (plicae), which further folds into a

  • microscopic structure, villi.

  • The epithelial cells of each villi form smaller folds

  • microvilli.

  • Due to the folding plus villi and microvilli, the

  • total surface area is 2200 ft2, (otherwise 3.6 ft2)

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Structure of Villus (Fig. 18.12)

A finger-like fold of mucosa that projects into the

intestinal lumen.

Each villus is covered by columnar epithelial cells

with goblet cells in between.

In the center: connective tissue, capillary vessels,

and a lacteal.

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The absorbed carbohydrates and amino acids enter

capillaries in the center of the villus, while lipids

enter the central lacteal.

Microvilli are brush like structure (brush border)

formed by epithelial cells on the surface of villus.

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Functions of small intestines:

1), Receiving the digestive juice from pancreas

and liver (enzymes and bile salts).

2), Secretion:

Intestinal juice, 1.8 L/day. It moistens chyme

and neutralize it’s pH.

Before absorption, pH of the food content must

be neutralized.

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a. NaHCO3 - is secreted by pancreas and small


NaHCO3  Na+ + HCO3-

H+ + HCO3  H2CO3  CO2 + H2O

Duodenum is the major location of buffering action.

Duodenum (10 inches long), receives chyme

from stomach and neutralize it.

The pH of chyme here changes from 1 to 7.5.

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pH of the rest of the small intestines is ~ 7.5,

which is optimal for most enzymes here.

b. Enzymes

Disaccharides, trisaccharides, dipeptides and

tripeptides are further broken down by enzymes

in the small intestines.

Brush border enzymes stick to the cell surface and

are exposed to the intestinal content.

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Enterokinase is a brush border enzymes that

activates trypsin that is produced by pancreas.

Sucrase, maltase, and lactase are also produced

by intestinal epithelial cells.

(Lactase gene may stop expressing in adults.)

Intestinal epithelial cells also make dipeptidase

to cleave dipeptides into single amino acids which

can then be absorbed.

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c. Small intestines also produce hormonethat

regulate secretion of stomach, pancreas and


The arriving of chyme to duodenum stimulates

the secretion of:

Cholesytokinin (CCK)

Chyme contianing lipids and proteins secretion

of CCK, which stimulates contraction of gall

bladder and secretion of pancreas.

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3). Regulation:

Arrival of chyme into duodenum also causes

local production of secrenin, which stimulates

the secretion of alkaline buffer, Na2HCO3, to

neutralize the acidic chyme.

Secrenin also inhibits gastric secretion and


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4). Digestion and absorption

The absorption of carbohydrate, lipids, Ca, Fe

mainly occurs in duodenum and jejunum.

Bile salts, water and electrolytes are absorbed

in ileum.

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Movement of small intestines:

Segmentation is the major movement form in

small intestines.

Small, periodic, ringlike contractions cut chyme

into segments and move it back and forth.

Peristalsis is weak in the small intestines.

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The smooth muscles in the small intestines are

controlled by vagus.

Smooth muscle cells have their own pace maker

to produce slow rhythmic contraction even when

there is no neural stimulation.

Stimulation from parasympathetic system can

accelerate both peristalsis and segmentation.

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If small intestines are overdistended or irritated,

a strong peristaltic rush may pass through the

entire length, sweeping the contents of the small

intestines into the large intestines so quickly, that

water, electrolytes and other substances that

would normally be absorbed are excreted through

frequent defecations.

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  • Anatomy:

  • It has larger lumen than small intestines and

  • contains 4 parts:

  • 1.)Cecum, the beginning part of large intestines

  • and is a short blind pouch.

  • The nerrow tube hanging down - appedix.

  • Appendix is an immune organs, which contains

  • many lymphatic nodules and lymphocytes.

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    2). Colon: from right to left, it includes

    ascending, transverse, and descending colons.

    3). Rectum

    4). Anal canal.

    The main function of large intestines is the

    reabsorption of water and electrolytes.

    There is 1500 ml of materials enters the large

    intestines each day, but only 200 ml becomes


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    Other functions of large intestines:

    Reabsorb and excrete bile salts.

    E.coli that normally residing in the large intestines

    produce vitamin K and some B vitamins, which

    can be absorbed in the large intestines.

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    G. Liver, Gall bladder andPancreas

    (accessory organs):

    Liver located at the upper right corner of the


    Gall balder is attached to the liver between its two


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    Liver is full of blood supply. Liver cells are

    called hepatocytes, which form hepatic plates

    that are one to two cells thick.

    The spaces between hepatic plates are called

    Sinusoids, whichmimic capillary vessels.

    However, hepatic plates are more permeable.

    They have large pores and lack basement membrane, which allow passage of proteins, fat

    and cholesterol.

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    Within each hepatic plate, there are thin bile tubes

    that collects bile juice produced by hepatocytes.

    Kupffer cells ?

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    Hepatic portal system

    All blood leaving the small intestines is transported

    to liver through hepatic portal vein.

    The blood is filtered in the liver and then enters

    hepatic veins, which drains to inferior vena cava.

    Hepatic portal system:

    vein  capillaries  vein.

    The purpose is to extract nutrients and to remove

    toxins before they enter systemic circulation.

    (The liver also receives blood from hepatic artery.)

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    • Functions of Liver

    • 1. Bile production and secretion

    • The liver produces 250 - 1500 ml of bile per day.

    • Major constituents of bile: bile pigments, bile

    • salts, phospholipids, cholesterol and ions.

    • Bile pigment includes billirubin and billiverdin.

    • Both are metabolic products of heme.

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    • 2. Detoxication

    • The liver can remove drugs and toxic molecules

    • by

    • excretion into small intestines;

    • phagocytosis by the Kupffer cells;

    • chemical alteration of these molecules.

    • For example, ammonia produced in protein

    • metabolism is converted into less toxic urea,

    • which is then excreted by the kidneys.

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    Another example, alcohol is hydrolyzed and

    broken down by alcohol dehydrogenase in the


    Also, liver converts lactic acid into glucose;

    and toxic purines into uric acid.

    Disadvantage: drugs can be deactivated in the


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    3. Participating the metabolism of glucose,

    protein and lipids

    While being filtered by liver, blood glucose level

    is monitored.

    If blood glucose is high, liver absorbs some of it

    for the synthesis of glycogen.

    When blood glucose is low (fasting), glycogen

    is broken down in to glucose, which is released

    into blood.

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    The liver monitors amino acids levels in the


    It removes excessive amino acids and use them

    for the synthesis of new plasma proteins or

    convert them to glucose or lipids to be stored.

    When blood amino acids are low, the liver will

    break down glycogen and triglycerides to

    synthesize and release amino acids to circulation.

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    Similarly, liver controls blood levels of triglyceride,

    fatty acids and cholesterol.

    When these molecules are low, liver breaks

    down the fat storage to release them to blood.

    When they are high, they are removed from

    blood to be stored in the liver as glycogen or fat.

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    4. Storage:Besidesglycogen and fat, vitamins

    (A, D, E, K and B12) and Fe can also be stored

    in the liver.

    Over ingestion of the lipid soluble vitamins can

    cause liver damage.

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    5. Biosynthesis

    Liver is a biochemical factory where many plasma

    proteins are synthesized.

    Albumin accounts for 70% of plasma proteins

    and is important for maintaining blood osmotic


    Thrombin and other clotting factors;

    Transport proteins: LDL and HDL;


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    Bile is produced by hepatocytes. It contains bile

    salts, billirubin, billiverdin, cholesterol and other


    Bile is continuously produced by liver cells

    but is not continuously released to small


    After being produced, bile is stored in gallbladder.

    It is only released into duodenum after meals.

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    When chyme reaches duodenum, CCK will be

    produced which stimulates gallbladder to contract,

    and bile will be ejected.

    When the small intestine is empty, the sphincter

    of ampulla at the end of common bile duct closes,

    forcing bile into systic duct and gallbladder to

    be stored.

    Pancreas duct and bile duct join into one tube that opens

    to duodenum (duodenal papilla).

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    Bile juice produced by hepatocytes

    hepatic duct  cystic duct  gallbladder 

    cystic duct  common bile duct  duodenum.

    The function of bile salts is to assist the digestion

    of fat.

    • Bile salts break the clumped fat molecules into

    • small droplets, which can be mixed with water

    • emulsification.

    • After emulsification, the fat splitting enzyme

    • - lipase can work on the fat molecules easily.

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    Pancreas - endocrine and exocrine gland

    Located in the upper right abdomen behind


    The endocrine function is carried out by

    pancreatic islet cells.

    Pancreas also secretes pancreas juice into


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    Components of pancreatic juice (Table 18.5)

    contains water, bicarbonate and many digestive


    a.Pancreatic amylase digest starch;

    b,Pancreatic lipase that splits triglyceride

    into fatty acids and glycerol.

    c,Proteinases (trypsin, chymotrypsin, and

    peptidase) cleave polypeptides into

    dipeptides and tripeptides.

    d, Pancreatic nuclease cleaves nucleotide

    chains into single nucleotides.

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    Digestion and Absorption in Small Intestines

    • Carbohydrate digestion

  • The digestion of starch occurs in the mouth and

  • small intestines.

  • Pancreatic amylase, which cleaves starch into

  • disaccharides and trisacharides that are later

  • digested by brushborder enzymes:

  • Sucrase: sucrose  glucose + fructose

  • Maltase: maltose  2 glucose

  • Lactase: lactose  galactose + glucose

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  • initiated in the stomach is completed in the

  • small intestines.

  • Polypeptide chains generated by pepsin are

  • further digested by: tripsin, chymotripsin and

  • elastase into dipeptides, tripeptides and free

  • amino acids.

  • Dipeptides and tripeptides are digested by

  • dipeptidase and tripepdidase (brush border)

  • into single amino acids.

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    Lipid Digestion:

    Bile is secreted in response to the arrival of lipids

    into duodenum.

    After emulsification, pancreatic lipase hydrolyzes

    triglycerides and phospholipids into glycerol, fatty

    acids and lysolecithin.

    These small subunits then associate with bile salts

    and cholesterol to form mixed micelles.

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    These micelles are little particles that have

    hydrophilic portion outside and hydrophobic

    portion in the middle, which can be easily

    absorbed by epithelial cells.

    All digestive enzymes are listed in table 18.8.

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    Absorption of six categories of nutrients mainly

    occurs in the small intestines

    Monosaccharides (glucose) and amino acids are

    absorbed into the intestinal epithelial cells and

    secreted into capillaries through facilitated

    diffusion and active transport.

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    Fatty acids, lysolecithin, cholesterol and glycerol

    contained in micelles can diffuse into epithelial


    Within the cytoplasm of epithelial cells,

    triglyceride is resynthesized.

    Triglycerides, together with cholesterol,

    phospholipids are then coated with proteins to

    form chylomicrons, which are secreted by

    epithelial cells into lacteals through exocytosis.

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    Lymphatic fluid containing the absorbed lipids

    will eventually join the blood circulation at

    subclavical vein.

    Water soluble vitamins are mostly absorbed in

    small intestines via diffusion.

    Fat soluble vitamins are mixed with triglycerides

    and are absorbed together with them.

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    Absorption outside of the small intestines:

    Stomach can absorb alcohol (diffusion), small

    amount of water (by osmosis), vitamin B12

    (active transport, bound with intrinsic factor),

    Fe, and Ca (active transport).

    Absorption in the large intestines:

    Water (hand out), bile salts, Vitamins K and

    Biotin (produced by E.coli).