Transcript Lecture 36. Physiology of absorption

Physiology of absorption.
Determine of notion “absorption”
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Absorption is a complex of processes, which
are provide transport of substances from
digestive tract into internal surroundings of
organism (blood, lymph, intercellular
substances)
Main types of transport of nutritive substances in
internal surroundings of organism
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1. Passive;
2. Active.
1. Passive transport include diffusion and osmosis. This
transport do not need presents of energy. In this case
substances transport through the mucus shell by help of
concentrative gradient. This way of transport have water, water
disolved vitamins (C, B6, B2).
2. Active transport include pinocytosis and active transport by
help of protein and energy. Active transport need energy of
ATP. This way characteristic of amino acids, monosaccharids,
vitamin B12, ions of calcium, enzymes. Pinocytosis – by help
of pynocytic bulb, where secreted enzymes for proteins
hydrolysis. Products of hydrolysis adsorbed by cell.
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The total quantity of fluid that must be
absorbed each day is equal to the ingested fluid
(about 1,5 liters) plus that secreted in the
various gastrointestinal secretions (about seven
liters). This comes to a total of approximately
8 to 9 liters. All but 1,5 liters of this is
absorbed in the small intestine, leaving only
1,5 liters to pass through the ileocecal valve
into the colon each day.
Absorption in the mouth cavity and
stomach
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In the mouth cavity absorbed water, water soluble
medicines (validol, nitroglycerin). In our oral cavity,
under the tongue present a big quantity of vessels.
That is why all water soluble substances absorbed.
They go to the bloodstream, and have immediately
action on our receptors. They do not go through the
liver, and do not desintoxicated, that is why may be
toxic effect of some substances. In stomach absorbed
alcohol and small quantity of other substances.
stomach
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The stomach is a poor absorptive area of the
gastrointestinal tract because it lacks the
typical villus type of absorptive membrane and
also because the junctions between the
epithelial cells are tight junctions. Only a few
highly lipid-soluble substances, such as
alcohol and some drugs like aspirin, can be
absorbed in small quantities.
Absorption in intestines
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Small intestines has a big length – 3-8 meters; large
quantity of substances absorbed in a middle part of
small intestines; common surface of intestines – 200
m2. The structure bases of absorption in small
intestines are villi. In large intestines absorbed water
and completed absorption of a small quantity of
hydrolitic substances of carbohydrates, proteins and
lipids. By help of this way may absorbed components
of nutritive elements (glucose, vitamins, water etc).
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The Absorptive Surface of the Intestinal
Mucosa – The Villi. The absorptive surface of
the intestinal mucosa, showing many folds
called valvulae conniventes (or folds of
Kerckring), which increase the surface area of
the absorptive mucosa about threefold. These
folds extend circularly most of the way around
the intestine and are especially well developed
in the duodenum and jejunum, where they
often protrude as much as 8 mm into the
lumen.
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The intestinal epithelial cells are characterized
by a brush border, consisting of about 600
microvilli 1 μm in length and 0,1 μm in
diameter protruding from each cell. This
increases the surface area exposed to the
intestinal materials another 20-fold. Thus, the
combination of the folds of Kerckring, the
villi, and the microvilli increases the
absorptive area of the mucosa about 600-fold,
making a tremendous total area of about 250
square meters for the entire small intestine –
about the surface area of a tennis court.
ABSORPTION IN ORAL CAVITY,
ESOPHAGUS AND STOMACH
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In the mouth cavity absorbed water, water soluble medicines
(for example, validol, nitroglycerin, adelphan, furosemid,
corinfar and others). In our oral cavity, under the tongue
present a big quantity of vessels. That is why all water soluble
substances absorbed in this place. They go to the bloodstream,
and have immediately action on our receptors. They do not go
through the liver, and do not desintoxicated, that is why may
be toxic effect of some substances, for example products of
food, drugs.
In esophagus do not absorbed nutritive substances as a rule.
In stomach absorbed alcohol, water and small quantity of other
substances.
ABSORPTION IN THE SMALL
INTESTINE
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Normally, absorption from the small intestine each
day consists of several hundred grams of
carbohydrates, 100 or more grams of fat, 50 to 100
grams of amino acids, 50 to 100 grams of ions, and 7
to 8 liters of water. However, the absorptive capacity
of the small intestine is far greater than this as much
as several kilograms of carbohydrates per day, 500 to
1000 grams of fat per day, 500 to 700 grams of amino
acids per day, and 20 or more liters of water per day.
In addition, the large intestine can absorb still more
water and ions, though almost no nutrients.
ABSORPTION IN THE LARGE
INTESTINE
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Approximately 1500 ml of chyme pass through the
ileocecal valve into the large intestine each day. Most
of the water and electrolytes in this are absorbed in
the colon, usually leaving less than 100 ml of fluid to
be excreted in the feces. Also, essentially all the ions
are also absorbed, leaving only about 1 mEq each of
sodium and chloride ions to be lost in the feces.
Most of the absorption in the large intestine occurs in
the proximal half of the colon, giving this portion the
name absorbing colon, whereas the distal colon
functions principally for storage and is therefore
called the storage colon.
Methods of absorptions’
investigation
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1. Angiostoma.
2. X-ray investigation.
3. Biochemical method of investigation.
Regulation of absorption
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Absorption regulated by nervous system, for
example by cortex; by vegetative nervous
system; endocrine glands, for example
adrenocorticotropin, aldosteron,
glucocorticoids, insulin, thyroxin, local factors
of food, for example pepper, mustard.
Peculiarities of absorption of organic
and inorganic substances
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Water and mineral salts
Products of proteins hydrolyses
Products of carbohydrates hydrolyses
Products of fats hydrolyses
Water and mineral salts
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Main quantity of water absorbed in small intestines,
small quantity – in stomach and large intestines.
Water absorbed by help of osmotic gradient. Water
absorbed together with amino acids, carbohydrates,
salts; vitamins absorbed with water. Main role of
water transport through the membrane have sodium
and chloride. Sodium transport in two stages: passive
(sodium go through the apical part of enterocytes
membrane from chyme) and active (sodium go by
help of Na+,K+-ATPase on baso-lateral part of
enterocytes membrane in blood). Chloride and
HCO3- go together with sodium. Ca2+, Mg2+
absorbed actively.
Products of proteins hydrolyses
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Proteins absorbed in free amino acids,
dipeptides, threepeptides appearance. Their
absorption into cell through the apical part of
membrane connects with secondary active
sodium transport. Most actively absorbed
arginine, methionine, leucine; L-form of
aminoacids. Then they transport through the
basolateral part of membrane in blood by help
of concentrative gradient.
Products of carbohydrates
hydrolyses
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Carbohydrates absorbed in monosaccharides
appearance. Most actively absorbed glucose
and galactose. Their absorption into cell
through the apical part of membrane connects
with secondary active sodium transport. Then
glucose transport through the baso-lateral part
of membrane in blood by help of concentrative
gradient. Mannose and fructose transport by
help of diffusion.
That the transport of most monosaccharides through the
intestinal membrane is an active process is demonstrated by
several important experimental observations:
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1. Transport of most of them, especially glucose and galactose, can be
blocked by metabolic inhibitors, such as iodoacetic acid, cyanides, and
phlorhizin.
2. The transport is selective, specifically transporting certain
monosaccharides without transporting others. The order of preference for
transporting different monosaccharides and their relative rates of transport
in comparison with glucose are:
3. There is a maximum rate of transport for each type of monosaccharide.
The most rapidly transported monosaccharide is galactose, with glucose
running a close second. Fructose, which is also one of the three important
monosaccharides for nutrition, is absorbed less than half as rapidly as either
galactose or glucose; also, its mechanism of absorption is different, as will
be explained below.
4. There is competition between certain sugars for the respective carrier
system. For instance, if large amounts of galactose are being transported,
the amount of glucose that can be transported simultaneously is
considerably reduced.
Products of fats hydrolyses
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In small intestines developed micellas – connection of
fat acids, cholesterol, monoglycerides, phospholipids
with bile acids salts. Bile salts acids act transport role
of micella to apical part of membrane. Then by help
of diffusion fat acids, cholesterol, monoglycerides,
phospholipids penetrate into enterocyte, where
synthesis new triglycerides. Inside enterocyte develop
chilomicrones. It consists from triglycerides,
phospholipids, cholesterol and other lipids. They
penetrate in lymph vessels by help of pinocytosis.