Download Absorption Function of the Small Intestine - 1

Absorption Function of the Small Intestine 1
Each day, the alimentary canal processes up to 10 liters of food, liquids,
and GI secretions; of which, only about one liter enters the large
intestine. Almost all ingested food and drink, 80 percent of electrolytes,
and most of the water are absorbed in the small intestine. The entire
small intestine is involved in absorption, but most absorption occurs
before chyme reaches the ileum. Absorption in the ileum primarily
involves the recycling of bile salts. The absorptive capacity of the
alimentary canal is amazing. By the time chyme passes from the ileum
into the large intestine, it is essentially indigestible food residue (mainly
plant fibers like cellulose) some water, and millions of bacteria.
The absorption of most nutrients through the mucosa of the intestinal
villi occurs via active transport mechanisms that are driven directly or
indirectly by metabolic energy. These nutrients enter the capillary blood
in the villus and travel to the liver via the hepatic portal vein. An
exception is certain lipids, which undergo passive absorption via
diffusion and then enter the modified lymph duct in the villus (called
alacteal), to be transported to the blood in lymphatic fluid. Substances
cannot be absorbed between theepithelial cells of the intestinal
mucosa, because these cells connect with tight junctions. This is why
substances can only enter blood capillaries by passing through the
epithelial cells and into the interstitial fluid.
Carbohydrate Absorption
All carbohydrates are absorbed in the form of monosaccharides. The
small intestine is highly efficient at this, absorbing monosaccharides at
an estimated rate of 120 grams per hour. All normally digested dietary
carbohydrates are absorbed. However, indigestible fibers including
cellulose, glucose polymers made by plants which is not broken down in
humans, are eliminated in feces. Common protein carriers via secondary
active transport, which requires ATP transport the monosaccharides
glucose and galactose –the products of the breakdown of starch and
disaccharides – into the epithelial cells). They leave these cells via
facilitated diffusion and enter the capillaries through intercellular clefts.
The monosaccharide fructose (which is in fruit) is absorbed and
transported by facilitated diffusion alone. Monosaccharides combine
with the transport proteins, which lie very near the disaccharide
splitting enzymes on the microvilli, immediately after disaccharides are
broken down.
Protein Absorption
Active transport mechanisms, primarily in the duodenum and jejunum,
absorb most proteins as their breakdown products, amino acids. Almost
all (95 to 98 percent) protein is digested and absorbed in the small
intestine. The type of carrier that transports an amino acid varies. Most
carriers are linked to the active transport of sodium. Short chains of two
amino acids (dipeptides) or three amino acids (tripeptides) are also
transported actively. But after they enter the absorptive epithelial cells,
they are broken down to their amino acids before leaving the cell and
entering the capillary blood via diffusion.
Lipid Absorption
Despite being hydrophobic, the small size of short-chain fatty acids
enables them to be absorbed in intestinal chyme, enter absorptive cells
via simple diffusion, and then take the same path as monosaccharides
and amino acids into the blood capillary of a villus. The large and
hydrophobic long-chain fatty acids and monoglycerides are not so easily
suspended in the watery intestinal chyme. However, bile salts and the
phospholipid lecithin (also found in bile) help make them more soluble
by surrounding them and creating tiny spheres called
micelles. Micelles are macromolecular structures made up of
aggregates of fatty elements and bile salts that create a polar
(hydrophilic) end that faces the water and a nonpolar (hydrophobic)
core. The core also includes cholesterol molecules and fat-soluble
vitamins. Micelles can easily squeeze between microvilli and get very
near the luminal cell surface. At this point, lipid substances exit the
micelle and are absorbed via simple diffusion. Without their "vehicles"
(i.e., the micelles), lipids would sit on the surface of chyme, like oil on
water, and would never come in contact with the absorptive surfaces of
the epithelial cells. About 95 percent of lipids are absorbed in the small
intestine. Bile salts not only speed up lipid digestion, they are also
essential to the absorption of the end products of lipid digestion.
The free fatty acids and monoglycerides that enter the epithelial cells are
reincorporated into triglycerides, which are then mixed with
phospholipids and cholesterol and surrounded with a protein coat,
creating chylomicrons. Chylomicrons are milky-white droplets of
water-soluble lipoprotein that are processed by the Golgi apparatus for
release from the cell. Chylomicrons are too big to pass through the
basement membranes of blood capillaries. Instead, they enter the much
larger pores of lacteals. This means that most fats are first transported
in the lymphatic vessels and do not enter the venous blood until they
reach the thoracic duct in the neck region. In the bloodstream, the
enzyme lipoprotein lipase breaks down the triglycerides of the
chylomicrons into free fatty acids and glycerol. These two breakdown
products can then pass through capillary walls to become energy used
by our cells or to be stored in adipose tissue as fats. Liver cells combine
the remaining chylomicron material with proteins, forming "new"
lipoproteins that transport cholesterol in the blood.
Nucleic Acid Absorption
Special carriers in the villus epithelium via active transport transport the
products of nucleic acid digestion –pentose sugars, nitrogenous bases,
and phosphate ions –across the epithelium.
Food/Breakdown
Products
Absorption
Mechanism
Entry to
Bloodstream
Destination
Carbohydrates
Glucose
Co-transport
with sodium
ions
Capillary blood Liver via
in villi
hepatic portal
vein
Galactose
Co-transport
with sodium
ions
Capillary blood Liver via
in villi
hepatic portal
vein
Fructose
Facilitated
diffusion
Capillary blood Liver via
in villi
hepatic portal
vein
Protein
Amino acids
Co-transport
with sodium
ions
Capillary blood Liver via
in villi
hepatic portal
vein
Lipids
Long-chainfatty
acids
Diffusion into
intestinal cells,
where they are
combined with
proteins to
create
chylomicrons
Lacteals of villi
Systemic
circulation via
lymph in
thoracic duct
Monoglycerides
Diffusion into
intestinal cells,
Lacteals of villi
Systemic
circulation via
Food/Breakdown
Products
Absorption
Mechanism
where they are
combined with
proteins to
create
chylomicrons
Entry to
Bloodstream
Destination
lymph in
thoracic duct
Short-chainfatty
acids
Simple
diffusion
Capillary blood Liver via
in villi
hepatic portal
vein
Glycerol
Simple
diffusion
Capillary blood Liver via
in villi
hepatic portal
vein
Nucleic acids
Active
transport via
membrane
carriers
Capillary blood Liver via
in villi
hepatic portal
vein