 In small intestine, carbohydrate, protein, fat,
electrolytes, vitamins and water are absorbed.
 Absorption of calcium or iron is adjusted
according to the body needs.
 Most
absorption occurs in duodenum and
jejunum and very little in the ileum [as most
absorption has already taken place].
 About 50% of small intestine can be removed with
a little interference with absorption.
2

VITAMIN B12 & BILE SALTS are absorbed in terminal ileum [if
the terminal ileum removed, their absorption will be effected].
 The mucous lining of the small intestine is well adapted to the
absorptive function of the small intestine for two reasons:
1. Has a very large surface area.
2. The epithelial cells in the mucous lining have a variety of
specialized transport mechanism.
 During absorption, a substance must pass through the epithelial
cell, diffuse through the connective tissue and then cross the wall
of the capillary or lymph vessel.
 Intestinal absorption could be passive or active.
3
Structure of a Villus:
Each villus has:
1) Epithelial cells: joined by tight junction at the lateral border
which limit the passage of the luminal content between the
cells. At the luminal border the epithelial cells have carriers for
absorption of nutrients and electrolyte.
2) Connective tissue core: formed by the lamina propria.
3) A capillary network: Each villus supplied by arteriole that
break up into a capillary network within the villus core. The
capillaries rejoin to form a venule that drains away the villus.
4) A terminal lymphatic vessel: Each villus supplied by a single
blind ended lymphatic vessel known as the central lacteal,
which occupies the center of the villus core.
Crypts of Lieberkuhn
• Dipping down into the mucosal surface between the villi
are shallow invaginations known as crypts of Lieberkuhn.
• Crypts of Lieberkuhn secrete water and electrolyte along
with the mucus secreted by the cells on the villus surface
forming succus entericus.
• The
crypts
function
as
nurseries.
New
cells
are
continuously produced in the crypts migrate to up the villi.
• Usually the epithelial lining is replaced every three days.
Sodium and Water Absorption
 Sodium absorbed by both passive and active transport.
 Sodium enters the epithelial cells across the luminal border
passively through the sodium channels or by secondary
active transport by Na+ - Cl- symporter, Na+ - H+ antiport or
Na+ - glucose symporter.
 Sodium is pumped out of the cell by Na+ - K+ pump.
 Water absorption follows the Na+.
 Sodium absorption results in movement of the water by
osmosis.
Carbohydrate Absorption
• Glucose and galactose are both absorbed by
secondary active transport, in which the
symport carriers such as sodium and glucose
cotransporter (SGLT) on the luminal membrane.
• The glucose or galactose leave the cells down
their concentration gradient via glucose
transporter GLUT-2 to enter the blood.
• Fructose is absorbed by Facilitated diffusion.
It enters the epithelial cells via GLUT-5 and
enter the blood via GLUT-2.
Proteins Absorption
 The end product of protein digestion is amino acid and
small peptide.
 Amino acids are absorbed in a similar way to glucose
and galactose via amino acid symporters. Amino acid
symporters are selective for different amino acids.
 Small
peptides
are
absorbed
with
tertiary
active
transport.
 Glucose, galactose, amino acids all get a free ride on the
energy expended for Na+ transport.
 Small peptides break down into amino acids by the
action of brush border enzymes.
 Amino acids enter capillary net work within the villus.
Fat Absorption
 The
end product of fat digestion are monoglycerides
and free fatty acids.
 The micelles are water soluble particles carry the
monoglycerides and free fatty acids within their lipid
soluble interiors.
 When the micelle approaches the absorptive epithelial
surface, monoglycerides and free fatty acids leave the
micelle and passively diffuse through the lipid bilayer of
the luminal membranes.
Fat Absorption
* The end products of fat digestion are monoglycerides
and free fatty acids. monoglycerides and free fatty acids
are resynthesized into triglycerides inside epithelial cells.
* These triglycerides aggregate and are coated with a
layer of lipoprotein from the endoplasmic reticulum to
form water-soluble Chylomicrons. Chylmicrons are
extruded through the basal membrane of cells by
exocytosis. Chylmicrons enter the lymphatic vessels.
Iron Absorption
• Only a portion of ingested iron is in a form that can be
absorbed, either heme iron or ferrous iron (Fe+2).
• Iron is absorbed by different energy-dependent carriers for
heme and Fe+2.
• Dietary iron needed for red blood cell production is transferred
into the blood by ion transporter Ferroprtin.
• Iron absorption is controlled with the hormone Hepcidin.
• In blood, iron is carried bound to plasma protein Transferrin.
• Absorbed dietary iron that is not needed immediately is stored
in the epithelial cells as Ferrtin.
• The unused iron is lost in the feces as the ferritin-containing
epithelia; cells are sloughed.
• Dietary ion that is not absorbed is lost in the feces.
Calcium Absorption
 Calcium
small
ions enter the luminal membrane of the
intestinal
epithelial
cells
down
its
electrochemical gradient through specialized Ca+2
channels.
 Calcium ions exit the basolateral membrane by
Ca+2 ATPase pump and Na+ - Ca+2 antiporter.
Vitamin D3 & parathormone enhance calcium
absorption.
Vitamins Absorption
• Water soluble vitamins are absorbed passively
with water from jejunum by simple diffusion.
•Vit B12 needs intrinsic factor for its absorption from
terminal ileum.
• Fat soluble vitamins are absorbed by simple
diffusion depending on fat absorption. Fat soluble
vitamins (A,K,E,D) are carried within the micelle.
• All nutrients absorbed through the small
intestine enter the liver through the hepatic
portal vein.
•After passing through the liver join the systemic
circulation.
• Fat enter the lymphatic system and bypass the
liver and directly join the systemic circulation.
• Harmful substances that may have been
absorbed are detoxified by the liver before
entering the systemic circulation.
VOLUMES ABSORBED BY SMALL AND LARGE
INTESTINE PER DAY
Volume entering the small intestine per day
Ingested Food
Fluid
1250 g
1250 ml
Sources
Secreted
from the
plasma
Saliva
Gastric juice
Pancreatic juice
Bile
intestinal juice
1500 ml
2000 ml
1500 ml
500 ml
1500 ml
Total volume = 9500 ml
 Volume absorbed by the small intestine per day = 9000 ml
 Volume entering the colon from the small intestine per day = 500 ml
 Volume absorbed by the colon per day = 350 ml
 Volume of feces eliminated from colon per day = 150 gm
MALABSORPTION SYNDROME
1- If ˃50% of intestine is removed  MALnutrition
as:
• Absorption of AA  body wasting & edema.
• Absorption of fat  fat soluble vit bleeding.
2- Malabsorption may be caused by MALdigestion as
in:
•Inadequate lipolysis ( pancreatic secretions)
•Obstructive jaundice   absorption of fats & vitamins.
3- Malabsorption due to abnormal mucosal transport as in:
• Non-specific defect:
1- As Celiac disease {Gluten Entropathy}
• A condition in which the small intestine is sensitive to gluten
(a protein constituent of wheat, barley, and rye).
• This condition is a complex immunologic disorder in which
exposure to gluten activates T-cell response that damage
the intestinal villi causing decrease of the surface area for
absorption. Therefore, absorption of all nutrients is impaired.
2- Also in Tropical sprue there is atrophy of villi.
• Specific:
Absence of lactase enzyme at brush border  milk
intolerance.
REFERENCES
 Human physiology by Lauralee Sherwood,
seventh edition
 Text book physiology by Guyton &Hall,11th edition
 Text book of physiology by Linda .s contanzo,third
edition
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