Mark Louie D. Lopez Department of Biology College of Science Polytechnic University of the Philippines DIGESTION and ABSORPTION ecture of Mark Louie D. Lop
Mark Louie D. LopezDepartment of BiologyCollege of SciencePolytechnic University of the Philippines
DIGESTION and
ABSORPTION
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SUMMARY OF DIGESTION
ANIMAL PHYSIOLOGY
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ABSORPTIVE SURFACES
ANIMAL PHYSIOLOGY
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ABSORPTIVE SURFACES
ANIMAL PHYSIOLOGY
Plicae circulares
In some books: spiral or circular concentric folds;
valvulae conniventes (or folds of Kerckring)
Large folds which increases the surface area of
the absorptive mucosa about threefold
Well-developed in the duodenum and jejunum
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ABSORPTIVE SURFACES
ANIMAL PHYSIOLOGY
Villi
Fingerlike projections of the mucosal surface that
enhance absorptive area another 10-fold.
Involved in actual absorption
Within the villi:
arrangement of the vascular system for absorption of fluid
and dissolved material into the portal blood
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ABSORPTIVE SURFACES
ANIMAL PHYSIOLOGY
Brush borders
Found at the apex of enterocytes
Layer of densely packed microvilli that is a
cylindrical protrusion of the cytoplasm that contains
actin filaments. It also increase the area between
intestinal surface and nutrients by 20-fold.
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ABSORPTIVE SURFACES
ANIMAL PHYSIOLOGY
Thus, the combination of plicae
circulares, villi, and microvilli
increases the total absorptive area of
intestinal mucosa by 600x
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CARBOHYDRATE DIGESTION AND ABSORPTION
50% of digestible carbohydrate derived from plants
and cereals as starch
Glycogen from animal meat – small amounts in
normal diet
Widely available, low in cost, stored easily
Raffinose – oligosaccharide in legumes, not
digested by humans, digested by bacteria in colon
= gas
ANIMAL PHYSIOLOGY
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DIETARY FIBERS
Dietary Fiber Class FunctionsCellulose:
main cell wall constituent
- Holds water, laxative, reduces colonic pressure, binds minerals
Noncellulose polysaccharides:
Hemicellulose, Pectin, Gums,
Mucilages, Algal substances
- Holds water, increases stool bulk, slows gastric emptying time, binds cholesterol and bile acids, fermentable material for colonic bacteria (volatile acids and gas)
Noncarbohydrate:
Lignins – woody part of plants
(sand in pears)
- antioxidant, binds bile acids and metals
ANIMAL PHYSIOLOGY
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FUNCTION OF CARBOHYDRATES
Energy
Glycogen reserves
Protein-sparing
Antiketogenic fat-sparing
Reserve fuel of heart
CNS function
ANIMAL PHYSIOLOGY
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CARBOHYDRATE DIGESTION
Mouth – Ptyalin (alpha-amylase)
Stomach – stops once food is mixed with HCl
Small intestine – lumen: Pancreatic amylases
digestion products = maltose
α-limit dextrin
lactose
sucrose
ANIMAL PHYSIOLOGY
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CARBOHYDRATE DIGESTION
ANIMAL PHYSIOLOGY
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CARBOHYDRATE DIGESTION
Small intestine - brush border enzymes
Maltase (glucoamylase) → maltose = glucose monomers
Lactase → lactose = glucose + galactose
Sucrase → sucrose = glucose + fructose
Isomaltase → α-limit dextrins = glucose
Trehalase → trehalose (from mushrooms) = glucose
ANIMAL PHYSIOLOGY
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CARBOHYDRATE DIGESTION
Small intestine – brush border enzymes: further digest the molecules into 3 sugar monomers
1. glucose (80%) Na+-glucose
2. galactose (5%) transporter
3. fructose (15%) - Fructose carrier
ANIMAL PHYSIOLOGY
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FAILURE TO ABSORB CARBOHYDRATES
Diarrhea – unabsorbed CHO will osmotically draw
fluids from the epithelium into the lumen ex: lactose
intolerance (absence of lactase) * As we grow
older, enzymes at the brush border are insufficient.
Hence, adults are more prone to have lactose
intolerance.
Intestinal gas – H2, methane, CO2, metabolic
products of intestinal flora
ANIMAL PHYSIOLOGY
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PROTEIN DIGESTION AND ABSORPTION
ANIMAL PHYSIOLOGY
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PROTEIN DIGESTION AND ABSORPTION
The pancreatic juice contains zymogens like trypsinogen,
chymotrypsinogen, and procarboxypeptidase.
The enterokinase present in the brush border is capable of
activating trypsinogen into trypsin.
Trypsin then activates the other zymogens into their active
forms (carboxypeptidase and chymotrypsin) so that long
peptides can be broken down into simpler forms (di-/tri-
peptides or amino acids).
Peptidases inside the enterocyte (cytoplasmic peptidases)
can transform di-/tri-peptides into amino acids.
ANIMAL PHYSIOLOGY
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PROTEIN DIGESTION AND ABSORPTION
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PROTEIN DIGESTION AND ABSORPTION
Amino acid transportes are symporters that carry their
substrate aa in conjuction with obligatory uptake of Na+
The small intestine is also notable for its ability to take up
short peptides (PepT1). Peptides taken up into enterocytes
are hydrolyzed by a series of cytosolic peptidases.
AA not required by the enterocyte are exported across the
basolateral membrane -> blood capillaries -> liver via portal
vein.
ANIMAL PHYSIOLOGY
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LIPID DIGESTION
Lipids: supply 40% of adult energy needs
Majority of fatty acids (FAs) in dietary triglyceride:
oleate and palmitate
Essential Fats
linoleic 2%
linolenic 0.5%
Arachidonic
Triglycerides
animals = long-chain saturated FAs
plants = polyunsturated FAs
Phospholipids – majority from bile, commonest ingested = lecithin (arachidonic and linoleic)
ANIMAL PHYSIOLOGY
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↓mitochondria in aging = ↓use of fat as fuel
(since mitochondria is the site of B-oxidation).
ANIMAL PHYSIOLOGY
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LIPID DIGESTION
ANIMAL PHYSIOLOGY
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LIPID DIGESTION
ANIMAL PHYSIOLOGY
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ANIMAL PHYSIOLOGY
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LIPID DIGESTION
ANIMAL PHYSIOLOGY
The more saturated fat (more H+), less safe
Polyunsaturated fats are good for the health (like
fish oils and linseed oils)
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LIPID ABSORPTION
Most dietary lipid absorbed in upper 2/3 of the
jejunum
Absorption influenced by the presence of other
foods and reduced by dietary fiber
Goes through 3 phases: water in the lumen, lipid
in the epithelial membrane and water in the
lymphatics & bloodstream
>95% ingested fat absorbed by adults
ANIMAL PHYSIOLOGY
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LIPID ABSORPTIONThe products of enzymatic digestion (free fatty acids, glycerol,
lysophospholipids, cholesterol) form micelles with bile acids
in the intestinal lumen.
The micelles interact with the enterocyte membrane and allow
diffusion of the lipid soluble components across the
enterocyte membrane into the cell.
The bile acids, however, do not enter the enterocyte at this
time. They remain in the intestine, travel further down, and
are then reabsorbed and sent back to the liver by the
enterohepatic circulation.
ANIMAL PHYSIOLOGY
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LIPID ABSORPTION
The intestinal epithelial cells will resynthesize triacylglycerol
from free fatty acids and 2-monacylglycerol and will package
them with a protein, apolipoprotein B-48, phospholipids, and
cholesterol esters into a soluble lipoprotein particle known as
a chylomicron.
The chylomicrons are secreted into the lymph and
eventually end up in the circulation, where they can distribute
dietary lipids to all tissues of the body.
ANIMAL PHYSIOLOGY
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LIPID ABSORPTION
The fat will be resynthesized in the SER and golgi apparatus
inserting some protein making it a chylomicrons for transport
into the circulation through the lymph.
Some of the fat (monoglycerides and MCFA) will be extruded
in the lacteals and go directly into the blood streams.
Some of the absorbed fat will got the lymph while the other
will go to the portal circulation
ANIMAL PHYSIOLOGY
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NUCLEIC ACID DIGESTION
ANIMAL PHYSIOLOGY
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NUCLEIC ACID DIGESTION
Pancreas release pancreatic nucleases that
converts nucleic acids into nucleotides
Small intestines has intestinal brush border
enzymes that converts nucleotides into N-
containing bases, ribose, deoxyribose and
phosphate
ANIMAL PHYSIOLOGY
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NUCLEIC ACID DIGESTION
ANIMAL PHYSIOLOGY
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ABSORPTION IN GASTROINTESTINAL TRACT
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SUMMARYCarbohydrate and Protein digestion - depends on:
combined effects of secreted enzymes in lumen, brush border and within the mucosa
close proximity at brush border of enzymes and sites of absorption of digestive products
Lipid digestion
depends on the presence of bile salts, pancreatic lipase and colipase at nearly neutral pH
Efficient digestion and absorption
salt and water secretion by jejunum
motor response of the gut
intestinal blood flow (may limit absorption)
ANIMAL PHYSIOLOGY
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INTESTINAL ABSORPTION
Water - most absorbed in jejunum & ileum
- absorbed against an osmotic gradient in the colon
Na+ - co-transport with glucose, amino acids & peptides
Cl- - flows passively down electrochemical gradient established by Na+
K+ - secreted in the colon
HCO3- - secreted partly in exchange for Cl-
ANIMAL PHYSIOLOGY
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BLOW FLOW IN GASTROINTESTINAL TRACT
ANIMAL PHYSIOLOGY
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BLOOD FLOW IN GASTROINTESTINAL TRACT
ANIMAL PHYSIOLOGY
Abdominal Aorta – supplies oxygenated blood to
the organs of the GIT
Most of the blood that goes to the liver is venous
in nature and is supplied via the portal vein from
the intestines
This limits the entry of harmful molecules(drugs/bacteria)
into the blood stream while receiving nutrients absorbed in
the GIT
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PANCREAS
ANIMAL PHYSIOLOGY
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PANCREAS
ANIMAL PHYSIOLOGY
Physiological Characteristics
Functions both as an exocrine (releases pancreatic juice
w/c contains enzymes, lipases, amylases, proteases, &
bicarbonate) and an endocrine (releases insulin &
glucagon) gland.
By bulk however, mostly exocrine in function.
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PANCREATIC JUICE COMPONENTS
ANIMAL PHYSIOLOGY
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PANCREATIC ENZYMES
ANIMAL PHYSIOLOGY
Proteases: digests proteins
Chymotrypsin, Trypsin: Proteins to Peptides
Carboxypolypeptidase: Peptides to amino acids
Amylase: digests carbohydrates
Starch to Maltose
Lipase: digests fat; needs to combine with bile
salts (secreted by liver)
Dietary fat triglycerides to monoglycerides + fatty acids
Pancreatic amylase: digest carbohydrates
(except cellulose)
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PANCREATIC BICARBONATE SECRETION
ANIMAL PHYSIOLOGY
Secreted by pancreatic duct cells
CO2 and H2O diffuses into the cell
Carbonic anhydrase facilitates H2CO3 formation
Carbonic acid (H2CO3) splits into H+ and HCO3-
Active transport of HCO3- to the lumen, associated with
Na+ diffusion into the lumen.
H+ is exchanged with Na+ from the blood by active
transport (this supplies process #4)
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PANCREATIC BICARBONATE SECRETION
ANIMAL PHYSIOLOGY
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PANCREATIC BICARBONATE SECRETION
ANIMAL PHYSIOLOGY
• How bicarbonate perform its neutralizing
action:
– HCl* + NaHCO3 NaCl** + H2CO3
– H2CO3 H2O + CO2***
*from duodenal contents
** NaCl solution neutralizes the acid contents emptied into the duodenum
*** goes to the bloodstream
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STIMULATION OF PANCREATIC SECRETION
ANIMAL PHYSIOLOGY
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STIMULATION OF PANCREATIC SECRETION
ANIMAL PHYSIOLOGY
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STIMULATION OF PANCREATIC SECRETION
ANIMAL PHYSIOLOGY
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LIVER
ANIMAL PHYSIOLOGY
A large, multi-lobed organ located in the abdominal
cavity
75% of the blood that passes it are deoxygenated
and are for filtration. 25% comes from hepatic vein
(100% oxygenated).
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LIVER
ANIMAL PHYSIOLOGY
Functions of the Liver:
Bile secretion – main digestive function
Storage of vitamins and minerals
Degrade and excrete hormones and other
wastes from the blood (e.g. bilirubin and excess
cholesterol)
Transform and excrete drugs and toxins
Disposal of ammonia from protein metabolism
by converting it to urea
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LIVER
ANIMAL PHYSIOLOGY
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GALL BLADDER
ANIMAL PHYSIOLOGY
Standard color: green
Store bile in between meals (25-50cc)
Bile flows into the gallbladder when the sphincter
of Oddi is closed.
Also concentrates bile by active transport of
sodium and the secondary absorption of H2O, Cl-,
and other constituents.
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BILIARY SYSTEM
ANIMAL PHYSIOLOGY
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BILE FLOW
ANIMAL PHYSIOLOGY
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REGULATORY MECHANISM OF BILIARY PHYSIOLOGY
ANIMAL PHYSIOLOGY
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GALL BLADDER EMPTYING
ANIMAL PHYSIOLOGY
Stimuli:
Cholecystokinin (major stimulus): initiated by fatty food
in the duodenum
Acetylcholine secreted by both vagus and ENS nerve
fibers (minor stimulus)
Stimuli results to rhythmical contractions of the
gallbladder wall and simultaneous relaxation of the
sphincter of Oddi.
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RECYCLING OF BILE
ANIMAL PHYSIOLOGY
Four ways that bile salts are recycled out of the
small intestines:
carrier-mediated active absorption in the terminal ileum
(most important route)
passive diffusion along the small intestine (minor role)
deconjugation to primary bile acids before being absorbed
passively or actively
conversion of primary bile acids to secondary bile acids,
then subsequent absorption of deoxycholic acid
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ANIMAL PHYSIOLOGY
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