lopez digestion and absorption mark louie d. lopez mark...

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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ANIMAL PHYSIOLOGY

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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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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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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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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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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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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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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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ANIMAL PHYSIOLOGY

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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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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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