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THE DIGESTIVE

SYSTEM (PART II)

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FUNCTIONS OF THE DS• Ingestion

• Secretion

• Mixing and propulsion

• Digestion

• Absorption

• Defecation

Ingestion• This process involves taking foods and liquids into

the mouth (eating).

Secretion• Cells within the walls of the GI tract secrete about

7 liters of water, acid, buffers, and enzymes into

the lumen of the GI tract daily.

Mixing and Propulsion• Alternating contraction and relaxation of smooth

muscle in the walls of the GI tract mix food and

secretions and propel them toward the anus.

• This is referred to as motility.

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Digestion• Mechanical digestion.

– The teeth cut and grind food.

– The smooth muscles of the stomach

and small intestine churn the food to

help it dissolve and mix with enzymes.

• Chemical digestion.– The complex carbohydrate, lipid,

protein, and nucleic acid molecules in

food are split into smaller molecules

by hydrolysis.

– Digestive enzymes produced by the

salivary glands, tongue, stomach,

pancreas, and small intestines

catalyze these catabolic reactions.

– Amino acids, cholesterol, glucose,

vitamins, minerals, and water can be

absorbed without chemical digestion.

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Absorption• Absorption is the entrance of ingested

and secreted fluids, ions, and small molecules that are products of digestion into the epithelial cells lining the lumen of the GI tract.

• The absorbed substances pass into the blood or lymph and circulate to all cells of the body.

Defecation• Substances that were not absorbed leave

the body through the anus in a process called defecation.

• These substances include wastes, indigestible substances, bacteria, cells sloughed from the GI tract, and digested materials that were not absorbed.

• The eliminated material is called feces.

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Gastric Emptying• Gastric emptying is the periodic release of

chyme from the stomach into the

duodenum.

• Stimuli such as distention of the stomach

and the presence of partially digested

proteins, alcohol, and caffeine initiate

gastric emptying.

• The enterogastric reflex ensures that the

stomach does not release more chyme than

the small intestine can handle.

• Stimuli such as distention of the duodenum

and the presence of fatty acids, glucose,

and partially digested proteins in the

duodenal chyme inhibit gastric emptying.

• Within 2 – 4 hours after eating a meal, the

stomach has emptied its contents into the

duodenum.

• Foods rich in carbohydrates spend the least

time in the stomach, proteins longer, and

fat-laden meals the longest.

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Vomiting• Vomiting or emesis is the forcible

expulsion of the contents of the upper GI tract (stomach and sometimes duodenum) through the mouth.

• Stimuli include the following:

– Irritation and distention of the stomach.

– Unpleasant sights.

– General anesthesia.

– Dizziness.

– Certain drugs (morphine, derivatives of digitalis).

Vomiting involves squeezing the stomach

between the diaphragm and abdominal

muscles and expelling the contents

through open esophageal sphincters.

Prolonged vomiting can lead to alkalosis

(higher than normal blood pH).

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THE SMALL INTESTINE

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THE SMALL INTESTINE

• The small intestine, averaging about 6 meters in length, is small in diameter.

• The first 25 cm is the duodenumthat receives bile from the gallbladder and pancreatic juice from the pancreas.

• Walls of the small intestine have finger-like projections called villiwhere nutrient molecules are absorbed into the cardiovascular and lymphatic systems.

• Villi have microvilli that increase the surface area available for absorption.

• The small lymphatic capillary in a villus is called a lacteal.

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PANCREAS• Stomach chyme passes into the duodenum

for chemical digestion. Activities of the

pancreas, liver, and gallbladder are

necessary for this chemical digestion to

occur.

• The pancreas is a retroperitoneal gland.

• It is about 12-15 cm long and 2.5 cm thick.

• It lies posterior to the greater curvature of

the stomach.

• The pancreas consists of a head, body and

tail.

• It is connected to the duodenum by 2 ducts.

– Pancreatic duct (duct of Wirsung) –

larger. In most people, the pancreatic

duct joins the common bile duct from the

liver and gallbladder and enters the

duodenum as the hepatopancreatic

ampulla (ampulla of Vater). The

ampulla opens at the major duodenal

papilla.

– Accessory duct (duct of Santorini) –

smaller.

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PANCREAS

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COMPOSITION OF

PANCREATIC JUICE• рН 7,8-8,4 1,5-2 l

Enzymes:

• Proteolytic (Trypsin, chymotrypsin,

elastase, a carboxypeptidase) – a

proteolysis and polypeptides to amino

acids

• Amylolytic (-amylase) – splitting of

carbohydrates to a glucose

• Lipolytic (lipase) – splitting of fats to

glycerinum and fatty acids

• Nucleic acid digesting enzymes

(ribonuclease, deoxyribonuclease).

Bicarbonates – buffers acidic juice in

chyme, stops the action of stomach

pepsin, and creates the proper pH for

digestive enzymes of the small intestine

Water – a dissolvent

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ENZYMES• The enterokinase transforms

trypsinogen to trypsin

• Trypsin transforms

chymotrypsinogen to

chymotrypsin

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CONTROL OF

PANCREATIC SECRETION

• Conditional (a view, a smell of food, etc.)

• Unconditional (mastication, swallowing)

• Nervous (VNS: PSVNS)

• Humoral – gastro hormones:SECRETIN and CHOLECYSTOKININ-PANKREOZIMIN (CCK) stimulate

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LIVER

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• The liver is the heaviest gland of the body,

weighing about 1.4 kg.

• It is the 2nd largest organ in the body after the

skin.

• It is inferior to the diaphragm and occupies most

of the right hypochondriac region and part of the

epigastric region.

• The liver is divided into a large right lobe and a

smaller left lobe by the falciform ligament.

• The falciform ligament also suspends the liver.

• The liver is almost completely covered by visceral

peritoneum.

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HISTOLOGY OF THE LIVER &

GALLBLADDER• The lobes of the liver are made up of

many functional units called lobules.

• The lobules contain hepatocytes

arranged in irregular, branching,

interconnected plates around a central

vein.

• The liver has large endothelial lined

spaces called sinusoids instead of

capillaries.

• Fixed phagocytes called stellate

reticuloendothelial (Kuppfer) cells destroy

worn out WBCs, RBCs, bacteria, and any

other foreign material in venous blood

draining from the GI tract.

• Bile is secreted from the hepatocytes and travels through the right and left hepatic ducts.

• These ducts merge to form the common hepatic duct, which later joins the cystic duct from the gallbladder.

• Bile is stored in the gallbladder for later release.

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FUNCTIONS OF THE

LIVER

• Carbohydrate metabolism.

• Lipid metabolism.

• Protein metabolism.

• Processing of drugs and

hormones.

• Excretion of bilirubin.

• Synthesis of bile salts.

• Storage.

• Phagocytosis.

• Activation of vitamin D.

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Gallbladder• The gallbladder is a pear-shaped sac that

is located in a depression of the posterior

surface of the liver.

• It is about 7-10 cm long and typically

hangs from the anterior inferior margin of

the liver.

• The parts of the gallbladder are the broad

fundus, the body (central portion), and

the neck (tapered portion).

• The gallbladder stores and concentrates

bile until it is needed in the small

intestine.

• In the concentration process, water and

ions are absorbed by the gallbladder

mucosa.

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SECRETORY FUNCTION OF

THE LIVER– The liver consists of secretory cells –

hepatocytes which produce bile.

– Bile is a yellow, brown, or olive-green liquid.

BILE COMPOSITION

рН 7.3-8.0 0.5-1 l

– Cholic pigments (bilirubin, a biliverdin)

– Cholic acids (cholic, chenodesoxycholic)

– Bicarbonates

– Water

– Cholesterol

BILE FUNCTIONS

– Bile salts play a role in emulsification, the

breakdown of large lipid globules into a

suspension of droplets.

– Dissolution of hydrolysates of fats. This also aids

in the absorption of lipids following digestion.

– These droplets increase the surface area

allowing pancreatic lipase to function more

efficiently.

– Activates lipases of an intestinal and a

pancreatic juice

– Activates peristalsis of an intestine

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Intestine

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Digestive Hormones

• Gastrin promotes secretion of

gastric juice, increases gastric

motility, and promotes growth of the

gastric mucosa.

• Secretin stimulates the secretion of

pancreatic juice and bile. Inhibits

secretion of gastric juice.

• Cholecystokinin stimulates

secretion of pancreatic juice and

causes ejection of bile from the

gallbladder. Enhances the effects

of secretin.

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SMALL INTESTINE (SI)• The major events of digestion and absorption

occur in the small intestine.

• The length of the SI provides great surface area for this to occur.

• Circular folds, villi, and microvilli also serve to increase the surface area of the SI.

• The SI begins at the pyloric sphincter of the stomach, coils through the central and inferior parts of the abdomen, and eventually opens into the LI.

• The SI is divided into 3 regions:

– Duodenum – shortest region,

retroperitoneal, starts at the pyloric

sphincter of the stomach.

– Jejunum – between the duodenum and

ileum.

– Ileum – the longest region, joins the large

intestine at the ileocecal sphincter.

• Circular folds – permanent ridges in the

mucosa. They enhance absorption by

increasing the surface area of the SI and by

causes the chyme to spiral, rather than move

in a straight line.

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FUNCTIONS OF THE SI

• Mixing chyme with digestive

juice and help food contact

with the mucosa for absorption.

• Peristalsis propulsion of food

through the SI.

• Complete digestion of

carbohydrates, proteins, and

lipids.

• Beginning and complete

digestion of nucleic acids.

• Absorption of approximately 90

% of nutrients and water.

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HISTOLOGY OF THE SI

• The same 4 basic layers that make up the rest of the GI

tract exist in the SI as well, with some basic exceptions.

• The mucosa forms a series of fingerlike villi (tufts of

hair) projections, which increase the surface area

available for absorption.

• Each villus contains an arteriole, a venule, a blood

capillary network, and a lacteal through which nutrients

are absorbed.

• The mucosa is simple columnar epithelium, which contains absorptive cells, goblet cells, enteroendocrine cells, and Paneth cells.

• The apical surface of the absorptive cells contains microvilli (bundles of actin filaments). Collectively, they are referred to as the brush border. They increase the surface area.

• The mucosa contains deep crevices lined with

glandular epithelium.

• Cells lining the crevices form the intestinal glands

(crypts of Lieberkuhn), which secrete intestinal juice.

• Paneth cells secrete lysozyme, a bactericidal enzyme.

• The lamina propria of the SI has an abundance of mucosa-associated lymphatic tissue (MALT).

• Solitary lymphatic nodules are present as well as groups of aggregated lymphatic nodules (Peyer’s patches).

• Duodenal (Brunner’s) glands of the submucosa secrete an alkaline mucus.

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Villi

Villi are finger-like projections in the small intestine.

Each villus is made up of connective tissue and

smooth muscles. The villi are lined by a single layer

of thin walled epithelial cells through which the

absorption takes place. The fats are absorbed into

the lymphatic system. In the centre of each villus, is

a lymphatic duct. It is also called sinus or lacteal. The

space between the lacteal and the epithelium is

criss-crossed by several small blood capillaries.

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INTESTINAL SECRETION

• Intestinal juice is a clear yellow fluid that contains water and mucus.

• It provides the absorption of substances from chyme.

COMPOSITION OF INTESTINAL JUICE

рН 7.2-8.6 1.5-2 l

Enzymes

• Proteolytic (enterokinase, a peptidase,aminopeptidase)

• Amylolytic (a maltose, a saccharase, alactase)

• Lipolytic (lipase)

Bicarbonates – provide of alkalinemedium

Water – as a dissolvent

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Brush Border

Enzymes

• The absorptive epithelial cells

synthesize several digestive

enzymes, called brush border

enzymes, and insert them into

the plasma membrane of the

microvilli.

• Consequently, some digestion

occurs at the surface of the

epithelial cells and not

exclusively in the lumen of the

SI.

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Mechanical Digestion

In The Small Intestine

• Segmentations – localized mixing contractions that occur in portions of the intestines distended by a large volume of chyme. Sloshes chyme back and forth.

• Migrating motility complexes –a type of peristalsis that moves the chyme down the length of the SI after segmentation has occurred.

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Chemical Digestion In The SI

• Chyme entering the small intestines contains partially

digested carbohydrates, proteins, and lipids.

• Pancreatic juice, bile, and intestinal juice complete the

effort of digestion.

• Pancreatic amylase, sucrase, lactase, and maltase complete the digestion of carbohydrates.

• These enzymes break complex carbohydrates into monosaccharides, which can be absorbed.

• Lactose intolerance occurs in people whose mucosal cells fail to produce enough of the enzyme lactase.

– Symptoms include siarrhea, gas, bloating, and abdominal cramps after the consumption of dairy products.

• Pepsin, trypsin, chymotrypsin, carboxypeptidase, elastase,

and peptidases complete the process of protein digestion.

• Proteins are broken down into single amino acids, which can

be absorbed.

• Lipases complete the process of lipid digestion in the SI.

• Bile salts increase the surface area of triglycerides through the process of emulsification. The globules are converted into droplets.

• Lipids are broken down into monoglycerides, which can then be absorbed.

• Pancreatic juice contains two nucleases: ribonuclease

(breaks down RNA) and deoxyribonuclease (breaks down

DNA).

• Brush border enzymes further break these down into

pentoses, phosphates, and nitrogenous bases, which can be

absorbed.

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TYPES OF INTESTINAL

DIGESTION• Cavitary – Provides initial hydrolysis of

alimentary substances to intermediate

products.

• It is characterized by that enzymes

synthesized in glandular cells are allocated as

a part of digestive juice in an intestine cavity

here again have the specific an effect on a

chyme.

• Near wall (membrane) – Provides hydrolysis

of intermediate products to monomers and

prepares transition for an absorption.

• It is carried out by the enzymes fixed on

cellular membranes of an intestine. Feature of

the given kind of digestion consists that it is

carried out on border of extracellular and

intracellular mediums.

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Figure . Localization of food

hydrolysis with various types of

digestion: (a) extracellular, (b)

intracellular, (c) membrane

mediated; (1) extracellular fluid,

(2) intracellular fluid, (3)

intracellular vacuole, (4)

nucleus, (5) cell membrane, (6)

enzymes

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ABSORPTION

– Represents transport process in a

blood and a lymph of various nutrients

through one or several layers of cells.

PASWAYS OF SUBSTANCES`

TRANSPORT

– Passive (Water, salts, low-molecular

bonds):

– Osmosis

– Filtration

– Diffusion

– The active:

– Primary transport (Amino acids)

– Secondary transport

(Monosaccharides)

– Endocytosis (Suspensions, emulsions)34

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Absorption In The SI• Forms that can be absorbed:

– Monosaccharides (glucose, fructose, and

galactose) from carbohydrates.

– Single amino acids, dipeptides, and tripeptides from

proteins.

– Fatty acids, glycerol, and monoglycerides from

triglycerides.

Mechanisms of absorption:

– Diffusion.

– Facilitated diffusion.

– Osmosis.

– Active transport.

• Passage of digested nutrients from the

gastrointestinal tract into the blood or lymph is

called absorption.

• About 90% of the nutrients are absorbed in the SI.

• The other 10% occurs in the stomach and the

large intestine.

• Any undigested or unabsorbed material passes

through to the LI.

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Absorption Of

Monosaccharides• All carbohydrates are absorbed as monosaccharides.

• They are absorbed via facilitated diffusion and active

transport.

• The SI can absorb up to 120 grams of carbohydrates

per hour.

Absorption Of Amino Acids,

Dipeptides, & Tripeptides• Most proteins are absorbed as amino acids via active

transport processes.

• About half of the amino acids come from food.

• The other half of the amino acids come from proteins in

digestive juices and dead cells that slough off the

mucosal surface.

Absorption Of Lipids• All dietary lipids are absorbed via simple diffusion.

• Adults absorb about 95% of the lipids present in the SI.

• Most dietary fatty acids require bile for adequate

absorption.

• When lipids are not absorbed properly, the fat-soluble

vitamins A, D, E, & K are not absorbed properly.

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Absorption Of Electrolytes• Most of the electrolytes absorbed by the SI come

from gastrointestinal secretions, and some come

from ingested foods and liquids.

• Active transport mechanisms are utilized to

absorb Na+ ions.

• Negatively charged bicarbonate, chloride, iodide,

and nitrate ions can passively follow Na+ or be

actively transported.

• Iron, potassium, magnesium, and phosphate ions

are absorbed via active transport.

Absorption Of Vitamins

• The fat-soluble vitamins A, D, E, & K are included with dietary lipids and absorbed via simple diffusion.

• Most water-soluble vitamins are absorbed via simple diffusion.

• Vitamin B12 combines with intrinsic factor and the combination is absorbed via active transport.

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Absorption Of Water• The volume of water in the SI (about 9.3 liters daily)

comes from ingested liquids and gastric secretions.

• The SI absorbs about 8.3 liters of it.

• 90% of the remaining water (about 0.9 liters) is

absorbed in the large intestine.

• Water absorption occurs via osmosis.

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Absorption Of Alcohol

• Alcohol is lipid soluble and begins

to be absorbed in the stomach.

• There is greater surface area for

absorption in the SI; therefore, the

longer alcohol remains in the

stomach, the more slowly blood

alcohol rises.

• Fatty acids in the chyme slow

gastric emptying; therefore, eating

fatty foods with alcohol will cause a

slower rise in blood alcohol.

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LARGE INTESTINE

(COLON)

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• The large intestine is the terminal portion of the GI

tract.

• It is divided into four principal regions.

Major Regions Of The LI

• Cecum.

• Colon.

• Rectum.

• Anal canal.

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LI Anatomy• The LI extends from the ileum to the anus.

• It is about 1.5 m long and 6.5 cm in diameter.

• It is attached to the posterior abdominal wall by

mesocolon.

• The iliocecal valve guards the opening to the LI

from the ileum.

• The cecum hangs inferior to the iliocecal valve.

• The appendix is a twisted, coiled tube hanging on

the cecum.

• The open end of the cecum merges with the

colon, which is divided into ascending,

transverse, descending, and sigmoid portions.

• The last 20 cm of the GI tract make up the

rectum.

• The anal canal is the termination of the rectum.

• The anus is the opening of the anal canal to the

exterior.

• This opening is guarded by an internal anal

sphincter (involuntary) and an external anal

sphincter (voluntary).

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Structure of colon

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• No villi or permanent circular folds are

found in the mucosa of the large

intestine.

• The epithelium contains mostly

absorptive and goblet cells. The

absorptive cells participate mainly in

water absorption.

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Histology Of The

Large Intestine• The submucosa is similar to that found in the rest

of the GI tract.

• The muscularis consists of an external layer of longitudinal smooth muscle and an internal layer of circular smooth muscle. This forms three conspicuous longitudinal bands called the teniae coli.

• Tonic contractions of the bands draw the LI together into pouches called haustra.

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Large Intestine Functions• Completion of absorption (water, ions, and

vitamins).

• Production of some B vitamins and vitamin K by

bacteria in the LI.

• Formation of feces.

• Expulsion of feces (defecation) from the body

through haustral churning and peristalsis.

• The iliocecal sphincter regulates the passage of

chyme from the ileum into the cecum.

• Normally, the valve remains partially closed to limit

passage of chyme.

• After a meal, the gastroileal reflex intensifies ileal

peristalsis.

• The hormone gastrin also relaxes the sphincter.

• Haustral churning – The haustra remain relaxed and become distended while they fill up. When the distention reaches a certain point, the walls contract and squeeze the contents into the next haustrum.

• Peristalsis occurs.

• Mass peristalsis is a movement that occurs at the middle of the transverse colon and quickly drives contents into the rectum.

• Food in the stomach initiates the gastrocolic reflex.

Mechanical Digestion In LI

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Motor activity of an intestine

Types of contractions:

1. The rhythmic

2. Like pendulum

3. Peristaltic (very slow, slow, fast, prompt)

4. The antiperistaltic

5. The tonic.

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Chemical Digestion LI

• Mucous is secreted by the glands

of the LI, but no enzymes are

secreted.

• The bacteria of the LI perform the

final stages of digestion.

• Bacteria ferment any remaining carbohydrates, which releases hydrogen, carbon dioxide, and methane gases. These gases constitute the flatus (gas) in the colon. When the gas is excessive it is termed flatulence.

• Bacteria convert any remaining proteins into amino acids.

• Some B vitamins and vitamin K are produced by the bacteria in the colon.

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Absorption & Feces

Formation In The LI

• Water is absorbed from the chyme over a period of 3-10 hours. As the water becomes absorbed it becomes solid or semi-solid and is termed feces.

• The feces consists of water, inorganic salts, sloughed off epithelial cells, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food.

• The LI absorbs water, vitamins and ions (I.e. sodium and chloride).

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Dietary Fiber

• Dietary fiber consists of indigestible plant

carbohydrates such as cellulose, lignin,

and pectin. These are found in fruits,

vegetables, grains, and beans.

• Insoluble fiber does not dissolve in water.

Insoluble fiber speeds up passage of

materials through the track.

– Woody or structural parts of plants

include the skins of fruits and

vegetables and the bran coating

around wheat and corn kernels.

• Soluble fiber dissolves in water and forms

a gel that slows passage of material

through the tract.

– Soluble fiber is found in beans, oats,

barley, broccoli, prunes, apples, and

citrus fruits.

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Defecation Reflex

• Mass peristalsis movements push

fecal material from the sigmoid

colon into the rectum.

• This causes distention of the rectal

wall, which stimulates stretch

receptors and initiates a

defecation reflex that empties the

rectum.

• This reflex opens the internal anal

sphincter.

• The external anal sphincter is

voluntarily controlled.