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The Digestive System

Physiology -IIPHL 226

1

1- Mouth to Esophagus

Dr. Dina A.A. Hassan

Associate Professor, Pharmacology

Email: [email protected]

Study objectives

• Identify the components of the digestive system.

• Describe the physiological processes involved in the digestive system.

• Explain how the body digests food sources

• Describe the control of digestive secretions, functions and composition of the different juices and enzymes of the GIT.

General Structure & Functions

of the Digestive System

I- Structure of the Digestive System :

Digestive system composed of 2 separate groups of

organs:

a- Alimentary canal:

It is a continuous tube that extends from the mouth to the

anus (about 9 –10 m).

It includes mouth, pharynx, esophagus, stomach, small

intestine and large intestine.

b- Accessory organs:

These are organs that help the digestive system in

digestion of food by:

1- Mechanical processes as teeth and tongue.

2- Secretion of digestive substances as salivary glands, liver,

gall bladder and pancreas.

3

II- Functions of the Digestive System:

1- Breakdown food into simpler molecules.

2- Absorb the nutrients which are necessary to life.

3- Removal of waste products from the body.

NOTES:

Food that is in the GIT is not really inside the body. To enter the body, food must be

broken down into simpler molecules then enter to blood stream.

Food is moved through the digestive tract by muscular contractions called peristalsis.

The peritoneum is a membrane that lines the abdominal organs. It produces a lubricating

fluid that reduces friction between organs.

4

Digestive Process

The digestive process involves 5 basic activities :

1. Ingestion – is the process of taking food into the mouth.

2. Propulsion - is the process of movement of food through the GIT.

It includes: a) Swallowing b) Peristalsis

3. Digestion- is the breaking down of food into simple components.

The process of digestion includes:

a) Mechanical digestion b) Chemical digestion

4. Absorption – is the passage of digested food from the digestive tract

to blood stream.

5. Defecation- is the elimination of indigestible substances (wastes)

through anus in the form of feces

5

Mechanical and Chemical digestions

1- Mechanical digestion

Mechanical digestion is the breakdown of food into smaller particles.

It prepares food for chemical digestion.

It begins in the mouth and continues into the stomach.

Mechanical digestion includes:

Mastication (Chewing) of food by teeth.

Mixing food with saliva by tongue.

Churning food in the stomach.

NOTE:

Churning = muscular contraction and relaxation of stomach.

6

2- Chemical digestion

Chemical digestion is the changes in the chemical composition of food with the aid of

digestive enzymes.

It converts complex molecules such as carbohydrates, proteins and fats into simpler

molecules that can be absorbed and used by cells.

It begins in the mouth and ends in small intestine.

Chemical digestion includes:

Conversion of carbohydrates into simple sugars.

Conversion of proteins into amino acids.

Conversion of fats into fatty acids and glycerol.

NOTES:

In mechanical digestion ----- No change in the chemical structure of food

In chemical digestion ------ There is a change in the chemical structure of food.

7

Oral Cavity (Mouth)

It is the gate of the GIT.

The tongue and teeth are the first “organs” of the digestive

tract.

The digestive process begins in the mouth by 2 ways:

1- Mechanical digestion

The teeth cuts and grinds food. This process is called mastication.

Tongue:

Mixes food with saliva to form a semisolid bolus that is easily

swallowed.

Pushes the bolus toward the pharynx.

8

2- Chemical digestion (by enzymes of saliva).

• Salivary glands secretes saliva (contain amylase enz) into the oral cavity.

• Salivary amylase begins to break down complex carbohydrates and starch found in foods such as bread, cereals, potatoes and pasta.

• Salivary amylase breaks the bonds between the polysaccharides, converting them to simple sugars such as fructose, maltose, glucose which can be absorbed.

• Salivary amylase hydrolyze dietary starch into disaccharides and trisaccharides which are converted by other enzymes to glucose to supply the body with energy.

NOTE:

Foods that contain large amounts of starch but little sugar, such as rice and potatoes, may acquire a slightly sweet taste as they are chewed because amylase digest some of their starch into sugar.

Function of mouth

1- Mechanical breakdown of food

2- Secretion of saliva (salivary amylase) which convert complex carbohydrates and starch into simple sugars

3- Tasting

9

Salivary Glands

They are 3 pairs:

1- Parotid glands (beneath the cheeks)

2- Submandibular glands (below the jaw bone)

3- Sublingual glands (below the tongue)

They secrete saliva.

10

Saliva

Saliva is clear, tasteless, odorless, viscid fluid, secreted from the parotid,

submandibular and sublingual salivary glands.

Volume of saliva secreted daily = 1.0 -1.5 L Most is produced during mealtime

Composition of Saliva

I- Water (99%) : To dissolve food easily so food can be tasted.

II- Solutes (1%):

1- Salivary amylase- an enzyme that helps break down complex carbohydrates and

starch into simple sugars such as fructose, maltose, glucose.

2- Mucous- lubricates food for easy swallowing.

3- Lysozyme- inhibit bacterial growth in the oral cavity and protect the teeth from decay.

4- Electrolytes- including sodium, potassium, chloride, phosphate, and bicarbonate ions

11

Functions of Saliva

1- Preparation of food for swallowing

Saliva moistens the food so converts it into a semisolid bolus that is easily

swallowed.

2- Digestive function

Amylase in saliva digest complex carbohydrates and starch into simple sugars

such as fructose, maltose, glucose.

3- Antibacterial action

Lysozyme in saliva inhibit bacterial growth in the oral cavity.

12

Pharynx

• It is the passageway of both food (bolus) and air

• Bolus passes through the oropharynx then

laryngopharynx to reach the esophagus

• Once a bolus of food reaches the pharynx, swallowing is

involuntary

Swallowing

Regulated by swallowing center in the medulla oblongata.

Stages of Swallowing

1- Buccal (Oral) stage

• It is a voluntary stage.

• The tongue pushes the bolus into the oropharynx.

13

2- Pharyngeal stage

• It is involuntary stage.

• It involves passage of bolus from pharynx to esophagus

• 3 actions blocks bolus and drink from re-entering the

mouth, the nasopharynx or larynx

a- The root of the tongue blocks the oral cavity

b- The soft palate rises to blocks the nasopharynx

c- Epiglottis close the airway that leads to the trachea

3- Esophageal Stage

• It is involuntary stage.

• It involves passage of bolus from the esophagus to stomach

14

http://en.wikibooks.org/wiki/File:Swallow_food.png

http://en.wikibooks.org/wiki/File:Swallow_food.png

Esophagus

Esophagus is a straight muscular tube About 25-30 cm long.

Function of Esophagus:

It transports bolus from the pharynx to the stomach by peristalsis in about 7 seconds.

Due to peristalsis, bolus can be swallowed in zero gravity, or even when the body is upside down.

Esophagus meets with the stomach at an opening called the Lower Esophageal Sphincter (LES) OR Cardiac Sphincter.

Physiologically, LES remains contracted in contrast to the middle and upper part of esophagus which normally remains completely relaxed.

Function of LES:

It prevents reflux of stomach contents into the esophagus

15

GERD

• LES located between the esophagus and stomach.

• LES relaxes to allow bolus to enter the stomach but prevents reflux of stomach contents into the esophagus

• If LES becomes weak, it fails to close completely, so the acidic juice in the stomach can flow back up or reflux into the esophagus.

• The stomach is protected from this acidic juice by a layer of mucus, while the esophagus does not have a barrier of mucus to protect it from acid.

As a result, reflux can cause inflammation and damage to the inner lining of the esophagus.

• This condition is known as heartburn (burning sensation behind the breastbone)

• Chronic condition of heartburn is known as gastroesophageal reflux disease, or GERD.

16

Conclusion & summary

references

• Books:

Human Physiology by Stuart Era Fox, McGill, NY, USA. Human Physiology by Arthur C. Guyton, 5th editin, W.B. Saunders Company, Philadelphia, London,Toronto.

• Essential References Materials:

Fox, SI 2010, Human physiology, 12th edn, McGraw Hill Higher Education, Boston.Hutchinson, M, Mallatt, J, Marieb, EN, Wilhelm, PB 2007, A brief atlas of the human body, 2nd edn,Pearson Benjamin Cummings, San Francisco.Martini, FH & Nath, JL 2009, Fundamentals of anatomy and physiology, 8th edn, Pearson Benjamin

Cunnings, San Francisco.• Websites:

• www.PubMed.com• www.cvphysiology.com• www.sciencedirect.com• http://macromolonline.8m.com/main.htm • http://www.med.umich.edu/lrc/Hypermuscle/Hyper.html • www.blackwell.com

http://www.pubmed.com/

http://www.blackwell.com/

19

The Digestive System

PHYSIOLOGY-2

PHL226

20




2- Stomach

Stomach

Stomach is a muscular elastic bag.

Volume of the empty stomach is only 50 mL,

but after a meal, its volume expands to about

1.5 L, SO stomach acts as a storage container

or food reservoir.

Food enters the stomach through the lower

esophageal sphincter (LES) and leaves it

through the pyloric sphincter.

The wall of the stomach has a number of folds

called gastric rugae which help the stomach

to expands during a large meal.

The inner surface of the stomach wall is lined

by a mucous membrane known as the gastric

mucosa. 21

The gastric mucosa contains gastric glands which secrete gastric juice.

These glands contain 4 types of secretory cells.

1- Mucus cells : Secrete mucus.

2- Parietal cells : Secrete hydrochloric acid (HCl) and intrinsic factor.

3- Chief cells : Secrete pepsinogen.

4- G-cells : Secrete gastrin hormone.

22

1- Mucus cells:

Mucus cells secrete mucus.

Mucus is a viscous alkaline fluid that covers the gastric mucosa.

Mucus is made up of a protein (mucin) and glycoproteins.

Functions of mucus:

It forms a coat over the gastric mucosa (about 1mm thick) to protects stomach

against the effect of HCl and enzymes SO, stomach wall will not be digested by

the acid or enzymes.

Mucus contains some bicarbonates, which helps to neutralize the stomach acid

over the gastric mucosa.

Mucus helps to lubricate food in the stomach.

23

2- Parietal cells:

Parietal cells secrete HCl and intrinsic factor.

a- HCl is responsible for the high acidity of stomach content (pH 1.5 – 2.0) at which

gastric enzymes work best.

Functions of HCl:

Denatures proteins SO inactivates salivary amylase and most enzymes in food.

Kills most of the microorganisms ingested with food.

Activates pepsinogen that secreted by chief cells into pepsin (proteolytic enzyme)

which starts protein digestion.

Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) that can be absorbed

and used for Hb synthesis.

Q- Carbohydrate digestion stops at the stomach level ….Why?

Salivary amylase which digest carbohydrates in the mouth at pH 6.7; transports

with the food bolus into stomach at which pH is 1.5 - 2.0 (due to HCl).

HCl of the stomach denatures salivary amylase enzyme (because salivary

amylase is a protein) so inhibits its activity.

24

Function of intrinsic factor:

It is essential for the absorption of vitamin B12 from the ileum in the small

intestine.

NOTE:

Vitamin B12 is necessary for the normal function of nerve cells and for the

formation of red blood cells.

Q: Which disease results due to a lack of intrinsic factor OR Vitamin B12?

A: Pernicious anemia.

25

3- Chief cells:

Chief cells secrete pepsinogen; an inactive proenzyme.

HCl activates pepsinogen into pepsin which is active proteolytic enzyme.

Pepsin functions most effectively at a strongly acidic pH of 1.5–2.0.

Chief cells in the stomach of newborn infants (but not of adults) secrete rennin

and gastric lipase enzymes which are important for the digestion of milk.

Rennin curdles milk proteins into casein to be digested by pepsin, while gastric

lipase initiates the digestion of milk fats.

Function of Pepsin:

Pepsin digests dietary proteins (mostly found in meat, fish, eggs and dairy

products) to a shorter peptide chain that will be completely digested in the small

intestine to amino acids.

26

4- G cells:

G cells secrete gastrin hormone.

Secretion of gastrin is stimulated by the distension of stomach with food. Caffeine is also

a stimulant.

Gastrin is carried by the blood stream to increase the stomach motility and to stimulate the

parietal cells to secrete HCl.

Gastrin secretion is inhibited by:

o Excess stomach acidity.

o Cholecystokinin (CCK).

Function of gastrin:

Increase the stomach motility.

Stimulates the production of HCl in stomach.

27

The gastric juice:

The gastric juice is a colorless, watery, acidic, digestive fluid secreted by the stomach

glands.

It has a pH ranging from 1.5 to 2.0 which is essential in activating many digestive

enzymes and in destroying most pathogens.

The gastric juice is made up of mucus, HCl, pepsinogen, pepsin and intrinsic factor. The

gastric juice also contains bicarbonates to neutralize the gastric acidity.

The gastric juice converts the semisolid bolus into a semifluid paste called chyme.

Chyme is semifluid paste consists of partially digested food and gastric juice.

28

Secretion of gastric juice occurs in 3 phases:

1- The cephalic phase:

It is the stage in which the stomach responds to sight, smell or taste of food.

About 30% of total gastric secretion occurs BEFORE food enters the stomach.

Sight, smell or taste of food stimulates the dorsal nucleus of the vagus nerve in

the medulla oblongata.

Vagus nerve (parasympathetic nerve) stimulate the gastric secretion.

2- The gastric phase:

It is the stage that begins with the arrival of the bolus into the stomach.

50-60% of total gastric secretion occurs DURING this phase.

Distension of stomach with food :

a- stimulates the dorsal nucleus of the vagus nerve in the medulla oblongata ……

b- stimulates the secretion of gastrin hormone from G-cells which, in turn, stimulates

the production of HCl in stomach.

29

3- The intestinal phase:

It is the stage that begins with the arrival of the chyme into the small intestine

Presence of chyme in the small intestine:

a- inhibits the dorsal nucleus of the vagus nerve, thus reduces vagal stimulation of

the stomach. SO reduces the gastric secretion.

b- stimulates duodenal enteroendocrine cells to release secretin (inhibits the chief

and parietal cells) and cholecystokinin (inhibits G-cells) thus reduces gastric

secretion.

NOTES:

The optimum pH for digestion in the Stomach is 1.5 - 2.0

Acid secretion is lowest in the morning before awakening and highest at night.

High acidity of the gastric juice may cause damage to the stomach wall, causing

gastric (peptic) ulcer.

Infection of the gastric mucosa with Helicobacter pylori are responsible for at

least 80% of peptic ulcers. So, treatment of gastric ulcers is commonly involves

the administration of antibiotic drugs.

30

o Stomach has 3 regions:

1. Fundus 2. Body 3. Pylorus

• Food bolus moves slowly from the fundus to the pylorus in 3- 4 h.

• During this period, food boluses are subjected to:

1- Mechanical digestion: By stomach contractions

• Contractions and relaxations of stomach help to churn the food bolus and mix it

with the gastric juice.

• Churning action of the stomach which occurs every 20 seconds converts the

semisolid bolus into a semifluid paste called chyme.

• The pyloric sphincter, keeps the food in the stomach until complete churning and

mixing with the gastric juice.

31

2- Chemical digestion: By stomach secretions.

Pepsin digest dietary proteins to a shorter peptide chain.

Once the gastric juice do its job, the pyloric sphincter opens and the chyme

passes gradually into the small intestine.

o Functions of the Stomach

1- The bulk storage of ingested food

2- Partial digestion of the food :

Pepsin: digest proteins to a shorter peptide chain.

3- Production of the intrinsic factor, that is required for the absorption of vitamin

B-12 in the small intestine.

32


references

• Books:








35

THE DIGESTIVE SYSTEM

PHYSIOLOGY-2

PHL226

36

3- Small Intestine to Large Intestine




Small Intestine (Bowel)

The small intestine is a long tube (6 m long), SO it fills most

of the abdominal cavity.

The small intestine is called small because the diameter or

the width of the tube is much less than the large intestine.

The small intestine plays the most important role in the

digestion and absorption of nutrients.

o The digestive process in the small intestine is completed

by bile, pancreatic juice and intestinal juice.

o 90% of nutrient absorption occurs in the small intestine,

and the other 10% occurs in the stomach and large

intestine.

The small intestine consists of 3 segments:

1- The duodenum (25 cm long).

2- The jejunum (1.5 m long).

3- The ileum (3.5 m long).

Each segment performs an important role in digestion and/ or

absorption.

37

The surface area of the small intestine is increased in

three ways.

1- Circular Folds (the plicae circulares):

The intestinal mucosa is not smooth, but arranged into

circular folds.

These folds occur from the duodenum to the middle of

the ileum.

The circular folds increases the surface area and force

the chyme forward in spiral movement causing more

contact with the intestinal mucosa SO helps in mixing

the chyme with the digestive juice.

2- Villi:

The intestinal mucosa forms finger like projections

(1 mm in length) called villi that are covered with the

epithelial cells.

The wall of villi is very thin; it is only one cell thick.

The deep cavities between villi are called intestinal

crypts (crypts of Lieberkühn).

The intestinal crypts contain the intestinal glands that

secretes the intestinal enzymes.38

3- Microvilli (brush border):

The epithelial cells which line villi forms little hair-like

projections called microvilli which gives brush-like

appearance.

The intestinal enzymes are embedded within the plasma

membrane of the microvilli SO, they are called brush

border enzymes.

NOTE:

The plicae circulares, villi and microvilli increase the

surface area SO increase the rate of digestion and

absorption.

39

The epithelial cells covering villi are of 2 types:

1- Absorptive epithelial cells (Enterocytes):

Enterocytes synthesize the intestinal digestive

enzymes which called brush border enzymes.

They transport the digested nutrient from the lumen

of the intestine to the circulatory system.

NOTE:

The chyme must contact the brush border for

digestion to occur.

2- Goblet cells:

Goblet cells secrete mucus that protects the

epithelial cells against digestion.

40

The process of digestion:

The duodenum is the shortest part of the small intestine

(25 cm long) in which most of the digestion of food

substances occur.

It receives the chyme from the stomach and digestive

secretions from the liver (gallbladder), the pancreas, and

from the intestinal glands.

The presence of fatty chyme in the duodenum, stimulates

the enteroendocrine cells to release cholecystokinin

hormone (CCK) into the bloodstream.

The presence of acid chyme in the duodenum, stimulates

the enteroendocrine cells to release secretin hormone and

gastric inhibitory peptide (GIP).

NOTE:

Enteroendocrine cells locate in the lower portion of the

intestinal crypts.

41

The main functions of cholecystokinin (CCK):

CCK inhibits the gastric motility indirectly; by inhibiting G-cells that produce gastrin.

It stimulates the liver to produce bile.

It stimulates the contraction of gallbladder and relaxes the sphincter of Oddi so, bile

releases into the duodenum.

It stimulates the exocrine regions of the pancreas to secret pancreatic enzymes.

The main functions of secretin:

Secretin inhibits both the chief and parietal cells of the stomach.

It stimulates the pancreatic ducts to secrete sodium bicarbonate which neutralizes the

acidic chyme in the duodenum.

It stimulates Brunner's glands (found only in the duodenum) to secrete alkaline mucus

that neutralizes the acidic chyme in the small intestine.

The main functions of gastric inhibitory peptide (GIP):

GIP has the opposite effects of gastrin.

It inhibits the gastric motility.

It inhibits the gastric acid secretion.

It stimulates the release of insulin in response to infusions of glucose. 42

Digestive juices in the SI:

I. Bile II. Pancreatic secretions III. Intestinal secretions

I. Bile:

Bile is a digestive juice that is secreted by the liver cells

(hepatocytes) and stored in the gallbladder.

The liver secretes 600 – 1000 mL of bile in a day.

It is golden yellow (or greenish) in color due to the presence of

bile pigments (bilirubin).

Bile is secreted by the liver continuously, but released into the

duodenum only under the stimulation of CCK.

Secretion of bile into the duodenum occurs according to the

amount of fat in the food.

Bile composition:

Water.

Bile salts (derivatives of cholic acid as sodium cholate).

Bile pigment (bilirubin).

Phospholipids and cholesterol.

Electrolytes as sodium, potassium, calcium, chlorine and

bicarbonate ions. 43

Functions of the bile:

1- Emulsification of lipids (fats):

In the duodenum, the bile salts act to separate thelarge fat globules into smaller droplets calledmicelles through a process called emulsification.

Emulsification is the process by which large globulesof fats are broken down into smaller droplets calledmicelles.

Micelles are water soluble droplets so they can mixwith water and digested by pancreatic juice.

2- The bile elevates the pH of the duodenal contents, toprovide an ideal neutral or slightly alkaline environmentfor the digestive enzymes in the small intestine.

3- The bile help the absorption of vitamin K from thediet.

44

NOTES:

Bile does not contain digestive enzymes but contain bile salts that have a digestive

function.

Bilirubin (bile pigment) is a metabolic breakdown product of hemoglobin that give bile

its characteristic greenish color. Bilirubin is further metabolized by bacteria in the colon

into stercobilin, which give the feces its characteristic brown color.

45

II. Pancreatic secretions:

At the same time that bile is released by the gall bladder, pancreatic

juices are secreted by the pancreas into the duodenum.

The pancreas is a large gland located below the stomach that

secretes pancreatic juice into the duodenum via the pancreatic duct.

The pancreas has 2 main functions :

A- Endocrine functions that regulates blood sugar.

B- Exocrine functions that helps in digestion.

A- Endocrine functions: include hormone secretion

1- Insulin: (50– 80%)

It is a hormone that is produced by beta cells that are locate in the

islets of Langerhans of the pancreas.

High blood glucose level stimulates beta cells to release insulin

Insulin decreases blood glucose level by stimulating the cells in the

liver and skeletal muscles to take up glucose from the blood.

In the liver and skeletal muscles, glucose is stored as glycogen.

46

2- Glucagon: (15 – 20%).

It is a hormone that is produced by pancreatic alpha cells.

Low blood glucose level stimulates alpha cells to release

glucagon.

Glucagon causes the liver to convert stored glycogen into

glucose, which is released into the bloodstream, so raises blood

glucose level

NOTE:

Effect of glucagon is opposite to that of insulin.

3- Somatostatin: (3– 10%).

It is a hormone that is produced by pancreatic delta cells.

Somatostatin suppresses the release of gastrointestinal hormones

(Gastrin, CCK, Secretin, GIP).

It suppresses the release of pancreatic hormones (insulin,

glucagon).

It suppresses the exocrine secretory action of pancreas.

47

B- Exocrine functions:

The exocrine functions of the pancreas include secretion of digestive juice called

pancreatic juice

CCK and secretin are digestive hormones that stimulates the pancreas to release the

pancreatic juice.

Components of the pancreatic juice :

1- Sodium bicarbonate 2- Pancreatic enzymes 3- Zymogens

1- Sodium bicarbonate:

Sodium bicarbonate neutralizes the acidic chyme entering the duodenum and elevates

the duodenal pH to about 7.4 - 7.8 which is the optimum pH for the pancreatic and the

intestinal enzymes.

The neutralization is important because the enzymes in the small intestine need a neutral

environment or slightly alkaline pH.

48

2- Pancreatic enzymes:

a- Pancreatic amylase :

All carbohydrates in the small intestine must be hydrolyzed to monosaccharides prior to

absorption.

Amylase enzyme hydrolyzes starch to the disaccharide; maltose.

Carbohydrates OR Starch𝑝𝑎𝑛𝑐𝑟𝑒𝑎𝑡𝑖𝑐 𝑎𝑚𝑦𝑙𝑎𝑠𝑒

maltose

b- Lipase enzyme:

It helps in the digestion of fats/ lipids into fatty acids and glycerol, which can be easily

absorbed by the intestinal surface.

Fats𝐿𝑖𝑝𝑎𝑠𝑒

Fatty acids + glycerol

49

c- Zymogens: Trypsinogen and Chymotrypsinogen.

Zymogens are digestive enzymes that synthesized inside the exocrine cells of the

pancreas and released into the duodenum in inactive forms (so called proenzymes).

If they are synthesized in an active form, they will digest the internal protein structures

of the exocrine cells (auto-digestion).

i. Trypsinogen is secreted from the pancreas in an inactive form, and then activated by the

intestinal enzyme enterokinase (enteropeptidase) into trypsin.

ii. Chymotrypsinogen is secreted from the pancreas in an inactive form, and then activated

by trypsin into chymotrypsin.

Trypsin and chymotrypsin are proteolytic enzymes (proteases). They are known as

endopeptidases. They are responsible for the initial breakdown of polypeptides into short

peptide chains (dipeptides).

The dipeptides are then digested to individual amino acids by carboxypeptidase enzyme

from the pancreas and aminopeptidase enzyme from the intestinal epithelium.

NOTE:

Trypsin and chymotrypsin are much more powerful than pepsins, so the greater part of

protein digestion occurs in the duodenum and upper jejunum.

50

III. Intestinal secretions:

The intestinal glands in the intestinal crypts secrete 1 to 2 L of intestinal juice per day in

response to acid and distension of the intestine.

The intestinal secretions contains:

a- Bicarbonate:

Cells in the duodenum produce large amounts of bicarbonate to completely neutralize any

gastric acid that passes down into the digestive tract.

b- Mucus:

Brunner's glands in the duodenum secrete alkaline mucus containing bicarbonate (in

response to secretin).

o This alkaline mucus in combination with bicarbonate from the pancreas and bile,

neutralizes the gastric acid in the incoming chyme.

Goblet cells in the duodenal mucosa also produce mucus.

o This mucus lubricates the intestinal contents and protects the duodenal wall against

the digestive juices.

51

c- Intestinal enzymes (brush border enzymes):

Enterokinase - activates trypsinogen into trypsin.

Aminopeptidase - hydrolyzes dipeptides into amino acids.

Enzymes that hydrolyzes disaccharides into monosaccharides are:

maltase hydrolyzes maltose into glucose.

sucrase hydrolyzes sucrose (common table sugar) into glucose and fructose.

lactase hydrolyzes lactose (milk sugar) into glucose and galactose.

NOTE:

Lactose becomes indigestible after age 4 in many humans due to decline in lactase

production (lactose intolerance).

52

NOTES:

Enzymes released into the duodenum are still active in the jejunum, and can digest

carbohydrates, fats and protein.

Digestion of starch starts in the mouth by salivary amylase and completed in the small

intestine by pancreatic amylase.

Digestion of protein starts in the stomach by pepsin and ends in the small intestine by

trypsin, chymotrypsin, carboxypeptidase and aminopeptidase enzymes.

Digestion of fat starts in the small intestine and ends in the small intestine by lipase

enzyme.

All proteolytic enzymes are secreted in an inactive form, to prevent auto-digestion, and

are activated in the lumen of the gut:

o by HCl in case of the stomach pepsin.

o by enterokinase (enteropeptidase) in case of trypsin.

o by trypsin in case of chymotrypsin.

Minerals, vitamins, and cholesterol are not broken down and are absorbed unchanged.

Peyer's patches are clusters of lymphatic nodules found in the ileum. They represent a

part of the gastrointestinal immune system that destroys the pathogenic microorganisms

in the GIT.

53

Absorption

Absorption is the movement of the digested material from the

GIT into the bloodstream.

Each villus contains both blood capillaries and a lacteal

(lymph vessel) for the absorption of nutrients.

The blood capillaries absorbs glucose, fructose, galactose,

amino acids in addition to vitamins, minerals and transport

them to the liver via the hepatic portal vein.

The lacteals absorbs the products of fat digestion (fatty acids

and glycerol) into the lymphatic system then to the

bloodstream.

54

The majority of nutrient absorption occurs in the duodenum

and jejunum.

The duodenum absorbs carbohydrates, proteins, and minerals

including calcium and iron.

The jejunum absorbs remaining carbohydrates and proteins

passed from the duodenum in addition to vitamin C,

vitamin B-2, B-6 and folic acid.

The ileum absorbs vitamine B12, bile salts, fat, cholesterol

and fat-soluble vitamins (A, D, E and K).

Vitamin B12 and bile salts are absorbed in the terminal ileum.

Removal of this portion of the ileum will necessitate

supplementary vitamin B12 doses for rest of the life (by

injection).

Water (90-95% of the daily fluid intake) is absorbed through

the small intestine.

55

Function of the small intestine:

1- Digestion:

Carbohydrates into monosaccharides.

Proteins into amino acids.

Fats into fatty acids and glycerol.

2- Absorption of monosaccharides, amino acids, fatty acids, glycerol as well as water,

vitamins and electrolytes into the bloodstream.

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In the liver:

1. The liver processes nutrients by filtering out any harmfulsubstances or wastes before the nutrients can be carried inthe blood to the rest of the body.

The processed nutrients are used by different organs:

a. To build the cells and tissues.

b. To provide energy.

2. Blood glucose levels are regulated.

Excess glucose is stored in the liver in the form ofglycogen in response to the hormone insulin.

When blood glucose level begin to drop, (eg. betweenmeals), the glycogen is re-converted to glucose inresponse to the hormone glucagon.

At the end of the SI, the non digested chyme (fibers, 3-5% of ingested protein and water) passes through the ileocecal valveto the large intestine.

Ileocecal valve is a sphincter that controls the flow of materials from the ileum into the cecum of the large intestine and prevent back flow of fecal contents from the large intestine into small intestine.

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The large intestine (Colon)

The large intestine is a muscular tube that connects the

small intestine to the rectum.

It forms the last section of the digestive tract, which is

about 1.5 m long.

The large intestine performs the vital function of

converting food into feces.

There is no secretion of digestive enzymes and any

digestion that takes place in the colon is carried out by

microbes.

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The LI is composed of 4 segments:

1- Cecum 2- Colon

3- Rectum 4- Anal canal (ends with anus).

The appendix is a small tube attached to the cecum.

The appendix contain a lymphoid tissue that is filled with

lymphocytes, but its removal has no any negative effect on

the immune system.

The cecum leads to the colon before entering the rectum.

The colon is consists of 4 sections:

1- Ascending colon (right).

2- Transverse colon.

3- Descending colon (left).

4- Sigmoid colon.

The colon is the major reservoir for intestinal bacteria.

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There are trillions of microbial population in the LI called the

microflora.

Importance of microflora:

1- Digestion of substances in the chyme that are not digestible by the SI

(bacterial fermentation).

NOTE:

Gases such as CO2 and methane are produced as a byproduct of

bacterial fermentation that lead to flatulence (passing of gases

through the anus).

2- Synthesis of some vitamins:

a- Vitamin K:

It is a fat-soluble vitamin that the liver needs in order to synthesize 4

clotting factors, including prothrombin.

Intestinal bacteria produces about half of our daily vitamin K

requirements.

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b- Vitamin B12:

It is a water-soluble vitamin that is necessary for the normal function of nerve cells in the

body and for the formation of red blood cells.

c- Biotin:

It is a water-soluble vitamin that is important to various reactions, especially those of

glucose metabolism.

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Functions of the LI:

1- Little absorption occurs in the large intestine

it absorbs most of the remaining water. The colon absorbs on average 1–1.5 liters of fluid every day.

it absorbs electrolytes, particularly sodium and potassium.

it absorbs vitamins produced by bacteria.

2- Bacterial digestion:

The large population of bacteria digest fiber content of the chyme through the process of fermentation to produce short chain fatty acids.

3- Storage of fecal material (stool) until defecation .

NOTES:

The stool itself is mostly food debris and bacteria.

Normal stool weight is approximately 250 g daily, of which 10 to 20% is bacteria. It contains indigestible fiber, water, electrolytes, and small amounts of protein and fat.

Q- What occurs when too much water is removed from stools in the large intestine?

A. Constipation

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The Digestive System - psau.edu.sa · which starts protein digestion. Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) that can be absorbed and used for Hb synthesis. Q-