2. Digestive System

8/2/2019 2. Digestive System

1/38

DIGESTIVE SYSTEM

INTRODUCTION

A. Food contains substances and energy the body needs to constructall cell components. The food must be broken down through

digestion to molecular size before it can be absorbed by the

digestive system and used by the cells.

B. The organs that collectively perform these functions compose thedigestive system.

C. The medical professions that study the structures, functions anddisorders of the digestive tract are gastroenterologyfor the upper

end of the system andproctologyfor the lower end.

OVERVIEW OF THE DIGESTIVE SYSTEM

A. Organization

1.The two major sections of the digestive system perform theprocesses required to prepare food for use in the body (Fig.

24.1).

2.The gastrointestinal tract is the tube open at both ends for thetransit of food during processing. The functional segments of

the GI tract include the mouth, esophagus, stomach, small

intestine and large intestine.


2/38

3.The accessory structures that contribute to the food processinginclude the teeth, tongue, salivary glands, liver, gallbladder and

pancreas.


3/38

B. Digestion includes six basic processes1. Ingestion is taking food into the mouth (eating).2. Secretion is the release by cells within the walls of the GI

tract and accessory organs, of water, acid, buffers and

enzymes into the lumen of the tract.

3. Mixing and propulsion result from the alternatingcontraction and relaxation of the smooth muscles within the

walls of the GI tract.

4. Digestiona) Mechanical digestion consists of movements of the GI

tract that aid chemical digestion.

b) Chemical digestion is a series of catabolic (hydrolysis)reactions that break down large carbohydrate, lipid

and protein food molecules into smaller molecules that

are usable by body cells.

5. Absorption is the passage of end products of digestion fromthe GI tract into blood or lymph for distribution to cells.

6. Defecation is emptying of the rectum, eliminatingindigestible substances from the GI tract.


4/38

LAYERS OF THE GI TRACT

A. The basic arrangement of layers in the gastrointestinal tract fromthe inside outward includes the mucosa, submucosa, muscularis

and serosa (visceral peritoneum) [Fig. 24.2].

B. The mucosa consists of an epithelium, lamina propria andmuscularis mucosa.

1. The epithelium consists of a protective layer ofnonkeratinized stratified cells, simple cells for secretion and

absorption , and mucus secreting cells, as well as some


5/38

enteroendocrine cells that put out hormones that help

regulate the digestive process.

2. The lamina propria consists of three components, includingloose connective tissue that adheres the epithelium to the

lower layers, the system of blood and lymph vessels through

which absorbed food is transported, and nerves and sensors.

a) The lymph system is part of the mucosa-associatedlymph tissues (MALT) that monitor and produce an

immune response to pathogens passing with food

through the GI tract.

b) It is estimated that there are as many immune cellsassociated with the GI tract as in all the rest of the

body.

3. The muscularis mucosa causes local folding of the mucosallayer to increase surface area for digestion and absorption.

C. The submucosa consists of aerolar connective tissue. It is highly

vascular, contains a part of the submucosal plexus (plexus of

Meissner), and contains glands and lymphatic tissue.

1. The submucosal plexus is a part of the autonomic nervoussystem.


6/38

D. Muscularis

1. The muscularis of the mouth, pharynx and superior part ofthe esophagus contains skeletal muscle that produces

voluntary swallowing. Skeletal muscle also forms the

external anal sphincter.

2. Through the rest of the tract, the muscularis consists ofsmooth muscle in an inner sheet of circular fibers and an

outer sheet of longitudinal fibers.

3. The muscularis also contains the major nerve supply to theGI tract the myenteric plexus (plexus of Auerbach) which

consists of fibers from both divisions of the ANS. This plexus

mostly controls GI tract motility.

E. The serosa is the superficial layer of those portions of the GI tract

that are suspended in the abdominopelvic cavity.

1. The esophagus is covered by an adventitia.2. Inferior to the diaphgram, the serosa is also called the

visceral peritoneum.

ESOPHAGUS

A. The esophagus is a collapsible, muscular tube that lies behind thetrachea and connects the pharynx to the stomach (Fig. 24.1).


7/38


8/38

B. The wall of the esophagus contains mucosa, submucosa andmuscularis layers. The outer layer is called the adventitia rather

than the serosa due to structural differences (Fig. 24.9).

C. Physiology of the Esophagusa) The esophagus contains an upper and a lower esophageal

sphincter.

b) During the esophageal stage of swallowing progressivecontractions of the muscularis push the bolus onward. There

propulsive contractions are termed peristalsis (Fig. 24.10).


9/38

D. Gastroesophageal reflux disease occurs when the loweresophageal sphincter fails to close adequately after food has

entered the stomach, resulting in stomach contents refluxing into

the inferior portion of the esophagus. HCl from the stomachcontents irritates the esophageal wall resulting in heartburn.

STOMACH

A. INTRODUCTION

1. The stomach is a J-shaped enlargement of the GI tract thatbegins at the bottom of the esophagus and ends at thepyloric sphincter (Fig. 24.11).

2. It serves as a mixing and holding area for food, beginsdigestion of proteins, and continues the digestion of

triglycerides, converting a bolus to a liquid called chyme. It

can also absorb some substances.

B. Anatomy of the Stomach1. Include the cardia,fundus, bodyandpyloris.2. When stomach is empty, the mucosa lies in folds.3. Pylorospasm andpyloric stenosis?


10/38

C. Histology of the Stomach1. Mucosa mucosa surface cells (Fig. 24.12).

- gastric pits and gastric glands

- gastric glands: 3 types ofexocrine glands :


11/38

- mucous neck cells (secrete mucus)

- chiefor ezymogenic cells (secrete

pepsinogen and gastric lipase)

-parietalor oxyntic cells (secrete HCl)Enteroendocrine cells G-cells

(secrete gastrin)

2. Submucosa3. Muscularis 3 layers of smooth muscle; longitudinal,

circular,oblique

4. Serosa


12/38

D. Mechanical and Chemical Digestion in the Stomach1. Mechanical digestion: peristaltic movement and mixing

waves.

2. Chemical Digestion- pepsin, HCl- gastric lipase (limited role in adult stomach)

3. The stomach is impermeable to most substances except

aspirin, alcohol , some electrolytes and water.

E. Regulation of Gastric Secretion and Motility1. Gastric secretion regulated by nerves and hormones (Fig.

24.13).

- Stimulation occurs in three overlapping phases:cephalic (reflex), gastric and intestinal .

2. Cephalic Phasea) consists of reflexes initiated by sensory receptors in

the head.

b) stimulates gastric secretion and motility.3. Gastric phase

a) Begins when food enters the stomach .b) Activation of stretch receptors and chemoreceptors

when stomach walls are distended, pH increases or

proteins entered the stomach Fig. 24.14).

- Peristalsis, continued flow of gastric juice.


13/38


14/38


15/38

c) Hormonal negative feedback regulates gastricsecretions.

1. Chemoreceptors and stretch receptors stimuli

the ANS to release ACh which stimulates G-cells

to release gastrin.

2. Gastrin stimulates growth of the gastri glands

and secretion of large amounts of gastric juice.

- Strengthens contraction of the loweresophageal sphincter

- Increases motility of the stomach- Relaxes the pyloric and ileocecal sphincters.

3. ACh and gastrin stimulate parietal cells to secrete

more HCl when histamine is present.

4. Intestinal Phase

a) The intestinal phase is due to activation ofreceptors in the small intestine.

b) Partially digested food in the small intestine triggers the enterogastric reflex

- secretion of GIP, secretin, CCK by the

intestinal mucosa. The effect;

inhibition of gastric secretion.


16/38

5. Regulation of Gastric Emptying

a) Gastric emptying is the periodic release ofchime from the stomach into the

duodenum Fig. 24.15).

b) is stimulated by nerves and gastrin.c) most food leaves the stomach 2-6 hours

after ingestion; CHO>protein>fats

d) inhibited by the enterogastric reflex andCCK, GIP

e) Vomiting


17/38


18/38

PANCREAS

A. Connected to the duodenum via the duct of Wirsung (pancreaticduct) and accessory duct (duct of Santorini) [Fig. 24.16].

B. Pancreatic islets (islets of Langerhans) secrete hormones and acini

secrete a mixture of fluid and digestive enzymes calledpancreatic

juice (Fig. 24.17).


19/38


20/38

C. Pancreatic juice

1. contains enzymes that digest starch (amylase), proteins

(trypsin, chymotrypsin carboxypeptidase), fats (pancreatic

lipase), and nucleic acids (ribonuclease, deoxyribonuclease).

2. contains sodium bicarbonate, return pH to 7.1-8.2, inhibiting

pepsin activity, promoting pancreating enzymes activity.

3. Pancreatitis trypsin digests pancreatic cells.

D. Pancreatic secretion is regulated by nervous and hormonal

mechanisms.

LIVER AND GALLBLADDER

A. Heaviest gland and second largest organ in the body after the

skin.

B. Gallbladder, a sac in a depression on the posterior surface of the

liver.

C. Lobes made up of lobules contain hepatic cells (hepatocytes),

sinusoids, stellate reticuloendothelial (Kupffers) cells and a

central vein.

D. Liver receives a double supply of blood from the hepatic artery

and the hepatic portal vein. Leaves via the hepatic vein.


21/38

E. Hepatocytes produce bile transported by a duct system to the

gallbladder for concentration and temporary storage.

1. Bile is partially an excretory product and partially a digestive

secretion.

2. Bile emulsifies triglycerides.

F. Bile secretion is regulated by nervous and hormonal mechanisms,

volume of hepatic blood flow and the concentration of bile salts

in the blood (Fig. 24.20).

G. The liver also functions in CHO, lipid and protein metabolism;

removal of drugs and hormones In blood; excretion of bilirubin;

synthesis of bile salts; storage of vitamins and minerals; and

activation of vitamin D.

H. Gallstones

the fusion of individual crystals of cholesterol.

- cause obstruction in any portion of the duct system.


22/38


23/38

3. CCK stimulates secretion of pancreatic juice rich in digestive

enzymes.

- stimulates ejection of bile into the duodenum (Fig. 24.21).

4. Other hormones secreted by and having effects on the GIT:

motilin, substance P, bombesin, vasoactive intestinal peptide

(VIP), gastrin releasing peptide, somatostatin and GIP.

SMALL INTESTINE

The major events of digestion and absorption occur in the small

intestine (SI).

The SI extends from the pyloric sphincter to the ileocecal sphincter.

The SI is divided into : duodenum, jejunum, ileum.

Projections called circular folds orplicae circularies enhance absorption

by increasing surface area (Fig. 24.22).

A. Histology of the Small Intestine

1. Mucosa forms villi(Fig. 24.22a).

a) embedded in the villus is a lacteal(lymphatic capillary)

for fat absorption.

b) cells of the mucosal epithelium include: absorptivecells, goblet cells, enteroendocrine cells, Paneth cells

(Fig. 24.22b).


24/38


25/38

c) microvilli, form brush border containing several

enzymes (Fig. 24.22c).

d) mucosa contains the intestinal glands (crypts of

Lieberkuhn).

2. Submucosa

- contains duodenal(Brunners) glands which secrete an

alkaline mucus.

- the submucosa of the ileum contains lymphatic

nodules (Peyers patches).


26/38

B. Mechanical Digestion in the Small Intestine

1. Segmentation ia localized contraction.

2. Peristalsis propels the chyme onward.


27/38

C. Chemical Digestion in the Small Intestine.

1. CHO are broken down into monosaccharides for absorption.

a) Intestinal enzymes break down:

- Starches into maltose, maltotriose and alpha-

dextrin (pancreatic amylase).

- Alpha-dextrins into glucose (alphadestrinase).

- Maltose to glucose (maltase).

- Sucrose to glucose and fructose (sucrose).

- Lactose to glucose and galactose (lactase).

b) Lactose intolerance failure of the intestinal mucosal

cells to produce lactase, the inability to digest the

sugar lactose in milk.

2. Protein digestion starts in the stomach.

a) Proteins are converted to peptides by trypsin and

chymotrypsin.

b) Peptides to amino acids by aminopeptidases.

3. Most lipid digestion in an adult occurs in the small intestine.

a) Emulsification bile salts break the globules of

triglycerides (fats) into droplets.


28/38

b) Pancreatic lipase hydrolyze triglycerides into fatty

acids and monoglycerides.

4. Nucleic acids are broken down into nucleotides for

absorption.

D. Regulation of Intestinal Secretion and Motility

1. The most important mechanism is the action of local

reflexes in response to chime.

2. Hormones VIP also assume a role.

3. Parasympathetic impulses increase motility.

Sympathetic impulses decrease motility.

E. Absorption in the Small Intestine.

1. Absorption is the passage of the end products of digestion

from the GIT into blood or lymph and occurs by diffusion,

facilitated diffusion, osmosis and active transport.

2. Absorption of Monosaccharides

a) Essentially all CHO are absorbed as monosaccharides

into blood capillaries (Fig. 24.24).

3. Absorption of Amino Acids, Dipeptides and Tripeptides.


29/38

Most proteins are absorbed as amino acids by active

transport processes into blood capillaries in the villus.

4. Absprption of Lipids

a) Dietary lipids are absorbed by simple diffusion.

b) Long-chain fatty acids and monoglycerides are

absorbed as part of micelles, resynthesized to

triglycerides and formed into protein-coated spherical

mass called chylomicron.

- Chylomicrons are taken up by the lacteal of avillus.

- From the lacteal chylomicrons enter thelymphatic system to CVS, finally reaching the

liver or adipose tissue.

c) The plasma lipids; fatty acids, triglycerides, cholesterolare insoluble in water and body fluids.

- the lipids combined with protein transporter

called lipoproteins to make them soluble.


30/38


31/38


32/38

5. Absorption of Electrolytes

a. Many absorbed electrolytes by the SI comes from

gastrointestinal secretions and some are part of

digested foods and liquids.

b. Absorption is primarily by active transport.

6. Absorption of Vitamins

a. Fat - soluble vitamins (A,D, E and K) are included along

with ingested dietary lipids in micelles and are

absorbed by simple diffusion.

b. Water soluble vitamins (B and C) are absorbed by

simple diffusion.

7. Absorption of Water (Fig. 24.25)

a. All water absorption in the GIT occurs by osmosis.

b. Absorption depends on electrolytes and nutrients to

maintain an osmotic balance with the blood.


33/38


34/38

LARGE INTESTINE

The large intestine (colon) extends from the ileocecal sphincter to the

anus.

The subdivisions : cecum, colon, rectum and anal canal(Fig. 24.26).

Inferior to the cecum is the appendix.

- Appendicitis (inflammation of the appendix).- A ruptured appendix results in gangrene or

peritonitis.

The colon is divided into the ascending, transverse, descending and

sigmoid portions.


35/38

A. Histology of the Large Intestine (LI)

1. Mucosa no villi or circular folds, present of globlet cells (Fig.

24.27).


36/38


37/38

2. Muscularis contains taeniae coli which contract and gather

the colon into a series of pouches called haustra.

B. Mechanical movements of the LI include haustral churning,

peristalsis and mass peristalsis.

C. The last stages of chemical digestion through bacterial rather than

enzymatic action. Some vitamins are synthesized by bacterial

action.

D. Absorption and Feces Formation in the Large Intestine

1. The LI absobs water, electrolytes and some vitamins.

2. Feces

3. The LI absorbs water to maintain the bodys water balance

although most water absorption occurs in the SI.

E. Defecation Reflex

1. Defecation.

- is a reflex action aided by voluntary contractions of the

diaphragm and abdominal muscles.

- the external anal sphincter is controlled voluntarily.

2. Diarrhea


38/38

3. Constipation

4. Dietary fiber : soluble (lower blood cholesterol) and

insoluble (protect colon cancer).

DISORDERS: HOMEOSTATIC IMBALANCES

1. Peptic ulcers

2. Diverticular diverticulitis

3. Tumors- colorectal cancer

4. Hepatitis A does not cause lasting liver damage)

Hepatitis B , C, D (cirrhosis, cancer, liver damage)

Hepatitis E (high mortality rate in pregnant women).

5. Anorexia nervosa (predominantly in young single females, may be

inherited, may die of starvation!).

2. Digestive System

Documents