Lecture Notes: Chapter 24

THE DIGESTIVE SYSTEM

 

I. Overview of the digestive system

A. Digestion includes six basic processes

1. 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 alternating contraction and relaxation of the

smooth muscles within the walls of the GI tract.

4. Digestion

a. 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

carbohydrates, lipid, and protein food molecules into smaller molecules that are usable by

body cells.

5. Absorption is the passage of end products of digestion from the GI tract into blood or

lymph for distribution to cells.

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

II. Layers of the GI tract

A. The basic arrangement of layers in the gastrointestinal tract from the inside outward include the

mucosa, submucosa, muscularis, and serosa (visceral peritoneum).

B. The mucosa consists of an epithelium, lamina propria, and muscularis mucosa.

C. The submucosa consists of areolar connective tissue. It is highly vascular, contains a part of the

submucosal plexus, and contains glands and lymphatic tissue.

D. The muscularis

1. The muscularis of the mouth, pharynx, and superior part of the 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 of smooth muscle in an inner sheet

of circular fibers (and fascicles) and an outer sheet of longitudinal fibers (and fascicles).

3. The muscularis also contains the major nerve supply to the GI tract--the myenteric plexus,

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 diaphragm, the serosa is also called the visceral peritoneum.

III. Mouth

A. Introduction

1. The mouth (oral or buccal cavity) is formed by the cheeks, hard and soft palate, lips, and tongue.

2. The vestibule of the oral cavity is bounded externally by the cheeks and lips and internally by

the gingivae ("gums") and teeth.

3. The oral cavity proper is a space that extends from the gums and teeth to the fauces, the

opening between the oral cavity and the pharynx (aka: throat).

B. Salivary Glands

1. The major portion of saliva is secreted by the major salivary glands, which lie outside the mouth

and pour their contents into ducts that empty into the oral cavity; the remainder of saliva

comes from minor aggregates of salivary glandular tissue (buccal glands) embedded in the

mucous membrane that lines the mouth.

2. There are three pairs of major salivary glands: Parotid, Submandibular, and Sublingual glands.

3. Saliva lubricates and dissolves food and starts the chemical digestion of carbohydrates.

It also functions to keep the mucous membranes of the mouth and throat moist.

4. Chemically, saliva is 99.5% water and 0.5% solutes such as salts, dissolved gases, various organic

substances, and, or major importance: enzymes (chiefly, salivary amylase)

5. Salivation is entirely under nervous control.

 


 

C. Tongue

1. The tongue, together with its associated muscle, forms the floor of the oral cavity. It is

composed of skeletal muscle covered with mucous membrane.

2. Extrinsic and intrinsic muscles permit the tongue to be moved to participate in food

manipulation for chewing and swallowing and in speech.

3. The lingual frenulum (aka: frenum)is a fold of mucous membrane that attaches to the

midline of the under surface of the tongue.

4. The upper surface and sides of the tongue are covered with papillae. Some papillae

(fungiform, foliate, and circumvallate; NOT filiform) contain taste buds.

D. Teeth

1. Teeth are composed primarily of dentin, a calcified connective tissue that gives the tooth

its basic shape and rigidity; the dentin of the crown is covered by enamel, the hardest

substance in the body, which protects the tooth from the wear of chewing.

a. The dentin of the root is covered by cementum, another bonelike substance, which

attaches the root to the periodontal ligament (the fibrous connective tissue lining the

tooth sockets in the mandible and maxillae).

b. The dentin encloses the pulp cavity in the crown and the root canals in the root.

2. There are two dentitions, or sets of teeth, in an individual's lifetime; deciduous (primary),

(milk teeth), or (baby teeth); and permanent (secondary) or (succedaneous teeth).

3. There are four different types of teeth based on shape; incisors (used to cut food), cuspids or

canines (used to tear or shed food), premolars (or bicuspids) (absent in the deciduous dentition

and used for crushing and grinding food), and molars (also used for crushing and grinding food).

E. Mechanical Digestion in the Mouth

1. Through mastication (chewing), food is mixed with saliva and shaped into a bolus that

is easily swallowed.

2. The enzyme amylase converts polysaccharide (starches) to disaccharides (maltose).

This is the only chemical digestion that occurs in the mouth.

IV. Pharynx

A. The pharynx is a funnel-shaped tube that extends from the internal nares to the esophagus

posteriorly and the larynx anteriorly

1. It is composed of skeletal muscle and lined by mucous membrane.

2. The nasopharynx functions in respiration only, whereas the oropharynx and

laryngopharynx have digestive as well as respiratory functions.

B. Deglutition, or swallowing, moves a bolus from the mouth to the stomach. It is facilitated

by saliva and mucus and involves the mouth, pharynx, and esophagus.

1. Deglutition consists of a voluntary stage, pharyngeal stage (involuntary), and

esophageal stage (voluntary).

2. Receptors in the oropharynx stimulate the deglutition center in the medulla and the lower

pons of the brain stem.

V. Esophagus

A. The esophagus is a collapsible, muscular tube that lies behind the treachea and connects

the pharynx to the stomach.

B. The wall of the esophagus contains mucosa, submucosa, and muscularis layers. The outer

layer is called the adventitia rather than serosa due to structural differences.

C. Physiology of the Esophagus

1. The esophagus contains an upper and lower esophageal sphincter.

2. During the esophageal stage of swallowing progressive contractions of the muscularis

push the bolus onward. These propulsive contractions are termed peristalsis.

VI. Stomach

A. Introduction

1. The stomach is a J-shaped enlargement of the GI tract that begins at the bottom of

the esophagus and ends a the pyloric sphincter.

2. It serves as a mixing and holding area for food, begins the digestion 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 Stomach

1. Four anatomical subdivisions of the stomach include the cardia, fundus, body, and pyloris.


2. When the stomach is empty, the mucosa lies in folds called rugae.

C. Histology of the Stomach

1. The surface of the mucosa is a layer of simple columnar epithelial cells called

mucous surface cells.

a. Epithelial cells extend into the lamina propria, forming gastric pits and gastric glands.

b. The gastric glands consist of three types of exocrine glands, mucous neck cells

(secrete mucus), chief or zymogenic cells (secrete pepsinogen and gastric lipase),

and parietal cells (secrete HCl and intrinsic factor)

c. Gastric glands also contain enteroendocrine cells, which are hormone producing cells.

G cells secrete the hormone gastrin into the bloodstream.

2. The submucosa is composed of areolar connective tissue.

3. The muscularis has three layers of smooth muscle: longitudinal, circular and an

oblique layer.

4. The serosa is part of the visceral peritoneum.

a. At the lesser curvature, the visceral peritoneum becomes the lesser omentum.

b. At the greater curvature, the visceral peritoneum becomes the greater omentum.

D. Mechanical and Chemical Digestion in the Stomach

1. Mechanical digestion consists of peristaltic movements called mixing waves.

2. Chemical digestion

a. Chemical digestion consists mostly of the conversion of proteins into peptides

by pepsin, an enzyme that is most effective in the very acidic environment (pH 2)

of the stomach. The acid (HCI) is secreted by the stomach's parietal cells.

b. Gastric lipase splits certain molecules in butterfat of milk into fatty acids and monoglycerides

and has a limited role in the adult stomach.

3. The stomach wall is impermeable to most substances; however, some water, electrolytes,

certain drugs (especially aspirin), and alcohol can be absorbed through the stomach lining.

E. Regulation of Gastric Secretion and Motility

1. Gastric secretion is regulated by nervous and hormonal mechanisms. Stimulation occurs in

three overlapping phases: cephalic (reflex), gastric, and intestinal.

2. Cephalic phase

a. The cephalic phase consists of reflexes initiated by sensory receptors in the head.

b. The cephalic phase stimulates gastric secretion and motility.

3. Gastric phase

a. The gastric phase begins when food enters the stomach.

b. When the stomach walls are distended or when pH increases because proteins have entered

the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors

are activated, resulting in waves of peristalsis and continual flow of gastric juice.

c. Hormonal negative feedback also regulates gastric secretions during the gastric phase.

(1) Chemoreceptors and stretch receptors stimulate the ANS to release acetylcholine, which

stimulates the release of gastrin by G cells.

(2) Gastrin stimulates growth of the gastric glands and secretion of large amounts of

gastric juice. It also strengthens contraction of the lower esophageal sphincter,

increases motility of the stomach, and relaxes the pyloric and ileocecal sphincters.

(3) Acetylcholine released by parasympathetic fibers and gastrin secreted by G cells stimulate

parietal cells to secrete more HCl when histamine is present.

4. Intestinal Phase

a. The intestinal phase is due to activation of receptors in the small intestine.

b. When partially digested food enters the small intestine, it triggers the enterogastric

reflex and the secretion of gastric inhibitory peptide (GIP), secretin, and cholecystokinin

(CCK) by the intestinal mucosa. The effect is inhibition of gastric secretion.

5. Regulation of gastric emptying

a. Gastric emptying is the periodic release of chyme from the stomach into the duodenum.

b. Gastric emptying is stimulated by two factors: nerve impulses in response to distension

of the stomach and stomach gastrin in response to the presence of certain types of foods.

c. Most food leaves the stomach 2-6 hours after ingestion. Carbohydrates leave earliest,

 


 

followed by proteins and then fats.

d. Gastric emptying is inhibited by the enterogastric reflex and by the hormones

CCK and GIP

e. Vomiting is the forcible expulsion of the contents of the upper GI tract

(stomach and sometimes duodenum) through the mouth.

VII. Pancreas

A. The pancreas is divided into a head, body, and tail and is connected to the

duodenum via the main pancreatic duct and accessory duct.

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

mixture of fluid and digestive enzymes called pancreatic juice.

C. Pancreatic juice

1. Pancreatic juice contains enzymes that digest starch (pancreatic amylase),

proteins (trypsin, chymotrypsin, and carboxypeptidase), fats (pancreatic lipase),

and nucleic acids (ribonuclease and deoxyribonuclease).

2. It also contains sodium bicarbonate, which converts the acid stomach contents

to a slightly alkaline pH (7.1-8.2), halting stomach pepsin activity and promoting

activity of pancreatic enzymes.

D. Pancreatic secretion is regulated by nervous and hormonal mechanisms. .

VIII. Liver and Gallbladder

A. The liver is the heaviest gland in the body and the second largest organ in the body after

the skin.

B. Anatomy of the Liver and Gallbladder

1. The liver is divisible into left and right lobes, separated by the falciform ligament.

Associated with the right lobe are the caudate and quadrate lobes.

2. The gallbladder is a sac located in a depression on the posterior surface of the liver .

C. The liver receives a double supply of blood from the hepatic artery and the hepatic portal

vein. All blood eventually leaves via the hepatic vein.

D. Hepatic cells (hepatocytes) produce bile that is transported by a duct system (the biliary

tree) to the gall bladder for concentration and temporary storage.

1 Bile is partially an excretory product (containing components of worn-out RBC.s

and partially a digestive secretion (NOT an enzyme; an emulsifier.)

2. Bile's contribution to digestion is the emulsification of triglycerides.

E. The rate of bile secretion is regulated by nervous and hormonal mechanisms as well

as by volume of hepatic blood flow and the concentration of bile salts in the blood. .

F. The liver also functions in carbohydrate, lipid, and protein metabolism; removal of

drugs and hormones from the blood; excretion of bilirubin; synthesis of bile salts;

storage of vitamins and minerals; phagocytosis; and activation of vitamin D.

IX. Small Intestine

A. Introduction

1. The major events of digestion and absorption occur in the small intestine.

2. The small intestine extends from the pyloric sphincter to the ileocecal sphincter.

B. Anatomy of the Small Intestine

1. The small intestine is divided into the duodenum, jejunum, and ileum.

2. Projections called circular folds, or plicae circularies, are permanant ridges in the mucosa

that enhance absorption by increasing surface area and causing chyme to spiral as it passes through

the small intestine.

C. Histology of the small intestine

1. The mucosa forms fingerlike villi that increase the surface area of the epithelium available

for absorption and digestion.

a. Embedded in the villus is a lacteal (lymphatic capillary) for fat absorption.

b. The cells of the mucosal epithelium include absorptive cells, goblet cells,

enteroendocrine cells, and Paneth cells.

c. The free surface of the absorptive cells feature microvilli, which increase surface area.

d. The mucosa contains many cavities lined by glandular epithelium. These cavities form the

intestinal glands (crypts of Lieberkuhn).

E. Mechanical digestion in the small intestine

1. Segmentation, the major movement of the small intestine, is a localized contraction in areas

r.nntaining food.


4. Peristalis propels the chyme onward through the intestinal tract. F. Chemical digestion in the small intestine.

1. Carbohydrates are broken down into monosaccharides for absorption.

a. Intestinal enzymes break down starches into maltose, maltotriose, and alpha dextrins (pancre~tic amylase); alpha-dextrins into glucose (alphadestrinase); maltose to glucose (maltase); sucrose to glucose and fructose (sucrase); and lactose to glucose and galactose (lactase).

2. Protein digestion starts in the stomach

a. Proteins are converted to peptides by trypsin and chymotrypsin. Also, enzymes break peptide bonds that attach terminal amino acids to carboxy ends of peptides (carboxypeptidases) and peptide bonds that attach terminal amino acids to amino ends of pep tides (aminopeptidases).

b. Finally, enzymes split dipeptides to amino acids (dipeptidase). 3. Most lipid digestion, in an adult, occurs in the intestine

a. Bile salts break the globules oftriglycerides (fats) into droplets, a process called emulsification.

b. Pancreatic lipase, due to the increase in exposed surface area of the droplets, can hydrolyze more triglycerides into fatty acids and monoglycerides.

4. Nucleic acids are broken down into nucleotides for absorption. G. Absorption in the small intestine

1. Absorption is the passage of the end products of digestion from the GI tract into blood or lymph and occurs by diffusion, facilitated diffusion, osmosis, and active transport.

2. Absorption of monosaccharides.

a. Essentially all carbohydrates are absorbed as monosaccharides. b. They are absorbed into blood capillaries.

3. Absorption of amino acids, dipeptides, and tripeptides.

a. Most proteins are absorbed as amino acids by active transport processes. b. They are absorbed into the blood capillaries in the villus. 4. Absorption of lipids

a. Dietary lipids are all absorbed by simple diffusion

b. Long-chain fatty acids and monoglycerides are absorbed as part of micelles, resynthesized t9 triglycerides, and fonned into protein-coated spherical masses called chylomicrons.

(1) Chylomicrons are taken up by the lacteal of a villus.

(2) From the lacteal they enter the lymphatic system and then pass into the cardiovascular system, finally reaching the liver or adipose tissue.

c. The plasma lipids--fatty acids, triglycerides, cholesterol--are insoluble in water and body

~

 

fluids.

 

(1) In order to be transported in blood and utilized by body cells, the lipids must be combined with protein transporters called lipoproteins to make them soluble.

(2) The combination of lipid and protein is referred to as a lipoprotein. 5. Absorption of electrolytes

a. Many of the electrolytes absorbed by the small intestine come from gastrointestinal secretions and some are part of digested food and liquids.

b. Active transport mechanisms are primarily used for electrolyte absorption. 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

a. All water absorption in the GI tract occurs by osmosis from the lumen of the intestines through epithelial cells and into blood capillaries.

b. The absorption of water depends on the absorption of electrolytes and nutrients to maintain an osmotic balance with the blood. X. Large Intestine

A. Anatomy of the large intestine

1. The large intestine (colon) extends from the ileocecal sphincters to the anus. 2. Its subdivisions include the cecum, colon, rectum, and anal canal. 3. Hanging inferior to the cecum is the appendix.

4. The colon is divided into the ascending, transverse, descending, and sigmoid portions.

 


B. Histology of the large intestine.

1. The mucosa of the large intestine has no villi or permanent circular folds. It does have a simple columnar epithelium with numerous goblet cells.

2. The muscularis contains specialized portions of the longitudinal muscles called taeniae coli, which contract and gather the colon into a series of pouches called haustia.

C. Mechanical movements of the large intestine include haustral churnig, peristalsis, and mass peristalsis.

D. The last stages of chemical digestion occur in the large intestine through bacterial, rather than enzymatic action. Substances are further broken down and some vitamins are synthesized by bacterial action and absorbed by the large intestine.

E. Absorption and feces formation in the large intestine

1. The large intestine absorbs water, electrolytes, and some vitamins.

2. Feces consist of water, inorganic salts, sloughed-off epithelial cells, bacteria, products of bacterial decomposition, and undigested parts of food. .

3. Although most water absorption occurs in the small intestine, the large intestine absorbs enough to .make it an important organ in maintaining the body's water balance.

 

LEA;RNING OBJECTIVES FOR CHAPTER 24

 

Following successful study of this chapter students will be able to:

 

1. Define or describe each of the following terms: gastrointestinal tract (alimentary canal), mucosa- associate lymphoid tissue (MAL]), parietal peritoneum, visceral peritoneum, peritoneal cavity, ascites, mesentery, lesser omentum, greater omentum, salivary gland; saliva, salivation, tongue extrinsic muscles, tongue intrinsic muscles, fungiform papillae, circumvallate papillae, filiform papillae, deciduous teeth, incisors, cuspids, molars, permanent teeth, mastication, bolus, salivary amylase, pharynx, deglutition, peristalsis, adventitia, upper esophageal sphincter, lower esophageal sphincter, cardia, fundus, body, pylorus, pyloric antrum, rugae, pyloric sphincter, lesser curvature, greater curvature, mucous neck cells, chief cells, parietal cells, gastric juice, G cell, mixing waves, chyme, pepsin, gastric lipase, acini, pancreatic juice, pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, pancreatic lipase, ribonuclease, deoxyribonuclease, gallbladder, hepatocytes, bile, emulsification, duodenum, jejunum, ileum, villi, lacteal, microvilli, alpha-dextrinase, maltase, sucrase, lactase, peptidases, nucleosidases, phosphatases, segmentation, pancreatic amylase, pepsin, lipases, micelles, enterohepatic circulation, chylomicrons, lipoprotein lipase, cecum, appendix,

taeniae coli, haustra, mass peristalsis, feces

2. List the functions of the digestive system (eg. ingestion, secretion, mixing, digestion, absorption,

defecation)

3. List and describe the four layers of the GI tract (eg. mucosa, submucosa, muscularis, serosa) 4. List and describe the stages of deglutition (eg. voluntary stage, pharyngeal stage, esophageal stage)

5. List and describe the phases of gastric secretion and motility (eg. cephalic phase, gastric phase, intestinal phase)

6. List and describe the functions of the liver.

7. Describe the absorption of carbohydrates, amino acids, lipids, electrolytes, vitamins, and water.