THE DIGESTIVE SYSTEM

What are the two reasons that food is vital to life?

1. It provides the energy that drives the chemical reactions of the body cells.2. It provides matter that is useful in building new tissues or repairing old

ones.

As it is normally consumed, food is not in a form that is immediately useful to the body. What must happen to the food before it is useful? What is this process called?

It must first be broken down into molecules small enough to be absorbed across cell membranes.

This breaking down process is called digestion. The organs that accomplish this process form the digestive system.

A. DIGESTIVE PROCESSES

Name and give a brief definition for each of the five basic processes of digestion.

ingestion -- taking food and drink by mouthmovement of food -- passage of food through the gastrointestinal tract by

peristalsisdigestion -- breakdown of foodstuffsabsorption -- passage of simple molecules from the digestive tract into

either blood or lymphdefecation -- elimination of indigestible substances from the system

Name and give a brief description for each of the two types of digestion.

mechanical digestion -- various movements specific for a particular organ that moves and churns food, breaking it down mechanically.

chemical digestion -- consists of a series of catabolic reactions that break down the foodstuffs enzymatically

B. ORGANIZATION

What are the two main groups of digestive organs?

The organs of the digestive system are divided into two main groups, the gastrointestinal (GI) tract (alimentary canal) and the accessory organs.

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List the organs of the gastrointestinal tract.

The organs of the GI tract include the mouth, esophagus, stomach, small intestine, and large intestine. This continuous tube contains the food from the time it enters the mouth until it exits via the anus.

How does the food move through this tube?

Muscular contraction of the wall of the GI tract break down the food physically and move it through the tract by peristalsis, a wave-like contraction of the smooth muscle.

What are the accessory organs of the digestive system?

The accessory organs are those structures which are not a direct part of the GI tract, but which aid in the breakdown of food, either mechanically or by adding secretions to the material as it moves down the tube (teeth, tongue, salivary glands, pancreas, liver, and gallbladder).

1. GENERAL HISTOLOGY OF THE GI TRACT

Name, and then describe, the four main layers of the GI tract.

In general, the wall of the GI tract, from esophagus to anus, has four main layers:1. mucosa (mucous membrane)2. submucosa3. muscularis4. serosa

a. MUCOSA

Describe the mucosa by listing and describing its three components.

The mucosa, the inner lining of the tract, consists of three parts, listed from innermost to outermost.1. epithelium2. lamina propria3. muscularis mucosa

Describe the epithelium.

The epithelium of the GI tract is in direct contact with the GI contents and is involved in protection, secre-tion of enzymes and hormones, and absorption of nutrients. From the stomach to the rectum, the

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epithelium is simple columnar. All other parts of the tract are stratified squamous.

Describe the lamina propria.

The middle layer of the mucosa is the lamina propria. This is a loose connective tissue rich in blood and lymphatic vessels and scattered lymphatic nodules.

Describe the muscularis mucosa.

The third layer of mucosa is the muscularis mucosae, a thin layer containing the smooth muscle cells arranged circumferentially around the tube. It throws the mucosa into folds that increase its surface area. This is particularly true in the stomach.

b. SUBMUCOSA

Describe the submucosa.

The second major level of the GI tract is the submucosa, a loose connective tissue that binds the mucosa to deeper structures. It is highly vascular and contains autonomic ganglia of the parasympathetic system, and may contain some exocrine glands that secrete into the gut.

c. MUSCULARIS

Describe the muscularis.

The third layer of the GI tract is the muscularis, generally consisting of inner circular and outer longitudinal smooth muscle. This muscle is responsible for GI motility (movement of food through the gut).

d. SEROSA

Describe the serosa.

The fourth, and outermost, layer of the GI tract is the serosa. This is a serous membrane composed of epithelium and connective tissue. It is also known as the visceral peritoneum.

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2. PERITONEUM

What is the peritoneum?

The peritoneum consists of a simple squamous epithelium called the mesothelium and an underlying connective tissue layer. It is divided into two layers with a potential space between them.

Differentiate between parietal peritoneum, visceral peritoneum, and the peritoneal cavity.

The parietal peritoneum lines the muscular wall of the abdominal cavity while the visceral peritoneum covers some of the organs as their serosa. The peritoneal cavity is the potential space between the two layers.

What does retroperitoneal mean?

Some organs of the abdominopelvic cavity lie between the parietal peritoneum and the muscular posterior abdominal wall. They are therefore said to be retroperitoneal.

What are the mesentery and the mesocolon?

In some areas, the peritoneum contains large folds that weave between the viscera and bind organs to the posterior wall, suspend-ing them within the abdominal cavity. The folds contain blood ves-sels, lymphatics, and nerves supplying the suspended organs. The mesentery surrounds the small intestine, and the mesocolon surrounds part of the large intestine.

C. MOUTH (ORAL CAVITY)

Describe the mouth as follows:

boundaries -- The mouth (oral or buccal cavity) is formed by the cheeks laterally, hard and soft palates superiorly, and the tongue inferiorly.

hard palate -- The hard palate is the anterior portion of the roof of the mouth. It is formed by the two maxillae and the two parietal bones. It also serves as the floor of the nasal cavity.

soft palate -- The soft palate is the posterior portion of the roof of the mouth. It consists of skeletal muscle covered by mucous mem-brane. Hanging from the middle of its free border is the uvula, a cone-shaped muscular process.

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palatal arches -- Extending from the uvula are two muscular folds. Anteriorly, the palatoglossal arches extend to the base of the tongue. Posteriorly, the palatopharyngeal arches project to the anterior surface of the pharynx.

mucosa -- The inside of the mouth is lined with nonkeratinized stratified squamous epithelium.

vestibule -- The vestibule of the mouth is the space bounded externally by the cheeks and lips and internally by the gums and teeth.

fauces -- The oral cavity itself extends from the gums and teeth, to the fauces, the passageway posterior to the palatopharyngeal arches that open into the oropharynx.

1. TONGUE

Describe the tongue as follows:

location and structure -- The tongue, with its associated muscles, forms the floor of the oral cavity. The tongue itself is com-posed of interlacing skeletal muscles, called the intrinsic muscles, covered by stratified squamous epithelium.

extrinsic muscles -- The extrinsic muscles of the tongue originate outside the mouth and insert into the tongue’s base. They are used to move the tongue side-to-side and in-and-out, and to help maneuver food and shape it within the mouth.

function -- Once a rounded mass of chewed food, called a bolus, has been formed, the intrinsic muscles bow the tongue so that the bolus is moved from the front of the mouth, through the fauces, and into the oropharynx. The tongue is also used to alter sounds in the creation of speech.

innervation -- The tongue muscles, both intrinsic and extrinsic, are innervated by cranial nerve XII, the hypoglossal nerve.

2. SALIVARY GLANDS

What is saliva?

Saliva is a fluid constantly secreted into the mouth by the salivary glands located in or near the mouth. There are four sets of salivary glands.

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Describe each of the following salivary glands?

buccal -- The buccal glands are the small glands located in the mucous membrane of the mouth itself. They are responsible for keeping the mouth and pharynx moist, even when food is not in the mouth.

parotid -- The paired parotid glands are located inferior and anterior to the ears, between the skin and masseter muscle. They empty their secretions into the mouth via the parotid (Stensen’s) ducts that open into the vestibule of the mouth opposite the upper second molars.

submandibular -- The paired submandibular glands are located just inside the mandible and beneath the base of the tongue on either side. They empty via the submandibular (Wharton’s) ducts on either side of the lingual frenulum, opposite the two lower central incisors.

sublingual -- The paired sublingual glands are located superior to the submandibular glands just under the mucosa of the floor of the mouth. They empty via a number of small ducts directly into the floor of the oral cavity proper.

a. COMPOSITION OF SALIVA

What are the basic components of saliva?

Saliva is composed of 99.5% water, used to dissolve foods, and 0.5% solutes.

Name, and then describe, each of the four basic solutes or solute groups?

salivary amylase --This enzyme is used to initiate carbohydrate chemical digestion by catalyzing the breakdown of starch (a polymer of glucose) to the disaccharide maltose (glucose-glucose).

ions – Na+, K+, C-l, HCO3-, and HPO4-2 are the main ions

found in saliva. They are used to activate salivary amylase and to buffer acidic foods, keeping salivary pH between 6.35 - 6.85.

mucous -- Mucin mixed with water forms mucous. It is used to lubricate food so that it can be easily swallowed and to facilitate speech.

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lysozyme -- Lysozyme is a nonspecific antimicrobial enzyme used to maintain some degree of cleanliness in the mouth.

b. SECRETION OF SALIVA

How is salivation controlled?

Salivation is completely under the control of the parasympa-thetic nervous system, via cranial nerves VII (facial) and IX (glossopharyngeal).

Name, and then describe, the three types of stimuli that initiate salivation.

psychic -- The anticipation, sight, sound, smell, or memory of food, working through the cerebral cortex, initiates reflexive salivation. (think Pavlov).

chemical -- Stimulation of taste buds by food components or the ingestion of irritants, working through the taste buds, initiates reflexive salivation.

tactile -- Stimulation of touch receptors within the mouth and pharynx initiates reflexive salivation. (Think about holding a pencil in your mouth.)

3. PHYSIOLOGY OF DIGESTION IN THE MOUTHa. MECHANICAL DIGESTION

Describe the mechanical digestion that occurs in the mouth.

Through chewing (mastication), food is reduced to a soft, shredded, and ground moist mass. This mass is shaped by the combined action of the cheeks, tongue, and teeth into a flexible bolus that can be easily swallowed.

b. CHEMICAL DIGESTION

Describe the chemical digestion that occurs in the mouth.

Salivary amylase enzymatically converts starch (a polymer [a chemical compound consisting essentially of repeating structural units] of glucose) into the disaccharide maltose (GLU-GLU). There is a lingual lipase, a lipid- digesting enzyme but its effects are very minor. Other dietary sugars, as well as proteins and fats, are unaffected chemically.

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4. PHYSIOLOGY OF DEGLUTITION

Define swallowing, and then describe the three phases by which the process occurs.

Swallowing (deglutition) moves food from the mouth, through the pharynx and esophagus, to the stomach. It involves three phases and requires 4-8 seconds for solids, 1 second for liquids.

In the voluntary stage, the bolus of food is forced to the back of the mouth and into the oropharynx by the movement of the tongue upward and back against the palate.

The pharyngeal stage begins when tactile receptors in the oropharynx are stimulated by the presence of the bolus. The sensory information is carried to the deglutition center of the medulla.

From the deglutition center, motor information via cranial nerves IX, X, XI causes contraction of the pharyngeal constrictors, contraction of the soft palate, and contraction of the laryngeal muscles.

As a result, food is moved down the pharynx to the esophagus, the soft palate blocks access to the nasal cavity, and the larynx is pulled up against the epiglottis, blocking access to the trachea.

The esophageal stage begins when the bolus is pushed into the proximal end of the esophagus. In response, the upper esophageal sphincter relaxes and the bolus moves into the esophagus.

Stretch of the esophagus initiates reflexive peristalsis, a wavelike contraction of the esophageal muscularis that pushes the bolus of food to the stomach.

At the distal end of the esophagus, the gastroesophageal sphincter (lower esophageal sphincter) is reflexively opened and the bolus is pushed into the stomach. The gastroesophageal sphincter immedi-ately closes.

D. ESOPHAGUS1. HISTOLOGY2. PHYSIOLOGY

Describe the esophagus as follows:

gross anatomy -- The esophagus is a muscular, collapsible 10-inch long tube that lies posterior to the trachea. It begins at the inferior end of the laryngopharynx, passes through the

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mediastinum anterior to the vertebral column, pierces through the diaphragm at the esophageal hiatus, and ends at the stomach.

sphincters -- There are two esophageal sphincters. The upper esophageal sphincter lies at the point where the esophagus exits from the laryngopharynx. The lower esophageal (gastroesophageal) sphincter lies at the distal end of the esophagus, at its junction with the stomach. They are normally closed and open only in response to the deglutition reflex.

mucosa/submucosa -- The mucosa of the esophagus is nonkeratinized stratified squamous for abrasion resistance. The submucosa contains numerous blood vessels and mucous-secreting glands.

muscularis -- The muscularis begins as skeletal muscle proximally, and grades into smooth muscle distally. The inner circular portions of the muscularis form the two sphincters of the esophagus.

function -- The esophagus plays no role in digestion other than as a conduit from the pharynx to the stomach, using peristalsis to move the food. There are no enzymatic secretions from the esophagus.

E. STOMACH1. ANATOMY

Describe the gross anatomy of the stomach as follows:

location -- The stomach is a J-shaped enlargement of the GI tract directly posterior to the diaphragm in the left upper quadrant (left hypochondriac region) of the abdominopelvic cavity. The superior end is a continuation of the esophagus; the inferior end becomes the duodenum.

subdivisions -- The stomach is divided into four major areas by imaginary lines: the cardia, fundus, corpus (body), and pylorus.

What is the cardia?

The cardia of the stomach is that portion that imme-diately surrounds the opening of the esophagus into the stomach.

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What is the fundus?

The fundus is the rounded portion lying above and to the left of the cardia. It is located above an imaginary line drawn in the horizontal plane between the open-ing of the esophagus and far left margin of the stomach.

What is the corpus?

The corpus (body) of the stomach is the large central portion below the fundus.

What is the pylorus?

The pylorus is the narrow inferior portion that tapers to funnel the corpus into the small intestine (duodenum).

pyloric sphincter -- At the termination of the pylorus, the inner circular layer of smooth muscle is thickened to form the pyloric sphincter which regulates the movement of food from the stomach into the duodenum.

curvature -- The concave medial border of the stomach is the lesser curvature and the convex lateral border is the greater curvature.

rugae -- When empty the mucosa of the stomach lies in large folds called rugae. As the stomach is filled during a meal, the rugae becomes flattened. This allows the stomach to increase its size without increasing its tension.

2. HISTOLOGY

Describe the histology of the stomach as follows:

epithelium -- There are two major differences in the histology of the stomach compared to the other GI structures. The epithe-lium of the stomach changes abruptly from the stratified squamous of the esophagus to simple columnar at the cardia. This epithelium contains many gastric pits; at the bottom of each are the gastric glands.

cells of the gastric glands -- The gastric glands consist of four different cell types:1. chief (zygomatic) cells2. parietal cells

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3. mucous neck cells4. G cells

What is the function of chief cells?

Chief cells secrete the principal gastric enzyme as an inactive precursor form called pepsinogen, and an enzyme of lesser importance, gastric lipase.

What is the function of the parietal cell?

Parietal cells secrete hydrochloric acid (HCl), which converts pepsinogen to its active form of pepsin, and intrinsic factor, which binds dietary vitamin B12.

What is the function of the mucous neck cells?

Mucous neck cells, located at the top (neck) of the gastric glands, secrete mucous which coats the mucosa, giving it protection from the HCl and pepsin.

What is the function of the G cells?

G cells secrete the hormone gastrin. As will be seen shortly, gastrin has a number of roles in GI physiology.

muscularis -- The other major change from the basic structure of the GI tract is that the muscularis has three layers of smooth muscle:1. inner oblique2. middle circular3. outer longitudinal

3. PHYSIOLOGY OF DIGESTION IN THE STOMACHa. MECHANICAL DIGESTION

Describe the mixing waves of the stomach.

Mixing waves are gentle, rippling peristaltic movements that begin in the upper corpus and move toward the pylorus of the stomach. They occur every 15 - 25 seconds and become more vigorous with continued stimulation from gastrin and parasympathetic input via the vagus nerve.

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What do mixing waves accomplish?

These movements macerate (to cause to become soft or separate into constituent elements) the food, mixing it with gastric juices to form a thin liquid paste called chyme.

As chyme becomes more liquid, some moves through the partially closed pyloric sphincter, while the rest is sloshed back into the corpus for more mixing.

b. CHEMICAL DIGESTION

Describe each of the following chemical activities in the stomach: pepsinogen, gastric lipase, and rennin.

HClpepsinogen ---> pepsin (requires pH1-3)

proteins ---> polypeptides

Gastric lipase has limited activity in lipid digestion while in the stomach because it requires pH 5-6. Therefore, its effects are negligible.

Rennin is present in the infant stomach only. Its function is to curdle milk so that it is retained in the stomach to allow time for protein digestion before passing to the small intestine.

4. REGULATION OF GASTRIC SECRETION AND MOTILITY

Overview of stomach control

There are three phases of control:1. cephalic phase2. gastric phase3. intestinal phase

a. CEPHALIC PHASE

Describe the endocrine and nervous mechanisms that occur during the cephalic phase of gastric control.

The activities of the stomach in digestion are controlled by hormonal and nervous mechanisms that occur in three overlapping phases: cephalic, gastric, and intestinal.

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The cephalic phase refers to reflexes initiated by sensory receptors of the head and are related to the same psychic stimuli that initiate salivation.

In addition, the feeding center of the hypothalamus, and the cerebral cortex, send impulses to the medulla that stimulate the vagus nerve (X).

Parasympathetic impulses from the vagus nerve stimulate the gastric glands to secrete their products and the smooth muscle of the stomach to increase its motility.

b. GASTRIC PHASE

Describe the endocrine and nervous mechanisms that occur during the gastric phase of gastric control.

The gastric phase of stomach control begins when food reaches the stomach. Sensory receptors in the stomach initiate both nervous and endocrine mechanisms causing gastric secretion and motility to continue.

Food stretches the stomach, stimulating the stretch receptors, while increased pH of chyme stimulates chemoreceptors. This sensory input initiates a neural negative feedback cycle.

The combined effects of this are a localized parasympathetic reflex that causes increased gastric gland secretion (lowering pH) and increased mixing wave rate and intensity.

In addition, distention of the stomach and the presence of proteins in chyme stimulate the G cells to secrete gastrin, which enters the blood and further stimulates the gastric glands.

Feedback system

controlled condition -- Food entering the stomach disrupts homeostasis by causing an increase in gastric juice pH AND stretch (distention) of the stomach wall.

receptors -- Chemoreceptors and stretch receptors in the stomach detect the changes in homeostasis and generate nerve impulses that pass to the control centers.

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control center -- The submucosal plexus in the wall of the stomach and neurons of the medulla generate parasympathetic impulses to the effectors.

effectors -- Parietal cells of the gastric mucosa secrete HCl and gastric smooth muscle contracts more vigorously (increased mixing waves)

responses -- In response, there is increased acidity in the stomach chyme and the mixing waves begin emptying of the stomach. An empty stomach is a return to homeostasis.

c. INTESTINAL PHASE

Describe the endocrine and nervous mechanisms that occur during the intestinal phase of gastric control.

With increased gastric secretion and motility, chyme becomes a thin liquid and some is eventually moved through the pyloric sphincter into the first part of the small intestine, the duodenum.

The intestinal phase of stomach control is due to activation of stretch receptors, as well as chemoreceptors, in the duodenum by chyme.

Stimulation of stretch receptors in the duodenum initiates the enterogastric reflex, in which sensory input to the medulla turns off the vagal output to the stomach, resulting in decreased gastric secretion and motility.

In addition, chyme stimulates enteroendocrine cells of the duodenum to secrete four hormones: gastrin, cholecystokinin (CCK), secretin, and gastric inhibitory peptide (GIP).

CCK, secretin, and GIP circulate through the blood and act to inhibit the gastric secretion and motility. These hormones will have other important effects at other locations in the GI tract.

The net effect of the intestinal phase is to inhibit the stomach until the duodenum can process the chyme it received and pass it on within the small intestine.

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5. REGULATION OF GASTRIC EMPTYING

Summarize the processes that stimulate and inhibit gastric emptying and describe their net result.

Gastric emptying is regulated by both neuronal (via the vagus nerve) and endocrine (via four hormones) mechanisms.

Stomach emptying is stimulated by the action of the vagus nerve, parasympathetic reflexes, and the hormone gastrin.

Stomach emptying is inhibited by the action of the enterogastric reflex, which inhibits vagal output, and three hormones (CCK, secretin, and GIP) that directly inhibit the stomach.

The net result is that the stomach is emptied slowly so that only a small amount of chyme enter the duodenum at any given time; as chyme is processed by the duodenum, the stomach becomes turned on again.

Within 2 - 8 hours after a meal, the stomach is completely empty of chyme. Meals rich in carbohydrates are emptied fastest, while meals rich in fat are emptied slowest.

6. ABSORPTION

Discuss absorption by the stomach.

The stomach mucosa is impermeable to most materials, so that most substances are not absorbed until they reach the small intestines.

Name 4 substances that are absorbed by the stomach.

water, electrolytes, certain drugs (aspirin), and alcohol

The next steps in digestion occur in the small intestine. Chemical digestion here depends not only on its own secretions, but also on the activities of three accessory organs of digestion.

Name the three accessory organs whose secretions are necessary to proper digestion within the small intestine.

pancreas, liver, and gallbladder.

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F. PANCREAS1. ANATOMY2. HISTOLOGY

Describe the pancreas as follows:

location -- The pancreas is an oblong gland about 5 inches long and 1 inch thick. It consists of a “head” end nestled into the bend of the duodenum and a “tail” lying next to the spleen. The gland lays posterior to the greater curvature of the stomach.

histology -- Histologically, 99% of the pancreas consists of small clusters (acini) of exocrine secretory cells that produce enzymes and sodium bicarbonate, then release them into the organ’s duct work. The other 1%of the cells are clusters of endocrine cells (islets of Langerhans) that secrete the hormones insulin and glucagon into the bloodstream.

ducts -- Exocrine pancreatic secretions pass from the secreting cells into small ducts which unite to form ever larger ducts. Eventually 2 large ducts are formed: the main pancreatic duct (of Wirsung) and the smaller accessory duct ( of Santorini).

hepatopancreatic ampulla -- In most people the main pancreatic duct exits the pancreas, joins with the common bile duct, and enters the small intestine about 4 inches distal to the pyloric sphincter as the hepatopancreatic ampulla(a flask with a globular body, a saccular anatomic swelling) (of Vater)

The smaller accessory duct, if present, opens into the duodenum about 1 inch above the ampulla of Vater.

3. PANCREATIC JUICE

Describe, and name if applicable, the function(s) of each of the following pancreatic juice components:

sodium bicarbonate -- (NaHCO3) buffers the pH of the small intestinal contents (chyme) to 7.1 - 8.2. This stops the action of pepsin and creates the proper environment for the enzymes that function in the small intestine.

carbohydrate-digesting enzymes -- Pancreatic amylase is the same as salivary amylase. It functions in the degradation of starch to maltose.

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lipid-digesting enzymes -- Pancreatic lipase and cholesterol esterase are fat-digesting enzymes.

nucleotide-digesting enzymes – Nucleases (DNAse and RNAse)

Give the names of the three inactive protein-digesting enzymes found in pancreatic juice.

trypsinogen, chymotrypsinogen, and procarboxypeptidase

Why are these enzymes secreted in an inactive form?

As with the stomach and pepsinogen, these protein-digesting enzymes are secreted in an inactive form to prevent self-digestion of the pancreas.

How, and where, are these enzymes activated?

Enterokinase, an enzyme secreted by the small intestinal mucosal cells, converts trypsinogen to trypsin, the active form. Trypsin, in turn, converts chymotrypsinogen to chymotrypsin and procarboxy-peptidase to carboxypeptidase.

The activation of the enzymes occurs in the small intestine where the protein substrates for the active enzymes are found. In this way, self-digestion of the pancreas is avoided.

4. REGULATION OF PANCREATIC SECRETIONS

Describe the control of pancreatic secretion as follows:

nervous mechanisms -- During the cephalic gastric phase of stomach control, parasympathetic impulses are conveyed to the pancreas via the vagus nerve, resulting in increased secretion of pancreatic juice.

endocrine mechanisms -- The hormones CCK and secretin, released during the intestinal phase of stomach control, affect the pancreas as well.

CCK stimulates the secretion of pancreatic enzymes.

Secretin stimulates the release of NaHCO3.

G. LIVER1. ANATOMY2. HISTOLOGY3. BLOOD SUPPLY

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Describe the liver as follows:

location -- The liver, the second largest organ of the body, is located in the right upper quadrant (right hypochondriac and part of the epigastric regions), tucked under the diaphragm.

lobes -- It is divided into two primary lobes, right and left, by the falciform ligament, a fold of the peritoneum, and two smaller lobes, the caudate and the quadrate, located between the right and left lobes.

bile canaliculi -- The liver is connected to the ampulla of Vater at the duodenum by a series of ducts through which the main liver secretion associated with digestion, bile, flows into the small intestine. Bile is secreted by rows of hepatic paren-chymal cells into tiny bile canaliculi to begin the journey out of the liver.

hepatic ducts -- Bile canaliculi join larger ducts, which eventually fuse to form ducts that exit the liver as the right and left hepatic ducts. These two fuse outside the liver to form the common hepatic duct.

common bile duct -- The common hepatic duct is joined by the cystic duct from the gallbladder, forming the common bile duct.

The common bile duct joins the main pancreatic duct at the ampulla of Vater, as described earlier. In this way bile makes its way to the small intestine.

Describe the liver lobule as follows:

cell arrangement -- The liver is composed of thousands of func-tional units called lobules. Each consists of specialized epithelial cells called hepatocytes arranged as plates of cells about the central vein. Adjacent plates of hepatocytes are separated by blood sinusoids lined with endothelium and reticuloendothelial cells (destroy RBCs and blood-borne bacteria).

bile flow -- Bile is secreted from the basal surfaces of the hepato-cytes, between two juxtaposed rows of cells, and into the bile canaliculi. It then flows outwards to the periphery of the lobule to eventually enter the hepatic ducts.

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blood supply -- The blood supply of the liver, which comes from two different sources, flows in the direction opposite to that of bile within the lobule (from the periphery towards the central vein in the center of the lobule).

The hepatic artery, coming from the celiac artery that rises from the aorta, brings oxygenated blood into the liver.

The hepatic portal vein carries the blood previously found in the small and large intestines where it absorbed nutrients from the diet.

portal triad -- Located in several locations around the periphery of a given lobule are distinct areas that contain a branch of the hepatic artery and a branch of the hepatic portal vein (delivering blood to the lobule), and a branch of a bile duct (taking bile away from the lobule). Each arrangement of these three tubes is called a portal triad.

blood flow -- Blood flows from the vessels of the portal triads, into the sinusoids, and towards the central vein, located in the center of each lobule. All central veins join to form the hepatic vein that leaves the liver and enters the inferior vena cava.

4. BILE

What is bile?

Bile is a yellowish-brown to olive-green liquid consisting of water, bile salts, bile pigments, cholesterol, and other substances. It is NOT an enzyme. Rather, it is a detergent.

How is bile used in the digestive process of the small intestine?

Bile is used in the small intestine for the process of fat emulsification. That is, the breakdown of large fat globules into smaller fat droplets, resulting in increased surface area for chemical digestion by the fat-digesting enzymes.

5. REGULATION OF BILE SECRETION

Describe the control of liver secretion of bile as follows:

nervous mechanisms -- As with the pancreas, increased vagal stimulation during the cephalic and gastric phases of stomach control, also leads to increased production of bile by two-fold over resting levels.

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endocrine mechanisms -- Secretin, released from the duodenum during the intestinal phase of stomach control, also stimulates the liver to increase its production of bile.

6. PHYSIOLOGY OF THE LIVER

Without giving specifics, can you list 9 functions of the liver including bile production?

1. carbohydrate metabolism2. protein metabolism3. lipid metabolism4. removal of drugs, hormones, and other chemicals from the

blood for detoxification5. synthesis of bile salts6. excretion of bile7. storage (particularly glycogen, ferritin, certain vitamins, and

toxic heavy metals that cannot be detoxified or excreted)8. phagocytosis of worn-out RBCs, WBCs, and some bacteria9. activation of vitamin D

Name, and then describe, the three main types of glucose metabolism that occurs in the liver.

glycogenesis -- In glycogenesis, hepatocytes convert glucose ( a monosaccharide) to glycogen (a polymer of glucose) when blood glucose level is high. This is one of the major means by which blood glucose is lowered back to within homeo-static range after a meal.

glycogenolysis -- In glycogenolysis, stored liver glycogen is broken down into its component glucose molecules and liberated into the blood for circulation in the body. This is one of the major means by which blood sugar level is brought back into homeostatic range during times of fasting.

gluconeogenesis -- In gluconeogenesis, hepatocytes convert amino acids and lactic acid to glucose when blood sugar level is low. This is another means used to raise blood glucose back into the homeostatic range during times of fast.

Name, and then describe, the three main types of protein metabolism that occurs in the liver.

deamination -- Hepatocytes remove the amino group (-NH2) from amino acids (deamination) so they can be used for ATP production or be converted to carbohydrates or fats. The

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amino groups are then converted to ammonia (NH3). Without this deamination process, death would occur within just a few days.

urea formation -- Hepatocytes convert toxic ammonia (NH3) into the much less toxic urea which is excreted from the body in urine.

plasma protein production -- Virtually all plasma proteins are produced by hepatocytes.

H. GALLBLADDER1. HISTOLOGY2. PHYSIOLOGY

Describe the gallbladder. Include its location, basic histology, how it is filled with bile, and how and when bile is removed from the gall bladder.

The gallbladder is a pear-shaped sac about 3-4 inches long, located in a fossa of the visceral (inferior) surface of the liver. It is connected to the biliary system of ducts by the cystic duct.

It consists of a mucosa of simple columnar cells that are thrown into rugae when the organ is empty. It has a typical muscularis of inner circular and outer longitudinal smooth muscle.

The functions of the gallbladder are to store bile excreted from the liver and concentrate it (up to 10-fold), by removing its water, until it is needed in the small intestine.

When no food is in the small intestine, bile is produced and excreted from the liver at a slow rate, filling the biliary system of ducts.

The bile does not flow into the small intestine because the ampulla of Vater has a sphincter, the sphincter of Oddi, which is normally closed. As a result, bile backs up in the ducts and backfills into the gall bladder.

When chyme enters the duodenum, CCK is secreted. In addition to its other roles, it also causes the sphincter of Oddi to relax and the smooth muscle of the gallbladder to contract.

As a result, bile is forcefully ejected from the gallbladder and it is drained from the biliary system into the duodenum, where it partici-pates in the emulsification of fats in the diet.

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I. SMALL INTESTINE1. ANATOMY

Describe the location of the small intestine and give the names of its three subdivisions.

The small intestine extends from the stomach to the large intestine, stretching some 21 feet in the cadaver. It is only about 7 feet long in the living state because of muscle tone. It is subdivided into three portions: the duodenum, the jejunum, and the ileum.

Describe the duodenum.

The duodenum, beginning distal to the pyloric sphincter of the stomach, is the C- shaped 10-inch long initial portion of the small intestine. It extends from the pyloric sphincter of the stomach to the second portion, the jejunum. It is retroperitoneal.

Describe the jejunum.

The jejunum, the second portion, is about 8 feet long (cadaver). It begins where the duodenum emerges from behind the peritoneum, is wrapped in the mesentery, and terminates at the ileum.

Describe the ileum.

The ileum, the third portion of the small intestine, is about 12 feet long (cadaver). It begins as a continuation of the jejunum and ends at the colon. Its distal-most portion forms the ileocecal (AKA--ileocolic) sphincter. Most absorption (90%) occurs in this section.

2. HISTOLOGY

Describe the small intestine as follows:

basic histology -- The small intestine has the same 4 tunics as the rest of the GI tract. The mucosa contains pits, like the stomach, that are lined with simple columnar epithelium forming simple tubular intestinal glands that secrete intestinal juice. The muscularis has typical inner circular and outer longitudinal smooth muscle layers.

mucosal specializations -- Because most absorption occurs in the small intestine, there are three structural adaptations of the mucosa that facilitate this process by increasing the mucosal surface area: plicae circulares, villi, and microvilli.

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What is the plicae circulares?

The plicae circulares is a circularly- arranged perman-ent longitudinal fold, about 0.4 inches high, that spirals through the length of the small intestine. It swirls the chyme as it passes through the tube.

What are villi?

Villi (singular = villus) are finger-like projections of the mucosa about 1 mm high (40 villi/ sq. mm or 26,000 / sq. in.), thus greatly increasing surface area for absorption. The center of each villus contains an arteriole, a capillary bed, a venule, and a single lymphatic capillary called a lacteal.

What are microvilli?

The apical (free) surface of each epithelial cell of the mucosa features microvilli. These are microscopic, finger-like processes of the cell membrane that further increase surface area. They are collectively called the brush border.

Peyer’s patches -- Peyer’s patches are lymphatic nodules found throughout the submucosa of the ileum in parti-cular. Their purpose is for protection from food-borne pathogens.

3. INTESTINAL JUICE AND BRUSH BORDER ENZYMES

What is intestinal juice?

Intestinal juice is a clear yellow fluid, pH 7.6, containing water and mucous. Together, pancreatic and intestinal juices, and bile (collectively known as the succus entericus), provide the medium for the completion of chemical digestion and absorption of the nutrients.

What are brush border enzymes? Name them.

Brush border enzymes are enzymes of the small intestine located on the membranes of the microvilli. They cause final chemical digestion of foodstuffs to occur on the surface of the absorptive cells, thus facilitating absorption.

Brush border enzymes include maltase, lactase, various peptidases, dextrinase, nucleosidases, and phosphatases.

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4. PHYSIOLOGY OF DIGESTION IN THE SMALL INTESTINESa. MECHANICAL DIGESTION

Describe the process of segmentation. How does it occur and what does it accomplish?

The major movement associated with mechanical digestion in the small intestine is segmentation, a strictly localized contraction of areas of the tract containing chyme.

Segmentation mixes chyme with all the digestive juices and up against the absorptive cells of the mucosa. It does not push chyme along the tract, but rather sloshes it back and forth.

Segmentation begins with contraction of the inner circular smooth muscle in an area containing chyme. It results in the formation of small segments in the length with the chyme.

Next, muscle fibers in the center of a segment contract while the other fibers relax, dividing the segment again. Finally, it repeats the sequence 12 - 16 times/minute, causing the segments to slosh together.

The second major movement of the small intestine is simple but weak peristalsis, contractions that move chyme forward through the tract. Chyme is propelled through the small intestine at a rate of about 1 meter (3 feet) per hour.

b. CHEMICAL DIGESTION

Review the chemical digestion of carbohydrates.

starch

salivary and pancreatic amylase

maltose

maltase

glucose + glucose

Starch, a polymer of glucose, is reduced to its glucose components during digestion. The monosaccharide can then be absorbed across the intestinal mucosa into the blood._____________________________________________________

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sucrose

sucrase

glucose + fructose

Sucrose, or table sugar, is reduced to its component monosac-charides by the enzyme sucrase. Glucose and fructose can them be absorbed across the intestinal mucosa into the blood._____________________________________________________

lactose

lactase

glucose + galactose

Lactose, or milk sugar, is reduced to its component monosaccharides by the enzyme lactase. Glucose and galactose can then be absorbed across the intestinal mucosa into the blood._____________________________________________________Review the chemical digestion of proteins.

proteins HClpepsin<---- pepsinogen

polypeptidesenterokinase

trypsin<---------------- trypsinogen trypsinchymotrypsin<-------- chymotrypsinogen

trypsincarboxypeptidase <-------- procarboxypeptidase

dipeptides

dipeptidases

amino acids

Amino acids can then be absorbed across the intestinal mucosa into the blood._____________________________________________________

Review chemical digestion of lipids.309

lipids

lingual lipase and gastric lipase (?)

lipids

bile

emulsified lipids

pancreatic lipasecholesterol esterase

fatty acidsglycerolsglyercides

The products of fat digestion are absorbed into the small intestinal cells, reassembled as chylomicrons, then absorbed into the lymph of the small intestine.

_____________________________________________________

5. REGULATION OF INTESTINAL SECRETION AND MOTILITY

Regulation of small intestinal secretion and motility is accomplished in three ways. Name and briefly them.

local reflexes -- Local reflexes are based on stretch of the intestinal wall and work through the parasympathetic ganglia (submucosal plexi) located there. (segmentation)

parasympathetic reflexes -- Parasympathetic reflexes via the vagus nerve are based on stretch of the intestinal wall. Stretch receptors send nervous impulses to the medulla, initiating an increase in rate of peristalsis (10 cm/sec) and increased intestinal gland secretion.

gastrin -- Gastrin is secreted by both the gastric and duodenal mucosae. It stimulated increased peristalsis and increased secretion from the intestinal glands.

6. PHYSIOLOGY OF ABSORPTION

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What is absorption?

Passage of digested nutrients from the GI tract into either the blood or lymph of the intestine is known as absorption. 90% of all absorption occurs in the small intestine. The remainder occurs in the stomach and the large intestine. Any undigested or unab-sorbed materials left in the small intestine are passed into the large intestine.

By what process(es) are carbohydrates and proteins absorbed?

For the most part, monosaccharides and amino acids are absorbed by either active transport or facilitated diffusion.

These products are absorbed into what?

All nutrients, except those derived from fat, are absorbed into the blood capillaries within the villi. This blood is eventually collected into the superior mesenteric vein for transport to the hepatic portal vein (formed by the union of the superior mesenteric vein and the splenic vein.)

By what process(es) are fats absorbed?

The products of fat digestion are absorbed into intestinal cells by simple diffusion, and then reassembled within the intestinal mucosal cell to form triglycerides that are coated with a protein, forming a chylomicron.

These products are absorbed into what?

Chylomicrons are exocytosed into the intestinal villi, where they move into a lacteal ( a specialized lymphatic capillary) and are eventually carried to the bloodstream by the thoracic duct. They are removed from the blood as it passes through the liver.

How is water absorbed by the small intestine?

Water, approximately 8 liters/day is absorbed by the gut by osmosis, following the osmotic gradients created as the various nutrients are moved from the gut into the mucosal cells, then into the blood or lymph.

J. LARGE INTESTINE311

1. ANATOMY

Describe the large intestine as follows:

general functions -- the overall functions of the large intestine are1. completion of absorption2. manufacture of certain vitamins using bacteria3. formation of feces4. expulsion of feces from the body

gross anatomy and location -- the large intestine is about 2.5 inches in diameter and about 5 feet long (cadaver). It extends from the ileocecal sphincter to the anus. It is attached to the posterior abdominal wall by the mesocolon.

parts -- The large intestine is divided into six portions. From the beginning to end they are the:

cecum -- The cecum, the beginning of the large intestine, is a blind pouch that receives the small intestine at the ileocecal sphincter. To it is attached the vermiform appendix.

ascending colon --The ascending colon is retroperitoneal. From the cecum, it ascends the right posterior abdominal wall, reaches the liver, and turns abruptly to the left at the hepatic (right colic) flexure.

transverse colon -- The transverse colon, extending from the hepatic flexure, hangs from the mesocolon as it crosses the abdomen anteriorly, then moves posteriorly to reach the spleen. It then turns downward at the splenic (left colic) flexure.

descending colon -- The descending colon extends retroperitoneally from the splenic flexure, along the left posterior abdominal wall to the left anterior superior iliac spine, where it moves toward the posterior midline as the sigmoid colon.

sigmoid colon -- The sigmoid colon moves into the posterior midline, lying on the anterior surface of the sacrum and terminates at the level of S-3 as the rectum.

rectum

Describe the following:

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rectum -- The rectum is the last 8 inches of the colon, extending from the sigmoid colon. It lies just anterior to the curve of the sacrum and coccyx, and terminates as the anal canal.

anal canal -- The anal canal is the terminal one inch of the GI tract. It is lined with a mucous membrane found in longitudinal folds called the anal columns. These folds contain branches of the hemorrhoidal arteries and veins. The anal canal opens to the outside of the body as the anus after passing through 2 sphincters.

internal anal sphincter -- The internal anal sphincter is formed by the inner circular smooth muscle of the muscularis at the bottom of the anal canal. It is innervated by parasympathetic outflow from the sacral spinal cord (involuntary control).

external anal sphincter -- The external anal sphincter is formed by the skeletal muscles of the pelvic diaphragm (muscular floor of the pelvis). It is controlled by the sacral spinal nerves (voluntary control as a learned response).

2. HISTOLOGY

Describe the histology of the large intestine as follows:

mucosa -- The mucosa has no villi or permanent plicae circulares so its surface area is greatly reduced from that of the small intestine. The epithelium is simple columnar with absorptive cells and a tremendous number of goblet cells.

muscularis -- The outer longitudinal layer of smooth muscle in the muscularis is modified to form the taenia coli, three bands of muscle that run the length of the colon. They serve to gather the colon into a series of pouches called haustra.

epiploic appendages -- The serosa of the large intestine is part of the visceral peritoneum. It has within it small fat-filled pouches called epiploic appendages which are used as energy reserves.

3. PHYSIOLOGY OF DIGESTION IN THE LARGE INTESTINE313

a. MECHANICAL DIGESTION

How is chyme moved into the large intestine?

In response to the gastroileal reflex and the hormone gastrin, the ileocecal sphincter relaxes. Increased motility of the small intestine causes chyme to move into the cecum.

Name and briefly describe the three types of mechanical digestion that occurs in the large intestine.

haustral churning -- Chyme entering the cecum initiates haustral churning. As each haustra fills, it stretches. This causes a local reflex that causes the haustra to contract. This pushes chyme to the next haustra, where the process is renewed.

mass peristalsis -- Stretch of the stomach initiates the gastrocolic reflex. Massive vagal output to the trans-verse colon initiates strong peristaltic contractions that begin in the middle of the transverse colon and extend to the rectum. This drives feces into the rectum and makes way for new chyme.

peristalsis -- Regular peristalsis, occurring as weak muscular contractions, sweep the colon at a rate of 3 - 12 times per minute, slowly moving chyme through the large intestine.

b. CHEMICAL DIGESTION

Describe the role of bacteria in the large intestine.

Chemical digestion in the large intestine is dependent upon the activity of bacteria that normally reside in the colon. 1. The bacteria ferment remaining carbohydrates in

colonic chyme, releasing gas (flatus). 2. The bacteria secrete vitamin K and some of the B

complex vitamins which are absorbed by the large intestinal mucosa.

4. ABSORPTION AND FECES FORMATION

What role does the large intestine play in absorption?

Simple molecules that escaped the small intestine without being absorbed or which were created by bacterial action, including vitamins, are absorbed by the large intestine. In addition, most of

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the remaining water in the chyme is absorbed, leaving only about 150 ml per day in the feces.

List the components of feces.

Water (about 150 ml/day), undigested foodstuff (plant fibers = cellulose), bacteria, products of bacterial decomposition, sloughed epithelial cells.

5. PHYSIOLOGY OF DEFECATION

Describe the defecation reflex.

Distention of the rectum when it is filled with feces results in stimulation of its stretch receptors. Sensory input to the sacral spinal cord causes parasympathetic motor output which elicits: (1) contraction of the descending colon, sigmoid colon, and rectum, and (2) reflex relaxation of the internal anal sphincter. With volun-tary relaxation of the external anal sphincter, the pressure gener-ated within the colon by contraction of muscularis causes the anus to open and the feces to be expelled.

K. REVIEW OF DIGESTIVE PROCESSES1. CEPHALIC PHASE OF GASTRIC CONTROL

Review the cephalic phase of gastric control.

1. The taste, smell, anticipation, thought, or sensation of food in the mouth promotes cerebral input to the vagal center in the medulla oblongata.

2. Parasympathetic sensations are carried by the vagus nerves to the stomach.

3. Postganglionic parasympathetics in the stomach wall stimulate the gastric juice secretion and the secretion of gastrin from the endocrine cells.

4. Gastrin is carried through the circulation to all parts of the stomach, where it also stimulates gastric juice secretion.

2. GASTRIC PHASE OF GASTRIC CONTROL

Review the gastric phase of gastric control.

1. Distention of the stomach by food initiates local stretch reflexes that stimulate gastric juice secretion.

2. At the same time, distention stimulates afferent vagal input to the vagal nuclei of the medulla

3. This results in efferent vagal impulses that further stimulate gastric juice secretion.

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3. INTESTINAL PHASE OF GASTRIC CONTROL

Review the intestinal phase of gastric control.

1. The presence in the duodenum of chyme with a pH > 3 or containing amino acids and peptides stimulate gastric secretions through vagus nerve pathways or through secretion of gastrin.

2. Stretch of the duodenum by chyme initiates the enterogastric reflex resulting in decreased gastric secretion and motility.

3. The presence of chyme in the duodenum which a pH of < 2 also inhibits the vagal pathway and stimulates secretion of three hormones that enter the circulation for delivery to the stomach, where they inhibit gastric secretion.

4. CONTROL OF BILE SECRETION

Review the control of bile secretion.

1. Secretin, produced by the duodenum and carried through the circulation to the liver, stimulates bile secretion by the liver.

2. Cholecystokinin, produced by the duodenum and carried through the circulation to the gallbladder and sphincter of Oddi, stimulates contraction of gallbladder smooth muscle and relaxation of the sphincter of Oddi. These actions allow bile to move from the gallbladder into the duodenum.

3. Vagal nerve stimulation of the liver causes the liver to secrete bile and the gallbladder to release bile to the duodenum.

5. CONTROL OF PANCREATIC SECRETIONS

Review the control of pancreatic secretion.

1. Secretin, secreted from the duodenum in response to chyme, travels through the circulation to the pancreas, where it stimulates release of a watery secretion rich in bicarbonate arrows.

2. Cholecystokinin, produced by the duodenum in response to chyme, travels through the circulation to the pancreas, where it stimulates release of secretion rich in digestive enzymes.

3. Vagal nerve stimulation of the pancreas causes it to secrete juices rich in digestive enzymes.

6. REFLEXES IN THE COLON AND RECTUM316

Review the reflexes in the colon and rectum.

1. The presence of food in the stomach (gastrocolic reflex) and chyme in the duodenum stimulate mass movement in the colon

2. Mass movements in the colon propel its contents towards the rectum

3. Mass movements are integrated by local reflexes throughout the length of the colon.

4. The presence of feces in the rectum stimulates local stretch reflexes as well as the parasympathetically-controlled defecation reflex which operates via the sacral spinal cord.

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