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Chapter 23: The Digestive System

Describe the function of the digestive system, and differentiate between organs of the alimentary canal and the accessory organs or structure.
Digestive organs fall into two main groups: the alimentary canal and the accessory organs.
• The alimentary canal, or GI tract, is the continuous muscular digestive tube that winds through the body digesting and absorbing foodstuff; its organs include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory digestive organs or structures aid digestion physically and produce secretions that break down foodstuff in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands, liver and pancreas.
List and define the major processes occurring digestive system activity.
• Ingestion: the simple act of putting food into the mouth

• Propulsion or Motility: moves food through the alimentary canal and includes both swallowing and peristalsis.

• Mechanical Breakdown or Digestion: the physical process of preparing the food for chemical digestion and involves chewing, mixing, churning and segmentation.

• Chemical Digestion: a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by the secretion of enzymes and various secretions (acid, bile, etc...)

• Absorption: the passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph.

• Defecation or Elimination: eliminates indigestibles substances from the body via the anus as feces.

❊ Segmentation: rhythmic local constrictions of the small intestine. Segmentation mixes food with digestive juices and makes absorption more efficient by repeatedly moving different parts of the food mass over the intestinal wall.
Describe stimuli and controls of digestive activity (basic functional concepts).
• Digestive activities within the GI tract are triggered by mechanical and chemical stimuli [e.g., stretching of organ by food, osmolarity, pH, etc.] and are detected by mechanoreceptors and chemoreceptors.

• Controls of digestive activity are both intrinsic ["in-house"] and extrinsic.
a. Neural mechanisms [Enteric Nervous System (ENS)]: the ENS has roughly as many neurons as the spinal cord, and as many neurotransmitters as the brain, but whose functions remain largely unknown...
❊ Short [myenteric] reflexes control smooth muscle contraction and glandular secretion as relatively localized activities involving small segments of the digestive tract; this mechanism is usually considered parasympathetic, but the plexus also contains sensory neurons, motor neurons and interneurons for local reflexes whose internal workings operate entirely outside the control of the CNS.
❊ Long reflexes involve interneurons and motor neurons in the CNS and provide a higher level of control over digestive and glandular activities.
b. Hormonal mechanisms involve as many as 18 hormones, affecting almost every aspect of digestive function.
c. Local mechanisms involve prostaglandins, histamines and other chemicals released into the interstitial fluid and affecting adjacent cells...
Describe the location and function of the peritoneum.
• The visceral peritoneum covers the external surfaces of most the digestive organs, and the parietal peritoneum lines the body wall of the abdominopelvic cavity.

• The peritoneal cavity is located between the visceral and parietal peritoneum and is filled with serous fluid.
Define retroperitoneal and name the retroperitoneal organs of the digestive system.
• Retroperitoneal organs are found posterior to the mesentery, lying against the dorsal abdominal wall.

• The retroperitoneal organs include most of the duodenum, the pancreas, the ascending and descending colon and the rectum.
Describe the function of the mesentery and cite specific examples of individual mesenteries.
• Mesentery is a double layer of peritoneum that extends to the digestive organs from the body wall.

• It allows blood vessels, lymphatics and nerves to reach the digestive organs, and holds the organs in place as well as stores fat.

• Some individual mesenteries include the greater omentum, the lesser omentum, the falciform ligament, the mesentery proper and the mesocolon.
Define splanchnic circulation and indicate the importance of the hepatic portal system.
• The splanchnic circulation serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation [hepatic, splenic, and left gastric arteries of the celiac trunk and the superior and inferior arteries]
Describe the tissue composition and the general function of each of the four layers of the alimentary canal.
1. Mucosa: the innermost, moist, epithelial membrane that lines the entire digestive tract. The mucosa consists of an epithelium, underlying lamina propria [areolar CT] and a thin muscularis mucosae [smooth muscle].
• It secretes mucus, digestive enzymes, and hormones
• Absorbs digestive end products into the blood
• Protects against infectious disease

2. Submucosa: a moderately dense connective tissue layer [areolar and dense irregular CT] containing:
• Blood and lymphatic vessels
• Lymphoid follicles
• Nerve fibers [submucosal nerve plexus or Meissner]
The lamina propria of the mucosa and the submucosa house MALT [Mucosa-Associated Lymphatic Tissue]; in the ileum [lasy section of the small intestine] the MALT is known as Peyer patches.

3. Muscularis externa: typically consists of smooth muscle and is responsible for peristalsis and segmentation. It consists of an:
• Inner circular layer
• Outer longitudinal layer
These muscular layers are controlled by the myenteric [Auerbach] plexus.

4. Serosa [adventitia]: the protective outer layer of the intraperitoneal organs, is the visceral peritoneum.
Identify the three major extrinsic salivary glands and their functions and describe the composition and functions of saliva, and explain how salivation is regulated.
Extrinsic Salivary Glands:
1. Parotid Gland: The main function of the parotid gland is to release saliva through the Stenson's duct, which opens opposite the upper second molar tooth. The parotid glands are found overlying the mandibular ramus and the inferior part of the external ear.

2. Sublingual Gland: The sublingual gland is located in the mouth. Its function is to produce mucin which is found in saliva. This gland is found on either side of the mouth underneath the tongue.

3. Submandibular Gland: The mucous cells of the gland are most active so it mainly produces saliva, about 70% made in the body. specifically, the serous cells produce amylase, which breaks down starches. also, the mucous cells produced help lubricate the throat

Saliva Composition: 1.0-1.5 liters/day, 97- 99.5% water, but also IgA, lysozyme, defensins, mucins and salivary amylase; production of a bolus.

Regulation: Parasympathetic [watery saliva] and sympathetic [more viscous saliva] stimulation.
Describe the anatomy of the esophagus and define GERD.
The Esophagus:
• Muscular tube about 25 cm (10 inches) long
• All four tunics
• Mucosa = nonkeratinized stratified squamous epithelium

GERD: Gastroesophageal Reflux Disease, is the burning, radiating substernal pain that occurs when the stomach acid regurgitates into the esophagus.
Describe the gross anatomy and functions of the stomach, including structural modifications of the wall of the stomach that enhance the digestive process.
1. Storage
2. Mechanical and chemical breakdown of food, forming chyme
3. Production of intrinsic factor [a glycoprotein required for absorption of vitamin B₁₂ in the small intestine]
4. Protection
5.Absorption [small nonpolar substances; e.g., alcohol and aspirin]

Gross Anatomy:
• The adult stomach varies from 15-25 cm long
• Its diameter and volume vary depending on the amount of food it contains
• The major regions of the stomach include:
1. Cardiac Region
2. Fundus
3. Body
4. Pyloric Region
• The convex lateral surface of the stomach is its greater curvature
• Its convex medial surface is its lesser curvature
• Extending from the curvatures are the lesser omentum and the greater omentum, which help to tie the stomach to other digestive organs and the body wall.
Name the cell types responsible for secreting the various components of gastric juice and indicate the importance of each in stomach activity.
The gastric glands of the stomach produce gastric juice, which may be composed of a combination of mucus, hydrochloric acid, intrinsic factor, pepsinogen, and a variety of hormones.
a. Surface mucous cells: continuously secrete an alkaline product containing mucin
b. Mucous neck cells: produce an acidic mucin
c. Parietal cells: hydrochloric acid
`Explain how gastric secretion and stomach motility are regulated [three reflexes] and define and account for the alkaline tide.
• Gastric secretion is controlled by both neural and hormonal mechanisms and act in three distinct phases:
1. Cephalic Phase
2. Gastric Phase
3. Intestinal Phase

• The reflex -mediated relaxation of the stomach muscle and plasticity of the visceral smooth muscle allow the stomach to accommodate food and maintain internal pressure.

• The interstitial cells of Cajal establish the stomach's basic electrical rhythm of peristaltic waves.

• Regulation of Gastric Secretion:
a. Phase 1: Cephalic [Reflex]
b. Phase 2: Gastric
c. Phase 3: Intestinal

• Alkaline Tide: Making of hydrochloric acid
Describe the gross anatomy and functions of the small intestine, including the structural modifications of the wall of the small intestine that enhance the digestive process.
The small intestine is the body's major digestive organ. Within its twisted passageways, digestion is completed and virtually all absorption.

Gross Anatomy:
• The small intestine extends from the pyloric sphincter to the ileococal valve where it joins the large intestine.
• Three Subdivisions:
1. Duodenum
2. Jejunum
3. Ileum

Modifications For Absorption:
• It is highly adapted for absorption with three microscopic modification:
1. Pilcae Circulares
2. Villi
3. Microvilli [brush border]
Differentiate among the roles of the various cell types of the intestinal mucosa.
Histology Of The Wall:
a. Simple columnar epithelial tissue with tight junctions and goblet cells
b. The intestinal crypts, or the crypts of Lieberkühn, secrete intestinal juice that serves as a carrier fluid for absorbing nutrients from chyme.
c. Enteroendocrine cells and the enterogastrones secretin and cholecystokinin
d. Intraepithelial lymphocytes [IELs]
e. Paneth cells: defensins and lysozyme
f. Peyer's patches: part of GALT
Describe the function local digestive hormones and paracrines.
1. Cholecystokinin (CCK):
• Potentiates secretin's actions on liver and pancreas
• Increases output of enzyme-rich pancreatic juice in pancreas
• Stimulates gallbladder to contract and expel stored bile
• Relaxes hepatopancreatic sphincter to allow entry of bile and pancreatic juice into duodenum

2. Gastrin:
• Increases HCI secretion in stomach (parietal cells)
• Stimulates gastric emptying (minor effect)
• Stimulates contraction of intestinal muscle in small intestine
• Relaxes ileocecal valve
• Stimulates mass movement in large intestine

3. Histamine:
• Activates parietal cells to release HCI in stomach

4. Intestinal Gastrin:
• Stimulates gastric glands and motility in stomach
Describe the structure and function of the gallbladder and liver and state the role of bile in digestion.
Gross Anatomy of the Liver:
• Largest gland in the body
• Located under the diaphragm and lies almost entirely within the rib cage
• Four Primary Lobes:
1. Right Lobe (largest lobe)
2. Left Lobe (separated by a fissure)
3. Caudate Lobe
4. Quadrate Lobe
• The falciform separates the right and left lobes and round ligament
• The hepatic portal vein and hepatic vein enter the liver at the porta hepatis ("gateway to the liver") and the common hepatic ducts, which runs inferiorly from the liver, all travel through the lesser omentum to reach their destination

Microscopic Anatomy of the Liver:
• The liver is composed of sesame seed-sized structure and functional units called liver lobules
• Liver lobules consist of hepatocytes (liver cells)
• Hepatocyte plates radiate outward from a central vein running in the longitudinal axis of the lobules
• Hepatocytes:
1. Process bloodborne nutrients in various ways (e.g. they store glucose as glycogen and use amino acids to make plasma proteins)
2. Store fat-soluble vitamins
3. Play important roles in detoxification, such as ridding the blood of ammonia by converting it to urea.
• Portal triad (portal tract region) contains:
1. A branch of the hepatic artery (supplying oxygen-rich arterial blood to the liver)
2. A branch of the hepatic portal vein (carrying venous blood laden with nutrients from the digestive viscera)
3. A bile duct
• Liver Sinusoids: between the hepatocyte plates

Function of the Liver:
• Hematological Regulation: Phagocytosis and antigen presentation [kupffer cells], synthesis of plasma proteins, removal of circulating hormones, removal of antibodies, removal or storage of toxins.
• Synthesis and secretion of bile [mostly water, with minor amounts of ions, bilirubin, cholesterol and an assortment of lipids know as bile salts, that function in the emulsification of lipids.

• Rests in a recess on the inferior surface of the right liver lobe
• Stores and concentrates bile

• contains:
1. yellow-green
2. alkaline solution containing bile salts
3. cholesterol
4. triglyceride
5. phospholipids
6. electrolytes
• Bile salts and phospholipids aid in digestive process
• bile salts are primarily cholic and chenodeoxycholic acid, are cholesterol derivatives
• They emulsify fats
Describe the structure and function of the pancreas and the role of pancreatic juice in digestion.
Structure of the Pancreas:
• soft, tadpole-shaped gland that extends across the abdomen its tail to its head, which is encircled by the duodenum
• most of the pancreas is retroperitoneal and lies deep to the greater curvature of the stomach

• produces enzymes that break down all categories of foodstuffs (exocrine product = pancreatic juice)
• secretion of a watery buffer solution of a pH of 7.5-8.8 and a "bucket full of enzymes" [pancreatic amylase, lipase, nuclease and proteases]

Pancreatic Juice:
• drains from the pancreas into the duodendum via the centrally located main pancreatic duct.
Describe how bile and pancreatic juice secretion into the small intestine are regulated.
1. Chyme entering duodenum causes release of CCK and secretin from duodenal enteroendocrine cells.
2. CCK and secretin enter the bloodstream.
3. CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3̄-rich pancreatic juice.
4. Bile salts and to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly.
5. CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum.
6. During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.
Describe the structure and functions of the large intestine and identify its specialized structure.
• Frames the small intestine on three sides and extends from the ileocecal.
• Teniae Coil: longitudinal muscular layer of its muscularis is mostly reduced to three bands of muscle
• Haustra: pocketlike sacs in the wall of the large intestine
• Epiploic Appendages: fat pocket pouches of the visceral peritoneum that hang from the surface of the lungs
• Subdivisions:
1. Cecum
2. Appendix
3. Colon
4. Rectum
5. Anal Canal
• Colon:
1. Ascending
2. Transverse
3. Sigmoid

• Absorption of water, bile salts, and bacterial-generated vitamins K, biotin [glucose metabolism] and vitamin B₃ (pantothenic acid0 [manufacture of steroids and some neurotransmitters]
Describe the motility and regulation of the large intestine.
• Haustral Contractions: slow segmenting movements lasting about 1min that occur 30mins or so.
• Mass Movements: (or mass peristalsis) are long slow-moving, but powerful contractile waves that move over large areas of the colon 3-4 times daily and force the contents toward the rectum.
• Gastrocolic Reflex: happens in the colon
List the enzymes involved in chemical digestion of proteins, carbohydrates and lipids.
• Brush Border Enzymes
• Salivary Amylase
• Pancreatic Amylase

• Pancreatic Enzymes
• Brush Border Enzymes
• Pepsin

• Pancreatic Lipases
• Emulsification by the detergent action of bile salts ducted in from the liver
Describes the process of absorption of breakdown products of foodstuffs that occurs in the small intestines.
• Virtually all of the foodstuffs and most of the water and electrolytes are absorbed in the small intestine.
• Most nutrients are absorbed by active transport processes,except for fat digestion , fat-soluble vitamins, and most water-soluble vitamins (which are absorbed by diffusion).
Describe some of the structural and functional properties of smooth muscle.