Biology 206: Chapter 23 Digestive System: Laurel Spring 2007

BIOLOGY 206: CHAPTER 23 DIGESTIVE SYSTEM: LAUREL SPRING 2007

I. Overview of the Digestive System

A. Overall Function

1. Digestive system takes in food, breaks it down into nutrient molecules, absorbs those molecules into the bloodstream and rids the body of indigestible remains.

B. Organization: 2 major components

1. GI tract/alimentary canal: organs through which food actually passes. In order includes: oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

2. Accessory organs: assist in digestion, but food does not pass through them.

a.  Include: teeth, tongue, salivary glands, pancreas, liver, and gall bladder.

II.  Digestive Process

A.  Overview

1.  Digestive tract is a disassembly line where food becomes less complex at each processing step.

2.  The 6 essential activities of digestion follow

B.  Ingestion

1.  taking of food into the body.

C.  Propulsion

1.  movement of food through the digestive system

2.  Peristalsis is the major means of propulsion and is involuntary waves of contraction and relaxation of smooth muscles of organ wall.

D.  Mechanical Digestion

1.  Prepares food for chemical digestion

2.  Includes chewing, mixing and segmentation (local rhythmic contractions)

E.  Chemical Digestion

1.  Chemical breakup of food into its smallest building blocks

2.  Accomplished by digestive enzymes

F.  Absorption

1.  passage of digested end products, vitamins, minerals and water from the lumen of the GI tract into the blood or lymph

2.  The small intestine is the major organ of absorption

G.  Defecation

1. elimination of indigestible substances through the anus as feces

III. Peritoneum = serous membrane of abdominopelvic cavity

A. Visceral peritoneum

1. covers external surface of most digestive organs

B. Parietal peritoneum

1. Continuous with visceral peritoneum

2. Lines abdominal and pelvic cavities.

3. Separated from visceral peritoneum by the peritoneal cavity with contains serous fluid.

C. Folds of the peritoneum

1.  Mesentery is a double layer of peritoneum that extends to the digestive organs from the body wall.

a.  route for blood and lymphatic vessels and nerves

b.  hold organs in place

c.  store fat

D. Retroperitoneal

1. refers to organs that adhere to the dorsal wall and lie behind the peritoneum.

2.  Includes portions of duodenum, pancreas and rectum

3.  May help the alimentary canal from kinking

IV. Histology of GI tract wall: 4 layer plan

A. Mucosa/mucous membrane:

1. moist innermost layer that lines the lumen.

2. Functions to 1) secrete enzymes, hormones and mucus, 2) absorption and 3) protection against disease

3.  Consists of 3 sublayers.

a.  epithelium of simple columnar epithelium and goblet cells with hormone and enzyme-secreting cells

b.  Lamina propria: areolar connective tissue with blood and lymph vessels and nodes.

c. . Muscularis mucosa: small amount of smooth muscle and creates local movement of mucosa and produces folds to increase surface area

B. Submucosa

1. Connective tissue that is deep to mucosa.

2. Connects mucosa to muscularis and contains nerves, lymph and blood vessels.

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C. Muscularis Externa

1. Mixes food and propels it through tract.

2. Mouth, pharynx, and upper third of esophagus = skeletal muscle for voluntary control for swallowing.

3. Rest has 2 layers of smooth muscle:

a. Circular muscle: innermost

b. Longitudinal: outermost

c. Stomach has a third layer of oblique muscles

4. In several places the circular layer thickens to form sphincters that act as valves to regulate food flow between organs.

D. Serosa (visceral peritoneum)

1. Outermost layer with protective function

2. areolar connective tissue and simple squamous epithelium

2. In the esophagus the serosa is replaced by an adventitia of fibrous connective tissue

V. Anatomy of Oral cavity/buccal cavity/mouth

A. General anatomy

1. Bounded by lips anteriorly and cheeks laterally which keeps food between teeth and assists in speech.

2. Bounded superiorly by palate.

a. Hard palate: composed of palatine and maxillary bones (palatine process) against which tongue forces food in chewing.

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b. Soft palate: posterior to hard palate, separate oral cavity from nasopharynx and closes off the nasopharynx in swallowing. Uvula is posterior portion of soft palate.

3. Vestibule: area between lips, cheeks, and gums.

4. Oral cavity proper lies within teeth and gums.

B. Tongue

1. Occupies floor of mouth

2. Composed of skeletal muscle, connective tissue, glands, and sense receptors.

3. Functions in speech, mixes food, forms food into bolus (ball) for swallowing, initiates swallowing.

4. Lingual Frenulum: fold of mucosa that secures tongue to floor of mouth and limits its posterior movement.

5. Labial frenulum is median fold that joins the internal aspect of each lip to gum.

6. Extrinsic muscles attach tongue to bone (hyoid, mandible, styloid process of temporal) and move tongue within the mouth.

7. Intrinsic muscles are not attached to bone and change the shape of the tongue (curl)

8. Papilla: projections of mucosa on superior surface

a. Aid in licking

b. Contain taste buds

C. 3 pairs of Salivary glands

1. Parotid: large, anterior and inferior to ears, superficial to masseter.

a. Inflamed by mumps

b. Ducts empty by upper molars.

2. Submandibular: lie medial to mandibular angle near base of tongue.

a. Ducts empty saliva at frenulum.

3. Sublingual: anterior to submandibular and under tongue

a.  Ducts empty along floor of mouth.

4.  Above 3 are extrinsic salivary glands because they are outside the oral cavity

5.  Intrinsic salivary glands are small and scattered throughout oral cavity

6. Saliva composed of

a. Water (99%) to moisten and dissolve food

b. with enzyme salivary amylase for starch digestion

c.  lysozyme to destroy bacteria. Are also other antibacterial agents

d.  Inorganic salts to buffer food for optimum pH for amylase

e.  Electrolytes such as Na+, Cl-, HCO3- etc.

f. Mucin: mixes with water to form mucus for lubrication

7. Functions:

a. Moisten food

b. Cleanse teeth

c. Dissolve food for tasting

d. Begin chemical breakdown of starch

D. Control of Salivation

1. Intrinsic glands secrete continuously to keep mouth moist but extrinsic are activated to deal with food.

2. Secretion of saliva is reflex response.: and controlled by the PARASYMPATHETIC division of the ANS

3. Salivation center between medulla and pons stimulated activated by chemoreceptors and pressoreceptors in mouth à increased parasympathetic firing down facial and glossopharyngeal nervesà increased salivary secretion rich in enzymes

4. Higher brain centers (smell, sight, hearing, thought of food) à increased salivation

5. Stomach and SI can also increase salivation, especially if irritated

6. Sympathetic nerves à decreased salivation as occurs in stage fright

VI. The Pharynx

(Review on your own the anatomy of the pharynx as learned in the Respiratory System)

VII. Anatomy of Esophagus

A. Structure and Function

1. 25 cm (10 in.) long muscular tube that acts as a passageway for food from pharynx to the stomach

2. Upper third: skeletal muscle, middle third: skeletal and smooth, lower third: all smooth.

3. Collapsed when not swallowing

4. Pierces diaphragm at esophageal hiatus

5. Joins stomach at cardiac orifice/ gastroesophageal sphincer

B. Deglutition: swallowing

1. Buccal phase is voluntary. A bolus is formed and pushed into oropharynx

2. Pharyngeal – esophageal phase: bolus moves through pharynx and esophagus by peristaltic waves via reflex action coordinated in the medulla and pons

a. Soft palate/uvula lifts to close opening to nasopharynx

b. Larynx rises so epiglottis closes glottis

C. Peristalsis

1. wave of contractions of smooth muscle that propels food forward through the GI tract.

VIII. Anatomy of the Stomach

A. Functions

1. Mechanical mixing to produce a creamy paste called chyme.

2. Enzymatic digestion

3. Storage of up to 4 L through distension

4. Absorption

5. Destroy foreign substances with HCl.

B. Gross Anatomy

1. J-shaped when empty

2. Rugae are longitudinal folds of mucosa to increase surface area.

3. Regions of the stomach

a. Cardiac region: small area that surrounds the cardiac orifice through which food enters from the esophagus.

b. Fundus: dome-shaped part projecting above the cardiac orifice

c. Body: main section

d. Pyloric region: funnel-shaped inferior end which ends in the pylorus where the stomach joins the duodenum through the pyloric sphincter

4. Greater curvature: convex lateral surface and anchors greater omentum

5. Lesser curvature: the on the concave medial surface and anchors lesser omentum

6.  Both omenta are mesentaries that tether the stomach to the body wall and other digestive organs.

7.  Lesser omentum: fold that connects stomach to liver.

8. Greater omentum: large fold that hangs from inferior portion of the stomach, interconnects stomach, duodenum, and large intestine.

a. Called fatty apron as has much adipose tissue

b. Also contains many lymph nodes that protects the peritoneal cavity and peritoneal organs

C. Microscopic Anatomy

1.  In addition to the circular and longitudinal smooth muscles that run through the digestive system, the stomach has an extra layer that runs obliquely. This allows for extra mixing, churning, and physical breakdown of food.

2.  The mucosa is composed of simple columnar epithelilum of alkaline mucous-producing goblet cells

3. Gastric pits are deep grooves in mucosa that lead into the gastric glands which produce the stomach secretion called gastric juice.

4. Glands composed of different types of cells as follows:

5. Parietal cells secrete:

a. HCl (pH = 1.5-3.5  = 2)

(1) Converts pepsinogen to pepsin

(2) Kills Microorganisms

(3) Disintegrates plasma membranes/disintegrates intermolecular bonds

b. Intrinsic factor: required for absorption of vitamin B12 and formation of RBCs.

6. Chief cells which secrete pepsinogen, a precursor of the enzyme pepsin.

7.  Enteroendocrine cells: secrete hormones such as gastrin, a hormone which increases gastric secretion and motility plus contracts the cardiac sphincter and relaxes the pyloric sphincter.

D.  Emesis

1.  vomiting

2.  Generally caused by excess stretching of stomach or small intestines or chemical irritants

3.  Force generated by the diaphragm and abdominal muscles

4.  Excess vomiting can lead to dehydration and acid-base imbalances

IX. Digestive Processes Occurring in the Stomach

A.  Activities

1.  Only enzymatic digestion in adults is protein digestion accomplished by the enzyme pepsin

2.  Recall the intrinsic factor is also secreted by stomach and prevents pernicious anemia

3.  Lipid-soluble substances like alcohol and aspirin are absorbed in stomach

B. Regulation of Gastric Secretions (recall parasympathetic is the stimulator!)

1. Gastric juices and motility regulated by nerves and hormones in 3 phases

2. Cephalic phase: nervous stimulation

a. Smell, taste, sight, thought of food stimulates cerebral cortex àfeeding center in hypothalamus à impulses to medullaà increases parasympathetic input to gastric glands via vagus nerve à increased rate of gastric secretion

3. Gastric phase: both neural and hormonal stimulation once food is in stomach

a. Nervous control begins with distension of stomach wall which activates stretch receptors à increased vagal firing which stimulate the medulla àincreased parasympathetic (vagal) stimulation of gastric glands à increased gastric secretions.

a.  The secretion of the hormone gastrin from stomach mucosa is stimulated locally by presence of food in stomach, especially proteins, caffeine, alcohol and rising pH . It circulates in blood stream, returns to stomach and increase gastric secretions, gastric motility and increase gastric emptying.

4  Intestinal phase

a. The first component is excitatory and mimics gastrin so the hormone is called enteric gastrin. It is initiated when chyme enters first part of the duodenum. Is very brief.

b.  The second component is inhibitory and is called the enterogastric reflex . It inhibits gastric secretions, gastric motility and decrease gastric emptying and is stimulated by chyme in lower duodenum (low pH, distension of duodenum, protein digestion products).

c. Allows more time for duodenal digestion and absorption.

d. Stimulation à increased vagal afferent input to medullaà increased sympathetic stimulation to stomach àdecreased gastric secretions, decreased motility, and decreased emptying

e. Chyme in duodenum also causes release of many intestinal hormones that also inhibit gastric peristalsis. 2 examples are

a. Secretin

b. Cholecystokinin (CCK)

X. Small Intestine and Associated Structures

A. Functions

1. Major organ of digestion and absorption

B. Gross Anatomy of the Small Intestine

1. Major organ of both digestion and absorption

2. 2 valves

a. Pyloric: between stomach and duodenum

b. Ileocecal: between ileum of Small intestine and Large Intestine

3. 6-7 m long (17-20') and 2.5 cm (1inch) in diameter.

4. It has 3 segments: duodenum, jejunum and ileum

5. Duodenum:

a. First straight section of 25 cm. (10 in.) and much is retroperitoneal

b. Receives chyme from stomach

c. Receives secretions from pancreas and gall bladder whose ducts dump into common area, the hepatopancreatic ampulla, and then enter the duodenum at the duodenal papilla, controlled by hepatopancreatic sphincter

6. Jejunum: second segment of 2.5 m (8')

7.  Ileum: last segment of 3.5 m (12') which joins Large Intestine at the ileocecal valve.

8.  The ileum and jejunum are sometimes called ileojejunum and its parts are suspended from the posterior abdominal wall by the fan-shaped mesentary

C. Microscopic anatomy of S.I is adapted for absorption and secretion

1. Plicae circulares: deep, permanent folds of mucosa and submucosa

2. Villi: finger like projections of mucosa approximately 1 mm tall.

a. Each contains a blood capillary bed and lymphatic capillary bed, a lacteal, for absorption.

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3. Microvilli: microscopic projections of plasma membranes which form a brush border

4. All 3 of the above structures function to increase surface area for absorption (600X).

5. Between the villi are openings into intestinal glands which secrete intestinal juice of water, mucus, lysozyme and digestive enzymes.

6. Peyer’s Patches are lymphoid structures in the submucosa and become more abundant toward the large intestine where there are huge numbers of bacteria.

D. Digestive Processes Occurring in the Small Intestine

1.  Food spends 3-6 hours in the small intestine

2.  Most digestion and absorption take place here

3. The major type of movement in the small intestine is segmentation

a. Rhythmic contractions form segments along SI and function to push the food back and forth a few centimeters at a time for maximum contact with the mucosa and digestive enzymes.