Chapter 24 Digestive System

I. of the Digestive System: a. The digestive tract is also called the alimentary tract or canal 1. gastrointestinal tract technically only refers to the stomach & intestines b. regions of the digestive tract include: 1. or oral cavity with accessory organs, & tonsils 2. (throat) with tubular mucus glands 3. esophagus with tubular mucus glands 4. stomach 5. small intestines – consists of: a. duodenum b. jejunum c. ileum d. liver e. gallbladder accessory organs f. pancreas 6. large intestines – consists of: a. cecum b. colon c. rectum d. anal canal 7. anus II. Functions of the Digestive System: 1. ingestion 2. mastication (chewing) 3. propulsion – movement from one end to the other of the digestive tract a. usually 24-36 hours b. deglutination – swallowing moves food & liquids (bolus) from the oral cavity into the esophagus c. Peristalsis – is responsible for moving material through most of the digestive tract. i. Muscular contractions occur in peristaltic waves. ii. Some parts of large intestine move material by mass movement, which all contractions that extend over much larger parts of the digestive tract. 4. Mixing – mix with digestive secretions to help break it into smaller parts. i. Segmental contractions – are mixing contractions that occur in the small intestine. 5. Secretion – secretions are added to lubricate, liquefy, and digest the food. i. Mucus – secreted the entire length of the digestive tract ii. Secretions also contain large amounts of water, which liquefies the food. iii. Enzymes – secreted by the oral cavity, stomach, intestines, & pancreas break large food molecules down. 6. digestion – is the breakdown of large organic molecules into: i. carbohydrates into monosaccharides ii. proteins into amino acids iii. triglycerides into fatty acids & glycerol a. consists of mechanical digestion (mastication & mixing of food), and chemical digestion (enzymes) 7. Absorption – is the movement of molecules out of the digestive tract & into the circulation or into the lymphatic system. a. molecules pass out of the digestion tract by: i. simple diffusion ii. facilitated diffusion iii. active transport iv. cotransport 8. elimination – is the process by which the waste products are removed from the body a. Feces – semi solid waste products that are eliminated from digestive tract by defecation.

III. Histology of the Digestive Tract: a. Figure 24.4

b. Digestive tract consists of 4 major layers or tunics c. 3 major types of glands are associated with the intestinal tract: 1. unicellular mucus glands 2. multicellular glands (accessory glands outside the digestive tract 2 3. multicellular glands in the mucosa & submucosa

A. Mucosa: a. Innermost tunic – consists of 3 layers i. Inner – mucous epithelium in the mouth, oropharynx, esophagus and anal canal. ii. Loose C.T. (lamina propria) iii. Pouter thin smooth muscle layer (muscularis mucosa)

B. Submucosa: a. Thick C.T. layer containing nerves, blood vessels & small glands. b. The nerve plexus of nerve cells form the submucosal plexus (Meissner’s plexus)

C. Muscularis: a. Consists of an inner layer of circular smooth muscle & an outer layer of longitudinal smooth muscle. 1. 2 exceptions: i. upper esophagus- mm striated ii. Stomach- 3 layers of smooth muscle. b. Myenteric plexus (Auerbach’s plexus) - is between the 2 muscle layers. c. Submucosal and myenteric plexus constitute the enteric plexus or intramural plexus. i. Important in the control of movement & secretion

D. Serosa or adventitia: a. 4th layer of digestive tract is a C.T layer called serosa or adventitia. b. Serosa is called the visceral

IV. Regulation of the digestive system: A. Nervous Regulation of the Digestive System: 1. local neuronal control of the digestive tract occurs with in the enteric nervous system (ENS) 2. 3 major types of enteric neurons: i. Enteric sensory neurons detect changes in the chemical compositions or detect mechanical changes. ii. Enteric motor neurons stimulate or inhibit smooth muscle contraction & glandular secretion. iii. Enteric interneurons connect enteric sensory & motor neurons. 3. ENS coordinates peristalsis & regulates local refluxes. Which control activities within specific, short regions. B. Chemical Regulation of the Digestive System: a. Digestive tract produces a number of hormones (gastrin, secretin) which are secreted by endocrine cells. b. Histamine are released locally with in the digestive tract.

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I. Peritoneum: 1. The serous membrane that covers the organs is the visceral peritoneum, & the one that covers the interior surface of the body wall is the parietal peritoneum. a. Peritonitis – is the inflammation of the peritoneal membranes; may result from chemical irritation or from infection. i. Ascites- accumulation of excess serous fluid in the peritoneal cavity; may accompany peritonitis, starvation, alcoholism or liver cancer. 2. Mesenteries- hold many of the organs in place within the abdominal cavity; 2 layers of serous membranes. 3. Retroperitoneal – abdominal organs that lie against the abdominal wall, have no mesenteries, and include the duodenum, pancreas, ascending colon, descending colon, rectum, kidneys, adrenal glands & the urinary bladder. 4. Lesser omentum – is the mesentery connecting the lesser curvature of the stomach & the proximal end of the duodenum to the liver & diaphragm. (AKA membrane of the bowels). 5. Greater omentum – the mesentery extending as a fold from the greater curvature & then to the transverse colon. a. Omental bursa- a cavity or pocket between both layers of mesentery. b. Fatty apron – large amount of fat that accumulates in the greater omental. 6. Coronary ligament attaches the liver to the diaphragm. 7. Falciform ligament attaches the liver to the anterior abdominal wall. 8. Mesentery proper- refers to the mesentery associated with the small intestine. 9. Transverse mesocolon – mesenteries of parts of the colon which extend from the transverse colon to the posterior body wall. 10. Mesoappendix is also known as the vermiform appendix. 11. Figure 24.5

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II. Oral Cavity: (mouth) a. Part of the digestive tract bounded by the anteriorly, the (throat) posteriorly, the laterally, the superiorly, and a muscular floor inferiorly. b. It is divided into 2 regions: 1. vestibule 2. oral cavity proper A. Lips & Cheeks: a. The lips (labia) are muscular structures formed mostly by the b. Figure 24.6

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c. One or more frenula (bridle) which are mucosal folds (upper & lower). d. The cheeks form the lateral walls of the oral cavity. 1. The substance of the includes the buccinator muscle, which flattens the cheek against the teeth, & the , which rounds out the profile on the side of the face. e. Lips & cheeks are important in the processes of mastication and speech. i. They help manipulate food with in the mouth & hold it while teeth crush it. ii. Also helps form words.

B. Palate & Palatine Tonsils: a. Palate consists of 2 parts: 1. (anteriorly) 2. (posterior, non-boney) b. Uvula (grape) – is the projection from the posterior edge of the soft palate. c. Palatine tonsils are located in the lateral wall of the fauces.

C. : a. Frenulum – the anterior part of the tongue is relatively free & is attached to the floor of the mouth by this thin fold of tissue. b. The muscles associated with the tongue are divided into 2 categories: 1. intrinsic muscles – with in the tongue itself 2. extrinsic muscles – are outside the tongue but attached to it. i. Protrude & retract the tongue ii. Move the tongue side-to-side iii. They change its shape c. terminal sulcus – a groove on the tongue i. lingual tonsil (lymphoid tissue) on the posterior 1/3 of the tongue: 1. moves food in the mouth 6 2. holds food in place during mastication 3. major role in the mechanism of swallowing 4. major sensory organ for taste 5. one of the primary organs of speech D. Teeth: a. 32 teeth, in 2 dental arches (maxillary arch & mandibular arch) b. teeth in each quadrant include: 1. one central & one lateral incisor 2. one canine 3. 1st & 2nd premolars 4. 1st, 2nd & 3rd molars i. 3rd molars are called wisdom teeth because they usually appear in late teens or early twenties.

c. Figure 24.7

d. teeth of the adult mouth are permanent or secondary teeth; most are replacements for primary or deciduous teeth (milk teeth) e. Each tooth consists of a crown with one or more cusps (points), a neck, & a root. i. Figure 24.8

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f. the clinical crown is that part of the tooth exposed in the oral cavity 1. anatomical crown is the entire enamel – covered part of the tooth g. Pulp cavity – center of the tooth, filled with blood vessels, nerves, & C.T. called pulp. h. Root canal – pulp cavity with in the root 1. Apical foramen – a hole at the point of each root where nerves & blood vessel enter & exit i. Dentin – is living, cellular, & calcified tissue with in the pulp cavity. 1. Enamel – covers the tooth crown, which protects the tooth against abrasion & acids produced by bacteria in the mouth. 2. – covers the surface of the dentin in the root, which helps anchor the tooth in the jaw. j. teeth are set in alveoli (sockets) 1. gingiva – cover the alveolar processes 2. periodontal ligaments secure the teeth in the alveoli k. play a role in mastication & speech l. dental diseases:

8 E. Mastication: 1. Anterior teeth, incisors & canines primarily cut & tear food; premolars & molars crush and grind food. 2. 4 pairs of muscles move the mandible during mastication: i. temporalis ii. masseter closes the jaw iii. medial pterygoid iv. lateral pterygoid – opens the jaw 3. Medial & lateral pterygoid & the masseter accomplish protraction and lateral & medial excursion of the jaw. 4. temporalis retracts the jaws

F. Salivary glands: a. 3 pair of large multicellular glands exist: 1. parotid 2. submandibular 3. sublingual i. Figure 24.9

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b. Also numerous small glands are located deep to the tongue (lingual glands); palate (palatine glands); cheeks (buccal glands), & lips (labial glands). c. All of the major large salivary glands are compound alveolar glands, which are branching glands. 1. The largest salivary glands, the parotid glands, are serous glands which produce mostly saliva & are located anterior to the ear. 2. Figure 24.9 (b)

3. Mumps- inflammation of the is called parotiditis & mumps, which is caused by a virus, is the m/c type of parotiditis. d. the submandibular glands – 1. Passes along inferior border of the posterior half of the mandible. e. The sublingual glands – are the smallest of the 3 paired salivary glands. f. Saliva is secreted at the rate of ~1-1.5L/day. 1. Serous part of saliva contains a digestive enzyme (salivary amylase) which is a starch-splitting enzyme. 2. Saliva prevents bacterial infection in the mouth by washing the oral cavity. It also contains lysozyme, which is an antibacterial and immunoglobin A. 3. The mucus secretions of the submandibular and sublingual glands contain mucin, which is a lubricant.

III. Pharynx a. Consists of 3 parts: 1. nasopharynx 2. oropharynx 3. laryngopahrynx 4. b. the posterior walls of the oropharynx & laryngopahrynx consist of 3 muscles (superior, middle, & inferior pharyngeal constrictors) IV. Esophagus: a. Is between the pharynx & stomach b. Hital hernia – is a widening of the esophageal hiatus; m/c in adults & allow part of the stomach to extend through the opening into the thorax. c. An upper esophageal sphincter & a lower esophageal sphincter regulate the movement of materials into and out of the esophagus.

10 V. Swallowing: (deglutition) a. Is divided into 3 separate phases: 1. Voluntary phase – a bolus is formed and pushed by the tongue to the posterior part of the mouth & into the oropharynx. 2. Pharyngeal phase – is a reflux that is initiated by stimulation of tactile receptors. a. Action potentials pass through trigeminal (V), glossopharyngeal (IX), vagus (X), and accessory (XI) nerves. b. The superior, middle & inferior pharyngeal constrictor muscles contrast at the same time c. This phase is unconscious & is controlled automatically. 3. Esophageal phase – is responsible for moving food from the pharynx to the stomach by peristaltic waves. a. Presence of food in the esophagus stimulates the enteric plexus, which controls the peristaltic waves; also stimulates tactile receptors.

VI. Stomach: A. Anatomy of the Stomach 1. Figure 24.11 (a)

2. The opening from the esophagus into the stomach is the gastroesophageal or cardiac opening. a. The lower esophageal sphincter (cardiac sphincter) surrounds the cardiac opening. b. Fundus – is a part of the stomach to the left of the cardiac region. c. The largest part of the stomach is the body, turns right creating a great curvature & a lesser curvature. d. Body narrows to form the pyloric region (gate keeper), which joins the small intestine.

11 1. Hypertrophic pyloric stenosis – common defect of the stomach I infants in which the pyloris is greatly thickened, not allowing normal stomach emptying. B. Histology of the Stomach: 1. The serosa (visceral peritoneum) is the outermost layer of the stomach. 2. the muscularis consists of 3 layers: i. outer longitudinal layer ii. a middle circular layer iii. inner oblique layer 3. The submucosa & the mucosa layers are thrown into large folds called rugae when the stomach is empty. 4. The epithelium forms gastric pits, which are the opening for the gastric glands. a. Figure 24.11 (b)

b. The epithelial cells of the stomach are of 5 types: i. 1st type – surface mucous cells line the gastric pits. ii. Mucous neck cells – produce mucus iii. Parietal cells (oxyntic) – produces hydrochloric acid & intrinsic factor iv. Chief cells (zymogenic) – produce pepsinogen. v. Endocrine cells – which produce regulatory hormones.

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C. Secretions of the Stomach: 1. Ingested food & stomach secretions, mixed together, form a semi fluid material called chyme. 2. Stomach secretions include: a. mucus (viscous & alkaline) b. hydrochloric acid (produces low pH & kills bacteria) c. gastrin d. histamine e. intrinsic factor (glycoprotein) f. Pepsinogen (inactive form of the protein-digesting enzyme pepsin). i. Chief cells secrete pepsinogen, which is packaged in zymogen granules, which are released by exocytosis. ii. Hydrochloric acid & previously formed pepsin molecules convert pepsinogen to pepsin. I. Regulation of stomach secretion: a. ~2-3L of gastric secretions are produced each day. i. The amount & type of food entering the stomach affects the secretion amount ii. Nervous & hormonal mechanisms regulate gastric secretion b. regulation is divided into 3 phases: 1. Cephalic phase – the sensations of taste & smell of food, stimulation of tactile receptors, & pleasant thought of food stimulate medulla oblongata. a. Parasympathetic stimulation of the stomach mucosa results in the release of the neurotransmitter acetylcholine, which increases the secretory activity of both the parietal & chief cells & stimulates the secretion of gastrin & histamine. 1. Histamine has the greatest stimulatory effect. 2. Gastric phase: a. Greatest volume of gastric secretion is produced in this phase. i. Presence of food in the stomach initiates this phase & the primary stimuli are distension of the stomach & presence of amino acids & peptides in the stomach. ii. The negative- feedback mechanism limits the secretion of gastric juice. 3. Intestinal phase: a. The entrance of acidic stomach contents into the duodenum of the small intestine controls the intestinal phase of gastric regulation. i. Causes the release of the hormone, secretin, which inhibits gastric secretion by inhibiting both parietal & chief cells. 13 ii. Acidic solutions also initiate a local enteric reflux, which inhibits gastric secretions. b. Fatty acids & certain other lipids in the duodenum and the proximal jejunum initiate the release of 2 hormones: i. Gastric inhibitory polypeptide- inhibits gastric secretions. ii. Cholecystokinin – inhibits gastric secretions c. The mechanism appears to involve the secretion of a hormone (enterogastrone). d. Table 24.3

e. Inhabitation of gastric secretions is also under nervous control. f. Distention of the duodenum wall, the presence of irritating substances in the duodenum, reduced pH, hypertonic & hypotonic solutions in the duodenum activate the enterogastric reflux.

D. Movements of the stomach: I. Stomach filling: 1. As food enters the stomach, the rugae flattens & the stomach volume increases. 2. The reflux inhibits muscle tone in the body of the stomach II. Mixing the Stomach Contents: 1. The mixing in the stomach is accomplished by gentle mixing waves, which occur about every 20 seconds (80%) 2. Peristaltic waves occur less frequently, are significantly more powerful (20%) III. Stomach Emptying: a. The amount of time food remains in the stomach depends on a number of factors: 1. type & volume of food 2. liquids exit with in 1 ½ -2 ½ hours after ingestion 3. After a typical meal, the stomach is usually empty within 3-4 hours.

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