Chapter 24: The Digestive System

Question 1

identify the organs of the digestive system.

Answer 1

A. Digestion

digestive system – body system that ingests food, breaks it down, processes it, and eliminates wastes from the body.

1. gastrointestinal (GI) tract or alimentary canal – a continuous tube running through the ventral body cavity extending from the mouth to anus.
2. accessory digestive organs – include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Teeth aid in physical breakdown of food, tongue assists in chewing and swallowing, the rest of the accessory digestive organs produce or store secretions that flow into the Gi tract through ducts.

primative gut = forerunner of gastrointestinal tract

Question 2

describe the basic processes performed by the digestive system.

Answer 2

basic processes

ingestion – taking food into mouth

secretion – release of water, acid, buffers, and enzymes into lumen of GI tract

mixing and propulsion – churning and movement of food through GI tract motility – the capability of the GI tract to mix and move material along its length

digestion – mechanical and chemical breakdown of food

1. mechanical digestion – the teeth cut and grind food before it is swallowed, and then smooth muscles of the stomach and small intestine churn the food to further assist the process
2. chemical digestion – the large carbohydrate, lipid, protein, and nucleic acid molecules in food are split into smaller molecules by hydrolysis

• *absorption** – passage of digested products from GI tract into blood and lymph
• *defecation** – elimination of feces from GI tract

1. feces – material discharged from the rectum. Made up of bacteria, excretions, and food residue.

Question 3

describe the structure and function of the layers that form the wall of the gastrointestinal tract.

Answer 3

layers of the GI tract – 4 layered arrangement of tissues (Deep to superficial) Mucosa, submucosa, serosa, and Adventitia (only in the esophagus)

Mucosa – deepest layer. The inner lining of the GI tract. Composed of 3 layers:
Epithelium – in direct contact with contents of the GI tract

1. Mainly nonkeratinized stratified squamous epithelium
2. Serves a protective function
3. In the stomach and intestines, it is simple columnar epithelium with tight junctions between.
4. Rate of renewal = 5-7 days

enteroendocrine cells – cell of the mucosa of the GI tract that secretes a hormone to govern function of the GI tract.

a. Several different types

lamina propria – a layer of connective tissue within the mucosa of the GI tract

1. Areolar connective tissue containing many blood and lymphatic vessels
2. Supports the epithelium and binds it to the muscularis mucosae
3. Contains the majority of the MALT cells

mucosa associated lymphatic tissue (MALT) - lymphatic nodules scattered throughout the lamina propria of mucous membranes of the GI tract (also in respiratory airways, urinary tract, and reproductive tract)

1. Contain immune system cells that protect against disease.
2. Present along the entire GI tract, especially in the tonsils, small intestine, appendix, and large intestine

muscularis mucosae – a thin layer of smooth muscle within the mucosa

1. Causes the mucous membrane of the stomach and small intestine to form many folds, increasing surface area for digestion and absorption
2. This layer moves, ensuring all absorptive cells are fully exposed to GI tract contents

Submucosa – 2nd deepest layer

1. Consists of areolar connective tissue that binds the mucosa to the muscularis
2. Contains many blood and lymphatic vessels that receive absorbed food molecules
3. Also contains the submucosal plexus (described shortly)
4. May also contain glands and lymphatic tissue.

submucosal plexus – extensive network of neurons Muscularis – third layer, 2nd from superficial

1. Contains skeletal muscle in mouth, pharynx, and superior to middle parts of esophagus to allow swallowing
2. External anal sphincter also contains skeletal muscle to control defecation
3. Smooth muscle in rest of tract, generally in 2 sheets:

Inner sheet of circular fibers Outer sheet of longitudinal fibers

d. Myenteric plexus – a second plexus of neurons between the layers of the muscularis

Serosa – most superficial layer

1. A serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium)
2. AKA visceral peritoneum because it forms a portion of the peritoneum

Adventitia – also most superficial layer but located only in the esophagus

1. Esophagus lacks a serosa
2. This is a single layer of areolar connective tissue

Question 4

describe the nerve supply of the GI tract.

Answer 4

neural innervation of GI tract – regulated by an intrinsic set of nerves called the ENS and an extrinsic set of nerves that are part of the ANS

enteric nervous system (ENS) - the part of the nervous system embedded in the submucosa and muscularis of the GI tract. Governs motility and secretions of the GI tract

1. Consists of about 100 million neurons that extend from esophagus to anus.
2. Arranged in two plexuses: myenteric and submucosal
3. both plexuses consist of motor neurons, interneurons, and sensory neurons.
4. The interneurons connect the myenteric and submucosal plexuses
5. Two major types of sensory receptors in the GI tract:

Chemoreceptors – detect chemicals in food present in the GI tract
Mechanoreceptors – ex. Stretch receptors, that are activated when food distends the wall of a GI organ

myenteric plexus (plexus of Auerbach) - located between the longitudinal and circular smooth muscle layers of the muscularis

a. Governs GI tract motility, particularly frequency and strength of contraction of the muscularis

submucosal plexus (plexus of Meissner) - found within the submucosa

a. The motor neurons supply the secretory cells of the mucosal epithelium, controlling secretions of the organs of the GI tract

autonomic nervous system – regulates neurons of the ENS

1. Vagus (X) nerve supplies parasympathetic fibers to most parts of the GI tract
2. Sacral spinal cord supplies parasympathetic fibers to the last half of the large intestine
3. Parasympathetic nerves that supply the GI tract form neural connections with the ENS.
4. In general, parasympathetic nerves of the GI tract cause an increase in GI secretion and motility when stimulated by increasing the activity of ENS neurons

gastrointestinal reflex pathways – regulate GI secretion and motility in response to stimuli present in the lumen of the GI tract.

a. Initial component – sensory receptors associated with the sensory neurons of the ENS

1. Axons of these sensory neurons synapse with other neurons located in the ENS, CNS, or ANS, informing these regions about the conditions in the GI tract
2. The neurons of the ENS, CNS, or ANS subsequently activate or inhibit GI glands and smooth muscle, altering GI secretion and motility.

Question 5

describe the peritoneum and its folds.

Answer 5

Peritoneum – the largest serous membrane of the body that lines the abdominal cavity and covers the viscera within it. Consists of two layers: parietal and visceral

parietal peritoneum – lines the wall of the abdominal cavity

visceral peritoneum – covers some of the organs in the cavity and is their serosa

peritoneal cavity – slim space between the parietal and visceral portions of the peritoneum that contains lubricating serous fluid.

Retroperitoneal – external to the peritoneal lining of the abdominal cavity

1. Some organs lie on the posterior abdominal wall and are covered by peritoneum on their anterior surfaces only.
2. These organs include the kidneys, ascending and descending colons of the large intestine, duodenum of the small intestine, and pancreas.

Peritoneal folds – 5 large folds that weave between the viscera.

1. The folds bind the organs to one another and to the walls of the abdominal cavity.
2. They also contain blood vessels, lymphatic vessels, and nerves that supply the abdominal organs.

greater omentum – the largest peritoneal fold. Drapes over the transverse colon and coils of the small intestine like a “fatty apron”

1. Is a double sheet that folds back on itself, totaling 4 layers
2. Attaches along the stomach and duodenum downward anterior to the small intestine, then turns upward and extends and attaches to the transverse colon
3. Considerable amount of adipose tissue. Can increase greatly with weight gain, and contributes to the “beer belly” in some overweight people
4. Contains many lymph nodes that contribute macrophages and plasma cells that help combat and contain infections within the GI tract.

falciform ligament – attaches the liver to the anterior abdominal wall.

lesser omentum – anterior fold in the serosa of the stomach and duodenum and connects them to the liver.

a. It is the pathway for blood vessels entering the liver and contains the hepatic portal vein, common hepatic artery, and common bile duct, as well as some lymph nodes.

Mesentery – a fan shaped fold of the peritoneum connecting the jejunum and ileum of the small intestine to the posterior abdominal wall.

1. This is the largest peritoneal fold
2. Typically laden with fat. Contributes extensively to the large abdomen in obese individuals
3. Extends from the posterior abdominal wall to wrap around the small intestine and then returns to its origin, forming a double-layered structure.
4. Between the two layers are blood and lymphatic vessels and lymph nodes

Mesocolon – two separate folds of the peritoneum that bind the transverse colon and sigmoid colon to the posterior abdominal wall.

a. Carries blood and lymphatic vessels to the intestines

Together the mesentery and mesocolon hold the intestines loosely in place, allowing movement as muscular contractions mix and move GI tract contents

Question 6

identify the locations of the salivary glands and describe the functions of their secretions.

Answer 6

mouth or oral cavity or buccal cavity – formed by the cheeks, hard and soft palates, and tongue

Cheeks – form the lateral wall of the oral cavity. Covered externally by skin, internally by a mucous membrane which consists of nonkeratinized stratified squamous epithelium
lip or labium (plural is labia) - fleshy folds surrounding the opening of the mouth.

1. Contain the orbicularis oris muscle
2. Covered externally by skin and internally by a mucous membrane

labial frenulum – midline fold of mucous membrane between the inner surface of the lip and the gums

1. Superior and inferior
2. The piece that can break when lip pushed up/downward

Vestibule – the space bounded externally by the cheeks and lips and internally by the gums and teeth
oral cavity proper – the space that extends from the gums and teeth to the fauces

Fauces – the opening between the oral cavity and oropharynx
Palate – a wall or septum that separates the oral cavity from the nasal cavity

1. Forms the roof of the mouth
2. Makes it possible to chew and breathe at the same time

hard palate – the anterior portion of the roof of the mouth

1. Formed by the maxillae and palatine bones and is covered by a mucous membrane
2. Forms a bony partition between the oral and nasal cavities

soft palate – forms the posterior portion of the roof of the mouth

a. An arch shaped muscular partition between the oropharynx and nasopharynx that is lined with mucous membrane.

Uvula – soft, fleshy mass that hangs from the free border of the soft palate

a. Drawn superiorly during swallowing to close off the nasopharynx and prevent swallowed foods and liquids from entering the nasal cavity

salivary glands – a gland that releases saliva into the oral cavity

a. Major salivary glands – secrete most of the saliva. 3 pairs of major salivary glands: parotid, submandibular, and sublingual

parotid glands – located inferior and anterior to the ears between the skin and masseter muscle

a. Connected to the oral cavity by a duct

parotid duct – the duct that pierces the buccinator muscle to open into the vestibule opposite the second maxillary molar tooth.

a. Connects the parotid glands to the oral cavity

submandibular glands – found in the floor of the mouth, inferior to the base of the tongue, medial and partly inferior to the body of the mandible. submandibular duct – run under the mucosa on either side of the midline of the floor of the mouth and enter the oral cavity proper lateral to the lingual frenulum

sublingual glands – situation in the floor of the mouth, deep to the mucous membrane and to the side of the lingual frenulum. Beneath the tongue, superior to the submandibular glands
lesser sublingual duct – duct of the sublingual gland. Opens into the floor of the mouth in the oral cavity proper.

Saliva – 99.5% water and 0.5% solutes including sodium, potassium, chloride, bicarbonate, and phosphate ions, as well as some dissolved gases and various organic substances including urea, uric acid, mucus, immunoglobulin A, lysozyme, and salivary amylase.

salivary amylase – an enzyme in saliva that initiates the chemical breakdown of starch in the mouth into maltose, maltotriose, and alpha-dextrin

Salivation – the secretion of saliva

1. Controlled by the ANS
2. Parasympathetic stimulation promotes ongoing secretion of saliva to moisten mouth and lubricate tongue and lips during speech.
3. Sympathetic stimulation dominates during stress, resulting in dryness of mouth.
4. If body becomes dehydrated, the salivary glands stop secreting saliva to conserve water, the resulting dryness in the mouth stimulates thirst.

Question 7

describe the structure and functions of the tongue.

Answer 7

Tongue – accessory digestive organ

• Skeletal muscle covered with mucous membrane
• Forms the floor of the oral cavity
• Divided into symmetrical lateral halves by a median septum
• Attached inferiorly to the hyoid bone, styloid process of the temporal bone, and mandible.
• Each half of the tongue has identical extrinsic and intrinsic muscles.

extrinsic muscles – originate outside the tongue and insert into connective tissues in the tongue. Attach to bones in the area.

• Include the hyoglossus, genioglossus, and styloglossus muscles
• Move the tongue from side to side and in and out to maneuver food for chewing, shaping into bolus, and moving food to back of mouth.
• Also form the floor of the mouth and hold the tongue in position

C. intrinsic muscles – originate in and insert into connective tissue within the tongue

• Alter the shape and size of the tongue for speech and swallowing
• Include the longitudinalis superior, longitudinalis inferior, transversus linguae, and verticalis linguae muscles

D. lingual frenulum – a fold of mucous membrane in the midline of the undersurface of the tongue

Attached to the floor of the mouth and aids in limiting the movement of the tongue posteriorly

Papillae – projections of the lamina propria covered with stratified squamous epithelium.

• Cover the upper surface and lateral surfaces of the tongue
• Many papillae contain taste buds, others lack but contain receptors for touch and increase friction between food and tongue.

F. lingual glands – in the lamina propria of the tongue

Secrete both mucous and a watery serous fluid containing the enzyme lingual lipase

G. lingual lipase – acts on as much as 30% of dietary triglycerides and converts them to simpler fatty acids and diglycerides

Question 8

identify the parts of a typical tooth and compare deciduous and permanent dentitions.

Answer 8

tooth or dentis (plural is dentes) - accessory digestive organs located in sockets of the alveolar processes of the mandible and maxillae.

Gingivae – AKA gums. Extend slightly into each socket. periodontal ligament (or membrane) - lining in each root socket, consists of dense fibrous connective tissue that anchors teeth to the socket walls and acts as a shock absorber during chewing

Crown – visible portion of the tooth above the level of the gums root(s) - one to three roots embedded in the socket

Neck – the constriction junction of the crown and root near the gum line Dentin – internal majority portion of the tooth.

1. Consists of calcified connective tissue that gives the tooth its basic shap and rigidity.
2. Harder than bone because of higher content of hydroxyapatite (70% vs 55%)

Enamel – covering over the dentin

1. Primarily calcium phosphate and calcium carbonate
2. Also harder than bone because of even higher amount of calcium salts (95%)
3. the hardest substance in the body.
4. Protects the tooth from wear and tear of chewing
5. Also protects against acids that can easily dissolve dentin

Cementum – covers the dentin of the root

1. Bonelike substance that attaches the root to the periodontal ligament

pulp cavity – enlarged part of the enclosed space of a tooth within the dentin.

a. Within the crown and is filled with pulp

Pulp – connective tissue containing blood vessels, nerves, and lymphatic vessels. root canals – narrow extensions of the pulp cavity that run through the root of the tooth.

apical foramen – opening at the base of each root canal through which blood vessels, lymphatic vessels, and nerves enter a tooth

Dentitions – sets of teeth.

a. 2 sets in humans, deciduous and permanent.

deciduous teeth or primary teeth or milk teeth or baby teeth

a. Begin to erupt around 6 months of age, with 2 per month until all 20 are present.

central and lateral incisors – chisel shaped and adapted for cutting into food. 1 root

cuspids or canines – pointed surface called a cusp. Used to tear and shred food. 1 root

first and second molars – 4 cusps each. Maxillary molars have 3 roots, mandibular molars have two roots. Molars crush food and prepare it for swallowing

permanent teeth or secondary teeth – replace primary teeth between ages 6- 12. 32 teeth total.

incisors and cuspids – same as in primary.

first and second premolars or bicuspids – replace the primary decidous molars

a. Have 2 cusps each and one root. Used for crushing and grinding.

first molars, second molars and third molars (or wisdom teeth) - erupt as the jaw grows to accommodate them. Do not replace primary teeth.

1. 1st permanent molars = 6 year molars
2. 2nd permanent molars = 12 year molars
3. 3rd permanent molars = wisdom teeth, around age 17 or not at all.

mechanical and chemical digestion in the mouth

Mastication – chewing. Food is moved by the tongue, ground by the teeth, mixed with saliva.

Bolus – soft, flexible, easily swallowed mass of food after chewing

salivary amylase – secreted by the salivary glands

a. Initiates the breakdown of starch. Breaks starch into smaller molecules such as maltose, maltotriose, and alpha-dextrins.

lingual lipase – secreted by the lingual glands in the tongue

1. Becomes activated in the acidic stomach and starts working on food after it is swallowed.

Breaks down triglycerides into fatty acids

Question 9

describe the location and digestive function of the pharynx.

Answer 9

Pharynx – funnel-shaped tube that extends from the internal nares to the esophagus posterior and to the larynx anteriorly.

Composed of skeletal muscle and lined by mucous membrane Divided into 3 parts: nasopharynx, oropharynx, and laryngopharynx

1. Nasopharynx – functions only in respiration
2. Oro and laryngopharynx have both respiratory and digestive functions and diglycerides.

Question 10

describe the location, anatomy, histology, and functions of the esophagus.

Answer 10

Esophagus - hollow muscular tube that connects the pharynx and stomach. Posterior to the trachea.

esophageal hiatus - opening in the diaphragm through which the esophagus passes.

histology of the esophagus

• Mucosa - nonkeratinized stratified squamous epithelium. Also has a lamina propria (areolar connective tissue), and a muscularis mucosae (smooth muscle) Near the stomach, the mucosa contains mucous glands.
• Submucosa - contains areolar connective tissue, blood vessels, and mucous glands.
• Muscularis **LAYER OF THE GI TRACT THAT CONTAINS SKELETAL MUSCLE- of the upper 1/3 consists of skeletal muscle. Middle 1/3 consists of skeletal and smooth muscle, and inferior 1/3 is all smooth muscle
• upper esophageal sphincter - skeletal muscle sphincter formed by the muscularis becoming more prominent Regulates movement of food from the pharynx into the esophagus
• lower esophageal (cardiac) sphincter - consists of smooth muscle, is near the heart, also formed by the muscularis becoming more prominent. Regulates movement of food from esophagus to stomach
• Adventitia - superficial layer of the esophagus. The areolar connective tissue of this layer is not covered by mesothelium and the connective tissue merges with the connective tissue of surrounding structures of the mediastinum as it passes through. The adventitia attaches the esophagus to surrounding structures.

physiology of the esophagus - the esophagus secretes mucous and transports food into the stomach.

Question 11

describe the three phases of deglutition.

Answer 11

Deglutition - movement of food from mouth to stomach. AKA swallowing

1 voluntary stage - the bolus is passed into the oropharynx

2 pharyngeal stage - the involuntary passage of the bolus through the pharynx into the esophagus

deglutition center - in the medulla oblongata and lower pons of the brain stem.

• Impulses sent by receptors in the oropharynx and return causing the soft palate and uvula to move upward to close off the epiglottis, preventing food/liquid from entering the nasal cavity and trachea.

3 Esophageal stage – involuntary passage of the bolus through the esophagus into the stomach
Peristalsis - progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis that push the bolus onward.

Question 12

describe the location, anatomy, histology, and functions of the stomach.

Answer 12

Stomach - J-shaped enlargement of the GI tract directly inferior to the diaphragm in the abdomen. Connects the esophagus to the duodenum. Serves as a mixing chamber and holding reservoir. The most distensible part of the GI tract. Can accommodate a large quantity of food. Starch and triglyceride digestion continues, protein digestion begins, bolus is converted to liquid, certain substances are absorbed.

Anatomy - 4 main regions: cardia, fundus, body, pyloric part

Cardia - surrounds the opening of the esophagus into the stomach

Fundus - rounded portion superior and to the left of the cardia food can stay here for an hour before being mixed with gastric juices

Body - inferior to the fundus; the large central portion

Pyloric Part – divided into 3 regions:

1. pyloric antrum - connects to the body of the stomach
2. pyloric canal - leads to the third region
3. Pylorus - connects to the duodenum

Rugae - large folds that can be seen with the unaided eye. Present when stomach is empty
pyloric sphincter - a thickened ring of smooth muscle through which the pylorus of the stomach communicates with the duodenum allows greater distenstion for food storage

lesser curvature - concave medial border of the stomach

greater curvature - convex lateral border

Histology - same 4 basic layers with certain modifications.

1. Mucosa - a layer of simple columnar epithelial cells. Also contains a lamina propria and a muscularis mucosae.

• surface mucous cells - a layer of simple columnar epithelial cells in the mucosa of the stomach.
• lamina propria - areolar connective tissue
• muscularis mucosae - smooth muscle
• gastric glands lining gastric pits - glands in the mucosa of the stomach composed of cells that empty their secretions into narrow channels called gastric pits. Formed by the epithelial cells extending down into the lamina propria where they form columns of secretory cells.
• mucous neck cells - secrete mucous. Located in the gastric glands. Mucus - is mucus. What else to say?
• parietal cells - secretory cells in gastric glands that produce intrinsic factor and hydrochloric acid.

hydrochloric acid - a component of gastric juice. Kills microbes in food, denatures proteins, and converts pepsinogen into pepsin intrinsic factor - needed for absorption of vitamin B12.

chief cells - secreting cell of a gastic gland that produces pepsinogen and gastic lipase.

Pepsinogen gastric lipase

• gastric juice - the secretions of the mucous, parietal, and chief cells together. Totals 2000-3000mL per day.
• G cells - a type of enteroendocrine cell, located mainly in the pyloric antrum, secretes the hormone gastrin into the bloodstream.

Gastrin - hormone that stimulates several aspects of gastric activity.

1. Submucosa - composed of areolar connective tissue.

muscularis (three layers including oblique layer) - an outer longitudinal layer, a middle circular layer, and an inner oblique layer. The oblique layer is limited primarily to the body of the stomach.

1. Serosa - composed of simple squamous epithelium (mesothelium) and areolar connective tissue.

• The portion of the serosa covering the stomach is part of the visceral peritoneum.
  At the lesser curvature of the stomach, the visceral peritoneum extends upward to the liver as the lesser omentum.
• At the greater curvature of the stomach, the visceral peritoneum continues downward as the greater omentum and drapes over the intestines.

mechanical and chemical digestion - several minutes after food enters the stomach, peristalsis begins, passing over the stomach every 15-25 seconds. Very few in the fundus, which serves as a storage area, mostly begin in the body and intensify as they reach the antrum

Propulsion - movement of gastric contents from the body of the stomach down to the antrum

Retropulsion - forcing of stomach contents back into the body of the stomach because they are too large to fit through the narrow pyloric sphincter.

Chyme - the semifluid mixture of partially digested food and digestive juice found in the stomach and small intestines during digestion of a meal. chyme cells - secretes the hormone that promotes the production of hydrochloric acid.

gastric emptying - once food particles in chyme are small enough, they can pass through the pyloric sphincter. A slow process, only about 3mL of chyme moves through the pyloric sphincter at a time.

proton pumps - powered by H+ –K+ ATPases actively transport H+ into the lumen while bringing potassium ions into the cell. Simultaneously, Cl- and K+ diffuse out into the lumen through channels in the apical membrane. The H+ and the Cl- combine to form HCl

Chymotrypsin pancreatic enzyme acts on peptide bonds

Pepsin - protein-digesting enzyme secreted by chief cells of the stomach in the inactive form pepsinogen, which is converted to active pepsin by HCl

1. It is the only proteolytic (protein digesting) enzyme in the stomach.
2. Very active in the acidic environment of the stomach. Becomes inactive at a higher pH.
3. Severs certain peptide bonds between amino acids, breaking down a protein chain of many amino acids into smaller peptide fragments.

gastric lipase - another stomach enzyme.

1. Splits triglycerides (fats and oils) in fat molecules into fatty acids and monoglycerides (a glycerol molecule attached to one fatty acid molecule)
2. This enzyme has a limited role in the adult stomach
3. Operates best at a pH of 5-6.

Question 13

describe the location, anatomy, histology, and function of the pancreas.

Answer 13

Pancreas - soft, oblong retroperitoneal organ gland lying along the greater curvature of the stomach and connected by a duct to the duodenum. Is both an exocrine gland (secreting pancreatic juice) and an endocrine gland (secreting insulin, glucagon, somatostatin, and pancreatic polypeptide)

Anatomy

1. Head - the expanded portion near the curve of the duodenum
2. Body - superior to and to the left of the head.
3. Tail - tapering end beyond the body.
4. pancreatic duct or duct of Wirsung - large of two ducts within the pancreas; drains pancreatic juice into the duodenum Joins the common bile duct from the liver and gallbladder and enters the duodenum as a dilated common duct called the hepatopancreatic ampulla or ampulla of Vater

e. major duodenal papilla - an elevation of the duodenal mucosa where the ampulla opens.

Lies about 10cm inferior to the pyloric sphincter of the stomach.

sphincter of the hepatopancreatic ampulla or sphincter of Oddi - a mass of smooth muscle surrounding the ampulla that regulates the passage of pancreatic juice and bile through the hepatopancreatic ampulla into the duodenum of the small intestine

accessory duct or duct of Santorini - the other major duct of the pancreas, deads from the pancreas and empties into the duodenum about 2.5cm superior to the hepatopancreatic ampulla

Histology - made up of small clusters of glandular epithelial cells called acini

a. Acini - groups of cells in the pancreas that secrete digestive enzymes.

About 99% of the acini constitute the exocrine portion of the gland.

b. pancreatic islets or islets of Langerhans - the remaining 1% of the clusters that form the endocrine portion of the pancreas

Secrete hormones glucagon, insulin, somatostatin, and pancreatic peptide.

composition and functions of pancreatic juice - about 1.2-1.5L per day Pancreatic juice – a clear, colorless liquid consisting mostly of water, some salts, sodium bicarbonate, and several enzymes
pancreatic amylase - starch digesting enzyme

Trypsin - digests proteins into peptides

Chymotrypsin - also digests proteins into peptides

carboxypeptidase - also digests proteins into peptides

elastase - also digests proteins into peptides

pancreatic lipase – the principal triglyceride-digesting enzyme in adults ribonuclease and

deoxyribonuclease - nucleic acid digesting enzymes that digest RNA and DNA into nucleotides.

Trypsinogen - the inactive form of trypsin, secreted so the enzyme does not digest the pancreas itself.

Enterokinase - activating brush-border enzyme that splits off part of the trypsinogen molecule to form trypsin.

• In turn, trypsin acts on the inactive precursors chymotrypsinogen, procarboxypeptidase, and proelastase to produce the protein digesting enzymes listed above.

regulation of pancreatic secretions - I got nothing from this textbook on this.

Question 14

describe the location, anatomy, histology, and functions of the liver and gallbladder.

Answer 14

A. Liver - large organ under the diaphragm.

• heaviest gland of the body, weighs about 1.4kg or 3lbs in an average adult. Second only to the skin in size.
• Inferior to the diaphragm, occupies most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity.
  
  1. the distribution of blood vessels) but more appropriately belong to the left lobe.

Function of the liver:

a) Conversion of carbohydrates

b) Protein metabolism

d) Phagocytosis

e) Storage of vitamins

(NOT c) Storage of bilirubin)

Round ligament - found in liver and is a remnant of the umbilical cord

Lots of functions, listed below.

B. Gallbladder - a small pouch, located inferior to the liver, that stores bile and empties by means of the cystic duct.

Pear shaped sac located in a depression of the posterior surface of the liver 7-10cm and typically hangs from the anterior inferior margin of the liver.

C. anatomy of the liver - almost completely covered by a dense irregular connective tissue layer that lies deep to the peritoneum

• right lobe - larger lobe – divided from left by the falciform ligament which is a fold of the mesentary.
• left lobe - smaller lobe
• quadrate lobe - inferior; included in right lobe based on internal morphology (primarily
• caudate lobe - posterior, same as quadrate.
• ligamentum teres - in the free border of the falciform ligament. A remnant of the umbilical vein of the fetus. This fibrous cord extends from the liver to the umbilicus.
• gall bladder - 3 portions
• *Fundus** - broad portion that projects inferiorly beyond the inferior border of the liver
• *Body** - central portion. Projects superiorly.
• *Neck** - tapered portion. Projects superiorly

Layers of the wall: mucosa – simple columnar epithelium arranged in rugae resembling those of the stomach. No submucosa. Muscular coat consists of smooth muscle fibers. The outer coat is the visceral peritoneum.
Functions – store and concentrate the bile produced by the liver (up to 10x) until it is needed in the intestine.

J. histology of the liver and gallbladder

Hepatocytes - the major functional cells of the liver

1. Perform a wide array of metabolic, secretory, and endocrine functions.
2. Specialized epithelial cells with 5-12 sides that make up about 80% of the volume of the liver
3. Form complex 3-D arrangements called hepatic laminae, plates of hepatocytes one cell thick bordered on either side by the endothelial- lined vascular spaces called hepatic sinusoids. They are highly branched, irregular structures.

• *bile canaliculi** - small ducts between hepatocytes that collect bile produced by hepatocytes
• *bile ducts** - merging bile canaliculi turn to bile ductules and then bile ducts which eventually merge

right and left hepatic ducts - larger ducts that unite and exit the liver as the common hepatic duct
common hepatic duct - the exit point of the bile ducts from the liver
cystic duct - the duct that carries bile from the gallbladder to the common bile duct

common bile duct - the final duct which is formed by the union of the common hepatic duct and the cystic duct

a. Empties bile into the duodenum at the hepatopancreatic ampulla

Sinusoids - highly permeable blood capillaries between rows of hepatocytes that receive oxygenated blood from branches of the hepatic artery and nutrient-rich deoxygenated blood from branches of the hepatic portal vein.

central vein - hepatic sinusoids converge and deliver blood into a central vein. From the central veins, the blood flows into the hepatic veins, which drain into the inferior vena cava.
lobules—hepatic and portal; hepatic acinus - methods of organizing the hepatocytes, bile duct system, and hepatic sinusoids into anatomical and functional units.

1. Hepatic lobule – each hepatic lobule is shaped like a hexagon. at its center is the central vein, radiating out from it are rows of hepatocytes and hepatic sinusoids. located at 3 corners of the hexagon is a portal triad. This model is based on adult pig liver. Humans are different
2. Portal lobule – emphasizes the exocrine function of the liver, bile secretion. the bile duct of a portal triad is at the center. The portal lobule is triangular in shape and is defined by 3 imaginary straight lines that connect three central veins that are closed to the portal triad. This model is not widely accepted. So why learn it?
3. Hepatic acinus – the preferred structural and functional unit of the liver. Smallest structural and functional unit of the liver. Provides a logical description and interpretation of patterns of glycogen storage and release, toxic effects, degeneration, and regeneration relative to the proximity of the acinar zones to branches of the portal triad. Each hepatic acinus is an approximately oval mass that includes portions of two neighbouring hepatic lobules. the short axis of the hepatic acinus is defined by branches of the portal triad (branches of the hepatic artery, vein, and bile ducts) that run along the border of the hepatic lobules. The long axis is defined by two imaginary curved lines which connect the two central veins closest to the short axis. Hepatocytes in the hepatic acinus are arranged in three zones around the short axis, with no sharp boundaries between them

zone 1 – closes to the branches of the portal triad, the first to receive incoming oxygen, nutrients, and toxins from incoming blood. Also the first cells to take up glucose and store it as glycogen after a meal and break down glycogen to glucose during fasting. Also the first to show morphological changes following file duct obstruction or exposure to toxic substances. The last cells to die if circulation is impaired and the first to regenerate

Zone 2 – structural and functional characteristics intermediate to zones 1 and 3

Zone 3 – farthest from branches of the portal triad. Last to show the effects of bile obstruction of exposure to toxins. The first to show effects of impaired circulation, the last to regenerate.

K. blood supply of the liver - receives blood from two sources: hepatic artery (oxygenated blood) and hepatic portal vein (deoxygenated blood containing newly absorbed nutrients, drugs, microbes, and toxins from the GI tract)

• Branches of the hepatic artery and the hepatic portal vein carry blood into hepatic sinusoids where nutrients, oxygen, and certain toxins are taken up by the hepatocytes.
  Products manufactured by the hepatocytes and nutrients needed by other cells are secreted back into the blood, which then drains into the central vein and eventually passes into a hepatic vein.
• Blood from the Gi tract passes through the liver as part of the hepatic portal circulation, so the liver is often a site of metastasis for cancer that originates in the GI tract.

L. Bile - secretion of the liver that contributes to digestion. Yellow, brownish, or olive green. PH of 7.6-8.6

Role - Emulsifies lipids prior to their digestion.

Composition – consists mostly of water, bile salts, cholesterol, lecithin, bile pigments, and several ions

• Bilirubin - the principal bile pigment. The orange pigment that is one of the end products of hemoglobin breakdown in the hepatocytes and is excreted as a waste material in bile.
• Emulsification - the breakdown of large lipid globules into smaller, uniformly distributed suspension of small lipid globules in the presence of bile.

functions of the liver

Secreting bile – needed for absorption of dietary fats

Carbohydrate metabolism – maintains a normal blood glucose level. When blood glucose is low, the liver breaks down glycogen and releases it to the bloodstream. The liver can also convert certain amino acids and lactic acid to glucose, and can convert other sugars such as fructose and galactose into glucose. When blood glucose is high, the liver converts glucose to glycogen and triglycerides for storage

Lipid metabolism – hepatocytes store some triglycerides, break down fatty acids to generate ATP, synthesize lipoproteins which transport fatty acids, triglycerides, and cholesterol to and from body cells, synthesize cholesterol, and use cholesterol to make bile salts.

Question 15

describe the location and structure of the small intestine.

Answer 15

A. small intestine - located central and inferior part of abdominal cavity

Anatomy - divided into 3 regions

1. Duodenum - shortest region.
   
   1. Retroperitoneal
   2. Starts at the pyloric sphincter of the stomach
   3. Forms a 10inch C-shaped tube that merges with the jejunum
2. Jejunum - middle portion
   
   1. About 1 meter long
   2. Extends to the ileum
3. Ileum - final and longest region of the small intestine

a. About 2m long

Protein metabolism – hepatocytes deaminate amino acids so that the amino acids can be used for ATP production or converted to carbohydrates or fats. The resulting toxic ammonia (NH3) is then converted to urea, which is excreted in urine. hepatocytes also synthesize most plasma proteins, such as alpha and beta globulins, albumin, prothrombin, and fibrinogen.

processing of drugs and hormones – the liver can detoxify substances such as alcohol and excrete drugs such as penicillin, erythromycin, and sulfonamides into bile. Can also chemically alter or excrete thyroid hormones and steroid hormones such as estrogens or aldosterone

Excretion of bilirubin – bilirubin is absorbed by the liver from the blood and secreted into bile. Most of the bilirubin in bile is metabolized in the small intestine by bacteria and eliminated in feces

Synthesis of bile salts – bile salts are used in the small intestine for the emulsification and absorption of lipids

Storage – prime storage site for certain vitamins (A, B12, D, E, K) and minerals (iron and copper), which are released from the liver when needed elsewhere in the body. (Also stores glycogen)

Phagocytosis – the stellate reticuloendothelial (Kupffer) cells of the liver phagocytize aged red blood cells, white blood cells, and some bacteria. Activation of vitamin D – the skin, liver, and kidneys participate in synthesizing the active form of vitamin D.

b. Joins the large intestine at the ileocecal sphincter

1. ileocecal sphincter - a fold of mucous membrane that guards the opening from the ileum into the large intestine
2. Histology - wall consists of 4 same layers that make up most of the GI tract

a. Mucosa - a layer of epithelium, lamina propria, and muscularis mucosae i. Epithelial layer consists of simple columnar epithelium with many types of cells

• absorptive cells - release enzymes that digest food and contain microvilli that absorb nutrients in small intestinal chyme
• goblet cells - secrete mucus
• intestinal glands or crypts of Lieberkuhn - secrete intestinal juice
  
  1. The intestinal glands are formed by many deep crevices in the small intestinal mucosa that are lined with glandular epithelium
  2. The cells lining the crevices form the intestinal glands
• Paneth cells - secrete lysozyme and are capable of phagocytosis
• 3 types of enteroendocrine cells in the small intestinal glands: S cells, CCK cells, K cells
• Secretin - a hormone secreted by S cells
• cholecystokinin (CCK) - hormone secreted by CCK cells
• glucose dependent insulinotropic peptide (GIP) - hormone secreted by K cells
• MALT – mucosa-associated lymphoid tissue – abundant in the lamina propria of the small intestinal mucosa
• solitary lymphatic nodules - most numerous in the distal part of the ileun
  aggregated lymphatic follicles or Peyer’s patches - clusters of lymph nodules that are most numerous in the ileum

Submucosa - of the duodenum it contains duodenal glands

• duodenal glands or Brunner’s glands - glands in the submucosa of the duodenum that secretes an alkaline mucus to protect the lining of the small intestine from the action of enzymes and to help neutralize the acid in chyme.
• Muscularis - consists of two layers of smooth muscle

1. Outer thinner longitudinal layer Inner thicker circular layer

Serosa - completely surrounds the small intestine except for a major portion of the duodenum which is retroperitoneal.

circular folds - folds of the mucosa and submucosa

• Permanent, deep, transverse folds
• Increase the surface area for absorption
• AKA plicae circulares
• Begin near the proximal portion of the duodenum and end around the midpoint of the ileum
• Some extend all the way around the circumference of the intestine, others only part way around.
• Enhance absorption by increasing surface area and causing chyme to spiral rather than move in a straight line.

villus (plural is villi) - fingerlike projections of the mucosa that are 0.5- 1mm long

• Large number, 20-40 per square mm
• Greatly increase surface area available for absorption and digestion
• Gives the intestinal mucosa a velvety appearance
• Each is covered by epithelium and has a core of lamina propria 1. Within the connective tissue of the lamina propria are an arteriole, a venule, a blood capillary network, and a lacteal

Lacteal - a lymphatic capillary

• Absorb triglycerides and other lipids from digested food
  m. microvillus (plural is microvilli) - microscopic fingerlike projections of the plasma membranes of cells that increase surface area for absorption
• 1 micrometer long cylindrical membrane covered projection that contains a bundle of 20-30 actin filaments
• Too small to be seen individually
• Estimated 200 million microvilli per square mm of small intestine
• Greatly increase the surface area of the plasma membrane, so larger amounts of digested nutrients can diffuse into absorptive cells in a given period

brush border - fuzzy line seen on a light microscope that is the microvilli of the small intestine

• Contains several brush-border enzymes that have digestive functions. (described shortly)
  o. intestinal juice - clear yellow fluid
• About 1-2 liters secreted daily
• Contains water and mucus
• Slightly alkaline (pH 7.6) due to high concentration of bicarbonate ions
• Together with pancreatic jioce, provides a liquid medium that aids absorption of substances from chyme in the small intestine
  p. brush border enzymes: dextrinase, maltase, sucrase, lactase, peptidases (including aminopeptidase and dipeptidase), nucleosidases and phosphatases
• Synthesized by the absorptive cells of the small intestine Inserted into the plasma membrane of microvilli
• Thus some enzymatic digestion occurs at the surface of absorptive cells that line the villi, rather than in the lumen exclusively

4 carbohydrate digesting enzymes: alpha-dextrinase, maltase, sucrase, lactase

Protein digesting enzymes called peptidases (aminopeptidase and dipeptidase)

Two nucleotide-digesting enzymes: nucleosidases and phosphatases

Also, as absorptive cells slough off into the lumen, they break apart and release enzymes that help digest nutrients.

mechanical digestion - two types of movement in the small intestine that are governed mainly by the myenteric plexus

Segmentations - localized mixing contractions that occur in portions of intestine distended by a large volume of chyme

1. Mix chyme with the digestive juices and bring the particles of food into contact with the mucosa for absorption
2. Do not push the contents along the tract
3. Contractions of circular muscle constrict the intestine into segments
4. Then muscle fibers in the midportion of each segment relax and divide the intestine further
5. Next the first contractions relax and the chyme sloshes back and forth between segments
6. Occur about 12x/min in the duodenum, slow to 8x/min in the ileum

migrating motility complex (MMC) - the type of peristalsis that occurs in the small intestine

1. Begins in the lower portion of the stomach, and pushes chyme forward along a short stretch of small intestine before dying out.
2. Slowly migrates down the small intestine, reaching the end of the ileum in 90-120 minutes
3. Then another MMC begins in the stomach
4. Altogether, chyme remains in the small intestine for 3-5 hours

chemical digestion

a. enzymes that digest carbohydrates
* Salivary amylase in the mouth is destroyed by the acidic pH of the stomach, thus only a few starches are broken down by the time chyme leaves the stomach
* Pancreatic amylase cleaves starches not already broken down into maltose, maltotriose, and alpha-dextrins in the small intestine
1. Acts on both glycogen and starches but has no effect on cellulose, an indigestible plant fiber referred to as roughage in the digestive system.

• After amylase has split starch into smaller fragments, alpha- dextrinase acts on the resulting alpha-dextrins and clips off one glucose unit at a time.
  Sucrose, lactose, and maltose are not acted on until they reach the small intestine

1. 3 brush-border enzymes digest these disaccharides into monosaccharides
2. Sucrase – breaks sucrose into a molecule of glucose and a molecule of fructose
3. Lactase – digests lactose into a molecule of glucose and a molecule of galactose
4. Maltase – splits maltose and maltotriose into two or three molecules of glucose, respectively

Digestion of carbohydrates ends with the production of monosaccharides that can be absorbed by the digestive system

enzymes that digest proteins

Begins in the stomach, where proteins are fragmented into peptides by pepsin
4 enzymes in pancreatic juice continue to break down proteins into peptides:

1. Trypsin, chymotrypsin, and elastase all cleave the peptide bond between a specific amino acid and its neighbor
2. Carboxypeptidase splits off the amino acid at the carboxyl end of a peptide

Protein digestion is completed by two peptidases in the brush border:

c.enzymes that digest lipids

1. Amimopeptidase – cleaves off the amino acid at the amino end of a peptide
2. Dipeptidase splits dipeptides into single amino acids

Enyzmes that split triglycerides and phospholipids are called lipases

1. 3 types of lipases that can participate in lipid digestion
2. Lingual lipase
3. Gastric lipase
4. Some lipid digestion occurs in the stomach through lingual and gastric lipases
5. Pancreatic lipase
6. Most lipid digestion occurs in the small intestine through the action of pancreatic lipase
7. Pancreatic lipase breaks triglycerides down into fatty acids and monoglycerides
8. Before a large lipid globule containing triglycerides can be digested in the small intestine, it must first undergo emulsification – a process in which the large lipid globule is broken down into several small lipid globules
9. Large lipid globules are emulsified by amphipathic bile salts, breaking them apart into several small lipid globules about 1 micrometer in diameter
10. The small lipid globules provide a large surface area for pancreatic lipase to function more effectively

d. enzymes that digest nucleic acids

Pancreatic juice contains two nucleases

1. Ribonuclease – digest RNA
2. Deoxyribonuclease – digest DNA

Brush-border enzymes further digest the nucleotides resulting from the nucleases

1. Nucleosidases 2. Phosphatases
2. These enzymes digest nucleotides into pentoses, phosphates, and nitrogenous bases which are absorbed via active transport

regulation of intestinal secretion and motility - motility governed by the myenteric plexus. Intestinal secretion – would it be governed by stomach/intestinal contents, hormonal control, who knows?
absorption in the small intestine - all chemical and mechanical phases of digestion from mouth through small intestine are directed toward changing food into forms that can pass through the absorptive epithelial cells lining the mucosa and into the underlying blood and lymphatic vessels.

These forms are monosaccharides from carbohydrates, single amino acids, dipeptides, and tripeptides from proteins, and fatty acids, glycerol, and monoglycerides from triglycerides.

b. absorption of monosaccharides

All carbohydrates are absorbed as monosaccharides
Huge capacity for small intestine to absorb them, 120grams per hour
All dietary carbohydrates that are digested are normally absorbed, leaving only indigestible cellulose and fibers in the feces
Monosaccharides pass from the lumen through the apical membrane via facilitated diffusion (fructose) or active transport (glucose and galactose via secondary active transport coupled to active transport of Na+ ions)
Monosaccharides move out of absorptive cells through their basolateral surfaces via facilitated diffusion and enter the capillaries of the villi
Transported in the blood to the liver by way of hepatic portal system

c. absorption of amino acids, dipeptides and tripeptides

Many proteins are absorbed as amino acids via active transport processes that occur mainly in the duodenum and jejunum About half of all absorbed amino acids are present in food, the other half come from the body itself as proteins in digestive juices and dead cells that slough off the mucosal surface. Normally, 95-98% of the protein present in the small intestine is digested and absorbed.

Amino acids and peptides enter the absorptive cells by different transporters.
Amino acids move out of absorptive cells via diffusion and enter capillaries of the villus

Transported in the blood to the liver by way of hepatic portal system.

d. absorption of lipids and bile salts

All dietary lipids are absorbed via simple diffusion
Adults absorb about 95% of lipids present in small intestine. (infants only absorb about 85% due to lower production of bile) Small short-chain fatty acids (contain less than 12 carbon atoms) and are more water soluble. They can dissolve in chyme, pass through the absorptive cells via simple diffusion, and exit the basolateral surfaces and into a blood capillary of a villus
Large short chain fatty acids, long-chain fatty acids, and monoglycerides are larger and hydrophobic. They are not water soluble and have difficulty being suspended in the watery intestinal chyme. Bile salts help make these molecules more soluble.
Micelles - tiny spheres 2-10nm in diameter that include 20-50 bile salt molecules.

1. They are formed due to the amphipathic nature of bile salts: the hydrophobic regions of bile salts interact with the lipid molecules and the hydrophilic regions of bile salts interact with the watery intestinal chyme
2. The micelles move to the brush border of the absorptive cells where the lipid molecules can diffuse into the cell, and the micelle is left behind in the chyme
3. Micelles continually repeat this ferrying service as they move from the brush border back through chyme to the

interior of the lumen to pick up more large lipid

molecules.
4. Micelles also solubilize other large hydrophobic

molecules such as fat-soluble vitamins (A, D, E, K) and cholesterol that may be present in intestinal chyme and aid their absorption

Inside absorptive cells, long-chain fatty acids and monoglycerides are recombined to form triglycerides, which aggregate into globules along with phospholipids and cholesterol and become coated with proteins

e. Chylomicrons - large spherical masses about 80nm in diameter (described immediately above) that leave the absorptive cell via exocytosis and enter lacteals where they are transported by way of lymphatic vessels to the thoracic duct and enter the blood at the junction of the left internal jugular and left subclavian veins

the hydrophilic protein coat that surrounds each chylomicron keeps them suspended in blood and prevents them from sticking to each other.

f. lipoprotein lipase - enzyme attached to apical surface of capillary endothelial cells in the liver and adipose tissue that breaks down triglycerides in chylomicrons and other lipoproteins into fatty acids and glycerol.

The fatty acids diffuse into hepatocytes and adipose cells and combine with glycerol during resynthesis of triglycerides
2-3 hours after a meal, few chylomicrons remain in the blood.

g. enterohepatic circulation - the cycle of bile salt secretion by hepatocytes into bile, reabsorption in the ileum, and resecretion into bile

Insufficient bile salts, due to obstruction of bile ducts or removal of gallbladder can result in the loss of up to 40% of dietary lipids in feces due to diminished lipid absorption

h. absorption of electrolytes

Many electrolytes absorbed by the small intestine come from GI secretions, and some are part of ingested foods and liquids Sodium ions are actively transported out of absorptive cells by basolateral Na+-K+ pumps after they have moved into absorptive cells via diffusion and secondary active transport. Negatively charged ions can passively follow Na+ or be actively transported

Calcium ions are absorbed actively in a process stimulated by calcitriol
Other electrolytes are also absorbed via active transport

i. absorption of vitamins

Fat soluble vitamins are absorbed in micelles with lipids via simple diffusion
Water soluble vitamins such as most B vitamins and vitamin C are also absorbed via simple diffusion

Vitamin B12 combines with intrinsic factor produced by the stomach and the combination is absorbed in the ileum via an active transport mechanism

j. absorption of water

The small intestine absorbs about 8.3 liters of fluid daily
Total volume of fluid entering the small intestine each day is about 9.3 liters from ingestion of liquids (about 2.3 liters) and various GI secretions (about 7 liters)
Water absorption occurs via osmosis from the lumen of the intestines through absorptive cells and into blood capillaries. Water can move across the intestinal mucosa in both directions, so absorption of water from the small intestine depends on the absorption of electrolytes and nutrients to maintain an osmotic balance with the blood.

Question 16

describe the anatomy, histology, and functions of the large intestine.

Answer 16

A. large intestine - terminal portion of the GI tract, from ileum to anus

Function of the large intestine:

a) Mechanical digestion

b) Chemical digestion

c) Absorption

d) Feces formation

(NOT e) Regulation of blood glucose)

Anatomy - about 1.5m long, 6.5cm in diameter

1. Attached to the posterior abdominal wall by its mesocolon – a double layer of peritoneum
2. 4 major regions structurally: cecum, colon, rectum, anal canal

Cecum - small pouch about 6cm long at the proximal end of the large intestine that hangs inferior to the ileocecal valve
appendix or vermiform appendix - twisted, coiled tube attached to the cecum

a. The mesoappendix (mesentery of the appendix) attaches the appendix to the inferior part of the mesentery of the ileum

Colon - portion of the large intestine between the cecum and rectum.

ascending colon - ascends on the right side of the abdomen to the inferior surface of the liver, turns abruptly to the left at the right colic (hepatic) flexure

a. retroperitoneal

transverse colon - crosses the abdomen curving beneath the inferior end of the spleen on the left side as the left colic (splenic) flexure
descending colon - passes inferiorly from the left colic flexure to the level of the iliac crest

a. retroperitoneal

sigmoid colon - begins near the left iliac crest, projects medially to the midline, terminates as the rectum around the level of S3 vertebra
Rectum - about 15 cm in length, lies anterior to the sacrum and coccyx
anal canal - the last 2-3cm of the rectum, opens to the exterior through the anus anal columns - longitudinal fold in the mucous membrane of the anal canal that contains a network of arteries and veins

Anus - the opening of the anal canal to the exterior guarded by two sphincters that remain closed except during elimination of feces
internal anal sphincter - smooth muscle (involuntary)
external anal sphincter - skeletal muscle (voluntary)

Histology - same 4 layers in the wall of the large intestine:

a. Mucosa - consists of simple columnar epithelium, lamina propria, and muscularis mucosae.

Epithelium contains mostly absorptive and goblet cells Absorptive cells function primarily in water absorption

1. microvilli present on absorptive cells

The goblet cells secrete mucus that lubricates the passage of colonic contents
Both absorptive and goblet cells are located in long, straight, tubular intestinal glands that extend the full thickness of the mucosa

Solitary lymphatic

1. Submucosa - consists of aerolar connective tissue
2. Muscularis - external layer of longitudinal smooth muscle, internal layer

of circular smooth muscle

teniae coli - thickened portions of the longitudinal muscles unique to the large intestine.

1. Run most of the length of the large intestine.
2. Separated by portions of the wall with less or no

longitudinal muscle.

d. haustra (singular is haustrum) - series of pouches along the colon

Formed by tonic contractions of the bands gathering the colon into pouches.
Gives the colon a puckered appearance

e. Serosa - part of the visceral peritoneum.

Omental appendices – small pouches of visceral peritoneum filled with fat, attached to teniae coli

mechanical digestion

a. Ileocecal sphincter – regulates passage of chyme from ileum to cecum. regulates the flow of material into the colon

Normally partially closed so passage is slow
When the cecum is distended, the degree of contraction of the ileocecal sphincter intensifies
Gastrin relaxes the sphincter

1. gastroileal reflex - intensifies peristalsis in the ileum after a meal, forcing any chyme into the cecum.
2. haustral churning - process in which haustra are relaxed and become distended while filling before the walls contract and squeeze the contents into the next haustrum.

Peristalsis occurs at a slower rate, 3-12 contractions per minute

1. mass peristalsis - a strong peristaltic wave that begins about the middle of the transverse colon and quickly drives the contents of the colon into the rectum – the gastrocolic reflex
2. gastrocolic reflex - initiated by food in the stomach, usually takes place 3-4 times per day, during or immediately after a meal.

chemical digestion - the activity of bacteria within the lumen

1. Bacteria ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases (these contribute to flatus in the colon
2. Bacteria convert any remaining proteins to amino acids and break down the amino acids into simpler substances: indole, skatole, hydrogen sulfide, fatty acids

Some indole and skatole is eliminated in the feces, contributing to the odour. The rest is absorbed and transported to the liver where they are converted to less toxic compounds and excreted in the urine.

c. Bacteria decompose bilirubin to simpler pigments, including stercobilin which gives feces the brown colour.
d. Bacterial products absorbed in the colon include several vitamins needed for normal metabolism, ex. Some B vitamins and Vitamin K

absorption and feces formation - chyme remains in the large intestine for 3-10 hours and becomes solid or semisolid because of water absorption and is now called feces.

1. Feces consists of water, inorganic salts, sloughed-off epithelial cells from the mucosa of the GI tract, bacteria, products of bacterial decomposition, unabsorbed digested materials, and indigestible parts of food.
2. The large intestine absorbs 80% of the water that enters it, and also absorbs ions and some vitamins.

defecation reflex - stimulated by distention of the rectal wall stimulating stretch receptors; defecation reflex results in defecation.

1. Receptors in rectal wall send sensory nerve impulses to the sacral spinal cord
2. Motor impulses from the cord travel along parasympathetic nerves back to the descending colon, sigmoid colon, rectum, and anus
3. The resulting contraction of the longitudinal rectal muscles shortens the rectum, thereby increasing the pressure within it.
4. This pressure, along with voluntary contractions of the diaphragm and abdominal muscles, plus parasympathetic stimulation opens the internal anal sphincter.
5. The external anal sphincter is controlled voluntarily. If relaxed, it opens and defecation occurs. If voluntarily constricted, defecation can be postponed.

Diarrhea - frequent defecation of liquid caused by increased motility of the intestines. Less time for absorption = more liquid in feces

a. Can be caused by lactose intolerance, stress, microbes that irritate the GI mucosa

Constipation - infrequent or difficult defecation caused by decreased motility of the intestines. Feces remains in the colon for prolonged periods, causing excessive water absorption.

a. May be caused by poor habits (delaying defecation), spasms of the colon, insufficient fiber in the diet, inadequate liquid intake, lack of exercise, emotional stress, and some drugs.

Question 17

describe the three phases of digestion.

Answer 17

A. phases of digestion - 3 overlapping phases

cephalic phase - the smell, sight, thought, or initial taste of food activates the neural centers in the cerebral cortex, hypothalamus, and brain stem.

a. The brain stem then activates the facial (VII), glossopharyngeal (IX), and vagus (X) nerves. Facial and glossopharyngeal stimulate the salivary glands to secrete saliva. The vagus nerves stimulate the gastric glands to secrete gastric juice.

gastric phase - begins once food reaches the stomach.

a. neural regulation - food distends the stomach and stimulates stretch receptors in its walls.

Chemoreceptors in the stomach monitor the pH of the stomach chyme.
Stretch receptors and chemoreceptors are activated when stomach walls are distended or when pH increases because proteins have entered the stomach and buffered some of the stomach acid.

Nerve impulses propagate to the submucosal plexus, where they activate parasympathetic and enteric neurons.
The resulting nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from gastric glands.

The waves mix the food with gastric juice, when the waves become strong enough, a small amount of chyme undergoes gastric emptying into the duodenum.
The pH of the stomach chyme decreases and the distention of the stomach walls lessens because chyme has passed into the small intestine, suppressing secretion of gastric juice.

1. hormonal regulation - gastric secretion during the gastric phase is also regulated by the hormone gastrin
2. Gastrin - released from the G cells of the gastric glands in response to several stimuli: distention of the stomach by chyme, partially digested proteins in chyme, the high pH of chyme due to the presence of food in the stomach, caffeine in gastric chyme, and acetylcholine released from parasympathetic neurons.

Once released, gastrin enters the blood stream, makes a round trip through the body and reaches its target organs in the digestive system

Gastrin stimulates gastric glands to secrete large amounts of gastric juice
Also strengthens the contraction of the lower esophageal sphincter to prevent reflux of acid chyme into the esophagus Also increase motility of the stomach

Also relaxes the pyloric sphincter, promoting gastric emptying Gastrin is inhibited when the pH of gastric juice drops below 2.0 and is stimulated with the pH rises.

1. This negative feedback loop helps provide an optimal low pH for the functioning of pepsin, the killing of microbes, and the denaturing of proteins in the stomach.

intestinal phase - begins once food enters the small intestine. In contracts to cephalic and gastric phases, the reflexes that occur during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach, preventing the duodenum from being overloaded with more chyme than it can handle. Also, responses occuring during the intestinal phase promote the continued digestion of foods that have reached the small intestine.

1. Neuroregulation – distention of the duodenum by presence of chyme causes the enterogastric reflex
2. enterogastric reflex - stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata, where they inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach

As a result, gastric motility is inhibited and there is an increase in the contraction of the pyloric sphincter, which decreases gastric emptying.

c. hormonal regulation - mediated by 2 major hormones secreted by the small intestine: cholecystokinin and secretin.

cholecystokinin or CCK - secreted by the CCK cells of intestinal glands in the small intestine in response to chyme containing amino acids from partially digested proteins and fatty acids from partially digested triglycerides. stimulated by high levels of dietary fat in the small intestine

1. Stimulates secretion of pancreatic juice that is rich in digestive enzymes.
2. Also causes the contraction of the wall of the gallbladder, which

squeezes stored bile out of the gallbladder into the cystic duct and

through the common bile duct.

1. Also causes relaxation of the sphincter of the hepatopancreatic ampulla

(sphincter of Oddi), which allows pancreatic juice and bile to flow into the duodenum.

1. Also slows gastric emptying by promoting contraction of the pyloric sphincter.
2. Produces satiety by acting on the hypothalamus
3. Promotes normal growth and maintenance of the pancreas
4. Enhances the effects of secretin

Secretin - acidic chyme entering the duodenum stimulates the release of secretin from the S cells of the intestinal glands in the small intestine

counteracts the effec of gastric acid in the small intestine

1. Secretin stimulates the flow of pancreatic juice that is rich in bicarbonate ions to buffer the acidic chyme that enters the duodenum from the stomach.
2. Secretin also inhibits secretion of gastric juice, promotes normal growth and maintenance of the pancreas, and enhances the effects of CCK.
3. Overall, secretin causes buffering of acid in chyme that reaches the duodenum and slows production of acid in the stomach.

Question 18

Major disorders?

Answer 18

A. disorders

Aging:

a) Decreased secretory mechanisms

b) Decreased motility of the digestive organs

c) Loss of strength and tone of the muscular tissue

d) changes in neurosensory feedback regarding enzyme and hormone release

dental caries – tooth decay.

Involves a gradual demineralization of the enamel and dentin.

If untreated, microorganisms may invade the pulp, causing inflammation

and infection, with subsequent death of the pulp and abscess of the

alveolar bone surrounding the root’s apex, requiring a root canal.

Begin when bacteria acting on sugars produce acids that demineralize

the enamel.

Dextran – a sticky polysaccharide produced from sucrose that causes

bacteria to stick to the teeth.

Dental plaque – masses of bacterial cells, dextran, and other debris

adhering to teeth

Brushing teeth after eating removes the plaque from flat surfaces before

bacteria can produce acids. Flossing is also recommended to remove plaque between teeth.

periodontal disease – collective term for a variety of conditions characterized by inflammation and degeneration of the gingivae, alveolar bone, periodontal ligament, and cementum.

a. Often caused by poor oral hygiene, local irritants, or by a poor “bite”

peptic ulcer disease – ulcers that develop in areas of the GI tract exposed to acidic gastric juice.

Most common complication is bleeding, which can lead to anemia.

In acute cases, can lead to shock and death

3 distinct causes are recognized

The bacterium H Pylori. Most common cause of PUD.
NSAIDs
Hypersecretion of HCl, as occurs in Zollinger-Ellison syndrome, a gastrin-producing tumor usually of the pancreas.

d. Treatment – avoid cigarettes, alcohol, caffeine, NSAIDs. If caused by H. Pylori, treatment with antibiotics. Tums or Maalox to temporarily buffer gastric acid. H2 blockers or proton pump inhibitors to block secretion of H+ from parietal cells if caused by hypersecretion of HCl.

Diverticulitis – inflammation of diverticula

Diverticula are outpouchings of the wall of the colon.

Symptoms – pain, constipation or increased frequency of defecation,

nausea, vomiting, low-grade fever.

Treatment – high fiber diet, surgical removal of affected colon in severe

cases.

colorectal cancer – among the deadliest of malignancies, second to lung in men and third to lung and breast in women.

Contributing factors – genetics, alcohol and diets high in animal fats and protein, polyps

Signs and symptoms – diarrhea, constipation, cramping, abdominal pain, rectal bleeding

Screening includes FOB testing, digital rectal exam, sigmoidoscopy, colonoscopy, and barium enema.

Tumors may be removed endoscopically or surgically.

Hepatitis – inflammation of the liver that can be caused by viruses, drugs, and chemicals, including alcohol. Several types of viral are recognized:

a. Hep A – infectious hepatitis. Caused by the Hep A virus and is spread through fecal contamination of objects such as food, clothing, toys, and utensils.

Generally is a mild disease of children and young adults characterized by loss of appetite, malaise, nausea, diarrhea, fever, and chills. Eventually jaundice appears.
Does not cause lasting liver damage. Most people recover in 4-6 weeks.

b. Hep B – caused by the Hep B virus

Spread primarily by sexual contact and contaminated syringes and transfusion equipment
Can also be spread by saliva and tears
Can be present for years or a lifetime and can produce cirrhosis or liver cancer.

Vaccines produced through recombinant DNA are available to prevent Hep B infection.

c. Hep C – caused by the Hep C virus

Clinically similar to Hep B.
Can cause cirrhosis and possibly liver cancer

d. Hep D – caused by Hep D virus

Transmitted like Hep B and a person must have been co-infected by Hep B before contracting Hep D
Results in severe liver damage and has a higher fatality rate than infection with Hep B alone.

e. Hep E – caused by the Hep E virus. Does not cause chronic liver disease but has a very high mortality rate among pregnant women