chapter 15 + 16

the digestive system is a hollow tube consisting of:

the oral cavity, esophagus, stomach, small and large intestines and anus. it also includes the associated glands: salivary glands, liver and pancreas

the digestive system is also known as

the GI, (gastrointestinal) tract or alimentary canal

the digestive systems function is

the breakdown of ingested food into it’s nutrient molecules, the absorption of those molecules into the blood and lymphatic capillaries and the elimination of waste products

during digestion, proteins, complex carbohydrates, nucleic acids, and fats are

broken down into their small molecular subunits that are easily absorbed through the small intestine lining

most water and electrolytes are

absorbed through the large intestine

the structures within the digestive tract allow the following (8):

1. ingestion which is the introduction of food and liquid into the oral cavity
2. mastication or chewing, which divides solid food into digestible pieces
3. motility, the muscular movements of materials through the tract
4. secretion of lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile
5. hormone release for local control of motility and secretions
6. chemical digestion or enzymatic degradation of large macromolecules in food to smaller molecules and their subunits
7. absorption of the small molecules and water into the blood and lymph
8. elimination of indigestible, unabsorbed components of food

lip (6):

1. part of the oral cavity
2. covered by typical oral mucosa
3. opposite side covered by skin
4. vermillion zone, (the inner lip) contains very thin dermis, lightly keratinized, highly micro-vascularized connective tissue
5. lacks glands for oil and sweat
6. contains much striated muscle internally, and many minor salivary glands

all parts of the GI tract have certain structural features in common. the GI is a hollow tube with a lumen of variable diameter and a wall consisting of 4 main layers:

1. mucosa
2. submucosa
3. muscularis
4. serosa

these layers have several common features:

smooth muscle cells, rich in blood vessels and lymphatics, an epithelial layer and connective tissue

the GI tract also has

numerous free immune cells and lymphoid nodules in the mucosa and submucosa, (constitutes MALT), that provide immune protection against potential pathogens that may have been ingested with food or drink

the rich autonomic innervation of the enteric nervous system provides

an anatomic explanation of the well-known actions of the emotional stress on the stomach and other regions of the GI tract

that “gut feeling”, butterflies in your stomach, nauseated feeling you may get when you are

scared or extremely upset are examples of the role emotions play on your GI tract

the oral cavity, (mouth) consists of

the tongue, teeth and minor salivary glands

the ducts of the major salivary glands open in the

oral cavity, where the process of digestion begins

the oral cavity is lined with a

protective, non-keratinized, keratinized or partially keratinized stratified squamous epithelium

the keratinized cell layers resist damage from

abrasion and are best developed in the masticatory mucosa on the gingiva, (gum) and hard palate

the tongue is a

mass of striated muscle covered by mucosa, which manipulates ingested material during mastication, (chewing) and swallowing

the muscle fibers of the tongue are

oriented in all directions allowing for a high degree of mobility

the dorsal surface mucosa of the tongue has

projecting lingual papillae of 4 types: filiform, foliate, fungiform, and large vallate

the anterior 2/3rds of the dorsal surface of the tongue has

peg-like projections of the mucosa called lingual papillae which serve both mechanical and taste functions

filiform papillae, narrow conical structures cover 2/3rds of the tongue, there are

no taste buds, instead it has a rough surface that provides friction for the manipulation of food

foliate, fungiform, and vallate papillae all have taste buds on their sides,

with chemosensory gustatory cells with synapses to basal sensory innervation, support cells and an apical taste pore with microvilli

Von Ebner’s glands discharge serous secretions that

wash the taste buds allowing them to receive and process new stimuli

taste cells turn over rapidly and have a lifespan of

7-10 days and are replaced by stem cells located basally in the taste buds

adults have 32 teeth and each tooth has enamel covering

its crown and neck, and a vascularized and innervated central pulp cavity within the dentin that makes up the roots and extends into the neck

enamel is the hardest material in the body, and it is composed of

hydroxyapatite and is similar to the calcified matrix of the bone, but without collagen

teeth also have roots that

extend below the gum and covered with cementum, (a calcified CT)

odontoblasts line the pulp cavity and secretes dentin throughout life, however the

cells that produce enamel (ameloblasts) degenerate and any damage to the tooth has to be restored with an artificial substance

the esophagus is a straight muscular tube that transports

food from the oral cavity to the stomach via the pharynx, without significant metabolic changes

the wall of the esophagus has the same major layers

of the other organs of the GI tract

esophagus: the mucosa is lined with an abrasion resistant, non-keratinized stratified squamous epithelium.

it has mucous secreting esophageal cardiac glands in the lamina propria near the pharynx and the stomach, and the submucosa secretes mucous to lubricate the walls and aids in the passage of food

aggregates of lymphoid cells are common in the

submucosa near the gastroesophageal junction

the muscularis externa is composed of both

skeletal and smooth muscle. the proximal third is exclusively skeletal muscle; the middle third is both skeletal and smooth muscle; and the distal third is exclusively smooth muscle

therefore, transport of food down the esophagus begins

with voluntary peristalsis (skeletal muscle) and ends with involuntary peristalsis (smooth muscle). tonic contractions of the muscularis externa at the upper and lower ends of the esophagus create functional sphincters that prevent reflux into the pharynx from the esophagus and into the esophagus from the stomach, respectively

adventitia/serosa

the thin outer layer of the esophagus

the stomach is a highly distensible, temporary storage organ where

ingested food is mixed with gastric secretions to produce an acidic paste called chyme, then released into the duodenum in small aliquots via the pyloric sphincter

the stomach is both an endocrine and exocrine organ:

endocrine when it secretes hormones to regulate it’s function and exocrine when it secretes hydrochloric acid (HCL) and various digestive enzymes

the stomach is divided into 4 anatomic regions:

1. cardia
2. fundus
3. body
4. pylorus

the wall of the stomach is built from

the same 4 layers as the other organs of the GI, but with special features modified for their specific roles

the stomach is lined with

simple columnar epithelium composed entirely of mucous cells. this otherwise smooth lining is punctuated by millions of deep gastric pits

multiple branched, tubular glands empty into the pits. although both gastric pits and glands are formed by invagination of the epithelium into the lamina propria, these two regions can be distinguished as follows (2):

1. gastric pit. composed primarily of mucous cells and are relatively uniform throughout all regions of the stomach
2. glands. composed of various secretory and endocrine cells. the types of cells that comprise the glands, vary among stomach regions, reflecting important functional differences in the regions

the major functions of the gastric epithelium are:

secretion of HCL and enzymes for digestion and mucus secretion to protect the stomach from the corrosive effects of the acid and enzymes

mucous surface cells

cover the surface mucosa and line the pits as well as secreting viscous alkaline mucous trapping bicarbonate-rich fluid beneath protecting the stomach lining from the harsh acid it produces. these cells are replaced every 3 days

mucous neck cells

line the cardia and pylorus. they produce soluble acidic mucus that mixes with and lubricates chyme as it goes along the GI tract

parietal cells

are large eosinophilic cells that secrete HCL into the lumen. it secretes 1-1.5L of HCL and it has a pH of 2. they also secrete gastric intrinsic factor, which is essential for the absorption of vitamin B12 in the ileum

chief cells

located in the base of the gastric glands and has many eosinophilic secretory granules. chief cells produce proteolytic enzymes, mainly pepsinogen, which is converted to its active form, pepsin, by gastric acid, as well as the enzyme lipase and the hormone leptin

enteroendocrine cells of the stomach are

located in epithelium throughout the stomach and all other regions of the GI tract, and are considered part of the diffuse neuroendocrine system (DNES)

release their secretory products basally into the lamina propria, rather than apically into the lumen. products of the enteroendocrine cells in the stomach include the following (6):

1. gastrin. stimulates parietal cells to secrete HCL
2. glucagon. stimulates glycogenolysis in the liver
3. histamine. stimulates parietal cells to secrete HCL
4. somatostatin. inhibits the release of hormones produced locally; in the stomach this includes gastrin, glucagon and histamine
5. serotonin. increases gut motility
6. ghrelin. released when the stomach is empty and stimulates feelings of hunger

the small intestine is the

region of the GI tract where digestive processes are completed and nutrients are absorbed. the small intestine also produces hormones that regulate diverse digestive activities

the terminal digestion of food requires enzymes produced

by the pancreas and bile produced in the liver, as well as buffers and enzymes produced by epithelial cells and glands of the intestinal mucosa itself

the small intestine is the longest region of the GI tract (6-7 m long), extending from the pyloric sphincter to the large intestine, and is divided into 3 segments:

1. duodenum
2. jejunum
3. ileum

duodenum

the submucosa contains duodenal glands. the mucosal epithelial cells secrete almost all of the HCO3- that directly neutralizes the acidic chyme propelled from the stomach. when the mucus and HCO3-barrier fail, the intestinal wall can erode, resulting in a duodenal ulcer. enteroendocrine cells in brunner’s glands produce urogastrone, a peptide that inhibits HCL production by the parietal cells. the bile and pancreatic ducts open into the duodenum

jejunum

has the most highly developed pilcae circulares, and is the main absorptive site in the GI tract. however a typical histologic section of the jejunum is usually identified negatively-it lacks brunner’s glands (duodenum) or Peyer’s patches (ileum)

ileum

the lamina propria contains large aggregations of lymphoid follicles called Peyer’s patches. specialized epithelial cells, called M cells, are located over these patches. Peyer’s patches and goblet cells are more numerous toward the last part of the small intestine

the epithelium of the glands is composed of the following cells (8):

1. goblet cells: secrete glycoproteins that lubricate the luminal surface and retain antimicrobial molecules such as defensins and secretory IgA near the epithelial surface. goblet cells are least abundant in the duodenum and become more numerous toward the large intestine
2. paneth cells: located at the base of the intestinal glands and provides a protective function. paneth cells secrete antimicrobial agents such as lysozyme, defensins, and phospholipase A2. they are readily identified in histologic secretions by their large eosinophilic secretory granules
3. enteroendocrine cells: present in varying numbers throughout the small intestine and produce hormones that regulate diverse digestive activities, including the following:
4. cholecystokinin (CCK): stimulates the secretion of pancreatic enzymes; causes the gallbladder to contract and expel bile; and relaxes the hepatopancreatic sphincter to allow bile and pancreatic juice into the duodenum
5. gastric inhibitory peptide (GIP): inhibits secretion from the gastric glands
6. motilin: increases gut motility. secretion. stimulates HCO3-secretion by the pancreatic ducts; increases bile output
7. somatostatin: inhibits local release of CCK
8. peptide YY: released in response to eating and reduces appetite

the ileum delivers liquid

containing the indigestible remains of foodstuffs to the large intestine

the large intestine absorbs

water and electrolytes from the fluid, compacts and temporarily stores the waste, and eliminates it as feces. it also absorbs vitamins produced by intestinal bacteria and secretes mucus that lubricates the intestinal surface

the large intestine is subdivided into the

cecum, colon, rectum, and anus; the appendix is a small appendage arising from the cecum

the large intestine is characterized by

the absence of villi and pilcae circulares and by the presence of straight tubular glands and bands of smooth muscle called the taeniae coli

the large intestine has 3 major regions:

the short cecum, with the appendix; the long colon, with its ascending, transverse, descending, and sigmoid portions and the rectum

along it’s entire length, the mucosa of the large intestine has

millions of short simple tubular intestinal glands, lined with lubricating goblet cells and absorptive cells for the uptake of water and electrolytes

the muscularis of the colon has its

outer longitudinal layer subdivided into 3 bands of smooth muscle called teniae coli, which act in the peristaltic movement of feces into the rectum

pathophysiology of IBS (5):

1. altered gastrointestinal motility
2. visceral hypersensitivity, post infectious reactivity
3. brain-gut interactions
4. alteration in fecal micro flora, bacterial overgrowth
5. food sensitivity, carbohydrate malabsorption, and intestinal inflammation all have been implicated in the pathogenesis of IBS

esophagus (upper, middle, lower):

1. non-keratinized stratified squamous epithelium; cardiac glands at lower end
2. small esophageal glands (mainly mucous)
3. both layers striated muscle in upper region; both layers smooth muscle in lower region; smooth and striated muscle fascicles mingled in middle region
4. adventitia, except at lower end with serosa

stomach (cardia, fundus, body, pylorus):

1. surface mucous cells and gastric pits leading to gastric glands with parietal and chief cells, (in the fundus and body) or to mucous cardiac glands and pyloric glands
2. submucosa-no distinguishing features
3. 3 indistinct layers of smooth muscle (inner oblique, middle circular, and outer longitudinal)
4. serosa

small intestine (duodenum, jejunum, ileum):

1. plicae circulares; villi, with enterocytes and goblet cells, and crypts/glands with Paneth cells and stem cells; Peyer patches in ileum
2. duodenal (brunner) glands (entirely mucous); possible extensions of Peyer patches in ileum
3. muscularis-no distinguishing features
4. mainly serosa

large intestine (cecum, colon, rectum):

1. intestinal glands with goblet cells and absorptive cells
2. submucosa-no distinguishing features
3. outer longitudinal layer separated into 3 bands, the teniae coli
4. mainly serosa, with adventitia at rectum

anal canal:

1. stratified squamous epithelium; longitudinal anal columns
2. venous sinuses
3. inner circular layer thickened as internal sphincter
4. adventitia

at the anal canal the simple columnar epithelium lining

the rectum shifts abruptly to stratified squamous epithelium of the skin at the anus

near the anus the circular layer of the

rectum’s muscularis forms the internal anal sphincter, with further control exerted by striated muscle of the external anal sphincter

the rectum temporarily stores

feces, which the anus transports to the exterior. the rectum resembles the colon, expect that the glands of the rectum are shallower and it lacks teniae coli

the epithelium of the anus changes from

simple columnar near the rectum to non-keratinized stratified squamous, and then to keratinized near the external opening

the external orifice is controlled by

an internal sphincter composed of smooth muscle and an external sphincter composed of skeletal muscle, which are under autonomic and voluntary control, respectively

the appendix is a

small, blind-ending extension of the proximal cecum. it has the same basic structure as the large intestine but with a small, irregular lumen and shorter, less dense glands

the main distinguishing feature of the appendix is

the abundance of lymphoid follicles in the mucosa and submucosa, which are especially prominent in children and decrease with age. the appendix has no known function.

the organs associated with the digestive tract include:

1. the major salivary glands
2. the pancreas
3. the liver
4. the gallbladder

products of these organs facilitate transport and digestion of food within the gastrointestinal tract

salivary glands are

accessory organs of the digestive system that produce saliva, a viscous solution that moistens and lubricates the oral cavity, initiates digestion, and provides antibacterial protection

there are 3 pairs of salivary glands located outside of the oral cavity:

the parotid, submandibular, and sublingual

salivary glands produc approximately

1\.5 liters of saliva per day. they are composed of serous cells that secrete a watery mixture of digestive enzymes and ions, and mucous cells, which secrete mucous

salivary glands have

secretory units of either protein-secreting serous cells, usually organized in round or oval acini, or of mucin-secreting mucous cells in elongated tubules

parotid glands have only serous acini; sublingual glands are

mixed but have primarily mucous tubules, sone with serous demilunes; submandibular glands are also mixed but have mainly serous acini

salivary secretory units are

drained by simple cuboidal intercalated ducts, which merge as simple columnar striated ducts, which merge further as the larger interlobular or excretory ducts

cells of striated ducts have mitochondria-lined,

basolateral membrane folds specialized for electrolyte reabsorption from the secretion; excretory ducts are unusual in having striated cuboidal or columnar cells

the parotid glands are branched acinar glands located

in each check below the ear, are composed entirely of serous acini that produce alpha-amylase which initiates hydrolysis of carbohydrates and proline-rich proteins. proline-rich proteins have antibacterial properties

the submandibular gland are branched tubuloacinar glands located

on either side of the neck below the mandible. comprised of primarily serous acini and in addition to producing alpha-amylase and proline-rich proteins they also produce lysozyme, the enzyme that hydrolyzes bacterial walls

the sublingual gland are also branched tubuloacinar glands, located

on the floor of the mouth, and is composed of primarily mucous cells with a few serous cells

pancreatic islets of endocrine cells are embedded in

exocrine serous acinar tissue, which comprises most of the pancreas and in which the cells secrete hydrolytic digestive enzymes for delivery to the duodenum

each pancreatic acinar cell is

pyramidal, with secretory (zymogen) granules in the narrow apical end and Golgi complexes, much rough ER, and a large nucleus a the basal end

intercalated ducts draining pancreatic acini, including their

initial centro-acinar cells that insert into the acinar lumen, secrete bicarbonate ions (HCO3-) to neutralize chyme entering the duodenum from the stomach

hormones produced by cells in the islets of Langerhans play essential roles in glucose and lipid metabolism and regulation of blood glucose levels, as follows:

1. aCells (\~20% of cells). secrete glucagon, which stimulates glycogen breakdown and the release of glucose into blood
2. betaCells (\~70%). secrete insulin, which stimulates glucose uptake into tissues and also regulates lipid metabolism
3. gammaCells (5-10%). secrete somatostatin, which inhibits hormone release by the a and beta cells. other hormone-producing cells are present in small numbers

liver hepatocytes are

large epithelial cells with large central nuclei (polyploid and often bi-nucleated), much smooth and rough ER, and many small Golgi complexes

hepatocytes have many functions, including

endocrine (plasma protein secretion), exocrine (bile secretion), glucose storage (glycogen granules), and detoxification (using SER and peroxisomes)

in the liver, hepatocytes are

organized into irregular plates to form polygonal hepatic lobules in which the hepatocyte plates radiate toward a small central vein

each hepatic lobule is surrounded by

sparse connective tissue that is more abundant in the portal areas at the corners

portal areas or tracts contain a small lymphatic and the portal triad:

a portal venule branch from the portal vein, a hepatic arteriole branch of the hepatic artery, and a bile ductule branch of the biliary tree

in the lobules the portal venule and hepatic arteriole both branch into

irregular sinusoids between the hepatic plates where the nutrient-rich and O2-rich blood mixes, flows past hepatocytes, and drains to the central vein

the endothelium of the hepatic sinusoids is

discontinuous and fenestrated; between it and the hepatocytes is the perisinusoidal space (of Disse) where exchange occurs between the hepatocytes and blood plasma

the sinusoidal endothelium includes

many specialized stellate macrophages or Kupffer cells, which recognize and remove effete erythrocytes, releasing iron and bilirubin for uptake by hepatocytes

also present in the perisinusoidal spaces are

hepatic stellate cells (or ito cells) containing many small lipid droplets for storage of vitamin A and other fat-soluble vitamins

between adherent hepatocytes in the hepatic plates are

grooves called bile canaliculi, sealed by tight junctions, into which hepatocytes secrete water and bile components, including bilirubin and bile acids