the digestive system digestive system organs fall into two main groups: the alimentary canal and the...

THE DIGESTIVE SYSTEM

• Digestive system organs fall into two main groups: the alimentary canal and the accessory organs:– Alimentary canal, or the gastrointestinal (GI) tract,

is the continuous muscular digestive tube that winds through the body digesting and absorbing foodstuff; its organs include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine

– Accessory digestive organs aid digestion physically and produce secretions that break down foodstuff in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands, liver, and pancreas

ORGANS OF THE ALIMENTARY CANAL

THE DIGESTIVE SYSTEM• Digestive Processes:

– Ingestion is the simple act of putting food into the mouth– Propulsion moves food through the alimentary canal and includes both

swallowing and peristalsis (alternate waves of contraction and relaxation of muscles in the organ walls)

• Its main effect is to squeeze food along the tract– Mechanical digestion is the physical process of preparing the food for chemical

digestion by enzymes and involves chewing, mixing, churning, and segmentation (rhythmic local constrictions of the intestine)

• Segmentation mixes food with digestive juices and increases the efficiency of absorption by repeatedly moving different parts of the food mass over the intestinal wall

– Chemical digestion is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes secreted into the lumen of the alimentary canal

– Absorption is the passage of digested end products (plus vitamins, minerals, and water) from the lumen of the GI tract through the mucosal cells by active and passive transport into the blood or lymph

• The small intestine is the major absorptive site– Defecation eliminates indigestible substances from the body via the anus as

feces

GASTROINTESTINAL TRACT ACTIVITIES

PERISTALSIS

PERISTALSIS

• (a): Peristalsis: adjacent segments of the intestine (or other alimentary tract organs) alternately contract and relax, which moves food along the tract distally

• (b): Segmentation: nonadjacent segments of the intestine alternately contract and relax, moving the food now forward and then backward– Results in food mixing rather

than food propulsion

Basic Functional Concepts

• The lumen (cavity) of the GI tract is an area that is actually outside the body, and essentially all digestive tract regulatory mechanisms act to control luminal conditions so that digestion and absorption can occur there as effectively as possible

Basic Functional Concepts• Digestive activity is provoked by a

range of mechanical and chemical stimuli

• Sensors (mechanoreceptors and chemoreceptors) involved in controls of GI tract activity are located in the walls of the tract organs

• These sensors respond to several stimuli; the most important are stretching of the organ by food in the lumen, osmolarity (solute concentration) and pH of the contents, and the presence of substrates and end products of digestion

• When stimulated, these receptors initiate reflexes that:

– 1.Activate or inhibit glands that secrete digestive juices into the lumen or hormones into the blood

– 2.Mix lumen contents and move them along the tract by stimulating smooth muscle of the GI tract walls

Basic Functional Concepts• Controls of digestive activity are

both extrinsic and intrinsic• Many of the controlling systems of the

digestive tract are intrinsic—a product of local nerve plexuses (short reflexes) or local hormone-producing cells

• Long reflexes are initiated by stimuli arising inside or outside the GI tract and involve CNS centers and extrinsic autonomic nerves

• The stomach and small intestine also contain hormone-producing cells that, when appropriately stimulated, release their products to the extracellular space

– These hormones are distributed via blood and interstitial fluid to their target cells in the same or different tract organs, which they prod to secrete or contract

NEURAL REFLEX PATHWAYS

THE DIGESTIVE SYSTEM

• The digestive system creates an optimal internal environment for its functioning in the lumen of the GI tract, an area that is technically outside of the body– Digestive activities within the GI tract are

triggered by mechanical and chemical stimuli– Controls of the digestive activity are both

extrinsic and intrinsic (nervous and hormonal)

Digestive System Organs:Relationship and Structural Plan

• Relationship of Digestive Organs to the Peritoneum– Most digestive system organs reside in the abdominopelvic

cavity– All ventral body cavities are lined by serous membranes

• The peritoneum of the abdominopelvic cavity is lined by a slippery serous membrane

– The visceral peritoneum covers the external surfaces of most of the digestive organs, and the parietal peritoneum lines the body wall of the abdominopelvic cavity

– Peritoneal cavity is located between the visceral and parietal peritoneums and is filled with serous fluid

– The serous fluid lubricates the mobile digestive organs, allowing them to glide easily across one another and along the body wall as they carry out their digestive activities


• (a): Mesentery is a double layer of peritoneum (a sheet of two serous membranes fused back to back) that extends to the digestive organs from the body wall (intraperitoneal or peritoneal organs)– It allows blood vessels,

lymphatics, and nerves to reach the digestive organs, and holds the organs in place as well as stores fat


• (b): not all alimentary canal organs are suspended by a mesentery (retroperitoneal organs)– Some parts of the

small intestine adhere to the dorsal abdominal wall (pancreas and parts of the large intestine)

PERITONEUM

HOMEOSTATIC IMBALANCE

• Peritonitis: inflammation of the peritoneum– The peritoneal coverings tend to stick

together around the infection site• Localizes the infection, providing time for

macrophages to attack to prevent the inflammation from spreading

• If widespread, it is dangerous and often lethal

The Splanchnic Circulation

• The splanchnic circulation serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation– Normally receives ¼ of the cardiac output

• Increases after a meal– Hepatic portal circulation collects nutrient-rich venous

blood draining from the digestive viscera and delivers it to the liver

• The liver collects the absorbed nutrients for metabolic processing or for storage before releasing them back to the bloodstream for general cellular use

Histology of the Alimentary CanalMucosa

• The innermost, moist, epithelial membrane that line the entire digestive tract from mouth to anus

• It secretes mucus, digestive enzymes, and hormones

• Absorbs digestive end products into the blood• Protects against infectious disease

• Digestive mucosa consists of three sublayers:

– 1. Lining epithelium• Rich in mucus-secreting goblet cells• Slippery mucus protects certain digestive

organs from being digested themselves by enzymes working within their cavities and eases food passage along the tract

• Stomach and small intestine secretes both enzymes and hormones (kind of endocrine gland)

– 2. Lamina propria• Connective tissue• Capillaries nourish the epithelium and absorb

digested nutrients• Lymph nodes: mucosa-associated lymphatic

tissue (MALT) helps defend us against bacteria and other pathogens

– 3. Muscularis mucosae• Scant layer of smooth muscle cells that

produces local movements of the mucosa

Histology of the Alimentary CanalSubmucosa

• Moderately dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers

• Its extensive vascular network supplies surrounding tissues of the GI tract wall

Histology of the Alimentary CanalMuscularis Externa

• Typically consists of smooth muscle and is responsible for peristalsis and segmentation

• Typically has an inner circular layer and an outer longitudinal layer of smooth muscle

• In several places, the circular layer thickens, forming sphincters, that act as valves to prevent backflow and control food passage from one organ to the next

Histology of the Alimentary CanalSerosa

• Protective outer layer of the intraperitoneal organs

• It is the visceral peritoneum

FOUR BASIC LAYERS OF THE ALIMENTARY WALL

Enteric Nervous System of the

Alimentary Canal• The alimentary canal has its own nerve

supply made up of enteric neurons that communicate widely with each other to regulate digestive activity– Intrinsic nerve plexuses found in the walls of the

alimentary canal• Submucosal nerve plexus occupies the submucosa and

chiefly regulates the activity of glands and smooth muscle in the mucosa

– Linked to the CNS by afferent visceral fibers and by sympathetic and parasympathetic branches

• In general, parasympathetic inputs enhance secretory activity and motility, whereas sympathetic impulses inhibit digestive activities

Mouth

• Mouth is the only part of the alimentary canal involved in ingestion

• Mouth ( oral cavity, buccal cavity) is a stratified squamous epithelial mucosa-lined cavity with boundaries of the lips, cheeks, palate, and tongue

Mouth

• The lips (labia) and cheeks have a core of skeletal muscle covered externally by skin that helps to keep food between the teeth when we chew and plays a small role in speech

• The palate forms the roof of the mouth and has two parts: the hard palate anteriorly and the soft palate posteriorly– Projecting posteriorly from the

soft palate is the uvula• Rises reflexively to close off

the nasopharynx when we swallow

Tongue

• The tongue is made of interlacing bundles of skeletal muscle and is used to reposition food (bolus) when chewing, mix food with salvia, initiate swallowing, and help form consonants for speech– Lingual frenulum: secures

the tongue to the floor of the mouth and limits posterior movements of the tongue

ORAL CAVITY

Tongue

• Filiform papillae give the tongue surface a roughness that provides friction for manipulating food– They contain keratin, which

stiffens them and gives the tongue its whitish appearance

• Fungiform and circumvallate papillae contain taste buds

TONGUE

Salivary Glands

• Salivary glands produce saliva, which cleanses the mouth, dissolves food chemicals so that they can be tasted, moistens food, and contains chemicals that begin the breakdown of starches

Salivary Glands

• Most saliva is produced by extrinsic salivary glands that lie outside the oral cavity and empty their secretions into it

• Their output is augmented slightly by small intrinsic salivary glands, also called buccal glands, scattered throughout the oral cavity mucosa

Salivary Glands

• Salivary glands are composed of two types of secretory cells:– Serous cells produce a watery secretion

containing enzymes, ions, and a tiny bit of mucin (glycoprotein that forms a slimy solution in water)

– Mucous cells produce mucus, a stringy, viscous solution

Salivary Glands

• Parotid glands:– Paired – Lie anterior to the ear

between the masseter muscle and the skin

– Opens into the vestibule next to the second upper molar

– Only serous cells

Parotid Duct


• Mumps:– Inflammation of the parotid glands caused by

the mumps virus (myxovirus)– Spreads from person to person in saliva– In adult males, there is a 25% risk that the

testes may become infected leading to sterility

Salivary Glands

• Submandibular gland:– Paired – Lies along the medial

aspect of the mandibular body on each side

– Duct runs beneath the mucosa of the oral cavity floor and opens at the base of the lingual frenulum

– Equal numbers of serous and mucous cells

Salivary Glands

• Sublingual gland: – Paired – Lies anterior to the

submandibular gland under the tongue

– Opens via 10-12 ducts into the floor of the mouth

– Contains mostly mucous cells

• Numerous minor glands are located in the oral cavity

SALIVARY GLANDS

Saliva• 97-99.5% water• Hypo-osmotic• Slightly acidic (pH 6.75-7.00)• Contains:

– Solutes include electrolytes: Na+, K+, Cl-, PO4-, and HCO3

-

– Digestive enzyme: salivary amylase– Protein mucin: forms thick mucus when dissolved in water

• Lubricates the oral cavity and hydrates food– Lysozyme: bacteriostatic enzyme capable of destroying certain bacteria– IgA: immunoglobulin G; antibodies– Metabolic waste: urea and uric acid– Defensins: produced by WBC to destroy bacteria

• Friendly bacteria that live on the back of the tongue convert food-derived nitrates into nitrites which are converted into nitric acid (bactericidal)

Control of Salivation

• Controlled primarily by the parasympathetic division of the autonomic nervous system

• Chemoreceptors and pressoreceptors in the mouth send signals to the salivatory nuclei in the brain stem (pons and medulla)

• Parasympathetic nervous system activity increases and impulses sent via motor fibers in the facial (VII) and glossopharyngeal (IX) nerves trigger a increase output of watery (serous), enzyme-rich saliva

• Sympathetic nervous system causes release of a thick mucin-rich saliva

Teeth

• Teeth lie in sockets (alveoli) in the gum-covered margins of the mandible and maxilla

• We masticate, or chew, by opening and closing our jaws and moving them from side to side while continually using our tongue to move the food between our teeth

• The teeth tear and grind food, breaking it into smaller pieces

Dentition

• Generally, all the teeth of the permanent dentition but the third molars have erupted by the end of adolescence– The third molars (wisdom teeth), emerge between the

ages of 17 and 25 years

• There are usually 32 permanent teeth in a full set, but sometimes the wisdom teeth never erupt or are completely absent


• When a tooth remains embedded in the jawbone, it is said to be impacted

• Impacted teeth can cause a good deal of pressure and pain and must be removed surgically

• Wisdom teeth are most commonly impacted

Dentition• Teeth are classified according

to their shape and function:– Incisors:

• Chisel-shaped• Adapted for cutting or nipping off

pieces of food– Canines: cuspids or eyeteeth

• Conical or fanglike• Tear and pierce

– Premolars: bicuspids• Broad crowns with rounded cusps

(tips)• Best suited for grinding or

crushing – Molars: four or five cusps

• Broad crowns with rounded cusps (4-5)

• Best grinders

Dental Formula

• Shorthand way of indicating the numbers and relative positions of the different types of teeth in the mouth

• Written as a ratio, uppers over lowers, for ½ of the mouth– Since the other side is a mirror image, the total

dentition is obtained by multiplying the dental formula by 2

• The permanent dentition (two incisors, one canine, two premolars, and three molars)– 2I,1C,2PM,3M X (32 teeth)– 2I,1C,2PM,3M

Tooth Structure• Five major regions:• 1. Crown:

– Exposed part of the tooth above the gingiva (gum), which surrounds the tooth like a tight collar

• Where the gingiva borders on a tooth, It dips downward to form a shallow groove called the gingival sulcus

– In youth, the gingiva adheres tenaciously to the enamel covering the crown

– As the gums begin to recede with age, the gingiva adheres to the more sensitive cementum covering the superior region of the root

– As a result, the teeth appear to get longer in old age

– Outer surface is enamel

Tooth Structure• Enamel: an acellular, brittle material

that directly bears the force of chewing– Hardest substance in the body (96%

calcium salts)– Heavily mineralized with calcium

salts (calcium carbonate)– Densely packed hydroxyapatite

(mineral) crystals (form of calcium phosphate) are oriented in force-resisting columns perpendicular to the tooth’s surface

• Soluble in the acids of soft drinks• Becomes decay-resistant fluoroapatite

after combing with fluoride ions present in fluoridated water or fluoride toothpastes

– The cells that produce enamel degenerate when the tooth erupts

• Any decayed or cracked areas of the enamel will not heal and must be artificially filled

– Proteins amelogenins and enamelins (4%): role not fully understood but believed to aid in framework and support

Tooth Structure• 2. Root:

– Portion of the tooth embedded in the jawbone

– Canine, incisors, and premolars have one root

• First two upper premolars have two roots (upper jaw)

– Molars:• First two upper have three roots• Lower have two roots• Third molars vary: usually one

root– Cementum: outer surface of the

root• Calcified connective tissue• Attaches the tooth to the thin

periodontal ligament– Anchors the tooth in the bony

alveolus of the jaw, forming a fibrous joint called a gomphosis

Tooth Structure• 3. Neck:

– Connects the crown and root• 4. Dentin:

– Bonelike material– Underlines the enamel cap and forms the

bulk of a tooth– Dentinal tubules: each tubule contains cell

types that secretes and maintains the dentin (odontoblast)

• Formed throughout adult life• New dentin can be formed rapidly to

compensate for tooth damage or decay– Softer (less brittle) than enamel since less

mineralized (75% of enamel)– Surrounds a central pulp cavity

• Contains a number of soft tissue structures (pulp)

– Connective tissue, blood vessels, nerve fibers

– Supplies nutrients to the tooth tissues and provides for tooth sensation

– 5. Root canal:• At the proximal end of each root canal is an

apical foramen that provides a route for blood vessels, nerves, and other structures to enter the pulp cavity of the tooth

– Trigeminal nerve and maxillary artery


• Death of a tooth’s nerve and consequent darkening of the tooth is commonly caused by a blow to the jaw

• Swelling in the local area pinches off the blood supply to the tooth and the nerve dies

• Typically the pulp becomes infected by bacteria some time later and must be removed by root canal therapy

• After the cavity is sterilized and filled with an inert material, the tooth is capped

Tooth Structure

• Although enamel, dentin, and cementum are all calcified and resemble bone, they differ from bone in that they are avascular

• Enamel also differs from cementum and dentin because it lacks collagen as its main organic component

Tooth and Gum Disease• Dental caries (cavities):

– Result from gradual demineralization of enamel and underlying dentin by bacterial action

– Begins when dental plaque (film of sugar, bacteria, and other mouth debris) adheres to the teeth

• Bacterial metabolism of the trapped sugars produces acids, which can dissolve the calcium salts of the teeth

• Once the salts are leached out, the remaining organic matrix of the tooth is readily digested by protein-digesting enzymes released by the bacteria

• Calculus: tartar– Plaque on gums– Disrupts the seals between the gingivae and the teeth, putting the gums at risk

for infection– Gingivitis: early stages of infection

• Gums red, swollen, bleeding• If neglected: periodontal disease (periodontitis)

– The bacteria invade the bone around the teeth forming pockets of infection (dissolving bone)– Accounts for 80-90% of tooth loss in adults– Concerns about its relationship to heart disease

» Bacteria in the blood stimulating clot formations that enter the heart coronary circulation

CANINE TOOTH

Pharynx

• From the mouth, food passes posteriorly into the oropharynx, and then the laryngopharynx

• The pharynx (oropharynx and laryngopharynx) provides a common passageway for food, fluids, and air (nasopharynx has no digestive role)

• Constriction of the pharyngeal muscles propels food into the esophagus

Esophagus• (a): Muscular tube collapses

when not involved in food propulsion

• (b): Arrow shows the point of abrupt transition from the stratified squamous epithelium of the esophagus (top) to the simple columnar epithelium of the stomach (bottom)

• The submucosa contains mucus-secreting esophageal glands– As a bolus moves through the

esophagus, it compresses these glands, causing them to secrete mucus that lubricates the esophageal walls and aids food passage

ESOPHAGUS

Esophagus• Takes a fairly straight course

through the mediastinum of the thorax pierces the diaphragm at the esophageal hiatus to enter the abdomen

• After food moves through the laryngopharynx, it is routed into the esophagus posteriorly as the epiglottis closes off the larynx to food entry

• The esophagus provides a passageway for food and fluids from the laryngopharynx to the stomach where it joins at the cardiac orifices

Esophagus

• Joins the stomach at the cardiac (gastroesophageal) sphincter which is a physiological sphincter– Acts as a valve– A slightly thicken circular

muscle

• The muscular diaphragm, which surrounds this sphincter, helps keep it closed when food is not being swallowed


• Heartburn:– First symptom of gastroesophageal reflux disease

(GERD)– Burning, radiating substernal pain that occurs when

the acidic gastric juice regurgitates into the esophagus

• Hiatal hernia:– Structural abnormality in which the superior part of the

stomach protrudes slightly above the diaphragm• Since the diaphragm no longer reinforces the cardiac

sphincter, gastric juice may flow into the esophagus

Digestive Processes Occurring in the Mouth

• Ingestion of food• Begins mechanical digestion by chewing

initiates propulsion by swallowing• Initiates the chemical digestion of

carbohydrates (polysaccharides: starch and glycogen) by enzyme salivary amylase in saliva– Hydrolysis into smaller fragments of linked glucose

molecules– Except for a few drugs that are absorbed through the

oral mucosa (nitroglycerine), essentially no absorption occurs in the mouth

Digestive Processes Occurring in the Pharynx and Esophagus

• Merely serve as conduits to pass food from the mouth to the stomach

• Their single digestive function is food propulsion, accomplished by the role they play in swallowing

Mastication (Chewing)

• Mastication, or chewing, begins the mechanical breakdown of food and mixes the food with saliva

• Partly voluntary:– We put food in our mouth– We contract muscles

• Partly reflexive:– Response to stretch and pressure receptors

in cheeks, gums, and tongue

Deglutition (Swallowing)• Involves coordination of over 22

separate muscle groups• Complicated process that involves two

major phases:– 1. The buccal phase (a) is voluntary

and occurs in the mouth where the bolus is forced into the oropharynx

– Food enters the pharynx and stimulates tactile receptors passing out of our control and into the realm of involuntary reflex activity

– 2. The pharyngeal-esophageal phase is involuntary and occurs when food is squeezed through the pharynx and into the esophagus

• Controlled by the swallowing center located in the medulla and lower pons

– Motor impulses are transmitted via various cranial nerves, most importantly the vagus nerve, to the muscles of the pharynx and esophagus

DEGLUTITION

Deglutition (Swallowing)

• (b): Once food enters the pharynx, all routes except the desired one—into the digestive tract—are blocked off:– Tongue blocks the mouth and

presses on the epiglottis– Soft palate rises to close off

the nasopharynx (uvula)– Larynx rises so that the

epiglottis covers its opening into the respiratory passageways, and the upper esophageal sphincter relaxes (b)

Deglutition (Swallowing)

• Food is squeezed through the pharynx and into the esophagus by wavelike muscular peristaltic contractions (c-e)

• Upper esophageal sphincter contracts (c)

• (d): food is moved through the esophagus by peristalsis

• (e):Just before the peristaltic wave (and food) reaches the end of the esophagus, the gastroesophageal sphincter relaxes reflexively to allow food to enter the stomach

DEGLUTITION

Stomach

• Temporary storage• Initiation of chemical

breakdown of proteins• Food is converted to a

creamy paste called chyme

• Lies in the left hypochondriac, epigastric, and umbilical regions of the abdomen

ABDOMINAL REGIONS

Stomach

• The stomach is a temporary storage tank where the chemical breakdown of proteins is initiated and food is converted to chyme

• The adult stomach varies from 15-25 cm long, but its diameter and volume vary depending on the amount of food it contains– The major regions of the

stomach include the cardiac region, fundus, body, and the pyloric region

• Pyloric sphincter: controls stomach emptying

Stomach

• The convex lateral surface of the stomach is its greater curvature, and its concave medial surface is its lesser curvature– Extending from these

curvatures are two mesenteries (omenta)

Stomach• Lesser omentum: runs from the

liver to the lesser curvature of the stomach where it becomes continuous with the visceral peritoneum covering the stomach

• Greater omentum:– Drapes inferiorly from the greater

curvature of the stomach to cover the coils of the small intestine

– Dorsally and ventrally wraps the spleen, transverse portion of large intestine, blending with the mesocolon (secures colon to parietal peritoneum of the posterior abdominal wall)

– Riddled with fat deposits giving it an appearance of a lacy apron

– Contains large deposits of lymph nodes (protecting the peritoneal cavity and intraperitoneal organs)

Stomach

• Served by the autonomic nervous system:– Sympathetic fibers

from the thoracic splanchnic nerve relayed through the celiac plexus

– Parasympathetic fibers are supplied by the vagus nerve

SYMPATHETIC NERVOUS SYSTEM

PARASYMPATHETIC NERVOUS SYSTEM

Stomach

• Arterial supply provided by branches (gastric and splenic) of the celiac trunk

• Veins are part of the hepatic portal system and ultimately drain into the hepatic portal vein

ARTERIES OF THE ABDOMEN

VEINS OF THE ABDOMEN

STOMACH

StomachMicroscopic Anatomy

• Wall contains the four tunics typical of most of the alimentary canal ( mucosa, submucosa, muscularis externa, adventita), but its muscularis and mucosa are modified for the special roles of the stomach

– Mucosa lining is folded (rugae)• Besides the usual circular and

longitudinal layers of smooth muscle, the muscularis externa has an innermost smooth muscle layer that runs obliquely

– This arrangement allows the stomach not only to move food along the tract, but also to churn, mix, and pummel the food, physically breaking it down into smaller fragments


• The surface epithelium of the stomach mucosa is a simple columnar epithelium composed of goblet cells, which produce a protective two-layer coat of alkaline mucus in which the surface layer consists of viscous mucus that traps a layer of bicarbonate-rich fluid beneath it

• Mucosa is also dotted with millions of gastric pits which lead to gastric glands that produce the stomach secretion called gastric juice


Gastric Glands• Cardiac and pyloric

regions primarily secrete mucus

• Pyloric antrum secrete mucus and several hormones (gastrin)

• Fundus and body regions secrete:– Most chemical digestion– Majority of secretions– Variety of Secretory

Cells: Mucous, Parietal, Chief, Enteroendocrine

Gastric GlandsSecretory Cells

• Mucous neck cells: upper neck region of the gland

– Produces a different type of mucus (acidic) secreted by the goblet cells of the epithelium

• Parietal cells: middle region of the gland

– Scattered among the chief cells– Secrete HCl: makes stomach contents

extremely acidic (pH 1.5-3.5) • Condition necessary for activation

and optimal activity of pepsin• Harsh enough to kill many of the

bacteria ingested with foods• Denatures proteins• Breaks down cell walls of plants

• Chief cells: basal region of the gland– Pepsinogen: the inactive form of the

protein-digesting enzyme pepsin• Activated by HCl

– Lipases: small amount of fat-digesting enzymes

Gastric GlandsSecretory Cells

• Enteroendocrine cells ( G cells):– Secrete a variety of hormone or

hormonelike products directly into the blood and influence several digestive system target organs

• Gastrin: hormone– Stimulates gastric acid secretion– Causes the lower esophageal

sphincter to contract and the ileocecal valve to relax

• Histamine: hormonelike– Dilation of blood vessels

• Endorphins (natural opiates):– Hormonelike– Decreasing sensation of pain

• Serotonin: hormone– Vasoconstrictor

• Cholecystokinin: hormone– Stimulates contraction of gall

bladder and pancreas• Somatostatin: hormonelike

– Inhibits gastric acid secretion

Stomach Mucosa

• Exposed to some of the harshest conditions in the entire digestive tract

• Gastric juice is corrosively acidic and its protein-digesting enzymes can digest the stomach itself

• Produces a mucosal barrier to protect itself:– 1. Thick coating of bicarbonate-rich mucus– 2. Epithelial cells joined together by tight junctions that prevent

gastric juice from leaking into the underlying tissue layers– 3. Deep in the gastric glands, where the protective alkaline

mucus is absent, the plasma membranes are impermeable to HCl

– 4. Damaged epithelial cells are shed and replaced by division of undifferentiated stem cells

MICROSCOPIC ANATOMY OF STOMACH

HOMEOSTATIC IMBALANCE• Gastritis: anything that breaches the gel-like mucosal barrier

causes inflammation of the stomach wall– Persistent damage to the underlying tissues can promote gastric ulcers,

erosions of the stomach wall• Danger posed by ulcers is perforation of the stomach wall followed by

peritonitis (inflammation of the membranes that line the abdominal cavity) and, perhaps, massive hemorrhage

• Causes:– Hypersecretion of HCl– Hyposecretion of mucus– Aspirin: lipid soluble (absorbed easily through mucosa) – Coffee– Alcohol: lipid soluble (absorbed easily through mucosa) – Stress– (90%): Acid-resistant bacteria: Helicobacter pylori

» Burrow beneath the mucus and destroy the protective mucosal layer leaving denuded areas

» Release several chemical» PROBLEM: also found in healthy people and might be linked to cancer

Digestive Processes Occurring in the Stomach

• Protein digestion is initiated in the stomach and is essentially the only type of enzymatic digestion that occurs there

• Proteins are denatured by HCl produced by stomach glands in preparation for enzymatic digestion

• Only function essential for life is the secretion of intrinsic factor:– required for intestinal absorption of vitamin B12

• Needed to produce mature erythrocytes

– In its absence: pernicious anemia results

Regulation of Gastric Secretion

• Gastric secretion is controlled by both neural and hormonal mechanisms– Nervous control is provided by long (vagus nerve-

mediated) and short (local enteric) nerve reflexes• Parasympathetic nervous system stimulates secretion of the

glands via the cranial nerve (vagus)• Sympathetic nervous system depresses secretory activity

– Hormonal control of gastric secretion is largely controlled by gastrin which stimulates secretion of enzymes and HCl, and of hormones produced by the small intestine, which are mostly gastrin antagonists

Phases of Gastric Secretion

• Three distinct phases:– Cephalic– Gastric– Intestinal

Neural and hormonal mechanism that regulates release of gastric juice

Cephalic Phase• Reflex phase• Only a few minutes• Gastric secretion occurs before

food enters the stomach• Triggered by aroma, taste, sight,

or thought of food• Senses stimulate the

hypothalamus which in turn stimulates the vagal nuclei of the medulla oblongata, causing motor impulses to be transmitted via the vagus nerves to parasympathetic enteric ganglia resulting in stimulation of the stomach glands

• Conditioned reflex• Depression or lack of appetite

suppresses the cephalic reflex

Gastric Phase

• Initiated by local neural and hormonal mechanisms once food reaches the stomach

• 3-4 hours• Provides 2/3 of gastric

juice• Stretch receptors

stimulate additional neural responses

Gastric Phase• Hormone gastrin (secreted by

enteroendocrine: G cells) plays a greater role in stimulating gastric juice secretion

• The more protein in the meal, the greater the amount of gastrin and HCl released

• As proteins are digested, the gastric contents gradually become more acidic, which again inhibits the gastrin-secreting cells

– This negative feedback mechanism helps maintain optimal pH and working conditions for the gastric enzymes

Gastric Phase• Binding of histamine, gastrin, and

acetylcholine (Ach) to parietal cell membrane receptors initiates intracellular events (mediated by second-messenger systems) that lead to HCl secretion into the stomach lumen

• H+ and HCO3- (bicarbonate ions) are

generated from the dissociation of carbonic acid (H2CO3)

• As H+ / K+ ATPase pumps H+ into the lumen, K+ enters the cell, HCO3

- is pumped into the interstitial space in exchange for chloride ions (Cl-)

– Cl- is obtained from blood plasma• Cl- and K+ then diffuse into the lumen

through membrane channels

Regulation and mechanism of HCl secretion

Gastric Phase• G cells (enteroendocrine

cells) that produce gastrin are activated by both:– Chemical (amounts of

proteins, HCl, gastrin etc.) in negative feedback mechanism

– Neural reflexes stimulate or inhibit gastrin/gastric juice production

• Inhibitory:– Emotional upsets, fear,

anxiety, or anything that triggers the fight-or-flight response inhibits gastric secretion because (during such times) the sympathetic division overrides parasympathetic controls of digestion

Regulation and mechanism of HCl secretion

Intestinal PhaseExcitatory

• Two components—one excitatory and the other inhibitory– Excitatory:

• Set into motion as partially digested food fills the initial part (duodenum) of the small intestine

• Stimulates intestinal mucosal cells to release a hormone (intestinal gastrin) that encourages the gastric glands to continue to secrete gastric juice

• This stimulatory effect is brief because as the intestine distends with chyme containing large amounts of H+, fats, partially digested proteins, and various irritating substances, the inhibitory component is triggered in the form of the enterogastric reflex

Intestinal PhaseInhibitory

• Enterogastric reflex is actually a trio of reflexes that:– 1. Inhibit the vagal nuclei in the medulla– 2. Inhibit local reflexes– 3. Activate sympathetic fibers that cause the pyloric sphincter to

tighten and prevent further food entry into the small intestine• Gastric secretory activity declines

– Protects the small intestine from excessive acidity • Triggers the release of several intestinal hormones

(enterogastrones: inhibits gastric juice secretion and stimulates intestinal secretions)– Secretin– Cholecystokinin (CCK)– Vasoactive intestinal peptide (VIP)– Gastric inhibitory peptide (GIP)

Peristaltic Waves in the Stomach

• (a): moves toward the pylorus• (b): most vigorous peristalsis and mixing action occurs close to the pylorus• (c): pyloric end of the stomach acts as a pump that delivers small amounts

of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach, where it undergoes further mixing (3ml of chyme squirts through at any given moment and the valve closes propelling the stomach contents backward for further mixing) (back and forth pumping effectively breaks up solids in the gastric contents)

Peristaltic Rate

• Constant rate: 3 waves per minute– Can be modified

• Set by the spontaneous activity of pacemaker cells located in the longitudinal smooth muscle layer (interstitial cells of Cajal)– Establish the stomach’s basic electrical rhythm of peristaltic

waves– Depolarize and repolarize spontaneously three times each

minute

• The rate at which the stomach empties is determined by both the contents of the stomach and the processing that is occurring in the small intestine

Regulation of Gastric Emptying

• Usually 4 hours after a meal– Liquid faster– Solids slower

• Fatty foods slower

• Depends on the contents of the duodenum as on what is happening in the stomach

PERISTALTIC WAVES

Neural and hormonal factors Inhibiting Gastric Emptying

• Controls ensure that the food will be well liquefied in the stomach and prevent the small intestine from being overwhelmed

Neural and hormonal factors inhibiting gastric emptying

Small Intestine and Associated Structures

• The small intestine is the site of the completion of digestion and absorption of nutrients:– These vital functions cannot be accomplished without the

aid of secretions from the liver (bile) and pancreas (digestive enzymes)

– It extends from the pyloric sphincter to the ileocecal valve where it joins the large intestine

– It has three subdivisions: the duodenum, the jejunum, and the ileum

– It is highly adapted for absorption with three microscopic modifications: plicae circulares, villi, and microvilli

– The intestinal crypts, or the crypts of Lieberkuhn, secrete intestinal juice that serves as a carrier fluid for absorbing nutrients from chyme

DUODENUM

Small Intestine

• Body’s major digestive organ

• Virtually all absorption• A convoluted tube

extending from the pyloric sphincter in the epigastric region to the ileocecal valve in the right iliac region where it joins the large intestine

Small Intestine

Small Intestine

• Longest part of the alimentary tube, but is only about half the diameter of the large intestine, ranging from 2.5 to 4cm (1-1.6 in)– In a cadaver: 6-7 m long (approximately 20 feet)– In a living human: 2-4 m (approximately 8-13 feet)

• Has three subdivisions:– Duodenum: which is mostly retroperitoneal (space

behind the peritoneum – Jejunum: intraperitoneal (within the peritoneal cavity)– Ileum: intraperitoneal (within the peritoneal cavity)

Duodenum

• About 25 cm (10 inches) long

• It is the shortest intestinal subdivision

• Hepatopancreatic ampulla: union of two ducts which opens into the duodenum via the major duodenal papilla (secretions controlled by the hepatopancreatic sphincter)– Bile duct delivers bile from the

liver– Pancreatic duct delivers

pancreatic juice from the pancreas

Jejunum

• 2.5 m (8 feet) long• Extends from the

duodenum to the ileum

Ileum

• Approximately 3.6 m (12 feet) in length

• Joins the large intestine at the ileocecal valve

• Both the jejunum and ileum suspend from the posterior abdominal wall by the fan-shaped mesentery

the digestive system digestive system organs fall into two main groups: the alimentary canal and the...

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