CHP. 23 The digestive system

Extrinsic salivary glands

*Parotid*- lies anterior to the ear between the masseter muscle and the skin. *Parotid duct*- opens into the vestibule next to the second upper molar *Submandibular gland*- lies along the medial aspect of the mandibular body. *Submandibular duct*- opens at the base of the lingual frenulum *sublingual gland*- lies anterior to the submandibular under the tongue *sublingual duct*-it opens via 10-20 ducts into the floor of the mouth

gastric contractile activity

*Peristaltic* waves move toward the pylorus at the rate of 3 per minute -This *basic electrical rhythm [BER]* (slow waves) is initiated by *PACEMAKER CELLS [CELLS OF CAJAL* -most vigorous peristalsis and mixing occurs near the pylorus. -*Chyme is either*: delivered in small amounts into the duodenum *OR* forced backward into the stomach for further mixing due to the pyloric valve closing [*retropulsion*]

Regulation of pancreatic secretion

*Secretin and CCK* are released when fatty or acidic chyme enters the duodenum CCK and secretin enter the bloodstream Upon reaching the pancreas: CCK induces the secretion of enzyme-rich pancreatic juice *Secretin* causes secretion of bicarbonate-rich pancreatic juice -Vagal stimulation also causes release of pancreatic juice

Classification of teeth

*Teeth are classified according to their shape and function* -*incisors*chisel shaped teeth adapted for cutting and nipping -*Canines [eyeteeth]*- conical or fanlike teeth that pierce or tear *Premolars [bicuspids] and molars*- have broad crowns with rounded tips and are best suited for grinding and crushing. During chewing, upper and lower molars lock together generating crushing force

Microscopic anatomy of the small intestine

*The small intestine is highly adapted for absorbing nurtients* Structural modifications of small intestine wall increase surface area *Placey circulares*: deep circular folds of the mucosa and submucosa *Villi*- fingerlike extensions of the mucosa *Microvilli*- tiny projections on absorptive mucosal cells plasma membranes also referred to as *brush border*

Homeostatic imbalance: vomitting

*Vomiting [emesis]*- the stomach empties via a different route [oral] -Causes include: extreme stretching, irritants such as bacterial toxins, excessive alcohol, spicy foods, and certain drugs *The emetic enter of the medulla *initiates a number or motor responses -Diaphragm and abdominal wall muscle contract Cardiac sphincter relaxes and soft palate closes off the nasal passages -Excessive vomiting can cause dehydration and upset electrolyte and pH balance

homeostatic imbalance

*ankyloglossia [tongue tied]* is a congenital situation where the lingual frenulum is extremely short Corrected surgically by cutting the frenulum

small intestine

*bodies major digestive organ* within its twisted passage ways, digestion is completed and virtually all *absorption* occurs -Runs from the pyloric sphincter to the ileocecal valve. -Named not for its length, but for the diameter of its lumen. The sml intestine is much longer and the large intestine!

Gross anatomy of the stomach

*cardiac region*- surrounds the cardial orifice through which the food enters the stomach from the esophagus. *Fundus*-dome-shaped part, tucked beneath the diaphragm *body*- mid portion of the stomach *Pyloric region*-made up of antrum and canal which terminates at the pylorus. The pylorus is continuous with the duodenum through the pyloric sphincter *Greater curvature*- entire extent of the convex lateral surface *Lesser curvature*- concave medial surface *lesser momentum*- runs from the liver to the lesser curvature. *Greater omentum*- drapes inferiorly from the greater curvature to the small intestine.

Essential Activity of digestion: step 4

*chemical digestion* -catabolic breakdown of food molecules by enzymes secreted into the small intestine

Teeth

*dentis*= teeth Primary and permanent dentitions have formed by age 21 *primary*- 20 deciduous [baby] teeth that erupt at intervals between 6 months and 24 months *permanent*- enlarge and develop causing the root o the deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years All but the *third molars* have erupted by the end of adolenscense *Usually 32 permanent teeth*

external mechanism that regulate the digestive process

*external regulation of digestion involves:* - *mechanical and chemical stimuli*: sensors involved in controlling GI tract activity are within the walls of each organ. *stretch receptors* of the lumen *osmolality* water contents monitoring * presence of substrate in the lumen* -*Extrinsic control by CNS*- nerve plexus inside or outside and ANS [like external stimuli; smell] *long reflexes* -*intrinsic control by local centers* - "gut brain" [nerve plexus inside the organ] *short reflexes*

Accessory digestive organs

*function*: produce a wide variety of secretions that help breakdown food. - teeth, tongue, gallbladder, salivary glands, liver, and pancreas

Control of Salivation

*intrinsic glands* secrete just a sufficient enough amount to keep the mouth moist *Extrinsic salivary glands* secrete serous, enzyme-rich saliva in response to: -Ingested food which stimulates chemoreceptors and pressorrecptors. -The though of food -Strong sympathetic stimulation inhibits salivation and results in *dry mouth* salivary nuclei in the brain stem activated by the parasympathetic system

mucosa: lamina propria and muscularis mucosae

*lamina propia* "universa packing tissue* loose areolar and reticular CT [macrophages] -Nourish the epithelium and absorption nutrients -Contains lymph nodes [MALT] important in defense against bacteria *Muscularis mucosa*- smooth muscle cells that produce local movements of mucosa [when squeezed]

The pancreas

*location* -lies deep to the greater curvature of the stomach . The head is encircled by the duodenum and the tail abuts the spleen. *Exocrine function*: - secretes pancreatic juices which breaks down all categories of foodstuff -*Acini* [clusters of secretory cells] contain zymogen granules with digestive enzymes *Endocrine function*- release of insulin, glucagon, and somatostatin by the islets of langerhan's

Essential activity of digestion: step 3

*mechanical digestion* -increases the surface area of ingested food by physically preparing for digestion by enzymes * chewing, mixing [food by saliva], churning food in the stomach, and *segmentation* [mixing food with digestion juices with a back and forth motion in small intestine]

nerve and blood supply of the stomach

*nerve*- sympathetic and parasympathetic fibers of the autonomic nervous system [*vagus nerve* *arterial*- celiac trunk, and corresponding veins [part of the hepatic portal system]

Regulation of gastric secretion

*neural [vagus] and hormonal [gastrin] mechanisms* regulate the release of gastric juice -Stimulatory and inhibitory events occur in three phases: *Cephalic [reflex] phase: reflex phase* *Gastric phase*- once food enters the stomach *Intestinal phase*- as partially digested food enters the duodenum phases can happen once or all at the same time

The mouth

*oral or buccal cavity* -Its boundaries are; the lips, cheeks, palate, and tongue *Oral orifice* is its anterior opening and continuous with the oropharynx posteriorly To withstand abrasion: - The walls of the mouth are lined with stratified squamous -The gums, hard palate, and dormsum of the tongue are slightly keratinized [waterproof]

Relationship of digestion organs and peritoneum

*peritoneum*: serous membrane of the abdominal cavity *Visceral peritoneum*- covers the external surface of most digestive organs *parietal peritoneum* lines the body wall -Peritoneal cavity* space that contains secretion from the peritoneum and lubricates the digestive organs. Allowing them to slide across one another. *mesentery*- double layer of peritoneum. -Provides vascular and nerve supply to the viscera -holds organs in place -stores fat .

Essential activity of digestion: step 2

*propulsion* - moves food through the alimentary canal via swallowing [voluntary] and *peristalsis* [involuntary] *peristalsis* major means of propulsion. involves alternating waves of contraction and relaxation of muscles in the organ walls .

Deglutition [swallowing]

- complicated process that involves coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups. *Buccal phase*- occurs in the mouth and is *voluntary* Bolus is forced into the oropharynx. *Pharyngeal-esophageal phase*- controlled by the medulla and lower pons [involuntary swallowing] -All routes except into the digestive tract are sealed -Peristalsis moves food through the pharynx to the esophagus

Retroperitoneal organs

- organs outside the peritoneum reproductive organs, urinary bladder, ascending colon, descending colon, rectum, the kidneys, adrenal glands, the pancreas, and duodenum *REALLY BAD AC/DC ROCKER KIDS PARTY DOWN*

Digestive process: mouth to esophagus

-Food is ingested -Mechanical digestion begins [chewing] and bolus is formed* -*Propulsion* is initiated by swallowing - Salivary amylase [ptyalin] begins chemical breakdown of starch -The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach

Digestion in the small intestine

As *chyme enters the duodenum*: -Carbohydrates and proteins are only partially digested *No fat digestion has taken place* -Digestion continues in the small intestine *Chyme* is released slowly into the duodenum Because it is hypertonic and has low pH, mixing is required for proper digestion -Required substances needs are supplied by the liver [bile] *Virtually all nutrient absorption take place in small intestine*

Composition of bile

A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids [aid in digestive process], and electrolytes *Bile salts* are cholesterol derivatives that: -Emulsify fat -Facilitate fat and cholesterol absorption -Help solubilize cholesterol -Enterohepatic circulation recycles bile salts -The chief bile pigment is bilirubin, a waster product of hemme

Regulation of bile release

Acidic, fatty chyme moves from the stomach into the duodenum stimulating the release of cholecystokinen and secretin into the blood stream. -*Bile salts* and *secretin* transported in blood stimulate the liver to produce bile -Vagul stimulation causes weak contractions of the gallbladder *Cholecystokinin* causes: -the gallbladder to contrast -The hepatopancreatic sphincter to relax *As a result, bile enters the duodenum*

Essential activity of digestion: step 1

* Ingestion* -taking food into the digestive tract via * the mouth

Essential activity of digestion: step 5

*Absorption* -movement of digested end products and nutrients from the lumen through the mucosal cells into the blood or lymph

Essential activity of digestion: step 6

*Defecation* -Eliminates indigestible substances [like carbohydrates] from the body via feces

Gross anatomy of small intestine

*Duodenum*- "twelve fingers widths long" curves around the head of the pancreas and is about 25 cm or 10 inches long. -*The bile duct and main pancreatic duct*- Join the duodenum at the hepatopancreatic ampulla -controlled by the sphincter of Oddi which is also called the hepatopancreatic sphincter *JEJUNUM*- "empty" it is about 2.5 m or 8 feet in length. -Extends from the duodenum to the ileum *THE ILEUM*- twisted is 12 feet in length -joins the large intestine at the ileocecal valve

Intestinal phase of gastric secretion

*Excitatory event occurs* -low pH and partially digested food enters the duodenum stimulatory phase- distention of duodenum, presences of fatty, acidic, or hypertonic chyme and or irritants in the duodenum, triggering the *enterogastric reflex* *enterogastric reflex*: Initiates inhibition of local reflexes and vagal nuclei -Constricts the pyloric sphincter, preventing further food entry into the small intestine. -releases enterogastrones that inhibit gastric secretion

Tooth structure cont

*Gingival sulcus*- depression where the gingival gum borders the tooth *Dentin*- a protein rich bonelike material underlies the enamel cap and forms the bulk of the tooth. Acts as a shock absorber during chewing *Pulp cavity*- cavity surrounded by dentin that contains pulp *Pulp*- connective tissue, blood vessels, and nerves *Root canal*- portion of the pulp cavity that extends into the root *Apical foreman*- proximal opening to the root canal *Odontoblasts*- secrete and maintain dentin throughout life

Glands of the stomach fungus and body

*Glands of the stomach fungus and body*: where most digestion occurs are substantially larger and produce the majority of the stomach secretions . These glands contain a variety of secretory cells: Mucous neck parietal cheif enteroendocrine

Receptors of the GI tract

*Mechano and chemoreceptors respond to*: -stretching of the lumen by food -osmolarity [ solute concentration] -pH of the contents -presence of substrate or end products of digestion These receptors innate reflexes that -Activate or inhibit digestive glands -Mix lumen contents and move them along

Stomach lining

The stomach mucosa is exposed to the harshest conditions on the digestive tract. Gastric juices are convulsley acidic and its protein- digesting enzymes could digest the stomach itself. The mucosal barrier is the stomach counter attach that protects with: - a thick coat of bicarbonate- rich [basic] mucus on the stomach wall -Epithelia cells that are joined by tight junctions preventing gastric juices from linking into tissue -Gastric glands that have cels impermeable to HCl -Damaged epithelial cella are quickly replaced

Digestive process in the stomach

The stomach: Holds ingested food -further degrades food both physically and chemically -Delivers *chyme* to the small intestine -Enzymatically *digests proteins*with pepsin -Parietal cells secrete intrinsic factor required for absorption of vitamin b 12 [ mature erythrocytes]

The gallbladder

Thin walled, green muscular sac on the ventral surface of the liver. -Stores and concentrates bile that is not being immediately used by absorbing its water and ions - Releases bile via the cystic duct which flows into the bile duct

Muscularis externa

This layer is responsible for *segmentation*, *peristalsis*, and mass movements [ like fecal matter] two layers: inner circular and outer longitudinal

Valves and sphincters of the rectum and anus

Three valves of the rectum stop feces from being passed with gas. The anus has two spincters: *Internal anal sphincter*- involuntary and composed of smooth muscle *External anal sphincter*-voluntary and composed of skeletal muscle *These sphincters are closed except during defecation

Esophagus

a muscular tube from the laryngopharynx to the stomach -Travels through the mediastinum and pierces the diaphragm -Joins the stomach at the cardiac orifice [mimics heartburn]

Dental formula

a shorthand way of indicating numbers and relative positions of the different types of teeth. -Written as a ratio, uppers over lowers for the mouth *primary*- 2I [incisors], 1C[canine], 2M[molars] *permanent*- 2I,1C, 2PM [premolars], 3M

Microscopic anatomy of the large intestine

colon mucosa is *simple columnar* except in the anal canal [stratified squamous] mucosa has números deep crypts lined with goblet cells *Anal canal mucosa is stratified squamous.* Anal sinuses exude mucus and compress feces Superficial venous plexuses are associated with the anal canal Inflammation of these veins results in itchy varicosities called hemorrhoids [ hemorrhoidal veins]

Mucosa: epithelial lining

consists of columnar epithelia and mucus-secreting goblet cells -The *mucus secretions*: - protect digestive organs from digesting themselves -Ease food along the tract *stomach and small intestine* mucosa: -Enzyme-secreitng cells -Hormone secreting cells [ making them endocrine and digestive organs]

Defecation

distention of rectal walls caused by feces: -Stimulates contraction of the rectal walls -Relaxes the internal sphincter -Voluntary signals stimulate relaxation of the external anal sphincter and defecation takes place

Tooth structure

each tooth has two main regions: The crown and root *crown*- exposed part of the tooth above the gingiva [gum] *Enamel*- acellular, material composed of calcium salts and hyrdoaxyapatite crystals. *Harderst substance in the body* Encapsulates the crown of the tooth *Root*-portion of the tooth embedded in the jaw bone. *neck*- constricted tooth region that connects the crown and the root. *Cementum*- calcified connective tissue, connects the tooth to thin periodontal ligament and covers the root. *Peridontal ligament*- -Anchors the tooth in the alveolus of the jar [bony socket] -forms the fibrous joint called a *gomphosis*

Esophageal Characteristics

esophageal *mucosa*- nonkeratinized stratified squamous -The empty esophagus is folded longitudinally and flattens when food is present -Glands secrete mucus as bolus moves through the esophagus *Muscularis changes from skeletal [superiorly] to smooth [inferiorly]

Large intestine

extends from the ileocecal valve and to the anus Has three unique functions: *Teniae coli*- three bands of longitudinal smooth muscle in its muscularis *Haustra*- pocket-like sacs causes by the tone of the teniae coli *Epiploic appendages*- fat-filled pouches of visceral peritoneum -Is subdivided into the cecum ,appendix, colon, recutim, and anal canal. The saclike *cecum* lies below the ileocecal valve in the right iliac fossa contains wormlike appendix

Submucosa

external to the mucosa. -dense irregular tissue, containing elastic fibers, blood, and lymphatic vessels, lymph nodes and nerves supplying tissue of the GI tract wall. so abundant in elastic fibers that it allows the stomach to regain its shape after a large meal

Palate

forms the roof of the mouth. has two parts: *Hard palate*- underlain by palatine bones and palatine processes of the maxillae - Assists the tongue in chewing -slightly corrugated on either side of the raphe *Soft palate*- a mobile fold mostly of skeletal muscle -closes off the nasopharynx during swelling [involuntary] -*uvula* projects down from its free edge -Palatoglossal and palatopharyngeal arches form the borders of the fauces

Histology of the alimentary canal

from the esophagus to the anal canal, the walls of the GI tract have the same 4 layers [tunics] from the lumen outward: mucosa, submucosa, muscular externa, and serosa -*each tunic has a predominant tissue type and specific digestion function*

regulation of gastric emptying

gastric emptying is regulated by: -The *neural enterogastric reflex*: Hormonal [enterogastrome] mechanisms -These mechanisms inhibit gastric secretion and duodenal filling *Carbohydrate-rich chyme* moves through the duodenum quickly -*Fat laden chyme* is digested more slowly causing food to remain in the stomach longer -Fatty chyme must mix with bile in the duodenum before it can be absorbed. This process takes time

Salivary Glands

glands associated with the oral cavity that produce and secrete saliva. Saliva: -Cleanses the mouth -moistens and dissolves food chemicals -Aids in bolus formation -Contains enzymes that breakdown starch *Extrinsic glands*- parotid, submandibular, sublingual *Intrinsic salivary glands [buccal glands]*- scattered throughout the oral mucosa

Colon

has distinct regions: ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon - *transverse and sigmoid portions are anchored via mesenteries called mesocolons. -*The sigmoid colon joins the rectum The anal canal, the last segment of the large intestine opens to the exterior at the anus.

Microscopic anatomy of the liver

hexagonal shaped *liver lobules* are the structural and functional units of the liver - composed of *hepatocytes [liver cells]* plates radiating outward from the central vein - *portal traids [portal tract]* are found at each of the six corners of each liver lobule Portal traids consist of: - *hepatic artery*- supplies oxygen-rich blood to the liver *hepatic portal vein*- carries venous blood with nutrients form digestive viscera *Liver sinusiods*-enlarged, leaky capillaries located between hepatic plates *Kupffner cells*- hepatic macrophages found in liver sinusoids

Tongue

occupies the floor of the mouth and fills the oral cavity when mouth is closed. *Functions*: -gripping and reposting food while chewing -initiation of swallowing and speech -mixing food with saliva and forming *bolus* *Instrinsic muscles* change the shape of the tongue *Extrinsic muscles* alter the position of the tongue *Lingual frenulum* secure the tongue to the floor of the mouth

Intestinal motility: Mass movements

occur in the *large intestine* -designed to move rectal content during the defection process

Cephalic [reflex] phase of gastric secretion

occurs before food enters the stomach, *excitatory event occurs*: -sight or thought of food -Stimulation of taste or smell receptors *Inhibitory evens include*: -loss of appetite or depression -decrease in stimulation

Intestinal motility: Peristalsis

occurs in the *esophagus, stomach, small and large intestine -Other organs include: uterus, the ureters, and gallbladder.

Gastric phase of gastric secretion

once food reaches the stomach local neural and hormonal mechanisms initiate. *Excitatory event occurs:* -stomach distention -activation of stretch receptors [mechanoreceptors] -Activation of chemoreceptors by peptides, caffeine, and rising pH -Release of gastrin to the blood *Inhibitory events include*: -A pH lower than 2 -Emotional upset which overrides the parasympathetic division

Hepatocyte function

production of bile -processing of bloodborne nutrients -Storage of fat soluble vitamins -Detoxification of drugs *The liver preforms over 5000 known functions with more being discovered

gastic gland: mucous neck cell

scattered in the neck or more basal regions of the glands: -produce a thin, acidic mucus

gastric glands: Parietal oxyntic cell

secretes hydrochloric acid [HCL] for vitamin c absorption and intrinsic factor [ needed for intestinal absorption of vitamin B12]

gastic gland: Enteroendocrine cells

typically deep the gastric glands, release a variety of chemical messengers into the interstitial fluid of the lamina propria. [into the blood stream] secrete gastrin, histamine, endorphins, serotonin, somatostatin and cholecystokinin.

Composition and function of pancreatic juices

water solution of enzymes and electrolytes [primary bicarbonate] -neutralizes acid chyme -Provides optimal environment of pancreatic enzymes Enzymes are released in inactive form and activated in the duodenum Examples include: Trypsinogen is activated to trips Procarboxypeptidase is activated Active enzymes secreted Amylase, lipases, and nucleases secrete in an active form, but these enzymes require ions or bile for optimal activity

Intestinal juices

Secreted by intestine glads in response to distention or irritation of the mucosa -It is slightly alkaline and isotonic with blood plasma -Is largely, water, enzyme-poor but contains mucus

function of the digestive system

Takes food in, breaks it down into nutrient molecules, absorbs these molecules into the blood stream, and rtfs the body of indigestible remains.

Enteric nervous system

The alimentary canal's own internal nerve plexus, which communicate widely with one another to regulate digestive system activity composed of 2 major nerve plexus: * Submucosal nerve plexus*: regulates glands and smooth muscle in submucosa *Myenteric nerve plexus*: "intestinal muscle" -major nerve supply that controls GI tract motility -*segmentation and peristalsis* are largely automatic involving local reflex arcs [ triggered by something in the lumen] -*linked to the CNS via long autonomic relflex arc by neurons that enter the instrinsic wall*

Bacteria Flora

The bacteria flora of the large intestine consists of: -Bacteria surviving the small intestine that enter the cecum and those entering via the anus -These bacteria: colonize the colon -ferment indigestible carbohydrates -Releasing irritating acids and gases [flatis] -Synthesizes B complex vitamins and vitamin K for the liver

Two main groups of the digestive system

-Those of the *alimentary canal* [Gastrointestinal tract] - *accessory digestive organs*-

Lips and cheeks

-help keep food between the teeth when we chew. -composed of a core of skeletal muscle covered externally by skin *Lips: orbicularis oris* *Cheeks: buccinators* *Vestibule*- bounded by the lips and cheeks externally and teeth/gums internally "gum hiding spot" *Oral cavity proper* area that lies within the teeth and gums *Labial frenulum*- medial fold that joins the internal aspect of each lip to the gums

Saliva: Source and Composition

-secreted from two secretory cells: *serous and mucous cells* of the salivary glands -Saliva is largely water [97-99.5%], hypo-osmotic, slightly acidic solution containing [ to combat bacteria] -Electrolytes- Na+, K+, Cl, PO2 and HCO- -Digestive enzymes- salivary amylase [ptyalin] Proteins- mucin,lysozyme, defensins, and IgA Metabolic waste- urea and uric acid

Response of the stomach to filling

-stomach pressure remains constant until about *1L* of food is ingested -relative unchanging pressure results from *reflex mediated relaxation and plasticity* Reflex-mediate events include: *receptive relaxation*- as food travels in the esophagus, stomach muscles relax *Adaptive relaxation*- the stomach dilates in response to gastric filling *Plasticity*- intrinsic ability of smooth muscle to exhibit the stress-relaxation response. [ stretching without greatly increasing tension]

Peritoneal organs (intraperitoneal)

-surrounded by the peritoneum stomach, jejunum, ileum, transverse colon, spleen, liver, and galbladder.

Stomach

-temporary storage tank where chemical breakdown of proteins begins and food is converted to *Chyme*

Water Absorption

95% of water is absorbed in the small intestine by osmosis. -Water moves in both directions across intestinal mucosa -Net osmosis coccus whiner a concentration gradient is established by active transport of solutes into the mucosal cells -Water uptake is couple with solute uptake, and as water moves into mucosal cells, substances follow along their gradient

Pharynx

From the mouth, the oropharynx and laryngopharynx allow passage of: - foods and liquids to the esophagus -Air to the trachea *Histology*- lined with stratified squamous and mucus glands *has two skeletal muscle layers*: -Inner longitudinal -outer pharyngeal constrictors

Liver: associated structures

Lesser omentum anchors liver to stomach at the lesser curvature *hepatic blood vessels* enter the lover at the porta hepatic -The *gallbladder* rests in a recess on the inferior surface of the right lobe. - Bile leaves the liver via tiny microscopic bile ducts [ enlarge with distance and fuse into the R and L hepatic ducts] R and L hepatic duct fuse to form the common hepatic duct which fuses with the cystic duct [draining the gallbladder] * the common hepatic and cystic duct form the bile duct*

control of motility

Local enteric neurons of the Gi tract coordinate intestinal motility *local enteric neurons [short reflex]* cause: - contraction and shortening of the circular muscle layer -Shortening of longitudinal muscle -Distention of the intestine -Other impulses relax the circular muscle *the gastroileal reflex* -relax the ileocecal sphincter -Allow chyme to pass into the large intestine

Functions of the large intestine

Other than digestion of enteric bacteria, no further digestion takes place *Vitamins, water, and electrolytes are reclaimed* Its major function is propulsion of fecal material toward the anus and water reclamation.

histology of the small intestine wall

The epithelium of the mucosa is made up of: *absorptive cells and goblet cells* *Interspersed T cells [intraepithelial lymphocytes]*- unlike T cells IEL's do not need priming -Upon antigen encounter they immediately release cytokines and kill infected target cells *Paneth cells* release defenses and lysozyme which destroy bacteria and help determine which bacteria may colonize the intestinal lumen *Enteroendocrine cells*- the source of enterogastrones secretin and cholecystokinin to name two *intestinal crypts*- cells secrete intestinal juice *Brunners glands*- in the duodenum secrete alkaline mucus to coat the acidic secretion of the mucus neck cells of the stomach -found inferior to the muscularis mucosa *Crypts of Leiberkuhn* are found in the jejunum found superficial to the muscularis mucosae *peyers patches* are found in the submucosa of the distal ileum

The liver

The largest gland or organ in the body, digestive function is to produce bile for transport to the duodenum. Also filters all the bodies blood. -Superficially has four lobes- right, left, quadrate, caudate *The falciform ligament*- a mesentery that separates the right and left lobes anteriorly -suspends the liver from the diaphragm and anterior abdominal wall *Ligamentum teres [round ligament]:* - a remnant of the fetal umbilical vein Runs along the free edge of the falciform ligament.

Motility of the small intestine

The most common motion of the small intestine is *segmentation* -It is initiated by intrinsic pacemaker cells [CAJAL cells] Moves contents steadily towards the ileocecal valve *After nutrients is absorbed*: -Peristalsis begins with each wave starting distal to the previous Meal remnants, bacteria, mucosal cells and debris are moved into the large intestine

The alimentary canal (gastrointestinal tract)

is the continuous muscle tube that winds through the body from the mouth to the anus. *function*: *digests* food and *reabsorbs* the digested fragments through its lining into the blood This tube is open to the external environment at both ends

gastric glands: chief zymogenic cells

mainly in the basal region of the gastric glands produces *pepsinogen*, the inactive form of pepsin. The most important protein digesting enzyme pepsinogen is activated to pepsin: -HCl in the stomach reacts with pepsinogen -Pepsin itself by a positive feedback mechanism of amino acid breakdown

Intestinal motility: Segmentation

mixing contractions occurring in the *small intestine*. Lumen content is moved back and forth to ensure appropriate mixing of lumen contents with digestive secretions.

organs of the alimentary canal

mouth, pharynx, stomach, small intestine, and large intestine.

microscopic anatomy of the stomach

stomach wall contains 4 tunics *muscularis*- has an additional oblique layer that: -allows stomach to churn, mix, and pummel food physically -Beaks down food into smaller fragments Epithelial lining is composed of: Goblet cells that produce a coat of alkaline mucus [stomach is very acidic] -Gastic pits containing gastric glands that secrete: -Gastric juices -Mucus -Gastrin

superior tongue

superior surfaces of the tongue bear 3 types of papillae [physical structures] *Filiform*-give the tongue roughness and provide friction for licking semisolid foods. *Fungiform*-scattered widely over the tongues surface. Each has a vascular core giving it a reddish hue. *circumvallate*- located in a V-shapred row in back of the tongue *sulcus terminalis*-groove that separates the tongue into two areas: -Its body that resides in the oral cavity -Its root, posterior third residing in the oropharynx

digestive process

the Gi tract is an *assembly line* where the nutrients becomes more available to the body at each step. There are 6 essential activites for the processing of food by the digestive system

Blood supply: The splanchnic circulation

the arteries that branch off the abdominal aorta to serve the digestive organs and hepatic portal circulation. *Arterial supply* - hepatic [liver], splenic[spleen], and left gastric[stomach] branches of the celiac trunk -*Inferior and superior mesenteric*: serve small and large intestine *Hepatic portal circulation* collects nutrient rich venous blood drawing fro the digestive viscera and delivers it to the liver for metabolic processing and storage.

mucosa of the GI tract

the inner most layer, a moist epithelial layer that lines the lumen of the alimentary canal *functions*: - secrete mucus -Absorb the end products of digestion into the blood -Protection against infectious disease Has 3 layers: lining of epithelium[simple columnar], lamina propria [aerolar CT], and muscularis mucosae [smooth muscle]

Motility of the large intestine

the movements most seen in the colon are *haustral contractions* -slow segmenting movements lasting about one minute and occur every 30 minutes or so -Haustra sequentially contract as they are stimulated by distention. *Mass movements*- long slow moving, but powerful contractile waves that move over large areas of the colon 3-4 times daily and force contents toward the rectum. *Typically occur during or after eating* *Presence of food in the stomach*: -activates the gastrocolic reflex -Initiates peristalsis that forces contents toward the rectum

Serosa

the protective *visceral peritoneum* replaced by the fibrous adventitia on the esophagus, binds it to surrounding structures *retroperitoneal organs* have both an adventitia and serosa needing to reduce friction and attach to surrounding structures.