Ch. 23: The Digestive System

2 digestive system organ groups

1. Alimentary canal 2. Accessory digestive organs

Lg. intestine subdivisions

1. Cecum: sac-like, lies below ileocecal valve in right iliac fossa, 1st part lg. intestine. 2. Appendix: attached to posteromedial surface of cecum, blond worm-like. Contains lymphoid tissue and as part of MALT plays important role in immunity, also stores bacteria and recolonizes gut when needed. It's twisted and susceptible to blockage. 3. Colon 4. Rectum 5. Anal canal

3 concepts regulate digestive activity

1. Digestive activity provoked by mechanical and chemical stimuli 2. Effectors of digestive activity: smooth muscle and glands 3. Neurons (intrinsic and extrinsic) and hormones control digestive activity

6 processes of food digestion

1. Ingestion 2. Propulsion: swallowing, peristalsis 3. Mechanical breakdown: chewing, churning, segmentation 4. Chemical digestion: enzymes 5. Absorption 6. Defecation

GI tract layers (tunics)

1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa

Intrinsic nerve plexuses

1. Myenteric nerve plexus 2. Submucosa nerve plexus

Effectors of digestive activity: smooth muscle and glands

1. Smooth muscle: When stimulated, receptors initiate reflexes that stimulate smooth muscle in organ walls to mix lumen contents and move them along tract. 2. Glands: Reflexes can also activate/inhibit glands that secrete juices into lumen or hormones into blood.

Motility of sm. intestine

2 motility patterns. 1. Segmentation 2. Migrating motor complex 1. After meal: segmentation; ensures chyme is thoroughly mixed with bile and pancreatic and intestinal juices , also ensures absorbable products come in contact with mucosa. 2. B/w meals: peristalsis migration motor complex: housekeeping function that sweeps debris and bacteria toward lg. intestine. Segmentation wanes and duodenal mucosa releases motilin: peristaltic waves initiated in proximal duodenum every 9-120 minutes sweep slowly along intestine before dying out in process of migrating motor complex. 3. After meal: intrinsic pacemaker cells in circular sm. muscle layer initiate back and forth segmentation movements, intestine is altered by long and short reflexes, parasympathetic enhances, sympathetic decreases and by hormones Ileocecal valve control: most of the time valve is closed by the 2 mechanism cause it to relax and allow food residues to enter cecum when ileum motility increase: gastroileal reflex is long neural reflex triggered by stomach activity which increases force of segmentation and ileum and relaxes valve; gastrin released by stomach increases motility of ileum and relaxes valve - once chyme passes through exerts backward pressure closing flap preventing regurgitation int ileum

Lg. intestine features

3 features: 1. Teniae coli: except for terminal end longitudinal muscle layer of muscularis is mostly reduced to 3 bands of smooth muscle called teniae coli which puckers wall into pocket-like sacs called haustra 2. Haustra: created by puckering of teniae coli 3. Epiloic appendages: small fat-filled punches of visceral peritoneum that hang from surface of lg. intestine, significance unknown. Rectum and anal canal lack teniae coli and hausfrau, however rectum's muscularis layer is well developed consistent with role of generating strong contractions to expel feces

Histology sm. intestine

4 layers of GI tract, mucosa and submucosa are modified. 1. Mucosa: studded with tubular glands called intestinal crypts b/w villi; they decrease in number along length of sm. intestine, contain 5 major types of cells in mucosa of villi and crypts A. Enterocytes B. Goblet cells C. Enteroendocrine cells D. Paneth cells E. Stem cells Also has MALT in mucosa 2. Submucosa: areolar connective tissue with mucus-secreting duodenal cells that produce alkaline (bicarbonate-rich) mucus that helps neutralize acidic chyme from stomach if inadequate intestinal walls will erode and duodenal ulcers will result. 3. Muscularis externa: typical and b-ilayered. 4. Serosa: except for bulk of duodenum which is retroperitoneal and has adventitia, visceral peritoneum (serosa) covers external intestinal surface.

Omentum (Omenta)

A fold of peritoneum connecting the stomach w/ other abdominal organs. Greater and lesser.

Enteric nervous system

AKA "gut brain" In house nerve supply for alimentary canal. Creates optimal environment for functioning in lumen of GI tract an area that is actually outside the body. Consists of over 100 million neurons (more than the spinal cord).

Alimentary canal

AKA gastrointestinal tract From mouth to anus Digests food, breaks it down into smaller fragments, absorbs digested fragments through lining into blood Organs: mouth, pharynx, esophagus, stomach, sm. & lg. intestines Cadaver 30ft long but shorter in living b/c of muscle tone

Gastric inhibitory peptide (GIP)

AKA glucose-dependent insulinotropic peptide. Synthesized by K cells in mucosa of duodenum and jejunum. Stimulated by fatty chyme presence. Inhibits HCl production in the stomach and stimulates insulin release from the pancreas.

Lips

AKA labia Fleshy lips formed by the orbicularis oris muscle

Mucosa

AKA mucous membrane Epithelial membrane that lines GI tract lumen from mouth to anus. Functions: 1. Secrete: mucus, digestive enzymes, hormones 2. Absorb: end products of digestion into the blood 3. Protect: against infectious diseases 3 sublayers: 1. Lining epithelium 2. Lamina propria 3. Muscularis mucosae

Muscularis externa

AKA muscularis Responsible for peristalsis and segmentation Has inner circular layer and outer longitudinal layer of smooth muscle cells. Several places along tract circular layer thickens forming sphincters that act as valves to control food passage from one organ to the next to prevent back flow.

Mouth

AKA oral cavity AKA buccal cavity Boundaries are the lips anterior, cheeks laterally, palate superiorly and tongue inferiorly. Anterior opening is the oral orifice, posterior cavity is continuous with the oropharynx. Walls of mouth lined w/ thick stratified squamous epithelium which withstands friction. Epithelium on gums, hard palate, dorm of tongue: slightly keratinized squamous epithelium for extra protection against abrasion during eating

Retroperitoneal organs

ALL OF: Urinary: adrenal glands, kidneys, ureters Aorta, inferior vena cava GI: PORTIONS OF: esophagus, rectum, pancreas, duodenum, ascending and descending colon

Appendicitis

Acute inflammation of appendix results from blockage (often by feces) that traps infectious bacteria in its lumen. Unable to empty contents appendix swells squeezing off venous drainage that could lead to ischemia and necrosis (low blood flow and tissue death) of appendix. Appendix can rupture and spray feces containing bacteria all lover abdomen causing peritonitis. S/S: pain in umbilical region, loss of appetite, nausea/vomiting, pain delocalization to LRQ, need to remove via appendectomy. Most common during adolescence when entrance of appendix is widest.

Serous fluid

Allows mobile GI organs to glide easily across each other and across body wall as they function.

Peristalsis

Alternating waves of contraction and relaxation in muscles of organ walls moving food along tract. Adjacent segments of the GI tract organs alternately contract and relax.

Oral orifice

Anterior opening to the mouth

Sm. intestine blood supply

Arterial: superior mesenteric artery. Venous: veins parallel superior mesenteric artery and drain into superior mesenteric vein, nutrient-rich venous blood from sm. intestine drains into hepatic portal vein which carries it to the liver.

Dorsal mesentery

Attaches to posterior abdominal wall 1. Dorsal mesogastrium: portion of the dorsal mesentery that attaches to the greater curvature of the stomach 2. Mesocolon: part of the dorsal mesentery that suspends the colon

Digestion in sm. intestine

Before sm. intestine: carbs and protein partially degraded by fat digestion has only begun Needed (mostly imported from liver and pancreas - so anything that hinders these organs hinders ability to digest food and absorb nutrients): 1. Bile 2. Digestive enzymes except for brush border enzymes 3. Bicarbonate ions )to provide proper pH for enzymatic catalyst) Brush border enzymes: perform final digestion of food into simple components that can be absorbed by intestinal cells; brush border enzymes are not secreted, they remain bound to the brush border plasma membranes.

Sm. intestine

Body's major digestive organ. Twisted passageway where digestion is completed with help of bile and pancreatic enzymes. Longest part GI tract 20ft in cadaver 13ft in living. Half diameter lg. intestine (1-1.6 inches). Extends from pyloric sphincter to ileocecal valve (sphincter) where it joins lg. intestine. 3 divisions: 1. Duodenum 2. Jejunum 3. Ileum

Clostridium Difficile

C. diff. Anaerobic bacterium most common cause of antibiotic-associated diarrhea accounting for 14,000 deaths per year. Some people have it normally small fraction of guts bacteria. Others get it by feca-oral route (poor hand washing) especially in hospital and longterm care. When other bacteria wiped out by antibiotics C. diff flourishes in gut and can cause pseudomembranous colitis (inflammation of the colon) that leads to bowel perforation and sepsis. Hard to treat and need even more powerful antibiotics sometimes fecal transplant to restore competitive bacteria.

Hepatic portal circulation

Collects nutrient-rich venous blood draining from digestive viscera and delivers it to liver.

Ileostomy

Colon is removed, terminal ileum can be brought out to abdominal wall in procedure called ileostomy, from there food residues are eliminated in sac attached to abdominal wall.

Enteric neurons

Communicate widely with one another to regulate digestive system activity. Semiautonomous enteric neurons constitute bulk ot 2 major intrinsic nerve plexuses (ganglia interconnected by unmyelinated fiber tracts) found in the walls of the alimentary canal that interconnect like chicken wire along GI tract. 1. Submucosa nerve plexus 2. Myenteric nerve plexus

Mesentery

Double layer of peritoneum: a sheet of 2 serous membranes fused back to back, that extends to GI organs from the body wall. 1. Provides route for blood and lymph vessels, nerves to reach digestive viscera 2. Holds organs in place 3. Stores fat In most places it is dorsal and attaches to posterior abdominal wall

Defecation

Eliminates indigestible foodstuffs from body in form of feces or urine.

Parasympathetic input

Enhances digestive activity

Regulating chyme entry into sm. intestine

Enters duodenum hypertonic so if lg. amounts chyme rush into sm. intestine osmotic water loss from blood into intestinal lumen would result in dangerously low volume. Also low pH of chyme must be adjusted by mixing with bile and pancreatic juice. Feedback via enterogastric reflex and enterogastrones to stomach pylorus control food movement into sm. intestine and prevent duodenum from being overwhelmed. Mechs. that regulate chyme entry: 1. Enterogastric reflex 2. Enterogastrones

Muscularis mucosae

External to lamina propria. Scant layer of smooth muscle cells that produce local movements of mucosa that enhance absorption and secretion.

Submucosa

External to mucosa. Areolar connective tissue. Elastic fibers enable stomach to regain normal shape after temporarily storing large meal. Contains: 1. Rich supply blood and lymphatic vessels 2. Lymphoid follicles 3. Nerve fibers

Peritoneal ligaments

Folds of peritoneum that connect viscera to viscera or to the abdominal wall. Exs. Gastrocolic ligamenets connects stomach and colon.

Constipation

Food remains in colon for extended periods of time too much water is absorbed and stool becomes hard and difficult to pass, may result from insufficient fiber or fluid in diet, improper bowel habits (holding it too long), lack of exercise, laxative abuse

Lg. intestine

Frames sm. intestine on 3 sides and extends from ileocecal valve to anus. 7cm diameter greater sm. intestine but is shorter 1.5m vs. 6m. Absorbs remaining water from indigestible food residues, temporarily stores it then eliminates it as semisolid feces (stool), absorbs metabolites produced by resident bacteria and as they ferment carbs not absorbed in sm. intestine.

Irritable bowel syndrome (IBS)

Functional GI disorder not explained by anatomical or biochemical abnormalities. Affected individuals have recurring and persistent abdominal pain relieved by defection, consistency and frequency changes of stool, bloating, flatulence, nausea, depression; stress is common precipitation factor.

Intraperitoneal (Peritoneal) organs

GI organs that keep their mesentery and remain in the peritoneal cavity. Stomach, ileum, jejunum, transverse colon, appendix, sigmoid colon, spleen, liver. SOME parts of duodenum, rectum, pancreas.

Major intrinsic nerve plexuses

Ganglia interconnected by unmyelinated fiber tracts

Motility of lg. intestine

Haustral contractions: food enters colon through ileocecal valve, colon becomes motile contractions are sluggish short-lived haustral contractions slow segmentation lasting 1 minute every 30 minutes. Occur mainly in ascending and transverse colon. Mass movements: mass peristalsis; long slow moving powerful contractile waves over large areas of colon 3-4x day just after eating, presence of food in stomach activates gastroileal reflex in sm. intestine and propulsive gastrocolic reflex in colon Segmentation movements: in descending and sigmoid colon promote final drying out of feces until mass movements propel feces into rectum - fiber in diets strength colon contractions and soften feces. Semisolid feces delivered to rectum contains undigested food residues, mucus, sloughed-off epithelial cells, bacteria and just enough water to allow smooth passage. Defecation: mass movements stretch rectal wall initiating defecation reflex a parasympathetic spinal reflex causes sigmoid colon and rectum to contract and internal anal sphincter to relax, feces forced into anal canal messages to brain allow us to decide whether external voluntary anal sphincter should open, if defecation delayed reflex contractions end and rectal walls relax - during defecations muscles of rectum contract to expel feces we aid by closing glottis and contracting diaphragm and abdominal wall to increase intra-abdominal pressure (process called Valsalva's maneuver) also contract levator ani muscle which lifts anal canal superiorly leaving feces below anus and outside body; involuntary or automatic defecation (fecal incontinence) occurs in infants because don't have control over external anal sphincter yet. Also occurs with spinal cord transections

Adventitia

In esophagus which is located in thoracic not abdominal cavity the serosa is replaced by adventitia (dense connective tissue that binds esophagus to surrounding structures).

Sm. intestine cells

In mucosa of villi and crypts. 1. Enterocytes: form bulk of epithelium, simple columnar absorptive cells bound by tight junctions and have microvilli, absorb nutrients and electrolytes, secretory cells that secret intestinal juice water mix containing mucus that serves as carrier fluid for absorbing nutrients from chyme. 2. Goblet cells: mucus-secreting cells 3. Enteroendocrine cells: source of enterogastrones (secretin and CCK); found scattered in crypts some in villi 4. Paneth cells: found deep in crypts, secretory cells that fortify defenses by releasing antimicrobial agents like defenses and lysozyme 5. Stem cells: continuously divide in crypts, daughter cells become any of the other cells which all differentiate as they migrate to the villi except Paneth cells which migrate to bottom of crypt.

Hormone-producing cells

In stomach and sm. intestine; release their products into interstitial fluid in extracellular space - blood and interstitial fluid distribute these hormones to target cells of same or different GI organs

Splanchnic circulation

Includes arteries that branch off abdominal aorta to serve digestive organs and the hepatic portal circulation. Arterial supply (branches of celiac trunk) supply: spleen, liver, stomach Mesenteric arteries: serve sm. and lg. intestines These receive 1/4 of the CO, % increases after meal.

Mucosa-associated lymphoid tissue (MALT)

Includes lymphoid follicles and aggregated lymphoid nodules (Peyer's patches) which are located in lamina propria but sometimes protrude into submucosa below. Increasing abundance toward distal end of sm. intestine b/c contain lg. numbers of bacteria that must be prevented from entering blood stream. Lamina propria of mucosa contains lg. numbers of IgA-secreting plasma cells that help protect against intestinal pathogens.

Mechanical breakdown

Increase SA of ingested food preparing it for digestion by enzymes. Chewing, mixing food with saliva by tongue, churning food in stomach and segmentation.

Peritonitis

Inflammation of peritoneum. Peritoneal coverings tend to stick together around infection site localizing infection providing time for macrophages to prevent inflammation from spreading. Causes: 1. Piercing abdominal wound 2. Perforating ulcer that leaks stomach juices into peritoneal cavity 3. Poor sterile technique during abdominal surgery 4. Most common: burst appendix that sprays bacteria containing feces all over peritoneum

Sympathetic input

Inhibits digestive activity

Ileum

Intraperitoneal, 12 ft long, joins lg. intestine at ileocecal valve, hangs in sausage-like coils in central and lower part of abdominal cavity suspended from posterior abdominal wall by fan-shaped mesentery, lg. intestine circles more distal parts.

Jejunum

Intraperitoneal, 8ft long, hangs in sausage-like coils in central and lower part of abdominal cavity suspended from posterior abdominal wall by fan-shaped mesentery, lg. intestine circles more distal parts.

Long reflexes

Involves CNS integration centers and extrinsic autonomic nerves. Enteric NS sends info to CNS via afferent visceral fibers, it receives sympathetic and parasympathetic branches (motor fibers) of ANS that enter the intestinal wall to synapse w/ neurons in the intrinsic plexuses. Can be ignited by stimuli arising inside or outside of GI tract. ENS acts as way station for autonomic NS allowing extrinsic controls to influence digestive activity. Parasympathetic input enhances digestive activity and sympathetic impulses inhibits them.

Extrinsic controls

Involving long reflexes

Intrinsic controls

Involving short reflexes entirely w/in enteric NS

Myenteric nerve plexus

Large; "intestinal muscle" lies between circular and longitudinal muscle layers of muscularis external.

Intestinal gastrin

Made in duodenal mucosa. Stimulated by acidic and partially digested food in duodenum. Stimulates gastric glands and motility.

Short reflexes

Mediated by enteric nervous system plexuses in response to stimuli w/in GI tract. Sensory information from the digestive system can be received, integrated and acted upon by the enteric system alone. Control of patterns of segmentation and peristalsis is largely automatic involving pacemaker cells and reflex arcs b/w enteric neurons in the same or different organs.

Duodenum

Most retroperitoneal, relatively immovable, curves around head of pancreas, 10 inches long, hangs. Shortest subdivision but has most features. Major duodenal papilla.

Epithelium of mucosa

Mouth, esophagus, anus: stratified squamous epithelium Rest of GI tract: simple columnar epithelium rich in mucus secreting cells that protect organs from being digested by enzymes working within their cavities and to ease food passage along tract. Mucosa in stomach and sm. intestine: contains enzyme-synthesizing and hormone-secreting cells (the mucosa is a diffuse endocrine system + part of the digestive system in this case)

MALT

Mucosa-associated lymphoid tissue. Helps defend against bacteria and other pathogens which have free access to GI tract. Found in various submucosa membrane sites of the body, such as the gastrointestinal tract, thyroid, breast, lung, salivary glands, eye, and skin.

Neurons (intrinsic and extrinsic) and hormones control digestive activity

Nervous system controls digestive activity via: 1. Intrinsic controls: involving short reflexes entirely w/in enteric NS 2. Extrinsic controls: involving long reflexes 3. Hormone-producing cells: in stomach and sm. intestine; release their products into interstitial fluid in extracellular space - blood and interstitial fluid distribute these hormones to target cells of same or different GI organs

Retroperitoneal

Not suspended by mesentery. Ex. During development some regions of the sm. intestine didn't adhere to the dorsal abdominal wall so they lost their mesentery and came to lie posterior to the peritoneum.

Digestion in lg. intestine

Nutrients spend 12-24 hours in lg. intestine. Sm. amount of digestion occurs by enteric bacteria but no further breakdown occurs. Harvest vitamins made by bacterial flora, reclaims remaining water and some electrolytes (Na and Cl), nutrient absorption not major function, primary concern is propulsive activities forcing feces towards anus to defecate.

Submucosal nerve plexus

Occupies submucosa

Ventral mesentery

One that extends from liver to anterior abdominal wall. Is part of the peritoneum closest to the umbillicus.

Serosa

Outermost layer; is the visceral peritoneum. 1. Connective tissue (areolar tissue) 2. Epithelium (covered by mesothelium: single layer of squamous epithelial cells)

Lg. intestine bacterial flora

Over 1,000 different types of bacteria. Some colonize via anus others enter from s. intestine after running through antimicrobial defenses (lysozyme, defensins, HCl and protein-digesting enzymes) Help w/ recovering energy from indigestible foods and synthesize some vitamins: 1. Fermentation: ferment some indigestible carbs and mucin in gut mucus resulting in short-chain fatty acids that can be absorbed and used for fuel by body cells, produce gases like H2 N2 CH4 CO2, dimly sulfide is smell, 500mL flatus is produced each day more when eat foods like beans rich in indigestible carbs 2. Vitamin synthesis: B complex vitamins and some vitamin K the liver needs to produce several clotting proteins are synthesized by gut bacteria Immune system and bacterial flora live in dynamic equilibrium; immune destroys bacteria threatening to breach mucosa barrier gut bacteria instructs immune system not to overact to their presence in the lumen. Beneficial bacteria outcompetes harmful bacteria. Dendritic cells: sample microbial antigens in lumen then they migrate to nearby lymphoid follicles w/in gut mucosa MALT and trigger IgA antibody mediated response restricted to gut lumen preventing bacteria from straying into epithelium where they might elicit a systemic response

Sm. intestine nerve supply

Parasympathetic: vagus nerve Sympathetic: fibers from thoracic splanchnic nerves Both relayed through superior mesenteric and celiac plexus

Absorption

Passage of digested end products, vitamins, minerals, water from lumen of GI tract through mucosal cells by active or passive transport into blood or lymph.

Dorsal mesogastrium

Portion of the dorsal mesentery that attaches to the greater curvature of the stomach

Histamine

Produced by cells in stomach mucosa. Stimulated by food in the stomach. Stimulates parietal cells to release HCl.

Vasoactive intestinal peptide (VIP)

Produced by enteric neurons. Stimulated by chyme w/ partially digested food. 1. Intestinal smooth muscle relaxation 2. Inhibits pancreas and stomach secretions 3. Dilates intestinal capillaries 4. Stimulates buffer secretion in small intestine

Secretin

Produced in S cells of duodenum. Stimulated by acidic chyme. Regulates secretions of the stomach, pancreas, liver.

Somatostatin

Produced in stomach and duodenum. Stimulated by food in the stomach + sympathetic nerve fibers. INHIBITS: secretions, GI blood flow, intestinal absorption, bile release

Serotonin

Produced in stomach. Stimulated by food in stomach. Causes contractions of the stomach.

Gut bacteria in health and disease

Proportions of gut bacteria can influence our body weight, susceptibility to diseases like diabetes, atherosclerosis, fatty liver disease, mood Manipulating gut bacteria may become routine healthcare strategy in future.

Digestive activity provoked by mechanical and chemical stimuli

Receptors located in GI tract organ walls, they respond to stimuli like: 1. Stretching of organ by food in the lumen 2. Changes in osmolarity (solute concentration) 3. pH of contents 4. Presence of substrates and end products of digestion

Adventitia + Serosa

Retroperitoneal organs have both adventitia (on side facing dorsal body wall) and serosa (on side facing peritoneal cavity).

Segmentation

Rhythmic local constrictions of sm. intestine that mixes food with digestive juices making absorption more efficient by repeatedly moving different parts of the food mass over the intestinal wall. Non-adjacent segments of GI tract organs contract and relax; food moves forward then backward mixing and breaking down.

Motilin

Secreted by endocrine M cells in crypts of duodenum and jejunum. Stimulated by fasting and periodically released. Helps control "migrating motor complex" contractions.

Intestinal juice

Secretes 1-2L a day. Major stimulus for production comes from hypertonic or acidic chyme. Juice is slightly alkaline (7.4-7.8) and isotonic with blood plasma. Largely contains water but some mucus too which is secreted by duodenal glands and goblet cells of mucosa.

Chemical digestion

Series of catabolic steps where enzymes are secreted into the lumen (cavity) of the GI tract to breakdown complex food molecules into their building blocks.

Peritoneum

Serous membrane of abdominopelvic cavity. 1. Visceral peritoneum: covers external surfaces GI organs Continuous with 2. Parietal peritoneum: lines body wall

Colon

Several regions: 1. Ascending colon: travels up right side abdominal cavity to right kidney. 2. Right colic (hepatic) flexure 3. Transverse colon 4. Left colin (splenic) flexure 5. Descending colon 6. Sigmoid colon: where enters pelvis 7. Rectum: level of 3rd sacral vertebrae sigmoid joins rectum which has 3 lateral curves internally 3 transverse folds called rectal valves which stop feces from being passed along with gas (flatus) 8. Anal canal: lies in perineum, entirely external to abdominopelvic cavity, begins where rectum penetrates levator ani muscle of pelvic floor and opens to body exterior via anus Anal canal has 2 sphincters: 1. Internal anal sphincter: involuntary, smooth muscle part of muscularis 2. External anal sphincter: voluntary, skeletal muscle, acts like purse strings to open and close anus

Lg. intestine mucosa

Simple columnar epithelium except in anal canal. Mucosa is thicker, deep crypts with many goblet cells that produce mucus easing passage of feces and protecting walls from irritating acids and gases released by resident bacteria Anal canal mucosa: stratified squamous epithelium merges with true skin around anus, more abrasions; hangs in long ridges/folds called anal columns, anal sinuses recess b/w bolus and exude mucus when compressed by feces aiding in emptying; horizontal tooth-shaped line parallels margins of anal sinuses, is called pectinate line, superior to this line are visceral sensory fibers innervating mucosa which is relatively insensitive to pain, area inferior to pectinate line is very sensitive to pain b/c of somatic sensory fibers in it. Blood: 2 superficial venous plexuses associated with anal canal; 1 with anal columns and other with anus, if these hemorrhoidal beings become dilated and inflamed itchy varicosities called hemorrhoids result

Peritoneal cavity

Slit-like potential space containing slippery serous fluid secreted by the serous membranes. This space is b/w the visceral and parietal peritoneums.

Circular folds

Sm. intestine. Deep permanent folds of mucosa snd submucosa 1 cm tall that force chyme to spiral through lumen slowing its movement and allowing time for full nutrient absorption.

Villi

Sm. intestine. Finger-like projections of mucosa 1 mm high that narrow and shorten along length of sm. intestine. In core of each villus is dense capillary bed and wide lymphatic capillary called lacteal where digested foodstuffs are absorbed through epithelial cells into capillary blood and lacteal.

Microvilli

Sm. intestine. Long densely packed cytoplasmic extensions of absorptive cells of mucosa that give fuzzy appearance brush border which bear brush border enzymes which complete carb and protein digestion in sm. intestine.

Cholecystokinin (CCK)

Stimulates digestion of fat and protein: causes release of digestive enzymes from the pancreas and bile from the gallbladder. Stimulated by fatty chyme presence. Made and secreted by enteroendocrine cells of the duodenum.

Gastrin

Stimulates secretion of HCl by parietal cells of the stomach and also aids in gastric emptying and motility. Stimulated by food in stomach. It's released by G cells in the stomach mucosa.

Digestive system

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

Accessory organs

Teeth, tongue, gallbladder Salivary glands, liver, pancreas

Lamina propria

Underlies epithelium. Loose areolar connective tissue. Capillaries nourish epithelium. Has isolated lymphoid follicles part of MALT. Lg. collections of lymphoid follicles within pharynx (as tonsils) and in appendix

Sm. intestine microscopic

Wall (strictures that enhance absorptive SA 600x): 1. Circular folds: deep permanent folds of mucosa snd submucosa 1 cm tall that force chyme to spiral through lumen slowing its movement and allowing time for full nutrient absorption. 2. Villi: finger-like projections of mucosa 1 mm high that narrow and shorten along length of sm. intestine. In core of each villus is dense capillary bed and wide lymphatic capillary called lacteal where digested foodstuffs are absorbed through epithelial cells into capillary blood and lacteal. 3. Microvilli: Long densely packed cytoplasmic extensions of absorptive cells of mucosa that give fuzzy appearance brush border which bear brush border enzymes which complete carb and protein digestion in sm. intestine.

Diverticula

When diet lacks fiber and volume of residues in colon is small colon narrows and its contractions become more powerful increasing pressure on wall promoting formation of diverticula small herniations of mucosa through colon walls - this condition called diverticulosis is most common in sigmoid colon and common in people over 70 years old. Diverticulosis progresses to diverticulitis in which diverticula become inflamed and may rupture leaking into peritoneal cavity.

Diarrhea

watery stool from any condition that rushes food residue trough lg. intestine before has enough time to absorb remaining water Causes: irritation of colon by bacteria or prolonged physical jostling of digestive viscera (marathon runners) Prolonged diarrhea causes dehydration and electrolyte imbalance (acidosis and loss of potassium)