Ch. 23: digestive system

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digestive system

takes in food, breaks its down into nutrient molecules, absorbs these molecules into the bloodstream and then rids the body into the indigestible remains.

two main groups of the organs of the digestive system

1. those of the ailmentary canal
2. accessory digestive organs

alimentary canal

also called gastrointestinal (GI) tract or gut
the continuous muscular digestive tube that winds thru the body. it digests food (breaks it down into smaller fragments) and absorbs the digested fragments thru its lining into the blood.
the organs of the alimentary canal are: mouth, pharynx, esophagus, stomach, small intestine and large intestine. the large intestine leads to the terminal opening, the anus.
food material in this tibe is technically outside the body bc the canal is open to the external environment at both ends.

accessory digestive organs

the teeth, tongue, gallbladder and a number of the large digestive glands- the salivary glands, liver and pancreas.
the accessory digestive glands produce a variety of secretions that contribute to the breakdown of foodstuffs.

digestive processes

processing of food by the digestive system involves 6 essential activities: ingestion, propulsion, mechanical digestion, chemical digestion, absorption and defecation.
1. ingestion- taking food into the digestive tract via the mouth.
2. propulsion- moves food through the alimentary canal--includes swallowing (voluntary) and peristalsis which is the major means of propulsion involving alternate waves of contraction and relaxation of muscles in the organ walls. its main effect is to squeeze food along the tract some mixing occurs.
3. mechanical digestion- physically prepares food for chemical digestion by enzymes. includes: chewing, mixing of food with saliva by the tongue, churning food in the stomach and segmentation (rhythmic local contractions of the small intestine--mixes food w/ digestive juices and increases the effeciency of absoprtion).
4. chemical digestion- series of catabolic steps in which complex food molecules are broken down into their chemical blocks by enzymes secreted into the lumen of the alimetary canal. chemical digestion of food begins in the mouth and is complete in the small intestine
5. absorption- passage of digested end products (+ vitamins, minerals and water) from the lumen of the GI tract thru the mucosal cells by active/passive transport into the blood or lymph. small intestine is the major A site
6. defecation- elimination of indigestible substances from the body via the anus in the form of feces.

homeostatic concepts of the digestive system

the digestive system creates an optimal environment for its functioning in the lumen of the GI tract (an area outside of the body) and essentially all digestive tract regulatory mechanisms act to control lumina lconditions so that digestion and absorption can occur as effectively as possible.

facts that apply to the regulatory mechanisms of the digestive system:

1. digestive activity is provoked by a range of mechanical and chemical stimuli: sensors (mechanoreceptors and chemoreceptors) involved in the control of the GI tract are located in walls of the tract organs. MOST important are: stretching of the organ by food i nthe lumen, osmolarity (solute concentration) and pH of the contends and presence of substrates and end products of digestion.
2. controls of the digestive activity are both intrinsic and extrinsic: intrinsic means a produce of "inhouse" nerve plexuses or hormone producing cells- most of the control systems are this. (explained more 2 cards down)
the stomach and s intestine have hormone producing cells that release their product into the interstitial fluid of extracellular space which target the same of diff digestive tract organs.

what do the receptors of the digestive system do when stimulated?

the receptos initiate reflexes that activate or inhibit glands that secrete digestive juices into the lumen or hormones into the blood or they stimulate smooth muscles of the GI tract to mix lumen contends and move them along the tract.

explain more about the intrinsic mechanisms of the control of the digestive activity

in between the muscular layers of the alimentary canal is the gut brain that is made of nerve plexuses that spread along the entire length of the GI tract and influence each other in the same and diff digstive organs. bc of this two kinds of reflexes occur: short (mediated by the local gut plexuses (or enteric) in response to stimuli arising in the GI tract) and long reflexes (initiated by stimulus arising inside or outside the GI tract and involve CNS centers and extrinsic autonomic nerves.
nerve fibers that excite smooth muscle secrete acetylcholine or substance ; those that inhibit smooth muscle release vasoactive intestine peptide (VIP) or nitric oxide.

basic anatomy of the digestive membranes

most D organs are in the abdominoperliv cavity and all ventral bodiy cavities have slippery serous membranes.
peritomeum- the serous membrane of the abpelvic cavity; is the most extensive serous membrane.
visceral peritoneum- covers the external surfaces of most D organs and is continuous with the parietal peritoneum
parietal peritoneum- lines the body wall.
peritoneal cavity- between the two peritoneums; a slitlike proteintal space containing slippery fluid secreted by the serous membranes.
serous fluid- lubricates the mobile D organs
mesentery- a double layer of peritoneum (2 serous membranes fused back to back) that extends to the D organs from the body wall. provide routes for blood vessels, lymphatics and nerves to reach the D organs; holds the D organs in place and stores fat. most attaches to the posterior ab wall; not all alimentary canal organs are suspended by a mesentry.
retroperitoneal organs- organs that lose their mesentry and lie posterior to the peritoneum; pancreas and parts of the L intestine.
intraperitoneal/peritoneal organs- D organs that keep their mesentry and remain in the peritoneal cavity

blood supply: the splanchnic circulation

includes the arteries that branch off the ab aorta to serve the D organs and the hepatic portal circulation.
recieves 1/4 of cardiac output- this increases after a meal is eaten.
the artery supply includes: the hepatic, splenic, and left gastric branches of the celiac trunk that serve the spleen, liver, stomach and the mesenteric arteries that serve the S and L intestines.
the hepatic portal circulation collects blood that drains from the D organs and takes it to the liver. the liver absorbs the nutrients for storage and metabolic processing and then releases them back to the bloodstream

layers of the alimentary canal

tunics- mucosa, submucosa, muscularis externa and serosa

mucosa/mucous membrane

innermost layer; most moist epithelial membrane that lines the alimentary canal lumen from mouth to anus
functions: 1 secrete mucus, digestive enzymes and hormones 2 absorb the end products of digestion into the blood 3 protect against infectious disease
consists of 3 sublayers: 1 lining of epithelium 2 a lamina propria 3 a muscularis mucosae
the epithelium is simple columnar with mucus secreting cells; the mucus protects D organs from being digested can contain enzyme synthesizing and hormone secreting cells in the stomach and S intestine.
the lamina propria (under epithelium) is loose areolar tissue; the capillaries nourish the epithelium and absorb nutrients; has isolated lymphoid follices (part of MALT) that help defend against pacteria and other pathogens (occur a lot in the pharynx and appendix)
muscularis mucosae- layer of smooth muscle cells that produce local mvmnt of the mucosa; can increase surface area

submucosa

external to the mucosa
areolar conn tissue w/ rich supply of blood and lymphatic vessels, lymphoid follices and nerve fibers
has a lot of elastic fibers so stomach can regain its normal shape

muscularis externa

surrounds submucosa
also called muscularis
responsible for segmentation and peristalsis
has an inner circular layer and an outer longitudinal layer of smooth muscle cells
helps prevent backflow and controls food passage from one organ to the next

serosa

protective outermost layer
IS the visceral peritoneum; formed of areolar conn tissue w/mesothelium
is replaced by adventitus in the esophagus- fibrous conn tissue that binds the esophagus to surrounding strucutres
retroperitoneal organs have BOTH serosa and an adventitia

enteric nervous system of the alimentary canal

enteric neurons- the nerve supply of the alimentary canal; connunicate w/ one another to regulate digestive activity; constitute the bulk of the 2 major intrinsic nerve plexuses in the walls of the alimentary canal: submucosal and myenteric nerve plexuses
submucosal nerve plexus- in the submucosa; includes sensory and motor neurons; regulates the activity of glands and smooth muscle cells in the mucosa
myenteric nerve plexus- between the circular and longitudinal muscle layer of muscularis; provide the major nerve supply of the GI tract wall and control of GI tract mobility.
patterns of segmentation and peristalsis is automatic involving pacemaker cells and local reflex arcs between enteric neurons

enteric NS is linked to the CNS bc afferent visceral fibers and sympathetic and paras branches of the ANS that synapse with intrinsic plexuses: D activity is subject to extrinsic control.
paras enhances input secretory activity and miotility' symp inhibit D activities.

mouth

only part of alimentary canal involved in ingestion; begins the propulsion process of swallowing
a mucosa lined cavity; also called oral cavity/buccal cavity
boundaries: lips anteriorly, cheeks laterally, palate superiorly and tongue inferiorly; the anterior opening the the oral orifice and the oral cavity is continuous with the oropharynx posteriorly
walls of mouth are lined with thick stratified squamous epithelium that withstand friction
oral mucosa responds to injury by producing antimicrobial defensins

lips and cheeks

lips (labia) and cheese help keep food between the teeth as we chew
composed of sk muscles covered by skin
orbicularis oris muscle forms lips; cheeks formed by buccinators
what lies between the teeth and gums is the oral cavity proper. vestibule is between gums and teeth
lips extend from the inferior margin of the nose and to the superior boundary of the chin.
red margin is where you kiss someone; is poorly keratinized.
labia frenulum- fold that joins lip to the gums

palate

forms roof of mouth
two parts: hard palate anteriorly and soft palate posteriorly
hard palate- forms rigid surface that tongue forces food against during chewing. the mucosa on either side of the ridge helps create friction
soft palate- mobile fold of sk muscles that closes off the nasopharynx when we swallow. palatoglossal arches anchor soft to the tongue. the palatopharyngeal arches connect the soft palate to the oropharynx. there two together form the fauces and the uvula projects off th edge of it.

tongue

occupies the floor of the mouth; fills most of oral cavity
composed of interlacing bundles of sk muscle fibers
grips food and repositions it during chewing.
forms a bolus when it mixes food with saliva
initiates swallowing by pushing the bolus to the pharynx
has intrinsic and extrinsic sk muscle fibers
intrinsic muscles- confined to the tongue; arent attached to bone; fibers run in several diff planes allowing it to change shapes
extrinsic muscles- extend to the tongue from points of origin on bones of the skull/soft palate; alter the tongues position
lingual frenulum attaches the tongue to the floor of the mouth and limits posterior mvmnt
superior tongue surface- has papillae projections. filiforme papille give the surface roughness for licking and provide friction. are in parallel rows; contain keratin.
fungiform papillae- scattered each has a vascular core
circumvallate papillae (10-12)- V shaped on the back of the tongue
foliate papillae- on lateral aspects of posterior tongue
fungicofrm, vallate, foliate have taste buds
terminal sulcus- distinguishes portion of tongue that is in the oral cavity from that in the oropharynx

the salivary gnads

secrete saliva
saliva: cleanses the mouth, dissolves food chemicals so they can be tasted, moistens food and aids in compacting bolus and contains enzymes that begin the chemical breakdown of starchy foods.
extrinsic s glands- outside the oral cavity and empty their secretions into it; paired compound tubuloalveolar glands
intrinsic s glands- sugment extrinsic secretions. are scattered thru oral cavity mucosa; also called buccal glands; have s and m cells
parotid gland- large; lies anterior to ear between muscle and skin; contain only serous cells
submandibular gland- along medial aspect of manibular body. its ducts run below the mucosa and opens at the base of the lingual frenulum; s and m cells
sublingual gland- anterior to submandibular gland under the longue; 10-12 ducts in floor of mouth.
secretory cells that make up salivary glands: serous (produce watery secretion containing enzymes, ions, and mucin) and mucous cells (produce mucus)

composition of saliva

97-99.5% water and is hypoosmotic; saliva is slight acidic
solutes: electrolytes, digestive enzymes salivary amylase and lingual lipase, proteins: mucin, lysozyme and IgA, metabolic wastes (urea and uric acid).
microorganism protection provided by: IgA antibodies, lysozyme (bactericidal enzyme that inhibits bacterial growth and helps prevent tooth decay), a cyanide compound, defensins (call devensive cells for battle in the mouth)
friendly bacterial convert food derived nitrates into nitrites to nitric oxide (occurs around gums)
saliva serves as a medium to detect and monitor diseases.

control of saliva

intrinsic glands secrete continuously. when food enters the mouth extrinsic glands are activated.
1000-1500 ml per day
controlled primarily by the parasympathetic division of ANS. chemoreceptors/mechanoreceptors send signals to the salivary nuclei when we ingest food. paras NS activity increases and sends input to motor fibers in facial and glossopharyngeal nerves for an increase in output of watery enzyme rich saliva.
chemoreceptors are activated by acidic receptors. mechanoreceptors are acitvated by any mechanical stimulus in the mouth.
symp division also causes a release of thick mucin rich saliva. extreme symp stimulation constricts blood vessels to inhibit saliva release.

teeth

in sockets in gum covered margins of mandible and maxilla.
we chew by opening and closing our jaw/moving side to side

dentition

two sets of teeth- primary and permanent dentitions
primary consists of deciduous teeth (called milk/baby teeth)--20 in total. lower central incisors appear first at 6 months.
permanent teeth- underlying; all but the 3rd molars (wisdom teeth) emerge by end of adolescence. 32 permanent teeth
as perm enlarge the roots of the milk teeth are reabsorbed.
wisdom teeth emerge at age 17-25.

dental formula and teeth classification

teeth are classified according to their shape and function as: incisors, canines, premolars and molars.
incisors (cuspids)- chisel shaped; adapted for cutting or nipping off food pieces
canines- fanglike; tear and pierce
premolars (bicuspids) and molars- broad crowns with round cusps suited for grinding and crushing.
molars are best grinders

dental formula- indicates the numbers and positions of diff types of teeth; written for one half of the mouth. consists of 2 incisors (I), one canine (C), and two molars (M):
primary:
2I,1C,2M (upper jaw) x 2 (20 teeth)
2I,IC,2M (lower jaw)
permanent: (2 incisors, 1 canine, 2 premolars, 3 molars)
2I,1C,2PM,3M (upper jaw) x2 (32 teeth)
2I,1C,2PM,3M (lower jaw)

tooth structure

tooth has 2 major regions: crown and root
crown- exposed part of tooth above gingiva (or gum)
enamel- brittle ceramic material thick as a dim, bears the force of chewing; hardest substance in the body, has heavily mineralized calcium salts and densely packed crystals
root- portion embedded in the jawbone; canine, incisors and premolars have 1 root, first upper premolars have two, first two upper molars have 2, corresponding lower molars have 2.
neck- connects crown and root
cementum- covers the outer surface of the root; calcified conn tissue; attaches to thin periodontal ligament
peridontal ligament- anchors tooth in the alveolus of jaw forming a fibrous joint called gomphosis.
dentin- protein rich bonelike material that underlies enamel cap and forms bulk of tooth; shock absorber for forces acting against enamel; contain dentinal tubes that have odontoblast (cell type that secretes and maintains dentin)
pulp cavity- surrounded by dentin; have soft tissue structures collectively called pulp.
pulp- supplies nutrients to tooth tissues and provides tooth sensations
root canal- where pulp extends into the root
apical foramen- proxmial end; is a canal that allows blood vessels, nerves and other things to enter the pulp cavity

pharynx

mouth-->oropharynx-->laryngopharynx
histology: like oral cavity: mucosa has friction resistant epithelium with mucus producing glands. external muscular layer made of two SK muscle layers (inner are longitudinal, outer are pharyngeal constrictor muscles)
contractions of these muscles propel food into esophagus

esophagus

muscular tube thats collapsed when not involved in food propulsion
takes a straight route thru mediastinum of the thorax.
esophageal hiatus- where esophagus goes thru diaphragm to enter the abdomen
cardiac orifice- where esophagus joins stomach. is surrounded by gastroesophageal/cardiac sphincter (acts as a valve)

structural features of esophagus wall

1. esophageal mucosa has nonkeratinized strat. squamous epithelium. changes to simple squamous epithelium at esophagus-stomach junction.
2. when esophagus is empty, its mucosa and submucosa are in longitudinal folds. the fold are flattened when food is in transit.
3. submucosa has mucus secreting esophageal glands. glands are compressed as bolus moves thru. as it does, it causes the glands to secrete the mucus to aid food passage.
4. muscularis is sk muscle in its superior 1/3, mixture of sk and smooth muscle in its middle 1/3 and entirely smooth muscle in its inferior 1/3.
5. has a fibrous adventitis instead of serosa. is composed of conn tissue that blends with surrounding structures.

digestive processes: mouth to esophagus

mouth: ingests, begins mechanical digestion by chewing, initiates propulsion by swallowing, and starts the chem breakdown of polysaccharides.
salivary amylase-->main enzyme of saliva; digests starches and glycogen, liberating smaller glucose fragments.
lingual lipase- fat digesting enzyme in saliva
no absorption occurs in the mouth
pharynx and esophagus serve as conduits to pass food from mouth to stomach- digestive function is propulsion accomplished by swallowing.

mastication (chewing)

is the mechanical breakdown in mouth.
part voluntary, part reflexive. the pattern and rhythm of jaw movements are controlled by stretch reflexes and in response to pressure inputs from receptors of mouth

deglutition (swallowing)

involves coordinated activity of over 22 separate muscle groups.
two major phases: buccal and pharyngeal-esophageal.
buccal phase- in mouth; voluntary; place tip of tongue against hard palate and contract tongue to force bolus to oropharynx. when food enters the pharynx, tactile receptors are stimulated and then it is involuntary reflex activity
pharyngeal-esophogeal phase- involuntary; controlled by centers in brain stem. respiration is momentary inhibited. tongue blocks mouth, soft palate closes nasopharynx, epiglottis covers respiratory openings.
food is moved thru pharynx into esophagus by pressure gradients created by wavelike contractions.
gastropharyngeal sphincter relaxes to allow food to enter stomach.
solid foods pass in 8 seconds; liquids pass in 1-2 secs.

stomach

expansion of GI tract
storage tank where chemical breakdown of proteins begins and food is converted to chyme ("juice")
lies in the upper left quadrant of peritoneal cavity, hidden by the liver and diaphragm.

gross anatomy of stomach

empty stomach has a volume of 50 ml, can hold 4 L (1 gallon of food). rugae are large longitudinal folds of stomach
regions of stomach:
cardiac region- surrounds the cardiac orifice where food enters from the esophagus
fundus- dome shaped part
body- midprtion of stomach continuous inferiorly with funnel shaped pyloric region.
pyloric antrum- the wider and superior part of the pyloric region. narrows to form the pyloric canal which ends at the pylorus.
pylorus- continuous with duodenum (first part of s intestine) through the pyloric valve/sphincter that controls the stomach emptying.
greater curvature- convex lateral surface of stomach
lesser curvature- concave medial surface
two mesenteries called omenta that extend from the two curvatures that help tether the stomach to other D organs and the body wall
lesser omentum- runs from the liver to the lesser curvature where its continuous with the visceral peritoneum
greater omentum- draps inferiorly from the greater curvature to cover the coils of the s intestine. wraps the spleen and transverse portion of L intestine before blending with the mesocolon. has fat deposits. large collection of lymph nodes.

microscopic anatomy of the stomach

contains 4 tunics but mucosa and muscularis are modified for special roles. muscular has innermost layer of smooth muscle fibers that run obliquely as well as the longitudinal and circular s muscle.
the innermost s muscle that runs obliquely allows the stomach to mix, churn, move food along the tract and pummel the food to physically break it to smaller fragments and ram the food into the s intestine.
the lining epithelium of the stomach is entirely mucous cells that produce a cloudy protective 2-layer coat of alkaline mucus (the surface layer being viscous, insoluble mucus that traps bicarbonate rich fluid). the lining epitherlium has gastric pits that lead into gastric glands that produce gastric juice.

cells forming the gastric pits

primarily mucous cells but varies in diff stomach regions. glands in the stomach fundus and body (where most chemical digestion occurs) is larger and produce the majority of the stomach secretions.
4 types of secretory cells in those two regions:
1. mucous neck cells- found in the upper neck region of the glands; produce a thin soluble mucous different from mucous cell secretions. is acidic mucus; function not known
2. parietal cells- found in middle region of glands scattered with chief cells; secrete HCl and intrinsic factor simultaneously; have 3 prongs that bear dense microvilli providing huge surface area for secreting H+ and Cl- into the stomach lumen. HCl makes the stomach very acidic: necessary for activation and activity of pepsin; also helps food digestion by denaturing proteins and breaking down cell walls of plants food; kills many bacteria. intrinsic factor is a glycoprotein required for vit B12 absorption in s intestine.
3. chief cells- in basal regions of the gastric glands; produce pepsinogen (inactive form of protein digesting enzyme pepsin); and secrete lipases. the first pepsinogen molecules are activated by encountering HCl. once pepsin is present it catalyzes pepsinogen to pepsin: involves the removal of a small peptide fragment from pepsinogen; shape is changed and active site is exposed. is a + feedback limited by amounf of pepsinogen available.
4. enteroendocrine cells- located deep in the gastric glands; release a variety of chemical messengers into the interstitial fluid of the lamina propria including: histamine & serotonin (local paracrine agents); somatostatin (locally acting and hormones; diffuse into capillaries); gastrin (hormone; regulates stomach secretion and motility

gastric juice stuff

gastric juice is corrosively acidic
counterattack is the mucosal barrier
three factors of the barrier:
1. thick coating of bicarbonate rich mucus built up on stomach wall
2. epithelial cells of the mucosa are joined together by tight junctions that prevent gastric juice from leaking into underlying tissues.
3. damaged epithelial mucosal cells are shed and quickly replaced by division of undifferentiated stem cells that are where gastric pits join the gastric glands. surface epithelium of mucous cells are renewed every 3-6 days

stomach digestive processes

stomach is a holding area for ingested food, further degrades food physically and chemically and delivers chyme to the small intestine.
protein digestion is the only significant enzymatic digestion in the stomach and it begins here. dietary proteins are denatured by HCl (unfolds the amino acid chains). pepsin is the most important protein digesting enzyme produced by gastric mucosa (infants stomach secrete renin that acts of milk protein)
alcohol and aspirin (common lipid soluble substances) pass thru the stomach mucosa into the blood
intrinsic factor- required for intestinal absorption of vit B12 (needed to produce mature erythrocytes)

regulation of gastric secretion

neural and hormonal mechanisms control G secretions. gastric mucosa pours out 3 L of gastric juice. nervous control provides long (vagus-nerve mediated) and short (local enteric) nerve reflexes. when vagus nerve stimulates the stomach, secretory activity of all stomach glands increase. (activation of sympathetic nerves depres secretory activity). hormone control is done by gastrin that stimulates enzyme and HCl secretion
stimuli acting at the head, stomach and s intestine provoke/inhibit gastric secretory activity

three phases of gastric secretion: cephalic, gastric and intestinal phases
effector site is the stomach

cephalic (reflex) phase

occurs before food enters the stomach. triggered by the aroma, taste, sight or thought of food. gets the stomach ready for upcoming digestion. olfactory receptors and taste buds relay to the hypothalamus that stimulate the vagal nuclei causing motor impulses to the vagus nerves to the parasympathetic enteric ganglia which stimulate the stomach gland

gastric phase

once food reaches the stomach, gastric phase is initiated by local neural and hormonal mechanisms. lasts 3-4 hours and provides 2/3 of the gastric juice released.
most important stimuli of this phase: distension, peptides and low acidity
distension- activates stretch receptors and initiates long and short reflexes. both lead to ACh release that stimulates more gastric juice output.
gastrin plays a greater role in stimulating stomach gland secretion during gastric phase. digested proteins, caffeine and rising pH activate gastrin secreting cells called G cells. its main target is HCl secreting patietal cells that spew out more HCl. highly acidic (pH below 2) gastric contents inhibit gastrin secretion.
protein in the stomach acts as a buffer and ties up H+ and pH of gastric contents rise. this stimulates gastrin secretion and HCl release returning proper acidic conditions. this neg feedback loop maintains optimal pH.
G cells are also activated by neural reflexes. sympathetic division inhibits gastric secretion.
HCl is stimulated by 3 chemicals: ACh and gastrin by increasing intracellular Ca+ levels and histamine acting thru cyclic AMP. when all 3 bind, HCl pours extremely fast.

HCL formation

parietal cells are stimulated. H+ is pumped into the stomach lumen against a H+ K+ concentration gradient. Cl- follows H+ into the lumen as K+ goes out and then cycles back in. Cl- comes from blood plasma and H+ comes from the breakdown of carbonic acid.
as H+ is pumped from the cell and HCO3- (bicarbonate ion) accumulated within the cell, HCO3- is ejected into the capillary blood. blood draining from the stomach is more alkaline than that supplying it. this is called the alkaline tide. HCO3- and Cl- move in opposite directions.
K+ and Cl- move into the stomach lumen by diffusing thru membrane channels.

intestinal phase

two components: one excitatory and the other inhibitory.
excitatory is set into motion as partially digested food fills the initial part (duodenum) of the s intestine. this stimulates intestinal mucosal cells to release intestinal (enteric) gastrin (hormone that encourages the gastric glands to continue secretory actions); is brief bc the chyme has large amounts of H+, fats, partially digested proteins etc and that inhibityory component is stimulated then as the enterogastric reflex.
enterogastric reflex- (trio of reflexes) 1. inhibits vagal nuclei in the medulla 2. inhibits local reflexes 3. activates sympathetic fibers that cause the pyloric sphincter to tighten and prevent further food entry in the s intestine. resulting in a decline in gastric activity- this protects the small intstine from excessive acidity and matches its processing abilities to the amount of chyme at the time.
the excitatory and inhibitory factors trigger the release of intestinal hormones collectively called enterogastrones: secretin, cholecystokinin (CCK), and vasoactive intesteinal peptide (VIP)--these inhibit gastric secretion when the stomach is very active.

gastric motilibty and emptying

stomach contractions: accommodate its filling, causes its emptying, compress, knead, and mix the food with gastric juices to produce chyme. mixing movements are made by peristalsis.

response to stomach filling

stomach stretches to accomodate incoming food by internal pressure stays the same. until 1.5 L of the food has been injested then pressure rises.
the unchanging pressure is due to: 1. reflex mediated receptive relaxation of the stomach muscle 2. plasticity of visceral smooth muscle.
receptive relaxation- occurs in stomach fundus and body in anticipation and response to food mvmnt thru the esophagus; coordinated with swallowing
gastric accommodation- intrinsic ability of visceral smooth muscle to exhibit the stress-relaxation response; stretched without greatly increasing tension

gastric contractile activity

stomach exhibits peristalsis begins near the gastroesphoogeal sphincter where there is a gentle rippling mvmnt of the thin stomach wall. as contractions approach the pylorus, the contractions are much thicker (as is the muscle here). the contents near the fundus and body (food storage area) remain undisturbed whereas food in and around the pyloric antrun are pummeled and mixed.
the pyloric regions acts as a dynamic filter that allows only liquids and small particles to pass thru the barely open pyloric sphincter. each pyloric wave (is constant in rate-3 per minute) spits out 3 ml or less of chyme into the s intestine, the rest is propelled back into the stomach for further mixing (this back and forth breaks up solids in the chyme)
the rhythm of the the stomachs peristaltic waves are set by the spontaneous activity of pacemaker cells in the longitudinal smooth muscle layer. these are called interstitial cells of Cajal; they depolarize and repolarize 3 times each minute establishing the basic electrical rhythm (BER). the pacemakers set the max rate of contraction but dont initiate the contractions or regulate their force. their subthreshold depolarized waves are ignited by neural and hormonal factors. these factors are the same that enhanced gastric secretion activity. stomach distension activates stretch receptors and G cells that stimulate gastric smooth muscle and increase stomach motility. more food= more stomach mixing and emptying mvmnts

regulation of gastric emptying

empties completely within 4 hours after a meal
larger the meal, larger the stomach distension, more liquid in its contents=faster the stomach empties
fluid passes quickly thru the stomach, solids linger until they are well quid state. gastric juices and converted to the liquid state.
rate of gastric emptying depends on the centents of the duodenum and what is happening in the stomach.
as chyme enters the duodenum receptors signal it to stretch that intitate the enterogastric reflex and hormonal mechanisms that inhibit acid and pepsin secretion. these mechanisms inhibit gastric secretory action and prevent further duodenum filling.
t forms a meal moves thru the duodenum quickly, fat forms a oily layer on top of the chyme and digests slower in the small intestine. chyme entering the duodenum thats fatty it remains in the stomach for 6 or more hours

small intestine

bodys major digestive organ- where digestion is complete and all absorption occurs. is aided by secretions from theliver (bile) and pancreas (digestive enzymes)
extends from the pyloric sphincter to the ileocecal valve (sphincter) where it joins the large intestine. is the longest part of the alimentary canal; half the diameter of the large intestine; 20 ft long.
3 subdivisions: the duodenum (mostly retroperitoneal) and the jejunum and ileum (both intraperitoneal organs)
duodenum- imovable; shortest intestinal subdivision.
hepatopancreatic ampullaa point in the wall of the duodenum where the bile duct (delivering bile from the liver) and the main pancreatic dct (carrying pancreatic juice from the pancreas) unite. this opens into the duodenum at the major duodenal papilla. the hepatopancreatic sphincter is a smooth muscle valve that controls the entry of bile and pancreatic juice.
jejunum- long, between the duodenum and ileum
ileum- joints the large intestine
s intestine nerve fibers- parasympathetics from the vagus and sympathetics from the thoracic splanchnic nerves.
arterial supply- from the superior mesenteric artery; drained by the superior mesenteric vein which drains into the heptatic portal vein that carries the nutrient rich venous blood to the liver.

modifications for absorption

length of s intestine provides a huge surface area and its wall has 3 structural modifications- circular folds, villi and microvilli- that amplify its absorptive surface a lot.
most absorption occurs in the proximal part of the s intestine so the specializations decrease in number towards the distal end.
circular folds (plicae circulares)- deep permanent folds of the mucosa and submucosa that force chyme to speral thru the lumen, slowing the mvmnt and allowing time for full nutrient absorption.
villi- fingerlike projections of the mucosa. the epithelial cells of the villi are absorptive columnar cells. the core of each villus is a dense capillary bed and a wide lumen capillary called a lacteal. giested foodstuffs are absorbed thry the epithelial cells into the capillary blood and the lacteal. villi are large and leaflife in the duodenum and narrow and shorten along the length of the s intestine. smooth muscle of the villi allow it to alternately shorten and lengthen which 1. increases the contact btwn the villus and the contents of the intestinal lumen 2. milk lymph thru the lacteals.
microvilli- densely packed are what make up the brush border. the plasma membrane of the microvilli ahve brush border enzymes that complete the digestion of carbs and proteins in the s intestine

histology of the small intestine wall

four tunics; submucosa dn mucosa are modified to reflect the itnestine's functions in the digestive pathway.
epithelium of the villus mucosa is simple columnar absorptive cells richly endowed with microvilli; they absorb nutrients and electrolytes. also has many mucus-secreting goblet cells
between the villi, the mucosa has pits that lead to intestinal crypts (which are tubular glands). crypt epithelial cells are secretory cells of intestinal juice. scattered thry the crypt epithelium are enteroendocrine cells that are the source of entergastrones and T cells
. deep in the crypt cells are secretory cells called paneth cells that fortify the s intestines defenses by releasing antimicrobial agents.
crypts decrease in number along the length of the s intestine but the goblet cells become more abundant.
submucosa- areolar conn tissue; has peyers patches.
peyers patcrease towards the small intestine end; shows that the end is where there are huge #s of bacteria that must be prevented from entering the bloodstream
duodenal glands- mucus secreting; also called brunners galnds; makes an alkal neutralizes acidic chyme
muscularis is bilayered

intestinal juice

intestinal crypts secrete 1-2 distension stimulates it. or irritation of the intestinal mucosa by acidic chyme.
is slightly alkaline
largely water but contains some mucus.
secreted by duodenal glands and by goblet cells of the mucosa.
enzyme poor

gross anatomy of the liver

blood rich; largest gland in the body; has 4 primary lobes- largest is the right lobe, smaller left lobe and posterior caudate love and quadrate lobe
falciform libament- mesentry that separates the right and left lobes anteriorly and suspends the lover from the diaphragm and anterior ab wall.
round ligament (ligams) a fibrous remnant of the fetal umbilical vein. runs along the inferior endge of the falciform ligament.
the lesser omentum anchors the liver to the lesser curvature of the stomach. the hepatic artery, hepatic pertal vein and common hepatic duct run inferorily thry the lesser omentum.
biles leaves the liver thru bile ducts that fuse to form the common hepatic duct that travels to the duodenum. this fuses with the cystic duct that drains the gallbaldder. these two form the bile duct.

microscopic anatomy of the liver

composed of sesame seed sized structural and functional units called liver lobules. each lobule is a hexagonal structure consists of hepatocytes (plates of liver cells). the hepatocytes radiate outward from the central vein.
liver's main function is to filter and process the nutrient rich blo.
portal trial- at each 6 corners of the lobule; has a branch of the hepatic artery, a branch of the hepatic portal vein and a bile duct.
liver sinusoids are enlarged leak that are between the hepatocyte plates.
hepatocytes produce 900 ml of bile daily, pnutrients, store fat soluble vitamins, play important roles in detoxification.
liver is very regenerative.
hepatocytes secrete vascular endotheliAL GROWTH FACTOR (VEGF) that cause endothelial cells to proliferatea and release other GF.
bile flows thru bile canaliculi that run between adjacent hepatocytes towards the bile duct.

composition of bile

bile is a yellow green alkaline solution containing bile satls, bile pigments, cholesterol, triglycerides, phospholipids, and other electrolytes. only bile salts and phospholipids aid in the digestive process.
bile salts- cholic and chenodeoxycholic acids; are cholesterol derivatives. they emulsify fats (distribute them thruout watery intestinal contents). these separate fat droplets to provide larger surface areas for fat digesting enzymes to work on the fat. bile salts facilitate fat and cholesterol absoprtion and help solubilize cholesterol.
enterohepatic circulation- a recycling mechanism that conserves bile salts. the bile salts are reabsorbed into the blood, returned to the liver by the hepatic portal blood and resecreted into newly formed bile.
bilirubin- chief bile pigment. is a waste product of the heme of hemoglobin
stercobilin gives feces the brown color.

gallbaldder

thin walled green muscular sac.
stores bile that is not immediately needed for digestion and concentrates it by absorbing some of its water and ions.
has honeycomb like folds when empty that allow the organ to expand as it fills.
when the muscular wall contracts bile is expelled into the cystic duct and flows into the bile duct.

pancreas

soft tadpole shaped gland that extends across the ab. is encircled by the duodenum.
most is retroperitoneal and dies deep to the greater curvature of the stomach.
important in producing enzymes that break down all categories of foodstuffs, which the pancreas then delivesr to the duodenum.
pancreatic juice- exocrine product that drains from the pancreas by the main pancreatic duct. the pancreatic duct fuses with the bile duct as it enters the duodenum.
acini- clusters of secretory cells surroind ducts. are full of rough ER and have deep staining zymogen granules taht contain digestive enzymes they manufacture.
pancreatic islets are more lightly staining and scattered amid the acini

compostition of pancreatic juice

1200-1500 ml of clear pancreatic juice is made daily. contains mainly water and contains enzymes and electrolytes.
acinar cells produce the enzyme rich contents of the pancreatic juice. epithelial cells lining the ducts release bicarbonate ions that make it alkaline.
amount of HCl in stomach is balanced with bicarbonate secreted by the pancreas. this helps neutralize acid chyme entering the duodenum
pancreatic proteases (protein digsting enzymes) are produced and released in inactivated in the duodenum. protects the pacnreas from self digestion
trypsinogen is activated to trypsin by enteropeptidasectivates more tryspin and two other pancreatic proteases (procarboxypeptidase and chymotrypsinogen) to their active forms carboxypeptidase and chymotrypsin.
amylase, lipases and nucleases are other pancreatic enzymes

regulation of bile and pancreatic secretion and entery into the small intestine

regulated by neural stimuli and most importantly hormones (cholecystokinin and secretin)
secretin stimulates liver cells to secrete bile
when no digestion is happening, the hepatopancreatic sphincter is closed and bile backs up the cycstic duct to the gallbladder.
bile doesnt enter the s intestine until the gallbladder contracts. parasymp impulses delivered from the vagus nerve fibers is a minor stimulus for gallbladder contraction but choecystokinin (CCK)
CCK is an intestinal hormone relreased to the blood when acidic, fatty chyme enters the duodenum. CCK stimulates secretion of pancreatic juice and relaxes the hepatopancreatic sphincter so that bile and pnacreatic juice can enter the duodenum.
secretin and CCK promote the pancreas to secrete its juice. secretin (released in respond to HCl in the intestine) targets the pancreatic duct cells that results in a watery bicarbonate rich pancreatic juice. CCK (released in response to proteins and fats in chyme) stimulate the acini to release enzyme rich pancreatic juice.

digestive processes in the s intestine

carbs and proteins are partially degraded here but no fat digstion has occurred yet. food digestion in the s intestine takes 3-6 hours and its here that absorption of most of the water and all nutrients occurs.
the small intestine plays no part in ingestion or defecation
most substances required for chemical digestion- bile, digstive enzymes and bicarbonate ions- are imported from the liver and pancreas. absoprtion is accomplished by its absorptive cells with their rich crop of apical microvilli.
optimal digstive activity in the s intestine depends on a slow measured delivery of chyme from the stomach. entering chyme is hypertonic.
the low pH of entering chyme must be adjusted upward and the chyme must be well mixed with bile and pancreatic juice for digestion to continue. food mvmnt into the s intestine is carefully controlled by the pumping action of the stomach pylorus.

motility of the s intestine

segmentation is the most common motion of the s intestine. chyme is moved back and forth in the lumen a few cm at a time by alternating contraction and relaxation rings of smooth muscle. these segmenting movements are initiated by intrinsic pacemaker cells in the circular smooth muscle. the pacemakers of the duodenum depolarize more frequently than those in the ileum.
the intensity of segmentation is altered by long and shot reflexes- which parasymp activity enhances and symp decreases- and by hormones. more intense contractions, the greater the mixing effect.
true peristalsis occurs late in the intestinal phase only after most nutrients have been absorbed. segmenting movements wane and the duodenal mucosa begins to release motilin. as this increases, peristaltic waves are initiated in the proximal duodenum and sweep slowly along the intestine. each successive wave is initiated a bit more distally and this pattern is calle dmigrating motility complex (MMC).
duodenum to ileum takes two hours. the MMC repeats itself and is critical for preventing the overgrowth of bacteria that migrate from the L intestine to the S
impulses sent proximally by effector neurons cause contractions and shortening of the circular muscle layer
impulses sent distally to certain interneurson cause shortening of the longitudinal muscle layer and distension of the intestine.

ileocecal sphincter is normally constricted and closed

two mechanisms- one neural and the other hormonal- cause it to relax and allow food residues to enter the cecum.
1. enhanced activity of the stomach initiates the gastroileal reflex- a long reflex that enhances the force of segmentation in the ileum
2. gastrin released by the stomach increases the motility of the ileum and relexes the ileocecal sphincter. once chyme has passes, it exterts a backwards pressure that closes the valves glaps. ensuring that the contents of the previous meal are swept completely out of the stomach and s intestine.

large intestine

extends from the ileocecal valve to the anus; greater diameter than the small intestine but half as long.
its major digestive function is to absorb most of the remaining water from indegestible food residues, store the residues temporarily and then eliminate them from the body as semisolid feces.

gross anatomy of the large intestine

exhibits 3 features seen nowhere else- teniae coli, haustra, and epiploic appendages. the longitudinal muscle layer of its muscularis is mostly reduced to 3 bands of smooth muscle called teniae coli- this causes the wall of the large intestine to pucker into pocketlike sacs called haustra. the epiploic appendages are small fat filled pouches of visceral peritoneum that hang from its surface (significance not known).

subdivisions of the large intestine

divisions: cecum, appendix, colon, rectum, and anal canal
cecum- first part of the large intestine; has the vermiform appendix attached to its posteromedial surface.
appendix- contains masses of lymphoid tissues and is part of MALT; plays an important role in the bodys immunity; unfortunately its twisted structure is an ideal place for enteric bacteria to accumulate/divide.
colon- the ascending colon travels up the right side of the ab cavity to the right colic (hepatic) flexture (a right angle turn). the transverse colon travels across the ab cavity and bends acutely at the left colic (splenic) flexture. the descending colon descends down the left side of the posterior ab wall and enters the pelvis where it becomes the sigmoid colon.
colon- retroperitoneal except for its transverse and sigmoid parts (are intraperitoneal) which are anchored to the posterior ab wall by mesentry sheets called mesocolons.
rectum- runs in front of the sacrum; at the level of the 3rd sacral vertebra. has 3 lateral curves/bends that are three transverse folds internally called rectal valves. the valves separate the feces from flatus (stop feces from being passed with gas).
anal canal- last segment of L intestine; begins where the rectum penetrates the levator ani muscle; opens to the body exterior at the anus. has two sphincters of smooth muscle: internal anal sphincter (involuntary) of smooth muscleand a voluntary external anal sphincter of sk muscle. sphincters are ordinarily closed except during defecation.

microscopic anatomy of L intestine

colon mucosa is simple columnar except for the anal canal
most food is absorbed before it reaches the L intestine so there ar eno circular folds, no villi, an no cells that secrete digestive enzymes. mucosa is thick though and crypts are deeper. mucus is made by the tremendous numbers of goblet cells in the crypts taht eases the passage of feces and protects the intestinal wall from irrating acids and gases released by resident bacteria in the colon.
the mucosa of the anal canal is stratified columnar and shows the greater abrasion received here. it has anal columns superiorly that are long folds. anal sinuses are between the columns and exude mucus when compressed which aids in emptying the anal canal

bacterial flora in the L intestine

some bacteria entering the cecum are still alive. this bacteria and bacteria that enter the GI tract by the anus are called bacterial flora of the L intestine. they colonize the colon, metabolize some host derived molecules and ferment some of the indigstible carbs releasing irritating acids and a mixture of gases. 500 ml of gas is produced wach day.
also synthesize B complex vitamins and most of the vitamin K the liver requires to synthesize some of the clotting proteins.
the epithelium of the mucosa can recruit immune cells and they open tight junctions to sample the microbial antigens. the lymphoid follices (MALT) are what have T cells that prevents bacteria from straying

digestive processes in the L intestine

spends 12-24 hours there; there is a small amount of digstion but no further food breakdown occurs in the L intestine.
the L intestine harvests vitamins made by the bacterial flora and reclaims most of the remaining water and some of the electrolytes absorption is NOT a major function of the L intestine. the primary concerns are the propulsion acts that force the fecal material toward the anus and then eliminate it from the body.
the L intestine is not essential for life.

motility of the large intestine

the L intestine musculature is inactive most of the time. pressure in the ileum opens the ileoceal sphincter and closes it preventing backwards mvmnt of chyme. the colons contractions are sluggish and shortlived. the haustral contractions are slow segmenting mvmnts lasting 1 min that occur ever 30 mins or so. these are the mvmnts seen in the colon. they reflect local controls of sm muscle within the walls of the individual haustra. as a hastrum fills with food residue, the distension stimulates its muscle to contract which propels the luminal contents into the next haustrum. they mix and aid in h2o absoprtion.
mass mvmnts are long slow mving but powerful contractle waves that move over large areas of the colon 3 or 4 times daily and force the contents toward teh rectum. they occur during or after eating. bulk or fiber increases the strength of the colon contractions and softens stool, allowing the colon to act well.

defecation

mass mvmnts force feces into the rectum and stretches the rectal wall which initiates the defecation reflex. spinal cord mediated parasymp reflex causes the sigmoid colon and the rectum to contract; internal sphincter relaxes.
as feces go into the anal canal, messages reach the brain allowing us to decide if external (voluntary) anal spincter should be opened or remain constricted to stop feces.
if defecation is delayed, the reflex contractions ends within a few seconds and the rectal wall relaxes.
during defecation, the muscle sof the rectum contract to expel the feces. we aid this process by voluntarily by closing the glottis and contracting our diaphragm and ab wall muscles to increase the intra-ab pressure.

mechanism of chem digestion: enzymatic hydrolysis

chemical digestion is a catabolic process in which large food molecules are broken down to monomers which are small enough to be absorbed by the GI tract lining. it is accomplished by enzymes secreted by both intrinsic and accessory glands into the lumen of the alimentary canal.
hydrolysis is the enzymatic breakdown of any type of food molecules. it involes addition of water to each molecular bond to be broken

chemical digstion of carbs

we injest 200-600 grams of carb food each day
monosaccharides (simple sugars) are monomers of carbs; are absorbed immediately without further ado. 3 in our diet are: glucose, fructose and galactose.
more complex carbs that are broken down to monomers are disaccharides: sucrose (table sugar), lactose (milk sugar) and maltose (grain sugar) and the polysaccharides glycogen and starch.
most digestible carbs in our diets are starch.
indigstible polysaccharides do not nourish by help move the food along the GI tract by providing fiber.
chemical digestion of starch begin in the mouth. salivary amylase splits starch into oligosaccharides (smaller fragments of two or 8 linked glucose molecules). works best in a slightly acidic environment. starch digestion continues until amylase is inactivated by stomach acid and broken apart by the stomachs protein digesting enzymes. larger the meal, longer amylase continues to work in the stomach.
starchy foods and other digestible carbs that escape being broken down by salivary amylase are acted on by pancreatic amylase in the s intstine. within 10 mins of entering the s intestine, starch is entirely converted to various oligosaccharides.
intesinal brush border enzymes further digest these products to monosacchardies. dextrinase and glucoamylase (act on oligasaccharides composed of more than 3 simple sugars) and maltase, sucrase and lactase (hydrolyze maltose, sucrose and lactose) are the important brush border enzymes.
colon doesnt secrete digestive enzymes, chemical digstion OFFICIALLY ends in the small itnestine.

chemical digstion of lipids

30-50 g of fat are ingested daily.
triglycerides are the most abundant fats in our diets. the small intestine is essentially the sole site of lipid digestion bc the pancreas is the only significant source of fat digesting enzymes, lipases.
fats need special pretreatment with bile salts to be digested and absorbed in the watery environemtn of the s intestine. they form fat globules in water solutions. as fat globules enter the duodenum, they are coated with detergent like bile salts.
bile salts have nonpolar and polar regions. the nonpolar (hydrophobic) parts clong to the fat molecules and their polzr (ionized hydrophilic) parts allow them to repel each other and interact with water. fat droplets are pulled off the large fat globules and a stable emulsion (aqueous suspension of fatty droplets) is formed.
emulsification diesnt break down chem bonds, it reduces the attraction between fat molecules increases triglycerides exposure to pancreatic lipases.
pancreatic lipases catalyze the breakdown of fats by cleaving off 2 of the fatty acid chains, yielding free fatty acids and monoglycerides- glycerol with one fatty acid chain attached. fat soluble vitamins that ride with fat require no digestion.

chemical diegstion of nucleic acids

dna and rna are hydrolyzed to their nucleotide monomers by pancreatic nucleases present in pancreatic juice. the nucleotides are then broken apart by intestinal brush border enzymes (nucleosidases and phosphatases) which release their free bases, pentose sugars and phosphate ions.

absorption

all foodstudd, 80% of electrolytes and most water is absorbed in the s intestine. absorption occurs along the length of the s intestine, most of its completed by the time the chyme reaches the ileum.
the major absorptive role of the ileum is to reclain bile salts to be recycled back to the liver for resecretion.
most nutrients are absorbed thru the mucosa of the itnestinal villi by active transport processes driven directly or indirectly by metabolic energy (ATP) then they enter the capillary blood in the villus to be transported in the hapatic portal vein to the liver. the exception is some lipid digestion products which are absorbed passively by diffusion.

carb absorption

monosaccharides glucose and galactose are shuttled by the secondary active transport (cotransport with Na+) into the epithelial cells by common protein carriers. they then move out of these cells by facilitated diffusion and pass into the capillaries via intracellular clefts.
fructose moves entirely by facilitated diffusion.

protein absorption

several carriers transport the diff amino acids resulting from protein digestion. most carriers are coupled to the active transport of sodium. short chains of two or 3 amino acids are actively transported by H+ dependent cotransport and are then digested to their amin acids within the epithelial cells before entering the capillary blood by diffusion.

lipid absorption

just as bile sats accelerate lipid digestion, they also are essential for the absorption of its end products. as the water insoluble products of fat digestion are liberated by lipase activity, they become associated with bile salts and lecithin (a phospholipid found in bile)
micelles are collections of fatty elements clustered together with bile salts in such a way that the polar ends of the molecules face the water and the nonpolar portions form the core. they are similar to emulsion droplets but are smaller vehicles and easily diffuse between microvilli to come into close contact with the luminal cell surface.
upon reaching the epithelial cells, the various lipid substances leave the micelles and move through the lipid phase of the plasme membrane by simple diffusion. fat absorption is completed in the ileum in the absence of bile.
once inside the epithelial cells, the free fatty acids and monoglycerides are resynthesized into triglycerides by the smooth ER. the triglycerides are then recombined with lecthin and other phospholipids and cholesterol and coated with a skin of proteins to form water soluble lipoprotein droplets called chylomicrons. they are dispatched to the golgi apparatus where they are processed for extrusion from the cells.
the chylomicron containing vesicles migrate to the basolateral membrane and are extruded by exocytosis. they they enter the more premeable lacteals. most fat enters the lymphatic stream for distribution in lymph.
they are eventually dumped into venous blood of the neck.
while in the bloodstream, the triglycerides of the chylomicrons are hydrolyzes to free fatty acids and glycerol by lipoprotein lipase, an enzyme associated with the capillary endotheium of liver and adipose tissue. the fatty acids and glycerol can pass thry the capillary walls to be used by tissue cells for energy or stored as fats in adipose tissue.
passage of short fatty acids is done by simple diffusion into the portal blood for distribution

nucleic acid absorption

pentose sugars, nitrogenous bases and phosphate ions are transported actively across the epithelium by secial carriers in the villus epithelium they then enter the blood

vitamin absorption

small intestine absorbs dietary vitamins and the large intestine absorbs some of the K and B vitamins made by the enteric bacterial guests. fat soluble vitamins dissolve in dietary fats
most water soluble vitamins (B vitamins and C) are absorbed by diffusion or by a special active or passive transporters.
intrinsic factors bind to vit B12 which then binds to specific mucosal receptor sites in the terminal ileum, which trigger its active uptakes by endocytosis.

electrolyte absorption

most ions are actively absorbed along the entire length of the s intestine but iron and calium is limited to the duodenum.
potassium ions move across the intestinal mucosa passively by faciliated diffusion in presonse to changing osmotic gradients. greater water absorption, greater potassium absorption
for most nutrients the amount reaching the intestine is the amount absorbed, regardless of the nutritional state. absorption of iron and calcium is intimately related to the bodys need
ionic iron, is actively transported into the mucosal cells where it binds to the protein ferritin. when iron reserves are depleted iron uptake from the intestine and its release to the blood are accelerated. iron binds to transferrin to transport it in circulation
calcium absorption is closely related to blood levels of ionic calcium. it is locally regulated by the active form of vit D, which promotes active calcium absprotion. decreased blood levels of ionic calcium promote PTH release from the parathyroid glands.

water absorption

9 L of water enter the small intestine daily.
water in teh most adundant substance in chyme and 95% of the its absorbed in the s intestine by osmosis. most of the rest is absorbed in the l intestine leaveing .1 L to soften feces.
300-400 ml per hour is the normal rate.
water uptake is effectively coupled to solute uptake and affects the rate of absorption of substances that normally pass by diffusion.

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