Digestive

Diffusion: upon reaching the epithelial cells, the various lipid substances leave the micelles and move through the lipid phase of the

plasma membrane by simple diffusion.

Gastric phase- Once food reaches the stomach, local neural and hormonal mechanisms initiate the gastric phase. This phase lasts three to four hours and provides about two thirds of the gastric juice released. Stimulation: the most important secretory stimuli are distension, peptides, and low acidity.

2a: stomach distension activates stretch receptors and initiates both short and long reflexes. In the long reflexes, impulses tracelto the medulla and then back to the stomach via vagal fibers. 2b: chemical stimuli provided by partially digested proteins, caffeine, and rising ph directly activate gastrin-secreting enteroendocrine cells called G cells in stomach antrum, during this phase gastrin stimulates release of enzymes. But its main target is Hcl secreting parietal cells. It prods parietal cells to spew out hcl. 1 by acting directly on receptors on these cells, and 2 by stimulating enteroendocrine cells to release histamine.

Ingestion

Ingestion is the process by which food is taken into the alimentary canal. It includes the processes that take place while the food is in the mouth (mouth = 'buccal cavity'), such as chewing and grinding using the teeth, the lubrication and chemical effects of saliva released from the salivary glands, and swallowing of the food - which sends it onwards down the digestive tract.

Stomach got rugae, temporarily stores food and begins protein digestion. Lining epithelium of stomach mucosa=simple columnar epithelium composed entirely of mucus cells produce alkaline mucus two layer coats.

Ingestion, defecation doesn't have that. Holding area for ingested food, and stomach continues demolition job begun in oral cavity. By further degrading food both physically and chemically. It then delivers chyme, the product of its activity into small intestine.

Explain the process of digestion, giving specific detail as to the events occurring in each organ of the alimentary canal, and identifying and explaining the role of accessory organs involved

Mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Ingestion occurs only at the mouth: The mouth also begins the propulsive process of swallowing, which carries food through the pharynx and esophagus to the stomach.

The esophagus: not involved in food propulsion: As food moves through laryngopharynx, its routed into esophagus posteriorly because epiglottis closes off epiglottis closes off larynx to incoming food.

Pierces diaphragm at esophageal hiatus mediastinum of thorax. Joins stomach at cardiac orifice within abdominal cavity. The cardiac orifice is surrounded by gastroesophageal or cardiac sphincter which is a physiological sphincter. That is, acts as a sphincter, but only structural evidence of sphincter is slight thickening of circular smooth muscle at that point.

Effectors of digestive activity are smooth muscle and glands. When stimulates, receptors in the GI tract initiate reflexes that stimulate smooth muscle of the GI tract walls to mix lumen contents and move them along the tract.

Reflexes can also activate or inhibit glands that secrete digestive juices into the lumen or hormones into the blood.

Intestinal phase: the intestinal phase of gastric secretion begins with a brief stimulatory component followed by inhibition. .

Stimulation: the initial stimulatory pt of intestinal phase is set into motion as partially digested food fills the first part (duodenum) of small intestine. This stimulates intestinal mucosal cells to release intestinal (enteric) gastrin, a hormone that encourages the gastric glands to continue their secretory activity. This stimulatory effect is brief because it is overridden by inhibitory stimuli as the intestine fills.

Explain the role of pH or the mechanisms that balance pH in each organ

The short version is that parietal cells produce hydrochloric acid (as well as bicarbonate, to serve as a buffer). Nerves in the stomach sense the acidity and release acetycholine and gastrin to increase and decrease the amount of acid generated. There is also input from the brain, via the vagus nerve, which stimulates more acid when the stomach stretches (i.e. you're full).

Cephalic phase/reflex- occurs before food enters stomach.

Triggered by roma taste sight or thought of food. These triggers act via the vagus nerve to stimulate gastric glands, getting the stomach ready for its digestive chore.

Neural controls: consist of both long and short nerve reflexes, ACh released, stimulating gastric juice.

When stomach is stimulated by vagus nerves.

A mesentery is a double layer of peritoneum- a sheet of two serous membranes fused back to back-that extends to the digestive organs from the body wall. Mesenteries provide routes for blood vessels, lymphatics, and nerves to reach the digestive viscera; hold organs in place and store fat. In most places the mesentery is dorsal and attaches to the posterior

abdominal wall, but there are ventral mesenteries too, such as the one that extends from the liver to the anterior abdominal wall. Some digestive organ mesenteries have specific names (such as omenta) or are called ligaments (even though these peritoneal folds are nothing like fibrous ligaments that connect bones).

SMALL INTESTINE: Major site for digestion and absorption; Small intestine is bodys major digestive organ. Within its twisted passageway, digestion is completed (with help of bile and pancreatic enzymes) and virtually all

absorption occurs. Small intestine convoluted tube extending from pyloric sphincter to ileocecal valve (sphincter), where it joins lg intestine.

Distinguish between alimentary canal organs and accessory organs Organs in digestive system (DS) fall in 2 groups;

alimentary canal (nourish) and accessory digestive organs.

short reflexes: mediated entirely by enteric nervous system plexuses in response to stimuli within gi tract. Control of patterns of segmentation and peristalsis=

automatic involving pacemaker cells and reflex arcs between enteric neurons in same or different organs.

LARGE INTESTINE: few nutrients, 12-24 hrs more to spend there. Harvest vitamins made by

bacterial flora and reclaims most of the remaining water and some electrolytes (sodium and chloride). However nutrient absorption isn't major function. As mentioned the primary concerns of lg intestine are propulsive activities that force fecal material toward anus and eliminate from body (defecation).

Peristalsis-major means of propulsion involves alternating waves of

contraction and relaxation of muscle in organ walls. It main effect is to squeeze food along the tract but some mixing occurs as well. In fact, peristaltic waves are so powerful that once swallowed food and fluids will reach your stomach even if you stand on your head.

Lipids: Triglycerides are most abundant fats in the diet. The small intestine is primary site of lipid

digestion because pancreas is major source of fat digesting enzymes, or lipases.

segmentation (contractions of small intestine). Segmentation mixes food with digestive juices and makes absorption more

efficient by repeatedly moving different parts of food mass over intestinal wall.

Composition of Saliva: Saliva is 97% water= hypoosmotic. It's osmolarity depends on

glans that are active and stimulus for salivation. Saliva slightly acidic but pH varies.

Protein digestion begins in stomach and is main type of enzymatic breakdown that occurs there. Hcl produced by stomach glands denatures dietary proteins in preparation for enzymatic digestion. (unfolded amino acid chain more accessible to enzymes). Intrinsic factor is required for intestinal absorption of vitamin b12, needed to produce mature erythrocytes. In its absence, pernicious anemia results. However if vitamin b12, is administered .

injection , individuals can survive with minimal digestive problems even after total gastrectomy (stomach removal). Since we describe digestion and absorption later, here we will focus on events that regulate, 1 secretory activity of gastric glands 2 stomach motility and emptying

The walls of the mouth are lined with a thick stratified squamous epithelium which withstands considerable friction. The epithelium on the gums, hard palate and dorsum of tongue is slightly keratinized for extra protection against abrasion during eating. The walls of the mouth are lined w thick stratified squamous epithelium, withstands friction. The lips and cheeks: The lips (labia) and the cheeks, which help

keep food between the teeth when we chew. The tongue occupies floor of the mouth, mixes food with saliva, forming it into a compact mass called a muscle. The tongue has intrinsic and extrinsic muscles. THe intrinsic muscles are confined in tongue and attach to bone, allow tongue to change shape.

The large intestine produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the

large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral)

Bile salts are bile acids that help to break down fat. Bile acids are made in the

liver. When you eat a meal, bile is secreted into the intestine, where it breaks down the fats

Small intestine: has 3 subdivisions; duodenum, jejunum and ileum. Immovable duodenum which curves around head of pancreas including

major duodenal papilla mentioned earlier.

The teeth chew down food. Mastication aka chewing=

mechanical breakdown.

PHARYNX= oropharynx and laryngopharynx - common passages for food fluids and air. Stratified squamous epithelium well supplied mucus producing glands. The external

muscle layers consists of two skeletal muscle layers. The cells of the inner layer run longitudinally. Those outer layers, pharyngeal constrictor muscles encircle the wall like 3 stalked fists. Contractions of these muscles propel food into esophagus.

The major salivary glands are paired compound tubuloalveolar glands that develop from oral mucosa. The parotid gland has facial

nerves in it. Saliva glands made of 2 types of secretory cells - serous and mucous cells

Note the secretions that help balance incoming hyperosmotic foodstuffs Chyme has a low

pH that is countered by the production of bile, helping to further digest food. Chyme is part liquid and part solid: a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and small intestine during digestion

The extrinsic muscles alter tongues .

position

Some of these processes are job of a single organ. For example, only the mouth ingests and only the large intestine defecates. But most digestive system activities

require the cooperation of several organs and occur bit by bt as food moves along the tract as described later.

Motility of lg intestine: when food residue enter colon thru ileocecal valve, the colon becomes motile. The movements most seen in color are haustral contractions. Mass movements (mass peristalsis) are long slow contractile waves, stomach activates gastroileal reflex in small intestine and propulsive gastrocolic reflex in colon. Segmenting movements in descending and sigmoid colon dry out feces. Stretching of rectal wall initiates defecation reflex. This parasympathetic spinal reflex causes

sigmoid colon and rectum to contract and internal anal sphincter to relax.

Generally, fat absorption is completed in the ileum, but in the absence of bile (as might occur when a gallstone blocks the cystic duct), it happens so

slowly that most of the fat passes into the large intestine and is lost in feces.

When protein foods are in stomach, ph of gastric contents rise because proteins act as buffers to tie up H. this rise in ph

stimulates gastrin secretion and Hcl release which provides acidic conditions to direct proteins. More protein in meal, greater amount of gastrin and hcl released. As proteins are digested, gastric contents gradually become more acidic which again inhibits the gastrin secreting cells. This eng feedback mechanism helps maintain optimal ph and working conditions for gastric enzymes.

long reflexes: involve cns integration centers and extrinsic autonomic nerves. The enteric nervous system sends information to central nervous system via afferent visceral nerve fibers. It receives sympathetic and parasympathetic branches (motor fibers) of AND that enter the intestinal wall to synapse with neurons in the intrinsic plexuses. Long reflexes can be initiated by

stimuli arising inside or outside of GI tract. In these reflexes the entire nervous system acts as a way station for the ANS, allowing extrinsic controls to influence digestive activity. Generally speaking, parasympathetic inputs enhance digestive activity and sympathetic impulses inhibit them.

Hormonal controls: of gastric secretion is largely province of gastrin. Stimulates secretion of enzymes and HCL by the

stomach, and of hormones (mostly gastrin antagonists) by the small intestine.

Emulsification: because triglycerides and their breakdown products are insoluble in water fats need special pretreatment with bile salts to be digested in the watery environment of small intestine. In aqueous solutions, triglycerides aggregate to form large fat globules, and only the triglyceride molecules at surfaces of such fatty masses are accessible to water soluble lipase enzymes. However bile salts vastly increase

surface area exposed to pancreatic lipases by breaking large fat globules into many smaller droplets. Without bile, lipids could not be completely digested during the time food spends in small intestine.

The superior tongue surface bears papillae , peglike projections of the underlying mucosa. -

the conical filiform papillae roughen the tongue surface, helping us lick semisolid foods and providing friction for manipulating goods. These papillae contain keratin which stiffens them and gives them whitish appearance. -the mushroom fungiform papillae are scattered widely over the tongue surface. Each has a vascular core that gives it a reddish hue. -eight to twelve large vallate papillae are located in a V shaped row at back of tongue. They resemble fungiform papillae but have additional surrounding furrow. -platelike foliate papillae are located on the lateral aspects of the posterior tongue.

Chylomicron transport: the milky white chylomicrons are too large to pass through either the plasma membrane of the epithelial cell or the basement membrane of a blood capillary. Instead the chylomicron-containing

vesicles migrate to the basolateral membrane and are extruded by exocytosis. They then enter the more permeable laceterals. Thus most at enters the lymphatic stream for distribution in the lymph. Eventually, the chylomicrons are emptied into the venous blood via the thoracic duct which drains the lymphatics of the digestive viscera.

Chemical digestion involves breaking down the food into simpler nutrients that can be used by the cells. Chemical digestion begins in the mouth when food mixes with saliva. Saliva contains an enzyme (amylase) that begins the breakdown of carbohydrates Digestive enzymes are released, or secreted, by the organs of the digestive system. These enzymes include proteases that digest proteins, and nucleases that digest nucleic acids. Examples of digestive enzymes are:

Amylase, produced in the mouth. It helps break down large starch molecules into smaller sugar molecules. Pepsin, produced in the stomach. Pepsin helps break down proteins into amino acids. Trypsin, produced in the pancreas. Trypsin also breaks down proteins. Pancreatic lipase, produced in the pancreas. It is used to break apart fats. Deoxyribonuclease and ribonuclease, produced in the pancreas. They are enzymes that break bonds in nucleic acids like DNA and RNA.

Identify and explain the major Digestive Processes occurring in the digestive system SIX.

Ingestion- taking food into digestive tract aka eating Propulsion- moves food thru alimentary canal includes swallowing (voluntary), and peristalsis (involuntary). Peristalsis-major means of propulsion involves alternating waves of contraction and relaxation of muscle in organ walls. It main effect is to squeeze food along the tract but some mixing occurs as well. In fact, peristaltic waves are so powerful that once swallowed food and fluids will reach your stomach even if you stand on your head. Mechanical breakdown- increases the surface area of ingested food, physically preparing it for digestion by enzymes. Mechanical process include chewing, mixing food with saliva by the tongue, churning food in the stomach and segmentation (contractions of small intestine). Segmentation mixes food with digestive juices and makes absorption more efficient by repeatedly moving different parts of food mass over intestinal wall. Digestion- involves a series of catabolic steps in which enzymes secreted into the lumen cavity of the alimentary canal break down complex food molecules to their chemical building blocks. Absorption-passage of digested end products (plus vitamins mineral and water) from the lumen of the GI tract through mucosal cells by active/passive tranpsort into blood or lymph. Defecation-poop. Eliminates indigestible substances from the body via the anus in the from of feces.

electrolytes (na, k, cl, Po, hco3) -Digestive enzymes salivary amylase and lingual lipase( lingual lipase makes minor contribution to overall fat digestion) -proteins mucin, lysozyme, and IgA -

Metabolic wastes (urea and uric acid) When dissolved in water, the glycoprotein mucin forms thick mucous. SALIVA protects against microorganisms because it contains 1. IgA antibodies, 2. Lysozyme; a bactericidal enzyme that inhibits bacterial growth in the mouth and may prevent teeth decay. 3. Defensins . Besides acting as a local antibiotic, defensins function as cytokines to call defensive cells (lymphocytes,neutrophils) Into mouth for battle. In addition to these 3 protectors; friendly bacteria that live on back of tongue promote the conversion of food derived nitrates in saliva into nitric oxide in an acidic environment. This transformation occurs around the gums where acid producing bacteria tend to cluster and in hcl acid rich secretions of stomach, the highly toxic nitric oxide is believed to be bactericidal in these location.

Alimentary canal(Gi tract)- continuous muscular tube that winds thru the body from the mouth to the anus. It digest food-break it down into smaller fragments- and absorbs the digested fragments through its lining into the blood.

Mouth, pharynx,esophagus, stomach, small intestine, and large intestine. The large intestine leads to the terminal opening or anus. In a cadaver, the alimentary canal is approx 9 m long, but in living person much shorter. Food material in this tube is considered outside bc canal is open to external both ends.

The intraluminal pH is rapidly changed from highly acid in the stomach to about pH 6 in the duodenum. The pH gradually increases in the small intestine from pH 6 to about pH 7.4 in the terminal ileum. The pH drops to 5.7 in the

caecum, but again gradually increases, reaching pH 6.7 in the rectum.

While in bloodstream, the triglycerides of the chylomicrons are hydrolyzed to fatty acids and glycerol by lipoprotein lipase, an enzyme associated with

capillary endothelium. The fatty acids and glycerol cna then pass through the capillary walls to be used by tissue cells for energy or stored as fats in adipose tissue. Liver cells then endocytose and process the residual chylomicron material.

Assimilation is the process by which components / chemicals from food (incl. liquid refreshments such as milk drinks, fruit juices etc.) are taken into the

cells of the body - after the food/beverage has been digested and absorbed

Stimulus acting at 3 sites- head, stomach, and small intestine-provoke or inhibit gastric secretions. The 3 phases gastric secretion are called

cephalic, gastric, and intestinal phases. One or more phases may occur at same time.

The mouth- anterior opening is the oral orifice. Posteriorly, the oral cavity is

continuous with the oropharynx.

Identify and explain the Regulation of digestive activity (general and organ specific): intrinsic and extrinsic Basic concepts of regulating digestive activity: 3 key govern regulation of digestive: Digestive activity is provoke by a range of mechanical and chemical stimuli. Receptors involved in controlling GI tract activity are

located in the walls of the tracts organs. These receptors respond to visceral stimuli, most importantly stretching of the organ by food in lumen changes in osmolarity (solute concentration) and ph of the contents, and the presence of substrates and end products of digestion.

The salivary glands- SALIVA: -cleanses mouth, dissolves food chemicals so they can be tasted, moistens food into bolus, contains the enzyme amylase that begins the digestion of starchy foods. Most saliva is produced by

major or extrinsic salivary glands that lie outside oral cavity and empty their secretions into it. Minor or intrinsic salivary glands scattered throughout oral vacit mucosa augment the output slightly.

Summarize the overall function of digestive system Digestive system takes in food, breaks it down into nutrient

molecules absorbs these molecules into bloodstream and then ris the body of indigestible remains.

Enterogastrones: the enterogastrone hormones are released by a scattering of enteroendocrine cells in the duodenal mucosal epithelium. The two

most important enterogastrones are secretin and cholecystokinin. The enterogastrones inhibit gastric secretion and also play other roles.

The accessory digestive organs are the teeth, tongue, gall bladder, and number of large digestive glands, the salivary glands, liver and pancreas. The teeth and tongue are in the

mouth or oral cavity while the digestive glands and gallbladder are outside gi tract and connect to it by ducts. The accessory digestive glands produce variety of secretions that help break down foodstuffs.

Digestive System: Swallowing Pharynx and esophagus serve to pass food. Deglutition (SWALLOWING) Two major phases in swallowing:

Buccal phase- occurs in mouth and is voluntary. Ends when food bolus leaves mouth and stimulates tactile receptors in the posterior pharynx initiating the next phase. Pharyngeal- esophageal phase: is involuntary and is controlled by swallowing center in brain stem (medulla and lower pons). Various cranial nerves most importantly vagus nerves, transmit motor impulses from the swallowing center to the muscles of pharynx and esophagus. Once food enters pharynx respiration is inhibited.

Absorption / Assimilation Absorption is the uptake of fluids or other substances by the tissues of the body. Digested 'food' (which is referred to by other terms depending on its stage of passage through the digestive system - see transit through the alimentary canal) is

absorbed into the bodily fluids blood and lymph from the alimentary canal. Most of the absorption part of the digestive process occurs in the jejunum and the ileum of the small intestine, though alcohol is readily absorbed through the stomach. The small intestine is lined with minute finger-like processes (called 'villi', a single example being a 'villus'), that greatly increase its surface area, and hence the rate at which absorption can take place.

Inhibition: Enterogastric reflex: duodenum inhibits

acid secretion in stomach by short reflexes thru enteric nervous system by long reflexes involving sympathetic and vagus nerves.

The enzymes in the small intestine work best in alkaline conditions, but the food is acidic after being in the stomach. A substance called

bile neutralises the acid to provide the alkaline conditions needed in the small intestine

Micelle formation: just as bile salts accelerate lipid digestion, they are also essential for absorption of its end products. As the water insoluble products of fat digestion the monoglycerides and free fatty acids-are liberated by lipase activity, they quickly become associated with

bile salts and lecithin (phospholipid in bile) to form micelles. Micelles are collections of fatty elements clustered together with bile salts in such a way that the polar (hydrophilic) ends of the molecules face the water and the nonpolar portions form the core. Also nestled in the hydrophobic core are cholesterol molecules and fat soluble vitamins. Although micelles are similar to emulsion droplets, they're about 500x smaller and easily diffuse between microvilli to come into close contact with the apical cell surface. Without micelles, the lipids would simply float on the surface of the chyme (like oil on water), inaccessible to the absorptive surfaces of epithelial cells.

Control of HCL secreting parietal cells is multifaceted. Its stimulated by ach, gastrin and histamine. When ⅓ chemicals bind to parietal cell receptors, hcl secretion is scanty but when all 3

bind hcl pours forth. Antihistamines such as cimetidine that bind to and block the H2 (histamine) receptors of parietal cells are used to treat gastric ulcers caused by hyperacidity. Histamin is key player.

Our gut bacteria help us recovering energy from otherwise indigestible foods and synthesizing some vitamins. -fermentation: gut bacteria ferment some indigestible carbohydrates and mucin in gut mucus. The resulting short chain fatty acids can be absorbed and used for fuel by the bodys cells. Unfortunately fermentation also produces mixture of gases (including dimethyl sulfide, H, N, Ch, and Co) some of these gases, such as dimethyl sulfide, are quite odorous (smelly), about 500 ml of gas (flatus) is produced each day, much more when we

eat foods (such as beans) rich in indigestible carbs. -vitamin synthesis: B complex vitamins and some of the vitamin K the liver needs to produce several clotting proteins are synthesized by gut bacteria.

Elimination Elimination is the final stage of this 4-stage summary of digestion. In physiology more generally the word '

elimination' can also apply to the entire process of excretion of metabolic waste products, incl. from the blood via the kidneys and urinary tract (as described in the section about the Renal System).

Neurons (intrinsic and extrinsic) and hormones control digestive activity. The nervous system controls digestive activity via both intrinsic (involving short reflexes entirely within the

enteric nervous system as described above) and extrinsic controls (involving long reflexes).

Describe the Enteric Nervous System (gut) in house nerve supply of the alimentary canal. It's staffed by enteric neurons that communicate widely with one another to regulate digestive system activity. These semi autonomous

enteric neurons constitute the bulk of two major intrinsic nerve plexuses (ganglia interconnected by unmyelinated fiber tracts) found in walls of alimentary canal: submucosal and myenteric new plexuses. These plexuses interconnect like chicken wire all along Gi tract and regulate digestive activity throughout its length.

Inhibition: 4 main factors in duodenum cause it to put the brakes on gastric secretion. Distension of duodenum or the presence of acidic, fatty or hypertonic chyme all trigger both neuronal and hormonal signals to tell the stomach enough. These same 4 .

factors also decrease gastric emptying. These brakes on gastric activity protect the small intestine from excessive acidity. They also prevent a massive influx of chyme from overwhelming the digestive and absorptive capacities of the duodenum by matching the amount of entering chyme to processing abilities of small intestine

Bile salts have both nonpolar and polar regions. Their nonpolar (hydrophobic) parts cling to fat molecules, and their polar (ionized hydrophilic) parts allow them to repel each other and interact with water. As a result, fatty droplets are pulled off the large

fat globules forming a stable emulsion- an aqueous suspension of fatty droplets. Emulsification doesn't break chemical bonds. It just reduces the attraction between fat molecules so they can be more widely dispersed, just as dish detergent breaks up a pool of fat drippings.

Digestion: pancreatic lipases catalyze the breakdown of fats by splitting off two of the fatty acid chains yielding free

fatty acids and monoglycerides (glycerol with one fatty acid chain attached).

Inhibition: highly acidic ph below 2,

gastric contents inhibit gastrin secretion, stress, fear anything else inhibits gastric secretion because sympathetic overrides parasympathetic

Digestion Digestion is the process by which ingested (food) material is broken down in the earlier stages of the alimentary canal into a form that can then be absorbed and assimilated into the tissues of the body. Digestion includes two types of processes - Mechanical (e.g. chewing, grinding, churning, mixing), and Chemical (e.g. action of digestive enzymes, bile, acids, etc.). The mechanical processes include the chewing and grinding of food by the teeth and also the churning and mixing of the contents of the stomach. Chemical processes that contribute to digestion also begin in the

mouth with action of saliva on food. However, most of the chemical digestive processes occur in the stomach and small intestine - where the partly-digested materials are subjected to gastric juices, pancreatic juice, succus entericus and so on.

Serous cells= produce watery secretion containing enzymes ions and mucin. Mucous cells= produce

mucus, viscous solution. The parotid and submandibular glands contain mostly serous cells. Sublingual glands contain mucous cells.

With a pH of approximately 2, chyme emerging from the stomach is very acidic. The duodenum secretes a hormone, cholecystokinin (CCK), which causes the gall bladder to contract, releasing alkaline bile into the duodenum. CCK also causes the release of digestive enzymes from the

pancreas. The duodenum is a short section of the small intestine located between the stomach and the rest of the small intestine. The duodenum also produces the hormone secretin to stimulate the pancreatic secretion of large amounts of sodium bicarbonate, which then raises pH of the chyme to 7.

The large intestine encircles these more distal pts of small intestine. The arterial supply of small intestine is primarily from superior mesenteric artery. Veins parallel the arteries and typically drain into superior mesenteric vein. Nerve fibers serving small intestine include

parasympathetics from the vagus and sympathetics from thoracic splanchnic nerves both relayed through superior mesenteric and celiac plexus. Carbs and proteins are degraded but fat digestion has only begun. Process of digestion accelerates during chymes torturous. Since we cover actual chemistry of digestion and absorption in detail later, here we will examine source of enzymes. Chyme entering duodenum hypertonic.

Chylomicron formation: once the free fatty acids and monoglycerides enter the epithelial cells, the smooth ER converts them back into triglycerides. The triglycerides are then combined with lecithin and other

phospholipids and cholesterol, and coated with a skin of proteins to form water soluble lipoprotein droplets called chylomicrons. This series of ecens is quite different from the absorption of amino acids and simple sugars, which pass through the epithelial cells unchanged.

The submucosal nerve plexus occupies the submucosa and the large myenteric nerve plexus lies between circular and longitudinal layer f nuscukaris externa. Enteric neurons of these

plexuses provide major nerve supply to gi tract wall and control gi tract motility (motion). The enteric nervous system participates in both short and long reflex arcs.

Not all alimentary canal organs are suspended by mesentery. For example, during development some regions of the small intestine adhere to the dorsal abdominal wall. In so doing, they lose their mesentery and come to lie

posterior to the peritoneum. These organs which include most of the pancreas and duodenum (first part of the small intestine) and parts of the large intestine, are called retroperitoneal organs. By contrast, digestive organs (ike the stomach) that keep their mesentery and remain in the peritoneal cavity are called intraperitoneal and peritoneal organs.

Passage of short chain fatty acids is quite different from what we have just described. These fat breakdown products don't depend on the

presence of bile salts or micelles and are not recombined to form triglycerides within the intestinal cells. They simply diffuse into the portal blood for distribution.