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SMALL INTESTINE


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#1 from_india

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Posted 30 September 2005 - 04:40 PM

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ATPase ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme (in most cases) harnesses to drive other chemical reactions that would not otherwise occur. This process is widely used in all known forms of lifATPases import many of the metabolites necessary for cell metabolism and export toxins, wastes, and solutes that can hinder cellular processes. An important example is the sodium-potassium exchanger (or Na+/K+ATPase)ALSO CALLED SODIUM PUMPS, which establishes the ionic concentration balance that maintains the cell potential.Membrane potential Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cell's plasma membrane. ion channelAnother, unrelated ion channeling process is part of ion implantation.Ion channels are pore-forming proteins that help establish the small voltage gradient that exists across the membrane of all living cells (see cell potential), by controlling the flow of ions. They are present in the membranes that surround all biological cells.An ion channel is an integral membrane protein or more typically an assembly of several proteins.ion channels permit the passage of ions more or less indiscriminately sodium pump :sodium-potassium pump, the mechanism of active transport by which gradients of the ions sodium (Na + ) and potassium (K + ) are formed across the cell membrane; using the energy generated by the Na + ,K + -ATPase, sodium is extruded from the cell and potassium is brought in. The gradients are necessary for protein biosynthesis, maintenance of osmotic equilibrium, propagation of nerve impulses, and secondary transport of some molecules (eg, glucose) across cell membranes. Called also Na + -K + p. Mucus and MucinsMucus is a "slimy" material that coats many epithelial surfaces and is secreted into fluids such as saliva. It is composed chiefly of mucins and inorganic salts suspended in water. Mucus adheres to many epithelial surfaces, where it serves as a diffusion barrier against contact with noxious substances (e.g. gastric acid, smoke) and as a lubricant to minimize shear stresses; such mucus coatings are particularly prominent on the epithelia of the respiratory, gastrointestinal and genital tracts. Mucus is also an abundant and important component of saliva, giving it virtually unparalleled lubricating properties (try sticking a piece of apple skin between your molars without saliva). Mucus-secreting cells are widely distributed through the body. Goblet cells are abundant in the epithelium of the gastrointestinal and respiratory tracts, mucous glands in these same organs deliver their products through ducts into the intestine and respiratory tree, and many of the acinar epithelial cells in salivary glands secrete mucus. Mucins are a family of large, heavily glycosylated proteins. Although some mucins are membrane bound due to the presence of a hydrophobic membrane-spanning domain that favors retention in the plasma membrane, the concentration here is on those mucins that are secreted on mucosal surfaces and saliva. Absorption in the Small Intestine: General MechanismsVirtually all nutrients from the diet are absorbed into blood across the mucosa of the small intestine. In addition, the intestine absorbs water and electrolytes, thus playing a critical role in maintenance of body water and acid-base balance. It's probably fair to say that the single most important process that takes place in the small gut to make such absorption possible is establishment of an electrochemical gradient of sodium across the epithelial cell boundary of the lumen. Don't turn off on me now - this is a critical concept and actually quite interesting! Also, as we will see, understanding this process has undeniably resulted in the saving of millions of lives. To remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular sodium is maintained by a large number of Na+/K+ ATPases - so-called sodium pumps - embedded in the basolateral membrane. These pumps export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane. In rats, as a model of all mammals, there are about 150,000 sodium pumps per small intestinal enterocyte which collectively allow each cell to transport about 4.5 billion sodium ions out of each cell per minute (J Membr Biol 53:119-128, 1980). Pretty impressive! This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates. Aside from the electrochemical gradient of sodium just discussed, several other concepts are required to understand absorption in the small intestine. Also, dietary sources of protein, carbohydrate and fat must all undergo the final stages of chemical digestion just prior to absorption of, for example, amino acids, glucose and fatty acids. At this point, its easiest to talk separately about absorption of each of the major food groups, recognizing that all of these processes take place simultaneously. Water and electrolytes Carbohydrates, after digestion to monosaccharides Proteins, after digestion to small peptides and amino acids Neutral fat, after digestion to monoglyceride and free fatty acids Absorption of Water and Electrolytes in SMALL INTESTINEThe small intestine must absorb massive quantities of water. A normal person or animal of similar size takes in roughly 1 to 2 liters of dietary fluid every day. On top of that, another 6 to 7 liters of fluid is received by the small intestine daily as secretions from salivary glands, stomach, pancreas, liver and the small intestine itself. By the time the ingesta enters the large intestine, approximately 80% of this fluid has been absorbed. Net movement of water across cell membranes always occurs by osmosis, and the fundamental concept needed to understand absorption in the small gut is that there is a tight coupling between water and solute absorption. Another way of saying this is that absorption of water is absolutely dependent on absorption of solutes, particularly sodium: Sodium is absorbed into the cell by several mechanisms, but chief among them is by cotransport with glucose and amino acids - this means that efficient sodium absorption is dependent on absorption of these organic solutes.Absorbed sodium is rapidly exported from the cell via sodium pumps - when a lot of sodium is entering the cell, a lot of sodium is pumped out of the cell, which establishes a high osmolarity in the small intercellular spaces between adjacent enterocytes.Water diffuses in response to the osmotic gradient established by sodium - in this case into the intercellular space. It seems that the bulk of the water absorption is transcellular, but some also diffuses through the tight junctions.Water, as well as sodium, then diffuses into capillary blood within the villus. Examine the animation above and consider the osmotic gradient between the lumen and the intercellular space (inside the villus). As sodium (green balls) is rapidly pumped out of the cell, it achieves very high concentration in the narrow space between enterocytes. The osmotic gradient is thus formed across apical cell membranes and their connecting junctional complexes. The arrow that appears denotes movement of water across the epithelium. Water is thus absorbed into the intercellular space by diffusion down an osmotic gradient. However, looking at the process as a whole, transport of water from lumen to blood is often against an osmotic gradient - this is important because it means that the intestine can absorb water into blood even when the osmolarity in the lumen is higher than osmolarity of blood. This ability is best explained by the "three compartment model" for absorption of water and, like many aspects of gut permeability, varies along the length of the gut. The proximal small intestine functions as a highly permeable mixing segment, and absorption of water is basically isotonic. That is, water is not absorbed until the ingesta has been diluted out to just above the osmolarity of blood. The ileum and especially the colon are able to absorb water against an osmotic gradient of several hundred milliosmols Absorption of Monosaccharides IN small intestine{Monosaccharides A sugar (like sucrose or fructose) that does not hydrolyse to give other sugars; the simplest group of carbohydrates}Simple sugars are far and away the predominant carbohydrate absorbed in the digestive tract, and in many animals the most important source of energy. Monosaccharides, however, are only rarely found in normal diets. Rather, they are derived by enzymatic digestion of more complex carbohydrates within the digestive tube. Particularly important dietary carbohydrates include starch and disaccharides such as lactose and sucrose. None of these molecules can be absorbed for the simple reason that they cannot cross cell membranes unaided and, unlike the situation for monosaccharides, there are no transporters to carry them across. This section will focus on understanding the processes involved in assimilation of three important carbohydrates: starch, lactose and sucrose. The key concepts involved in all three cases are that: the final enzymatic digestion that liberates monosaccharides is conducted by enzymes that are tethered in the lumenal plasma membrane of absorptive enterocytes (so-called "brush border hydrolyases").glucose generated by digestion of starch or lactose is absorbed in the small intestine only by cotransport with sodium, a fact that has exceptionally important implications in medicine. Brush Border Hydrolases Generate MonosaccharidesPolysaccharides and disaccharides must be digested to monosaccharides prior to absorption and the key players in these processes are the brush border hydrolases, which include maltase, lactase and sucrase. Dietary lactose and sucrose are "ready" for digestion by their respective brush border enzymes. Starch, as discussed previously, is first digested to maltose by amylase in pancreatic secretions and, in some species, saliva. Dietary lactose and sucrose, and maltose derived from digestion of starch, diffuse in the small intestinal lumen and come in contact with the surface of absorptive epithelial cells covering the villi where they engage with brush border hydrolases: maltase cleaves maltose into two molecules of glucose lactase cleaves lactose into a glucose and a galactose sucrase cleaves sucrose into a glucose and a fructose At long last, we're ready to actually absorb these monosaccharides. Glucose and galactose are taken into the enterocyte by cotransport with sodium using the same transporter (secondary active transport). Fructose enters the cell from the intestinal lumen via facilitated diffusion through another transporter. Absorption of Glucose: Transport Across the Intestinal EpitheliumAbsorption of glucose, or any molecule for that matter, entails transport from the intestinal lumen, across the epithelium and into blood. The transporter that carries glucose and galactose into the enterocyte is the sodium-dependent hexose transporter, known more formally as SGLUT-1. As the name indicates, this molecule transports both glucose and sodium into the cell and in fact, will not transport either alone. The essence of transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose, and can be summarized as follows: the transporter is initially oriented facing into the lumen - at this point it is capable of binding sodium, but not glucosesodium binds, inducing a conformational change that opens the glucose-binding pocketglucose binds and the transporter reorients in the membrane such that the pockets holding sodium and glucose are moved inside the cellsodium dissociates into the cytoplasm, causing glucose binding to destabilizeglucose dissociates into the cytoplasm and the unloaded transporter reorients back to its original, outward-facing position The animation seen below depicts digestion of maltose and entry of the resulting glucose, along with sodium, into the enterocyte (actually, two sodium ions are transported for each glucose). Despite the simplicity of the diagram, you should easily be able to identify the sodium-dependent hexose transporter and "watch" its conformational changes. Also, imagine the corresponding process involving lactose and sucrose assimilation. Once inside the enterocyte, glucose and sodium must be exported from the cell into blood. We've seen previously how sodium is rapidly shuttled out in exchange for potassium by the battery of sodium pumps on the basolateral membrane, and how that process that maintains the electrochemical gradient across the epithelium. The energy stored in this gradient is actually what is driving glucose entry through the sodium-dependent hexose transporter described above. Recall also how the massive transport of sodium out of the cell establishes the osmotic gradient responsible for absorption of water. Glucose is tranported out of the enterocyte through a different transporter (called GLUT-2) in the basolateral membrane. Glucose then diffuses "down" its concentration gradient into capillary blood within the villus.{enterocyte: intestinal cell} Absorption of Amino Acids and Peptides in small intestine {Peptide: Two or more amino acids joined by a bond called a "peptide bond."}{amino Acid: The basic building block of protein. All amino acids contain an amino (NH 2 ) end, a carboxyl end (COOH) and a side group ®. In proteins, amino acids are joined together when the NH 2 group of one forms a bond with the COOH group of the adjacent amino acid. The side group is what distinguishes each of the amino acids from the others.}Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be digested into amino acids or di- and tripeptides first. In previous sections, we've seen two sources secrete proteolytic enzymes into the lumen of the digestive tube: the stomach secretes pepsinogen, which is converted to the active protease pepsin by the action of acid.the pancreas secretes a group of potent proteases, chief among them trypsin, chymotrypsin and carboxypeptidases. Through the action of these gastric and pancreatic proteases, dietary proteins are hydrolyzed within the lumen of the small intestine predominantly into medium and small peptides (oligopeptides). The brush border of the small intestine is equipped with a family of peptidases. Like lactase and maltase, these peptidases are integral membrane proteins rather than soluble enzymes. They function to further the hydrolysis of lumenal peptides, converting them to free amino acids and very small peptides. These endproducts of digestion, formed on the surface of the enterocyte, are ready for absorption. Absorption of Amino AcidsThe mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The lumenal plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino acids. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form. Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. Further, absorption of amino acids, like that of monosaccharides, contributes to generating the osmotic gradient that drives water absorption. The basolateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into blood. These are not dependent on sodium gradients. Absorption of PeptidesThere is virtually no absorption of peptides longer than three amino acids. However, it seems that there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed without dependence on sodium, probably by a single transport molecule. Once inside the enterocyte, the vast bulk of di- and tripeptides are digested into amino acids by cytoplasmic peptidases and exported from the cell into blood. Only a very small number of these small peptides enter blood intact. Absorption of Intact ProteinsAs emphasized, absorption of intact proteins occurs only in a few circumstances. In the first place, very few proteins get through the gauntlet of soluble and membrane-bound proteases intact. Second, "normal" enterocytes do not have transporters to carry proteins across the plasma membrane and they certainly cannot permeate tight junctions. One important exception to these general statements is that for a very few days after birth, neonates have the ability to absorb intact proteins. This ability, which is rapidly lost, is of immense importance because it allows the newborn animal to acquire passive immunity by absorbing immunoglobulins in colostral milk. In constrast to humans and rodents, there is no significant transfer of antibodies across the placenta in many animals (cattle, sheep, horses and pigs to name a few), and the young are born without circulating antibodies. If fed colostrum during the first day or so after birth, they absorb large quantities of immunoglobulins and acquire a temporary immune system that provides protection until they generate their own immune responses. The small intestine rapidly loses the capacity to absorb intact proteins - a process called closure - and consequently, animals that do not receive colostrum within the first few days after birth will likely die due to opportunistic infections. Absorption of Lipids in SMALL INTESTINEThe bulk of dietary lipid is neutral fat or triglyceride, composed of a glycerol backbone with each carbon linked to a fatty acid. Additionally, most foodstuffs contain phospholipids, sterols like cholesterol and many minor lipids, including fat-soluble vitamins. In order for the triglyceride to be absorbed, two processes must occur: Large aggregates of dietary triglyceride, which are virtually insoluble in an aqueous environment, must be broken down physically and held in suspension - a process called emulsification.Triglyceride molecules must be enzymatically digested to yield monoglyceride and fatty acids, both of which can efficiently diffuse into the enterocyte The key players in these two transformations are bile salts and pancreatic lipase, both of which are mixed with chyme and act in the lumen of the small intestine. Emulsification, Hydrolysis and Micelle FormationBile salts play their first critical role in lipid assimilation by promoting emulsification. As derivatives of cholesterol, bile salts have both hydrophilic and hydrophobic domains (i.e. they are amphipathic). On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile salts intercalate into the lipid, with the hydrophilic domains remaining at the surface. Such coating with bile salts aids in breakdown of large aggregates or droplets into smaller and smaller droplets. Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase. The activity of this enzyme is to clip the fatty acids at positions 1 and 3 of the triglyceride, leaving two free fatty acids and a 2-monoglyceride. Lipase is a water-soluble enzyme, and with a little imagination, it's easy to understand why emulsification is a necessary prelude to its efficient activity. Shortly after a meal, lipase is present within the small intestine in rather huge quantities, but can act only on the surface of triglyeride droplets. For a given volume of lipid, the smaller the droplet size, the greater the surface area, which means more lipase molecules can get to work. The drug orlistat (Xenical) that is promoted for treatment of obesity works by inhibiting pancreatic lipase, thereby reducing the digestion and absorption of fat in the small intestine. As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile salts and complex with other lipids to form structures called micelles. Micelles are essentially small aggregates of mixed lipids and bile salts suspended within the ingesta. As the ingesta is mixed, micelles bump into the brush border and the lipids, including monoglyceride and fatty acids, are absorbed. Absorption and Transport into BloodLipids are absorbed by a mechanism distinctly different from what we've seen for monosaccharides and amino acids. The major products of lipid digestion - fatty acids and 2-monoglycerides - enter the enterocyte by simple diffusion across the plasma membrane. A considerable fraction of the fatty acids also enter the enterocyte via a specific fatty acid transporter protein in the membrane. Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesize triglyeride! Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons. Remember where this is occurring - in the absorptive enterocyte of the small intestine. Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocyte. The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the space outside the cells. Because chylomicrons are particles, virtually all steps in this pathway can be visualized using an electron microscope, as the montage of images to the right demonstrates. Transport of lipids into the circulation is also different from what occurs with sugars and amino acids. Instead of being absorbed directly into capillary blood, chylomicrons are transported first into the lymphatic vessel that penetrates into each villus. Chylomicron-rich lymph then drains into the system lymphatic system, which rapidly flows into blood. Blood-borne chylomicrons are rapidly disassembled and their constitutent lipids utilized throughout the body. Another lipid of importance that is absorbed in the small intestine is cholesterol. Cholesterol homeostatis results from a balance of cholestrol synthesis, absorption of dietary cholesterol, and elimination of cholesterol by excretion in bile. Years ago it was shown that cholesterol, but not plant sterols, is readily absorbed in the intestine. More recently, a specific transport protein has been identified that ferries cholesterol from the intestinal lumen into the enterocyte. From there, cholesterol is incorporated into chylomicrons and shuttled into blood by the mechanisms described above. Review of Food Chemistry"I didn't fight my way to the top of the food chain to become a vegetarian" ProteinsThe diet of any animal contains hundreds if not thousands of different molecules, but the bulk of the ingested nutrients are in the form of huge macromolecules that cannot be absorbed into blood without first being reduced to much simpler and smaller forms - even table sugar (sucrose) cannot be absorbed without first being enzymatically ripped apart into glucose and fructose. The most important enzymatic reaction in digestion of foodstuffs is hydrolysis - the breaking of a chemical bond by the addition of a water molecule. ProteinsProteins are polymers of amino acids linked together by peptide bonds. Chain length varies tremendously and many dietary proteins have been modified after translation by addition of carbohydrate (glycoproteins) or lipid (lipoprotein) moieties. These modifications will be almost totally ignored in this text. Very short proteins, typically 3 to 10 amino acids in length, are called peptides. Although very small peptides can be absorbed to a limited degree, for all intents and purposes, proteins must be reduced to single amino acids before they can be absorbed. Enzymes that hydrolyze peptide bonds and reduce proteins or peptides to amino acids are called proteases or peptidases. LipidsFatty acids are present in only small amounts in animal and plant tissues, but are the building blocks of many important complex lipids. True fatty acids possess a long hydrocarbon chain terminating in a carboxyl group. Nearly all fatty acids have an even number of carbons and have chains between 14 and 22 carbons in length. The principle differences among the many fatty acids are the length of the chain (usually 16 or 18 carbons) and the positions of unsaturated or double bonds. For example, stearic acid (pictured below) has 18 carbons and is saturated. The so-called "short-chain" or volatile fatty acids are 2 to 4-carbon molecules of great importance in intermediary metabolism and as the mainstay of ruminant nutrition. They are represented by acetic, butyric and proprionic acids. The most abundant storage form of fat in animals and plants, and hence the most important dietary lipid, is neutral fat or triglyceride. A molecule of triglyceride is composed of a molecule of glycerol in which each of the three carbons is linked through an ester bond to a fatty acid. Triglycerides cannot be efficiently absorbed, and are enzymatically digested by pancreatic lipase into a 2-monoglyceride and two free fatty acids, all of which can be absorbed. Other lipases hydrolyse a triglyceride into glycerol and three fatty acids. CarbohydratesThe diversity of dietary carbohydrates necessitates discussion of several classes of these molecules, ranging from simple sugars to huge, branched polymers. Monosaccharides or simple sugars are either hexoses (6-carbon) like glucose, galactose and fructose, or pentoses (5-carbon) like ribose. These are the breakdown products of more complex carbohydrates and can be efficiently absorbed across the wall of the digestive tube and transported into blood. Disaccharides are simply two monosaccharides linked together by a glycosidic bond. The disaccharides most important in nutrition and digestion are: lactose or "milk sugar": glucose + galactose sucrose or "table sugar": glucose + fructose maltose: glucose + glucose Oligosaccharides are relatively short chains of monosaccharides which typically are intermediates in the breakdown of polysaccharides to monosaccharides. Polysaccharides are the most abundant dietary carbohydrate for all except very young animals. You should be familiar with three important polysaccharides, each of which is a large polymer of glucose: Starch is a major plant storage form of glucose. It occurs in two forms: alpha-amylose, in which the glucoses are linked together in straight chains, and amylopectin, in which the glucose chains are highly branched. Except for the branch points of amylopectin, the glucose monomers in starch are linked via alpha(1->4) glycosidic bonds, which, in the digestive tract of mammals, are hydrolyzed by amylases.Cellulose is the other major plant carbohydrate. It is the major constituent of plant cell walls, and more than half of the organic carbon on earth is found in cellulose. Cellulose is composed on unbranched, linear chains of D-glucose molecules, linked to one another by beta(1->4) glycosidic bonds, which no vertebrate has the capacity to enzymatically digest. Herbivores subsist largely on cellulose, not because they can digest it themselves, but because their digestive tracts teem with microbes that produce cellulases that hydrolyze cellulose.Glycogen is the third large polymer of glucose and is the major animal storage carbohydrate. Like starch, the glucose molecules in glycogen are linked together by alpha(1->4) glycosidic bonds. Small Intestinal MotilityCoordinated contractions of smooth muscle participate in several ways to facilitate digestion and absorption in the small intestine: foodstuffs are mixed with digestive enzymes from the pancreas and bile salts from the biliary system nutrient molecules in the lumen are constantly dispersed, allowing them to contact the epithelium where enzymatic digestion is completed and absorption occurs chyme is moved down the digestive tube, making way for the next load and also eliminating undigestable, perhaps toxic substances In most animals, the small intestine cycles through two states, each of which is associated with distinctive patterns of motility: Following a meal, when the lumen of the small intestine contains chyme, two types of motility predominate: segmentation contractions chop, mix and roll the chyme and peristalsis slowly propels it toward the large intestine.The interdigestive state is seen between meals, when the lumen is largely devoid of contents. During such times, so-called housekeeping contractions propagate from the stomach through the entire small intestine, sweeping it clear of debris. This complex pattern of motility is also known as the migrating motor complex and is the cause of "growling". Motility in the small intestine, as in all parts of the digestive tube, is controlled predominantly by excitatatory and inhibitory signmals from the enteric nervous system. These local nervous signals are however modulated by inputs from the central nervous system, and a number of gastrointestinal hormones appear to affect intestinal motility to some degree. Villi, Crypts and the Life Cycle of Small Intestinal EnterocytesIf examined closely, the lumenal surface of the small intestine appears similar to velvet due to its being covered by millions of small projections called villi which extend about 1 mm into the lumen. Villi are only the most obvious feature of the mucosa which houses a dynamic, self-renewing population of epithelial cells that includes secretory cells, endocrine cells and the mature absorptive epithelial cells which take up nutrients from the lumen and transport them into blood, fulfilling the basic function of the digestive system. Understanding how the small intestine functions requires looking at the structure of the mucosa in more detail. Epithelial Cell DynamicsThe mucosa of small intestinal mucosa is arranged into two fundamental structures: Villi are projections into the lumen covered predominantly with mature, absorptive enterocytes, along with occasional mucus-secreting goblet cells. These cells live only for a few days, die and are shed into the lumen to become part of the ingesta to be digested and absorbed. That's right, we're all really cannibals. Crypts (of Lieberkuhn) are moat-like invaginations of the epithelium around the villi, and are lined largely with younger epithelial cells which are involved primarily in secretion. Toward the base of the crypts are stem cells, which continually divide and provide the source of all the epithelial cells in the crypts and on the villi. The system described above is really quite elegant. Stem cells in the crypts divide to form daughter cells. One daughter cell from each stem cell division is retained as a stem cell. The other becomes committed to differentiate along one of four pathways to become an enterocyte, enteroendocrine cell, goblet cell or Paneth cell. Cells in the enterocyte lineage divide several more times as they migrate up the crypts, and as they migrate onto the villi, differentiate further into the mature absorptive cells that express all the transport proteins and enzymes characteristic of those cells. To put it another way, enterocytes are born at the bottom of the crypts, pass through childhood migrating up the walls of the crypts, then settle down briefly to enjoy an absorptive adulthood on the villi. Inside the VillusVirtally all nurtients, including all amino acids and sugars, enter the body across the epithelium covering small intestinal villi. As shown in the diagram above, each villus contains a capillary bed and a blunt-ended lymphatic vessel referred to as the "central lacteal". After crossing the epithelium, most of these molecules diffuse into a capillary network inside the villus, and hence into systemic blood. Some molecules, fats in particular, are transported not into capillaries, but rather into the lymphatic vessel, which drains from the intestine and rapidly flows into blood via the thoracic duct. Details of the transport of major nutrients into the capillaries or lymphatics in the villi are presented in subsequent sections. Gross and Microscopic Anatomy of the Small IntestineThe small intestine is the longest section of the digestive tube and consists of three segments forming a passage from the pylorus to the large intestine: Duodenum: a short section that receives secretions from the pancreas and liver via the pancreatic and common bile ducts.Jejunum: considered to be roughly 40% of the small gut in man, but closer to 90% in animals.Ileum empties into the large intestine; considered to be about 60% of the intestine in man, but veterinary anatomists usually refer to it as being only the short terminal section of the small intestine. In most animals, the length of the small intestine is roughly 3.5 times body length - your small intestine, or that of a large dog, is about 6 meters in length. Although precise boundaries between these three segments of bowel are not observed grossly or microscopically, there are histologic differences among duodenum, jejunum and ileum. A bulk of the small intestine is suspended from the body wall by an extension of the peritoneum called the mesentery. As seen in the image to the right, blood vessels to and from the intestine lie between the two sheets of the mesentery. Lymphatic vessels are also present, but are not easy to discern grossly in normal specimens. It is within the small intestine that the final stages of enzymatic digestion occur, liberating small molecules capable of being absorbed. The small intestine is also the sole site in the digestive tube for absorption of amino acids and monosaccharides. Most lipids are also absorbed in this organ. All of this absorption and much of the enzymatic digestion takes place on the surface of small intestinal epithelial cells, and to accomodate these processes, a huge mucosal surface area is required. If the small intestine is viewed as a simple pipe, its lumenal surface area would be on the order of one half of a square meter. But in reality, the absorptive surface area of the small intestine is roughly 250 square meters - the size of a tennis court! How is this possible? At first glance, the structure of the small intestine is similar to other regions of the digestive tube, but the small intestine incorporates three features which account for its huge absorptive surface area: Mucosal folds: the inner surface of the small intestine is not flat, but thrown into circular folds, which not only increase surface area, but aid in mixing the ingesta by acting as baffles.Villi: the mucosa forms multitudes of projections which protrude into the lumen and are covered with epithelial cells.Microvilli: the lumenal plasma membrane of absorptive epithelial cells is studded with densely-packed microvilli. The panels below depict the bulk of this surface area expansion, showing villi, epithelial cells that cover the villi and the microvilli of the epithelial cells. Note in the middle panel, a light micrograph, that the microvilli are visible and look something like a brush. For this reason, the microvillus border of intestinal epithelial cells is referred to as the "brush border".Secretion in the Small IntestineLarge quantities of water are secreted into the lumen of the small intestine during the digestive process. Almost all of this water is also reabsorbed in the small intestine. Regardless of whether it is being secreted or absorbed, water flows across the mucosa in response to osmotic gradients. In the case of secretion, two distinct processes establish an osmotic gradient that pulls water into the lumen of the intestine: Increases in luminal osmotic pressure resulting from influx and digestion of foodstuffs: The chyme that floods into the intestine from the stomach typically is not terribly hyperosmotic, but as its macromolecular components are digested, osmolarlity of that solution increases dramatically.Starch, for example, is a huge molecule that contributes only a small amount to osmotic pressure, but as it is digested, thousands of molecules of maltose are generated, each of which is as osmotically active as the original starch molecule.Thus, as digestion proceeds lumenal osmolarity increases dramatically and water is pulled into the lumen. Then, as the osmotically active molecules (maltose, glucose, amino acids) are absorbed, osmolarity of the intestinal contents decreases and water can be absorbed.Crypt cells actively secrete electrolytes, leading to water secretion: The apical or lumenal membrane of crypt epithelial cells contain a ion channel of immense medical significance - a cyclic AMP-dependent chloride channel known also as the cystic fibrosis transmembrane conductance regulator or CFTR. Mutations in the gene for this ion channel result in the disease cystic fibrosis. This channel is responsible for secretion of water by the following steps:Elevated intracellular concentrations of cAMP in crypt cells activate this channel, resulting in secretion of chloride ions into the lumen.Accumulation of negatively-charged chloride anions in the crypt creates an electric potential that attracts sodium, pulling it into the lumen across the tight junctions - the net result is secretion of NaCl.Secretion of NaCl into the crypt creates an osmotic gradient across the tight junction - water is drawn into the lumen.


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#2 from_india

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Posted 01 October 2005 - 08:58 PM

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#3 joolie

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Posted 02 October 2005 - 07:32 AM

thanks india, im sure everyone will agree your input on here is much appreciated. Posted Image Posted Image
joolie :)

#4 Guest_Gardentime1_*

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Posted 02 October 2005 - 02:13 PM

India,You seem quite knowledgeable about the small intestine. I have had 120 cm of the ileum removed due to adhesions and obstruction. Will the remaining ileum and small bowel compensate for the removal? I am asking because I am having problems gaining weight, I have some minor GI issues. Also dealing with burning pain in the lower left quadrant, which seems to be in the abdominal wall. Char

#5 SpAsMaN*

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Posted 02 October 2005 - 02:44 PM

Garden time,how have you been diagnose? Posted Image
---------------------------------
-->IBS INDUCED BY NSAIDS NAPROXEN
-->NSAIDS YOU SUCKS!!!
-Permanent discomfort/cecum&sigmoid stuck/trapped gas-
---->IBS-Type constipation normal transit time(diagnose with non-relaxing puborectalis december 2005)
--->Pubic nevralgia
***WORST PERMANENTLY SINCE RIFAXIMIN

#6 Guest_Gardentime1_*

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Posted 03 October 2005 - 04:39 AM

Hey Spasman, I was an emergency surgery twice in 2002, for small bowel obstruction. Later in 2002 I developed the lower left quadrant pain and was diagnosed by the pain symptoms and many diagnostic tests to rule out other internal problems. I have had 4 various ultra sounds, 4 CT's, and the barium x-rays. Most recent again, barium swallow, CT with contrast and another ultrasound. That annoying pain just does not go away and at times is very disabling.char

#7 Kathleen M.

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Posted 03 October 2005 - 05:29 AM

A lot of the time the remaining small intestine can compensate for what is missing, but other times it does not. A lot depends on how much and which part is missing.If weight problems are out of proportion with what you eat and your activity level it may be an indication that it isn't fully compensated for. Other conditions can cause weight issues like that so they may need to check you for things like thyroid issues and diabetes if you haven't had these things checked recently.Some people are missing enough they can't absorb all the bile they produce which can cause diarrhea (but that tends to be more a burning on exit rather than feel it while it is in the colon). Scar tissue from surgeries can be a source of pain. It could also be unrelated. It may take some investigation from the doctor to determine that.
My story of beating IBS: My Story with IBS
Ph.D in Biology

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Posted 03 October 2005 - 05:55 AM

Have had yearly blood work for thyroid and blood sugar - negative.You are right about the burning issue on exit, but that depends on what I am eating. 75 percent of the time I have no D problems or burning. I have been to many Doctors as the scar tissue is an ongoing problem. I do not want another surgery and that might be the final option for the adhesions. Lately I have had several trigger point injections, the first one was helpful and the second one only helped for 2 days. I am leary about the steroids in the injection.I have a high activity level, motivated to lose that annoying pain. I seem to eat enough, several sandwiches, snacks, large dinner, more snacks on a daily basis.Thanks for your input, Char

#9 SpAsMaN*

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Posted 03 October 2005 - 08:43 AM

quote:
Later in 2002 I developed the lower left quadrant pain and was diagnosed by the pain symptoms and many diagnostic tests to rule out other internal problems. I have had 4 various ultra sounds, 4 CT's, and the barium x-rays. Most recent again, barium swallow, CT with contrast and another ultrasound.
Which test has diagnose the adhesion and obsstruction?
---------------------------------
-->IBS INDUCED BY NSAIDS NAPROXEN
-->NSAIDS YOU SUCKS!!!
-Permanent discomfort/cecum&sigmoid stuck/trapped gas-
---->IBS-Type constipation normal transit time(diagnose with non-relaxing puborectalis december 2005)
--->Pubic nevralgia
***WORST PERMANENTLY SINCE RIFAXIMIN

#10 Guest_Gardentime1_*

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Posted 03 October 2005 - 11:49 AM

Hey Spasman,Wish there would have been a test to diagnose the obstruction before it happened; it woke me with violent pain and profound vomiting. I was an emergency surgery as x-ray and CT showed an obstruction, which after 6 hours of GI tube did not open up. My surgeon was concerned about the blood supply to the small bowel.The small bowel was obstructed because of adhesions; adhesions are not seen on x-ray or CT. Char

#11 no answers

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Posted 03 August 2006 - 07:52 PM

My husband has been hospilized 3 times in 3 months for bowel obstructions. Docs do not want to do surgery. Any suggestions?

#12 Guest_Gardentime1_*

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Posted 03 August 2006 - 09:41 PM

I actually do not have a answer for your hubby's bowel obstructions. Sometimes medical treatment works, it did not work for my obstructions which were caused by adhesions. Char





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