chapter 16 nots part 2: small intestine and etc.

Digested carbohydrates and proteins are both absorbed by

by secondary active transport and enter the blood. Absorption of digestion end products of both carbohydrates and proteins is accomplished by Na+ dependent symport and both categories of end products are absorbed into the blood.

Hepatocytes →

each liver cell. Performs metabolic and secretory tasks. Only thing not accomplished is phonetic activity

→ Bile salts are recycled through the

enterohepatic circulation:

Microvilli

(singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, cellular adhesion, and mechanotransduction. 1 Structure 1.1 Locations

→ biochemical balance among the stomach, pancreas, and the small intestine is normally maintained.

Production of gastric and pancreatic digestive secretions typically does not alter acid-base status of the body because the amount of H+ secreted by the gastric parietal cells is usually matched by the HCO3 secreted by the pancreatic duct cells. Body does not experience a net gain or loss of acid or base during digestion.

Large intestine:

The large intestine consists of the colon, ceecum, appendix, and rectum The cceum forms a blind-ended pouch below the junction of the small and large intestines at the ileocecal valve. The small, finterlike projections at the bottom of the ceecum is an apendix., finally mohid tissue that houses lymphocytes. The colon, which makes up most of the large intestine, is not coiled like the small intestine but consists of 3 relatively straight parts → the ascending colon, the transverse colon, and the descending colon. The end part of the descending colon becomes S shaped, forming the sigmoid colon, and then straightens out to form the rectum.

Lipid soluble

The maximum concentration of a chemical that will dissolve in fatty substances. Lipid soluble substances are insoluble in water. They will very selectively disperse through the environment via uptake in living tissue.

lecithin

a substance widely distributed in animal tissues, egg yolk, and some higher plants, consisting of phospholipids linked to choline.

Liver organization:

he liver is organized into functional units known as lobules which are hexagonal columns of tissue surrounding a central vein and delineated by vascular and bile channels. At each of the six outer corners of a lobule are 3 vessels: a branch of the hepatic artery, a branch of the hepatic portal vein, and bile duct. Blood flows from the branches of both the hepatic artery and the hepatic portal vein flows from the periphery of the lobule into large, expandable capillary spaces called sinuses. Which run between rows of live cells to the central vein like spokes on a bicycle wheel. The central veins of the liver lobules converge to form the hepatic vein which carries blood away from the liver. The thin, bile carrying channels, a bile canaliculus, runs between the cells within each hepatic plate. Hepatocytes continuously secrete bile into these thin channels which carry the bile to a bile duct at the periphery lobule. The bile ducts from the various lobes convert to eventually form the common bile duct, which transports the bile from the liver to the duodenum. Each hepatocyte is on contact with a sinusoid on one side and a bile canaliculus on the other side.

Mass movement propel feces long distances.

3 to 4 times a day, generally after a meal, a marked increase in motility takes place during which large segments of the ascienndg and transverse colon contract simultaneously, driving the feces on third to three third fourths of the length within a few seconds. The massive contractions appropriately called MASS MOVEMENTS, drive the colonic contents into the distal part of the large intestine where material is stored until dedication occurs. When food enters the stomach, mass movements are triggered in the colon primarily by the gastrocolic reflex, which is mediated from the stomach to the colon by parasympathetic innervation. In most people, this reflex is most evident after the first meal of the day and is often followed by the urge to defecate. When a new meal enters the digestive tract, reflexes are initiated to move the existing contents further along the tract to make way for the incoming food. The gastroileal reflex moves the remaining small-intestine contents into the large intestine, and the gastrocolic reflex pushes the colonic contents into the rectrum, triggering the defecation reflex.

→ The large intestine is primarily a drying and storage organ.

The most digestion already done in small intestine Indigestible food residues such as celluloses and nonabsorbable capillary components and the remaining fluid come here Extracts more water and salt, drying and compacting the contents to form a firm mass known as feces for eliming from the body. Primary function is to store feces before defecation. Cellulose and other indigestible substances in the diet provide bulk and help maintain regular bowel movements by contributing to the volume of colonic contents.

Liver blood flow:

Anatomical organization of the liver permits each hepatocyte to be in direct contact with blood from two sourcesL arterial blood coming from the heart and venous blood coming from the digestive tract. The hepatocytes receive fresh arterial blood via the arteries to supply their oxygen and delivers blood-norne metabolites for hepatic processing. Venous blood enters the liver by the hepatic portal system, a unique and complex vascular connection between the digestive tract and the liver. The veins draining the digestive tract do not directly join the inferior vena cava, a large vein that returns blood to heat. Atrial blood, which provides the lives O2 supply and carries blood-borne metabolites for hepatic processing, is delivered by the hepatic artery. Venous blood draining the digestive tract is carried by the hepatic portal vein to the liver for processing and storage of newly absorbed nutrients. Blood leaves the liver via the hepatic vein. The veins from the stomach and intestine enter the hepatic portal vein which carries the products absorbed from the digestive tract directly to the liver for processing storage, or detoxification before they gain access to the general circulation. Within the liver, the portal vein again breaks up into a capillary network (the liver sinusoids) to permit exchange between the blood and the hepatocytes before draining into the hepatic vein, which joins the inferior vena cava.

Protein absorption:

Both ingested proteins and endogenous (within the body) proteins that have entered the digestive tract lumen from the following sources are digested and absorbed : Digestive enzymes, all of which are proteins have been secreted into the lumen. Proteins within the cells that are pushed off from the villi into the lumen during the process of mucosal turnover. Small amounts of plasma proteins that normally leak from the capillaries into the digestive tract lumen About 20 to 40 of endogenous proteins enter the lumen each day from these three sources. This quantity can amount to more than the quantity of proteins in ingested food. All endogenous proteins must be digested and absorbed, along with the dietary proteins, to prevent depletion of the body's protein stores. The amino acids absorbed from both food and endogenous proteins are used primarily to synthesize new proteins in the body. The proteins presented to the small intestine for absorption are in the form of amino acids and a few small peptide fragments. Amino acids are absorbed into the intestinal cells by symporters,similar to glucose and galactose absorption. The sugar symporter are distinct from the amino-acid symporters, and the amino-acid symporter are selective for different amino acids. Small peptides gain entry by means of yet another Na+ dependent carrier in a process known as tertiary active transport in reference to a third linked step ultimately being driven by energy used in the first step. In this case, the symporter simultaneously transports both H+ and the peptide from the lumen into the cell, driven by H+ moving down its concentration gradient and the peptide moving against its concentration gradient. The H+ is established by an antiporter in the luminal membrane that is driven by Na= moving into the cell down its concentration gradient. The Na+ concentration gradient that drives the antiporter in turn it established by the energy-dependent Na -K+ pump at the basolateral membrane. Thus, Glucose, galactose, amino acids, expended for Na+ transport. The small peptides are broken down into their constituent amino acids by the aminopeptidase. Like monosaccharides, amino acids leave the intestinal cells by facilitated diffusion and enter the capillary network within the villus.

→ The mucosal lining experiences rapid turnover.

Deeping down into the mucosal surface between the villi, are shallowing invaginations known as the crypts of lieberkuhn. Unlike gastric pits, these intestinal crypts do not secrete digestive enzymes, but they do secrete water and salt, which along with the mucus secreted by the cells on the villus surface, constitute the succus entericus. Furthermore, the crypts function as nurseries. These epithelial cells lining the small intestine slough off and are replaced at a rapid rate as a result of high mitotic activity of the stem cells in the crypts. New cells that are continually being produced in the crypts migrate up the villi and in the process, push off the older cells at the tips of the villi and in the process, push off the older cells at the tips of the villi into the lumen. The entire trip from crypt to tip averages 3 days . So the mucosal epithelium is replaced every 3 days. The crypt stem cells are very sensitive to damage by radiation and anticancer drugs both of which may inhibit cell division. The new cells undergo several changes as they migrate up the villus, The concentrations brush-border enzymes increases the capacity for absorption improves, so the cells at the tip of the villus have the greatest digestive and absorptive capacity. Just at their peak, these cells are pushed off by the newly migrating cells. Thus, the luminal contents are constantly exposed to cells that are optimally equipped to complete the digestive and absorptive functions efficiently. Furthermore, just as in the stomach, the rapid turnover of cells in the small intestine is essential because of the harsh luminal conditions. Cells exposed to the abrasive and corrosive luminal contents are easily damaged and cannot live for long so they must be continuously replaced by a fresh supply of newborn cells. The old cells sloughed off into the lumen are not entirely lost to the body, these cells are digested with the cell constitutes being absorbed into the blood reclaimed for synthesis of new cells, among other things. In addition to stem cells, defensive PANETH cells, are found in the crypts. Paneth cells produce two chemicals that thwart bacteria. (1), lysosomes, the bacteria-lysing enzyme also found in the saliva. (2) defensins, small proteins with antimicrobial powers.

Carbohydrate absorption:

Dietary carbohydrates are presented to the small intestine for absorption mainly in forms of the disaccharides maltose, sucrose, and lactose (and to a lesser extent in the form of the short polysaccharide a-limit dextrin). The disaccharides located in the brush border membranes of the small intestine cells further reduce these disaccharides and polysaccharides into the absorbable monosaccharide units of glucose (mostly) galactose, and fructose. Glucose and galactose are both absorbed by secondary active transport, inn which symport carriers, such as the sodium and glucose cotransporter on the luminal membrane transport both the monosaccharide and Na+ from the lumen into the interior of the intestinal cell. The operation of these symporters which do not directly use themselves, depend on the Na+ concentration gradient established by the energy-consuming basolateral Na+-K+ pump. Glucose ( or galactose), having been concentrated in the cell by these symporters, leaves the cell down its concentration gradient by facilitated diffusion ( passive carrier-mediated transport) via the glucose transporter GLUT-2 in the basal border to enter the blood within the villus. In addition to glucose being absorbed through the cells by means of the symporter, recent evidence suggests that a significant amount of glucose cross the epithelial barrier though the leaky tight junctions between the epithelial cells. Fructose enters the epithelial cells from the lumen via GLUT-5 using facilitated diffusion. This process involves the higher concentration of luminal fructose driving the monosaccharide into the cell. Like the other monosaccharides, fructose exits via GLUT-2 and enters the blood.

intensive absorption by the small intestine keeps pace with secretion:

Digestive juices from the plasma is huge. It is the source of digestive secretion because the secretory cells extract from the psalms the necessary raw materials for the secretory product, Absorption must closely parallel secretion to keep the plasma volume from falling sharply. Only secretary products that escapes the body is bilirubin, a waste product that must be eliminated.

Haustra contraction slowly shuffle the colonic contents back and forth.

Haustra actively change location as a result of contraction of circular smooth muscle layer. Tendine colli→ shorter than the underlying circular smooth muscle and mucosal layers would be if these layers were stretched out flat. Movements of large intestine are slow and mnonproplsive. As is appropriate for its absorptive and storage functions. Main motility → haustral contractions initiated by the BER colonic smooth muscle cells. Facilitating ring like contraction similar to small intestine segmentation but occur less frequently. Location of the haustra sacs gradually changes as a relaxed segment that has formed a ssac slowly contracts while a previously contracted area simultaneously relaxes to form a new sac. Expose the colonic contents to the absorptive mucosa. Hausta contractions are largely controlled by locally mediated reflexes involved the intrinsic plexus.

Digested fat is absorbed passively and ters the lymph.

Fat absorption is different from carbohydrate and protein absorption because the insolubility of fat in water presents special problems. Fat must be transferred from the watery chyme through the watery body fluids, even though fat is not water soluble. Therefore, fat must undergo a series of physical and chemical transformations to circumvent this problem during its digestion and absorption. When the stomach contents are emptied into the duodenum, the ingested fat is aggregated into large, oily triglyceride droplets that float in the chyme. Recall that through the bile salt' detergent action in the small-intestine lumen, the large droplets are dispersed into a lipid emulsification of small droplets, exposing a greater surface area of fat for digestion by pancreatic lipase. The products of lipase digestion are also not very water soluble so little of these end products of fat digestion can diffuse through the aqueous chyme to reach the absorptive lining. However, biliary components facilitate absorption of these fatty end products forming micelles.

Liver function:

The liver is the largest and most important metabolic organ in the body; it can be viewed as the body's major biochemical factory. It's functions include the following. Secretion of bile salts, which aid fat digestion and absorption. This is the only liver function directly related to digestion. Metabolic processing of the major categories of nutrients ( carbs, proteins, lipids) after their absorption from the digestive tract. Detoxifying or degrading body wasted and hormones, as well as drugs and other forgein compounds. Synthesizing plasma proteins, including those needed for blood clotting, those that transport steroid and thyroid hormones and cholesterol in the blood and angiotensin important in salt conserving renin-angiotensin system. Storing glycogen, fats, iron, copper, and many vitamins

→ energy-dependent NA+ absorption drives passive h20 absorption.

Na+ may be absorbed both passively and actively. When the electrochemical gradient favors movement of Na+ from the lumen to the blood, passive diffusion of Na+ can occur by paracellular transport between the intestinal epithelial cells through the leaky tight junctions into the interstitial fluid within the villus. Movement of Na+ through the cells is energy dependent and involves different carriers or channels at the luminal and basolateral membranes, similar to the process of Na+ reabsorption across the kidney tubules. Na enters the epithelial cells across the luminal border either by itself passively through NA channels or in the company of another ion or a nutrient molecule by secondary active transport via three different carries : Na+-Cl- symporter, Na+-H+ antiporter, or Na+- glucose (or amino acid) symporter. Na+ is actively pumped out of the cell by the Na+ -K+ pump at the basolateral membrane into the interstitial fluid in the lateral spaces between the cells where they are not joined by tight junctions. From the interstitial fluid, Na+ diffuses into the capillaries. As with the renal tubules in the early part of the nephron, the absorption of Cl-,H20, glucose and amino acids from the small intestine is linked to this energy-dependent Na+ absorption. Cl- passibley follows down the electrical gradient created by Na+ absorption and also can be absorbed by secondary active transport if needed. Most H2O absorption in the digestive tract depends on the active carrier that pumps Na+ into the lateral spaces, resulting in a concentrated area of high osmotic pressure in that located region between the cells, similar to the situation in the kidneys, This localized high osmotic pressure induces H2O to move from the lumen through the cell into the lateral spaces. Water entering the space reduces the osmotic pressure but raises the hydrostatic(fluid) pressure. The elevated hydrostatic pressure flashes H2O out of the lateral space into the interior of the villus, where it is picked up by the capillary network. Meanwhile, more Na+ is pumped into the lateral spaces to encourage more H20 absorption.

Segmentation contractions mix and slowly propel the chym.

Segmentation, the small intestine's primary motility during digestion of a meal, both mixes and slowly propels the chyme. Segmentation consists of oscillating, ring like contraction of the circular smooth muscle along the small intestin's length; between the contracted segments are relaxed

Emulsification (in digestion)

The breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the fats into fatty acids and glycerol .

Small intestine:

The small intestine is the site where the most digestion and absorption takes place. The small intestine lies coiled within the abdominal cavity, extending between the stomach and the large intestine. It is arbitrary divided into 3 segments ( dudenu,, jejunum, and ileum). Small intestine motility includes ( segmentation and the migrating motility complex)

feces are eliminated by the defecation reflex

When mass movements move feces into the rectum, the resultant distension of the rectum stimulates stretch receptors in the rectal wall, initiating the defecation reflex. This caues the interanl anal sphicter to relax and the rectum and sigmoid colon to contract more vigorusly. If the external and anal sphomcters os a;ps re;axed, defectation cofurs. External is under voluntary control. The initial rectal distention is accompanied by the console urge to defecate. Constipation: occurs when the feces becomes too dry. Appendicitis → large material in large intestine not defecated '

-Diarrhea results in loss of fluid and electrolytes.

When secretions and absorption does not parallel however acid-base abnormalities can result because these normal naturalization process cannot take place. Diarrhea → secreted material is also lost. And other fluids→ causes dehydration, loss of nutrient materials and metabolic acidosis resulting from loss of HCO#.

---> The liver performs various important functions,

including bile production. Besides pancreatic juice, the other secretory product emptied into the duodenal lumen is Bile. The Biliary system includes the liver, gallbladder, and associated ducts.

Most absorbed nutrients immediately pass through the ___ for processing.

liver The venules that leave the small-intestine villi, along with those from the rest of the digestive tract, empty into the hepatic portal vein, which carries the blood to the liver. Consequently, anything absorbed into the digestive capillaries first must pass through the hepatic biochemical factory before entering the general circulation. Thus, the products of carbohydrates and protein digestion are channelled into the liver, where many of these energy-rich products are subjected to immediate metabolic processing. Furthermore, harmful substances that may have been absorbed are detoxified by the liver before gaining access to general circulation After passing through the portal circulation, the venous blood from the digestive system empties into the vena cava and returns to the heart to be distributed through the body carrying glucose and amino acids for use by the tissues. Fat, which cannot penetrate the intestinal capillaries is picked by the central lacteal and lymphatic system instead, bypassing the hepatic portal system. Contractions of the villi, accomplished by the muscularis mucosa, periodically compress the central lacteal and milk the lumps out of this vessel. The small lymph vessels converge and eventually form the thoracic duct, a large lymph vessels that empties into the venous system within the chest. Absorbed fat is carried by the systemic circulation to the liver and other tissues of the body, therefore the liver does not have a chance to act on the digested fat, but not until the fat has been diluted by the blood in the general circulatory system. This dilution of fat protects the liver from being inundated with more fat than it can handle at one time.

Fat absorption:

remember that micheles are water-soluble particles that can carry the end products of fat digestion within their lipid-soluble interiors. - once these michelels reach the luminal membrane of epithelial cells, monoglycerides and free fatty acids passively diffuse from the micheles through the lipid component of the epithelial cell membranes to enter the interior of these cells. - bile salts continuously repeat their fat-solubilizing function down the length of the small intestine until all fat is absorbed. Then the bile salts themselves are reabsorbed in the terminal ileum by special active transport. This is an efficient process because relatively small amounts of bile salts can facilitate digestion and absorption of large amounts of fat with each bile salt performing its ferrying function repeatedly before it is reabsorbed. - once within the interior of the epithelial cells, monoglycerides and free fatty acids are resynthesized into triglycerides. - these triglycerides conglomerate into droplets and are coated with a layer of lipoprotein ( synthesized by the endoplasmic reticulum of the epithelial cells)., which makes the fat droplets water soluble. The large, coated fat droplets, known as CHYLOMICRONS, are extruded by exocytosis from epithelial cells into the interstitial fluid within the villus. Chylomicrons are bigger than michaels. They enter the central lacteals rather than the capillaries because of the structural differences between these two vessels. Capillaries have a basement membrane that prevents the chylomicrons from entering but the lymph vessels do not. Thus, fat can be absorbed into the lymphatics but not directly into the blood. - the actual absorption of monoglycerides and free fatty acids from the chyme across the luminal membrane of the small-intestine epithelial cells is traditionally considered a passive process because the lipid-soluble fatty end products merely dissolve in and pass through the lipid part of the membrane. However, the overall sequence of events needed for fat absorption require energy. - for example, bile salts are actively secreted by the liver the resynthesis of triglycerides and formation of chylomicrons within the epithelial cells are

Bile is continuously

secreted by the liver and is diverted to the gallbladder between meals. The liver continuously secretes bile, even between meals. The opening of the bile duct into the duodenum is guarded by the sphincter of Oddi, which prevents bile from entering the duodenum except during digestion of meals. When this sphincter is closed, bile secreted by the liver hits the closed sphincter and is diverted back up into the gallbladder, a small, saclike structure tucked beneath but not directly connected to the liver. The bile is not transported directly from the liver to the gallbladder. Bile is stored and concentrated in the gallbladder between meals Active transport (salt) out of the gallbladder with water following osmotically, results in a 5-10 times greater concentration of the organic constitutes. After a mean, bile enters the duodenum as a result of the combined effects of relaxation of the sphincter of Oddi, gallbladder contraction and increased bile secretion by the liver. Because the gallbladder stores concentrated bile, it is the primary site for precipitation of concentrated bile constitutes into gallstones. We can live without gallbladder, bile will instead be stored in the common bile duct which becomes dilated.