Digestive System Summary PDF

Preview

Summary

Susset Hernandez Alcover NSU Miami DIGESTIVE SYSTEM MONOMERS: Fats Fatty acids and monoglycerides Proteins amino acids Carbohydrates, starches monosaccharides DIGESTION Breakdown of food into smaller pieces. Allows absorption to occur. There are two types: Mechanical digestion (stomach) and Chemical...

Description

Susset Hernandez Alcover – NSU Miami

DIGESTIVE SYSTEM

!

MONOMERS: Fats = Fatty acids and monoglycerides Proteins = amino acids Carbohydrates, starches = monosaccharides

DIGESTION Breakdown of food into smaller pieces. Allows absorption to occur. There are two types: Mechanical digestion (stomach) and Chemical digestion (intestine).

DIGESTIVE SYSTEM FUNCTIONS Motility: movement of food by peristalsis. Sphincters need to relax. Ingestion by mastication or deglutition. Secretion of exocrine [water, HCl, bicarb, digestive enzymes] and endocrines enzymes [hormones that aid in regulation].

LAYERS OF THE GI TRACT 1. MUCOSA Absorption and secretion. Lines lumen. Composed of simple columnar epithelium, lamina propia, muscularis mucosa. It is what gets in contact with the food you’re eating. 2. SUBMUCOSA It is a vascular connective tissue layer. Contains glands and nerve plexuses. The guts have their own neuron system called Enteric Nervous System, that contains the submucosa plexus and the myenteric plexus. Submucosa plexus is the submucosa that regulates primarily secretion of hormones, enzymes, etcetera. 3. MUSCULARIS EXTERNA It where segmentation and peristalsis occur to forward movement. Contains the Myenteric plexus is found between the muscle layers, and regulates motility. Myenteric plexus is the major nerve supply to GI tract, SNS inhibits the myenteric plexus, slows it down; while the PNS stimulates it, speeds it up. 4. SEROSA Which of the following is true regarding the myenteric plexus? 1. It is found in the submucosa 2. It helps regulate motility 3. It is innervated by alpha motor neurons 4. None of the above

Susset Hernandez Alcover – NSU Miami

ESOPHAGUS Connects pharynx to stomach. Peristalsis is the stomach motility that moves the bolus into the cardiac portion of the stomach [upper portion]. Circular muscle contracts behind, relaxes in front. Bigger bolus = bigger contraction.

GASOESOPHAGEAL SPHINCTER Terminal portion of the esophagus. It is where the stomach and the esophagus get in contact. Prevents backflow from the stomach. Closed until food pushed through from above. Not a true sphincter, doesn’t always work properly, doesn't’t always close to prevent backflow and people end up with esophageal ulceration. At times permits reflux, meaning that the stomach content finds its way up into the esophagus, and that is heartburn. Not fully functional in babies, that’s why they spit up after meals.

STOMACH Continuous with the esophagus. Empties into duodenum. Food storage. Begins proteins digestion process. Pepsin is the enzyme in the stomach that begins the protein digestion process. It has an extra layer to provide it with the strength to breakdown the food. Bactericidal functions. HCl [stomach acid] provides protection against ingested pathogens. Chyme production that aids in movement into the small intestine. [main] Which of the following is not a major function of the stomach? 1. Mechanical breakdown of food 2. Production of chyme 3. Absorption of nutrients 4. Food storage

STOMACH MOTILITY Receptive Relaxation Reflex [RRR] refers to, for example, when you sallow food, before it gets to the stomach, causes the stomach to extend and to relax so that it’s ready to receive the food, to accommodate the volume of food. Peristalsis of the stomach begins shortly after food gets to the stomach. It is regulated by the pacemaker cells in the muscularis externa. Becomes stronger with time. The goal is to produce the chyme that is going to move into the small intestines. Pyloric sphincter is what connects the stomach to the duodenum. Antrum, which is the widened area that ends at the pyloric sphincter, holds 30mL of chyme. As the wave of peristalsis come down, it ends up churning about 3 ml of the chyme into the small intestine, whatever doesn’t make it gets turned back it and gets churn again. The stomach is full of HCl, that chyme is very acid rich, the duodenum is protective by acidity by bicarbonate layer, but that bicarbonate layer is not very thick and it is also easily overwhelmed, so if the stomach were to send a bunch of chyme at a once, it’s going to overwhelm the buffering capacity very quickly and it will result in ulceration, so by sending just a little bit forward at a given time, we allow the enzymes to work on those substances more efficiently, and time for digestion and absorption to occur, before sending everything forward.

Susset Hernandez Alcover – NSU Miami If duodenum becomes overfilled, it will inhibit gastric motility. What effect would be expected if the stomach dumped all of its contents into the small intestines at once? 1. Increased absorption 2. Duodenal ulceration 3. Better mechanical breakdown 4. None of the above If the stomach dumped its contents all at once into the duodenum, how would this affect the activity of the enzymes within the small intestine? 1. Activity would decrease 2. Activity would increase 3. Activity would not change

DIGESTION AND ABSORPTION Stomach does not do much nutrient absorption, primarily just breakdown. Chemical digestion begins in the mouth under the action of salivary amylase in the saliva. It breaks down mostly carbohydrates, starches. Lingual lipase that breakdown fats, is also found in the mouth but it doesn't’t get activated until it gets to the stomach because it’s only activated during acidic conditions, so once this occurs, digestion of fats occurs in the stomach.

ENZYMES AND THEIR OPTIMAL pH. Enzymes have optimal pH, and when they are out of that optimal pH they won’t work. Enzymes active in the stomach have acidic pH, so when those enzymes that work in that acidic environment gets to the small intestines that is slightly alkaline, they are no longer active because they're too far away from their optimal pH to really be functional. But in the case of saliva, they work around a neutral environment, so when salivary amylase gets to the stomach, it’s no longer going to be functional, because the pH of the stomach is very low so it inactivates it. When pepsin moves to the SI is also inactivated. But once you get to the small intestine, there's a ton of enzyme that comes from the intestines as well and form the pancreatic secretion and they work in slightly alkaline pH because of the intestinal environment which its slightly alkaline to neutral, and will pick up the process once it gets there. If anything happens to make the duodenum too acidic, all of those enzymes are not going to work and it’s there in the small intestines under the action of most of those enzymes that the bulb of the digestion occurs so that absorption can occur later. And that is another reason for not sending a lot of stomach content forward to the small intestine at a given time, because you don’t only damage the mucosa of the intestinal tract but you create this very acidic environment, that the buffers can’t deal with and the enzymes that normally works there can’t do their functions.

RUGAE. CELL TYPES. Goblet cells: Produce mucous Parietal cells: Produce HCl and intrinsic factor Chief cells: Produce Pepsinogen [inactive form of pepsin] Enterochromaffin cells: Produce Histamine [important for proper production of stomach acid] and serotonin G cells: Produce Gastrin D cells: Produce Somatostatin Stem cells: Replaces epithelium

Susset Hernandez Alcover – NSU Miami

INTRINSIC FACTOR Intrinsic factor is required for the absorption of B12. B12 is not absorbed in the stomach but you have to have intrinsic factor in the stomach in order for B12 to be absorbed later on. The reason this becomes important is because B12 is necessary for proper red blood cell production. If people don’t have enough B12 in their diet or can’t produce enough intrinsic factor will end up with a type of anemia that is called Pernicious anemia. In most case, because B12 is prevalent in a lot of food, so usually the problem is not their nutrition is because of the fact that they are not producing enough intrinsic factor. Treatment is B12 injections, you can’t really orally supplement them unless you're doing it in really high doses because they're not really good at absorbing B12.

GASTRIC JUICE Composition: HCl [highly acidic] Functions: Exocrine secretions and water. H+ ions goes to the lumen of the stomach transport by H/K Proton Pump that interacts with Clproducing HCl.

ACID SECRETION Acid secretion stimulated by many factors such as Gastrin, histamine, neurotransmitters, exercise, caffeine, alcohol etc. Gastrin when stimulates acid secretion from parietal cells = weak production of HCl. When Gastrin stimulates Enterochromaffin cells, which causes strong production of histamine, which binds H2 Rs and acts on parietal cells that ultimately stimulates HCl production strongly, so it indirectly turns on acid production. Zantac, Tagamet, are H2 blockers, which slow down the actions of histamine and in consequence acid production. PNS release acetylcholine which stimulates Enterochromaffin and parietal cells that results in acid production. Which of the following would reduce acid secretion? 1. Parasympathetic stimulation 2. Gastrin secretion 3. Enterochromaffin cell stimulation 4. Blockade of proton pump 5. Elevated histamine

FUNCTION OF STOMACH ACID Denatures proteins [So, for example, when you eat meat, it will break down the protein, break down the connective tissue due to the fact that stomach acid will activate pepsinogen to pepsin] makes easier to digest. Provides optimal pH for pepsin [to act because pepsin acts in a very acidity pH, and protein digestion begins] Kills microbes [it provides an immune defense against ingested pathogens] *People that are in Proton Pump Inhibitors for long periods of time, one of the things that start happening is that start getting GI infections, so this role of stomach acid is very important.

Susset Hernandez Alcover – NSU Miami

PROTECTION OF STOMACH MUCOSA FROM ITS OWN ACID It has a thick, stable mucous layer that is adherent to the epithelium, produces epithelial cells that protects the stomach from its own stomach acid [HCl] and pepsin [pepsin breakdowns protein and the mucosa has a lot of protein so it’s good that it is protected against pepsin] Epithelial cells are replaced very quickly in fact the entire epithelium every 3 days, but this is not enough. Contains bicarbonate (alkaline) secreted from epithelial cells what keeps pH near surface of cells near neutral. When people get stomach ulcers is more because of the malfunction of the mucosa because it’s not produced properly or its degraded rather than because of hyper acidic conditions As long as the mucous coat is intact, the stomach is well protected from the acidic environment. Tight junctions between cells keep acid and pepsin within lumen.

ASA and NASIDS Ex: Ibuprofen, Naproxen or Aleve, aspirin, acetaminophen or Tylenol. Inhibit prostaglandins synthesis. Mucous production is stimulated by prostaglandins. Implications: the mucous coat doesn’t get produced or it gets degradative. So the stomach will not be protected from its acid and pepsin, producing ulcers. Inability of blood clot. Nonsteroidal anti-inflammatory drugs result in stomach ulceration by which of the following? 1. Stimulation of histamine production 2. Stimulation of the PNS 3. Stimulation of gastrin production 4. Inhibition of prostaglandin synthesis

DIGESTION AND ABSORPTION IN STOMACH Most of the chemical digestion occurs in Small Intestine. The digestion that occurs in the stomach is minimal. Proteins partially digested by the action of pepsin. Starch digestion stops because salivary amylase is no longer active in this environment. Alcohol and aspirin absorbed across stomach, due to lipid solubility.

ULCERS Peptic Ulcers: Erosions of stomach or duodenal mucous membranes. Stomach or gastric ulcers are caused by erosion of the mucous coat, which is what H pylori also does. Duodenum ulcers are caused by hyper acidic conditions, either the stomach is producing too much acid or sending too much acid back to the duodenum. Contributors: Smoking, Alcohol, NSAIDS, glucocorticoids [steroid for anti-inflammatory], stress, caffeine. Treatment: Proton Pump Inhibitors [inhibits stomach acid production], H2 blockers, antacids, antibiotics (H pylori)

Susset Hernandez Alcover – NSU Miami

GASTRITIS Inflammation of stomach mucosa caused by breakdown of protective barriers that results in histamine release which stimulates acid production. Treatment: H2 blockers such as Zantac and Tagamet, H/K Proton Pump Inhibitors.

SMALL INTESTINES Absorption and chemical digestion. Most absorption occurs in the duodenum and jejunum. It has its own enzymes and also receives other enzymes by pancreatic secretion, and the liver puts bile salts which emulsify fats. Emulsification is not a chemical breakdown, it is essentially taking a big fat droplets and putting them into smaller fat droplets so that the enzymes will have more surface area on which to act It greatly facilitates the chemical digestion process because the enzymes work more efficiently than they would do otherwise. The pancreatic secretion and the bile secretion are contained in a bicarb rich fluid which helps to neutralize some of the acid coming from the stomach, the duodenum has also enzymes that produces bicarb rich fluid and that helps in the neutralization process as well. Duodenum is highly folded to increase surface area but as you go on the other further regions, that folding decreases dramatically because they don’t need that increased surface area because most of the absorption has already happened. Ileum is the last part and empties its content into the large intestines via ileocecal valve (sphincter). B12, water, bile salts electrolytes are absorbed in the Ileum. Unlike other regions Villus [in the Villi of the SI] contains Goblet cells, that produce mucous and they contain lymphocytes, blood capillaries which takes up carbohydrates and amino acids, and lymphatic vessels, which contains the Central lacteal that is responsible for fat absorption.

INTESTINAL ENZYMES Microvilli plasma membrane contains enzymes called Brush Border Enzymes that are embedded in the Brush Border and not free into the lumen. In order for something to be able to be active in the brush border enzymes they have to come on contact w the brush border, so the segmentation that occurs has the responsibility to bring the content of the intestine in contact with the brush border so the enzymes can work and that enhances the action of those enzymes. A lot of the pancreatic enzymes come in as inactive precursors and you need something to activate them, and that something is Enterokinase, which activates trypsinogen to trypsin and then trypsin goes on and activates the rest of the pancreatic enzymes. Enterokinase is the first step on activating other enzymes. Lactase, present in kids under 4, becomes inactive in most adults. As we decrease the amount of milk products in our diets as we get older, we lose activity of that enzyme And the end result could be in most cases Lactose intolerance which is lactase deficiency. It doesn’t break down lactose enzymes, so it can’t be absorbed, results in an increased osmotic pressure, pulling water into the lumen of the intestine and that produces diarrhea.

Susset Hernandez Alcover – NSU Miami

INTESTINAL MOTILITY Once all of those content has been brought in contact w the brush borders and the pancreatic enzymes has had a chance to act, and the nutrients has been digested and then they get absorbed across the intestinal mucosa, then what you’re left with is material that needs to be moved forward because all the digestion that can occur in that region of the intestine has occurred, so once that happens PERISTALSIS starts [slow waves]. PERISTALSIS helps to move products forward and occur along the GI system. It starts from the esophagus to the intestine. In the small intestines, peristaltic waves begin in the duodenum. SEGMENTATION is a shorting time of motility that the small intestine does. Muscular constriction of lumen, occurs simultaneously in intestinal segments. Moves chyme forward. One of the reason for this is that this action brings the content in contact with the brush border of the intestine where the intestinal enzymes are located. Pacemaker cells in the walls of the intestines set rhythm for segmentation. Become less frequent distally and that’s because once you get to more distal regions most of the absorption has already occurs so you don’t need more of this to occur any longer. When all the segmentation and peristalsis are going on, there are some indigestive materials left, and can’t stay there because it can promote bacterial growth and end up in a bad GI infection and MMCs [Migratory Motor Complexes] is meant to get rid of everything that didn't’t move forward, is meant to clear the small intestine. Occur during fasting. Stops when we eat. Which of the following is not true regarding slow waves? 1. Exhibit automaticity 2. Every slow wave initiates a muscle contraction 3. Mediated via gap junctions

LARGE INTESTINES Ileocecal valve connects the small with the large intestines. From the ileocecal valve to anus [Ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal]. Primary function is absorption of water [but keep in mind that the small intestines does most of the water absorption], it also absorbs electrolytes and vitamins [B and K]. Chyme coming from the ileum enter the cecum, but when it gets here most of the nutrient have already been absorbed. The mucosa doesn’t have villi, because it doesn’t do as nearly as much absorption. Contains Haustras: when the haustras are distended is diverticulosis; and inflammation of that extension is diverticulitis.

INTESTINAL MICROBIOTA Produce vitamin K [deficiency: coagulation disorders] and folic acid. Ferment indigestible chyme molecules. Outcompete pathogens. Exert anti-inflammatory actions in gut. Protect mucosa. Antibiotic effects kill normal flora.

Susset Hernandez Alcover – NSU Miami

DIARRHEA When things move up too quickly there's not enough time for the body to reabsorb the water and people end up losing a lot of water in the feces. People can get dehydrated very quickly. *The absorption of water is passive (osmotic gradient due to active reabsorption of ion), as the nutrients get absorbs, the water follows. It can be caused by Enterotoxin, produced by cholera [less common]. Since its osmotic absorption, if you keep more salt into the lumen of the intestine, water remains as well [water follows the solute], water can’t be pull back into the body, it stays in the intestinal tract. It can also be caused by Lactose Intolerance, Lactose can’t be broken down, stays in the lumen, since exerts osmotic pressure, pulls water in, water can’t be pull back into the body, stays in the intestinal tract. Which of the following would be expected to result in diarrhea? 1. Increased sodium chloride in the intestine 2. Cholinergic antagonist 3. Decreased GI motility 4. None of the above

ACCESSORY GLANDS [LIVER, GALLBLADDER, PANCREAS] LIVER Largest intestinal organ. Gallbladder is found underside the liver. Liver produces the bile and the gallbladder stores it. Functions of the liver: bile production and secretion. It has a large capacity to regenerate itself because its mitotic division of hepatocytes. Division ceases once mass is restored. Kupfer cells, macrophages that resides in the liver.

LIVER FIBROSIS It can be caused by alcohol and viral hepatitis. It is an accumulation of collagen that can lead to cirrhosis. Cells lose capacity to repair, hepatocytes die and they lose their function.

Susset Hernandez Alcover – NSU Miami

HEPATIC PORTAL SYSTEM Everything that is absorbed by the GI tract before going to the blood stream pass by the liver and this one is going to inactivate a fraction of it.

FIRST PASS EFFECT Usually with respect to drugs. Drugs taken orally go to liver first. So...