Digestive System - notes PDF

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

o 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. o 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. o 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.

o 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. o 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. o 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. 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.

Digestion and Absorption Fat-soluble vitamins  Vitamins A, D, E, K • Absorbed with fats Water-soluble vitamins  Vitamins B and C—diffuse into blood Electrolytes  Absorbed by active transport or diffusion Drugs are primarily absorbed in the intestine.  Various transport mechanisms  Some (e.g., aspirin) absorbed in the stomach Carbohydrates  Digestion starts in mouth.  Followed by digestion in the small intestine Proteins  Digestion starts in stomach, continues in small intestine. Lipids  Emulsified by bile prior to chemical breakdown  Action of enzymes form monoglycerides and free fatty acids  Formation of chylomicrons

Vomiting refluxes Deep inspiration Closing the glottis, raising the soft palate Ceasing respiration  Minimizes risk of aspiration of vomitus into lungs Relaxing the gastroesophageal sphincter Contracting the abdominal muscles  Forces gastric contents upward Reversing peristaltic waves  Promotes expulsion of stomach contents

o 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 wat Large-volume diarrhea (secretory or osmotic)  Watery stool resulting from increased secretions into intestine from the plasma  Often related to infection  Limited reabsorption because of reversal of normal carriers for sodium and/or glucose Small-volume diarrhea  Often caused by inflammatory bowel disease  Stool may contain blood, mucus, pus.

 May be accompanied by abdominal cramps and tenesmus Steatorrhea—“fatty diarrhea”  Frequent bulky, greasy, loose stools  Foul odor  Characteristic of malabsorption syndromes Celiac disease, cystic fibrosis  Fat usually the first dietary component affected Presence interferes with digestion of other nutrients.  Abdomen often distended Dehydration and hypovolemia are common complications of digestive tract disorders. Electrolytes  Lost in vomiting and diarrhea Acid-base imbalances  Metabolic alkalosis • Results from loss of hydrochloric acid with vomiting  Metabolic acidosis • Severe vomiting causes a change to metabolic acidosis because of the loss of bicarbonate of duodenal secretions. • Diarrhea causes loss of bicarbonate. Radiography  Contrast medium may be used. Ultrasound  May show unusual masses Computed tomography (CT) Magnetic resonance imaging (MRI) CT and MRI may use radioactive tracers.  Can be used for liver and pancreatic abnormalities Fiberoptic endoscopy used in upper GI tract  Biopsy may be done during procedures. Sigmoidoscopy and colonoscopy  Biopsy and removal of polyps may be done. Laboratory analysis of stool specimens  Check for infection, parasites and ova, bleeding, tumors, malabsorption Blood tests  Liver function, pancreatic function, cancer markers...