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GI 3 - The Intestines (Digest, Absorb, Defecate).doc

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Department
Health Sciences
Course
HTHSCI 1H06
Professor
Cale Zavitz
Semester
Winter

Description
G.I. 3: The Intestines (Digest, Absorb, Defecate) Reverse Peristalsis (Vomiting, emesis) • Why do we vomit? ⁃ Extreme stretch ⁃ Irritants ⁃ Bacterial toxins ⁃ Excessive EtOH (ethanol) ⁃ Certain foods and drugs • Involves ⁃ Afferent stimulation of medulla- irritant fibres within gastric mucosa will send signals up to medulla to stimulate vomiting centre ⁃ Vomiting centre- has outputs which involve somatic neurons to cause effects such as contracting diaphragm (take deep breath in) and then abdominal muscles- puts pressure on stomach- need to relax LES- built up pressure in stomach can propel contents up through esophagus and out oral cavity- must close soft palate, otherwise food stuffs will go into nasal cavity • Ipecac- syrup will induce vomiting- stimulates irritant receptors lining gastric mucosa Pyloric Stenosis • Narrowing of the pyloric sphincter • The only place for gastric contents to go is up the esophagus and past the gums (projectile vomiting) • It is most common in first male babies • If mild, wait… if severe, treatment by slitting the pyloric sphincter Overview of the Digestive Process • Ingestion- putting food into oral cavity • Mechanical digestion- mastication (chewing), continued in blender of stomach, segmentation in small intestine- circular layer of muscle lining wall of small intestine will contract at alternating places- helps to break up food occupying lumen of small intestine • Chemical digestion- begins in mouth with secretions of amylases in saliva- lingual lipase works in pH of stomach- continues in stomach with gastric juices which contain HCL to denature proteins, and pepsin to break down proteins- continues in small intestine, which receives enzymes from accessory organs of GI tract (liver- digesting fats, pancreas- secretes a plethora of enzymes which break down proteins, sugars, and fats)- small intestine is also where we see absorption come into play • Absorption- middle to distal segments of small intestine will begin absorbing nutrients- many nutrients go directly into blood stream (sugars, short chain fatty acids, amino acids)- will be absorbed directly into capillary beds surrounding wall of GI tract- exception is complex fats (long chain fatty acids, monoglycerides)- absorption continues into large intestine where fecal matter (nutrients we couldn't break down or absorb across intestinal wall) is developing- can still pull electrolytes (sodium, chloride) and water from left over matter Small Intestine: Basic Plan • Alterations for the Small Intestine: ⁃ Villi (microvilli- brush border)- finger-like projections in mucosal layer- on villi, each individual cell's apical surface has a lot of folding- increases surface area- looks fuzzy, called brush border (technical term- microvilli) ⁃ Plicae circularis- alteration within folding of mucosa- underneath mucosa is muscular mucosa (helps mucosa fold)- along length of small intestine, there are intense folds in a helical fashion- help to increase surface area (have absorption, so greater the surface area the better)- helical fashion helps to mix up chyme with all of the enzymes being introduced in small intestine from liver and pancreas • Small intestine suspended from posterior abdominal wall by mesentery, which is a perfect highway for blood vessels, nerves, and lymphatics servicing wall of intestine • Two layers of enteric plexus ⁃ Submucosal- in submucosa underneath glands ⁃ Myenteric- in mucosa or outer layer of gut wall- resides between inner circular and outer longitudinal layer of muscles- helps to coordinate muscular actions Small Intestine: Blood Supply • Largely supplied by Superior Mesenteric Artery- very first segment receives blood from Celiac Trunk (higher up on abdominal aorta) • Superior Mesenteric Artery also supplies ascending and half of transverse large intestine • Inferior Mesenteric Artery- supplies rest of large intestine (last bit of transverse, descending segment, sigmoid portion, rectum, and anus) Small Intestine: Venous Drainage • Blood coming back from gut must go through hepatic portal vein- needs to be filtered through liver, nutrients need processing • Pancreaticoduodenal vein- drains duodenum and pancreas (in curve of duodenum)- dumps into superior mesenteric, which dumps into hepatic portal vein • Jejunum and ileum drained by superior mesenteric artery • Veins of Small Intestine: Pancreaticoduodenal, Superior Mesenteric, Hepatic Portal Divisions of the Intestine • Small Intestine (bowel): ⁃ Duodenum (25 cm)- means "12"- 12 finger lengths in width ⁃ 4 parts: superior, descending, inferior, ascending ⁃ Largest lumenal diameter ⁃ Most dense Plicae Circularis- accepting enzymes from pancreas and liver, have to mix them with chyme from stomach- need density of Plicae Circularis to mix these together ⁃ Specialized mucosa in initial (superior) portion- accepting acidic contents from stomach, has special glands that produce a lot of bicarb ⁃ Pancreatic duct and bile duct dump into descending portion via Hepatopancreatic Ampulla ⁃ Jejunum (1m) (40%) ⁃ Thick wall, wide lumen ⁃ Denser mucosa ⁃ Ileum (2m) (60%) ⁃ Smallest diameter ⁃ Few mucosal folds ⁃ Ends at Ileocaecal valve ⁃ As you go further down, size of lumen decreases • Large Intestine (colon): ⁃ 6 regions Small Intestine: Villi • Finger-like projections- can see with naked eye- look like sea anemones • Help to increase surface area • Absorptive cell- most abundant cell on villi- Each cell will have own microvilli- plasma membranes are put into folds- absorbs nutrients from lumen of gut • Goblet cells- produce mucous- contains bicarb to neutralize acidic chyme- also lubricates • Enteroendocrine cell- secretes hormones (secretin, CCK)- this is how duodenum can modify actions of stomach- "entero-" for "intestines"- "endocrine" b/c it creates a hormone that is dumped into bloodstream • Paneth cells- at base of villi- secrete lysozyme- protective protein associated w/ innate immune system that non-specifically degrades proteins, which are sometimes pathogens • Blood vessels go to tip of villi- must be able to put nutrients in bloodstream • Lacteal- specialized lymphatic vessel- villi also responsible for absorbing fat Small Intestine: Duodenum • In initial segment of duodenum (before area where secretions of liver and pancreas reach gut), must have specialized mucosa • Duodenal glands (Brunner's glands)- produce bicarb-rich mucous to protect duodenum from acidic chyme- will not find them past area where secretions of liver and pancreas reach gut as they are no longer necessary- only in initial segment Small Intestine: Jejunum • Plicae Circularis- circular folds Small Intestine: Ileum • In wall associated w/ mucosa, will find modified lymphoid tissues (MALT- Mucosal Associated Lymphoid Tissue)- Payer's Patches- contain adaptive immune system cells- T- and B- cells sit ready for any pathogens coming across gut wall- will initiate adaptive response at level of gut wall- do not need lymph nodes- ingest a lot of bad things, want immune system sitting ready Digestion Continues… • Protecting the small intestine from acidic chyme ⁃ Small amounts of chyme pass through pylorus (3 mL at a time) ⁃ Rapidly neutralized by bicarbonate-rich mucus (Brunner's glands) ⁃ Chyme passes the pancreatic ampulla (ampulla of Vater- not Vader)- hepatopancreatic ampulla because secretions from liver and pancreas come trough common duct to get into the lumen of small intestine ⁃ Bicarbonate-rich pancreatic juice completes neutralization of acidic chyme • Protection of the Small Intestine and continued digestion ⁃ Enteroendocrine cells in lining of mucosa of small intestine (particularly duodenum)- will release hormone components- Secretin and CCK (cholecystokinin)- works on two primary organs to cause secretion of bicarb-rich secretions- can stimulate liver to release bicarb-rich bile- stimulates pancreas to secrete bicarb-rich juices- help to neutralize acidic chyme 1) Acidic chyme entering duodenum causes the enteroendocrine cells of the duodenal wall to release secretin, whereas fatty, protein-rich chyme induces release of CCK- tells the liver to contract the gallbladder, which releases a lot of bile salts that are needed to help emulsify fats and break them into smaller globules 2) CCK and secretin enter bloodstream 3) Upon reaching pancreas, CCK induces the secretion of enzyme-rich pancreatic juice- help break down fats, proteins, and sugars; secretin causes copious secretion of bicarb-rich pancreatic juice Small Intestine: Motility • Two Types- changes during different states: ⁃ Fed Pattern- food occupying lumen of gut- during feeding, irregular contractions mix contents with digestive juices- segmentation- alternating regions of circular muscles will contract ⁃ Fasting- once food has been absorbed, is occupying less space in lumen- residual is left- need to get rid of it- slow, migrating motor complex (peristalsis) begins at duodenum and propels residual material into colon (prevents bacterial growth)- if you don't have constant movement in that direction, could have bacteria from large intestine go up into small intestine Digestion Continues… Again • Pancreas releases plethora of enzymes: ⁃ Proteases- help break down proteins ⁃ Trypsinogen (inactive)- Trypsin ⁃ Chymotrypsinogen (inactive)- Chymotrypsin ⁃ Procarboxypeptidase (inactive)- Carboxypeptidase ⁃ "-gen" indicates that it is inactive, needs to be cleaved ⁃ "Pro-" indicates precursor as well, needs to be cleaved ⁃ Pancreas needs to release inactive enzymes, otherwise it would be eating itself ⁃ Having another protease to cleave them will activate them
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