Unit 3 - Chapter 41 Structure and fx digestive system Flashcards
GI tract major fx
- mechanical + chemical breakdown of food and the absorption of digested nurtrients
GI tract
- hollow tube that extends from mouth to anus
Walls of GI tract
- mucosa (innermost) -> basement membrane
- muscularis mucosa (peristalsis)
- submucosa (dense connective tissue that supports mucosa as well as joins mucosa to the bulk of underyling smooth muscle + contains submucosal glands that secrete mucus)
- muscularis (circular muscle and longitudinal layer) — peristalsis
- serosa (final layer - composed of connective tissue and gives strength to the digestive tract)
What parts of GI are controlled voluntarily and involuntarily?
- swallowing + defecation => voluntary
- GI tract iself controlled by extrinsic automatic nerves (vagus, parasympathetic, splanchinic, and sympathetic nerves)
- ALSO intrinsic autonomic nerves (enteric nervous sytem) + intestinal hormones
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Parasympathetic nerves – stimulate motility ie ‘makes you poo’
* The parasympathetic nerves originate in the medulla with the brain
* Innervation of ascending colon and half transverse colon is via the vagus nerve
* Innervation of the descending colon, rectum and anus is via the sacral nerve roots S2-S4 though the pelvic nerves, these para sympathetic nerves stimulate the rectum and anal canal to contract and the internal anal sphincter to relax, this allows faeces to pass though the anus
Sympathetic nerves – inhibit motility ie ‘stops your poo’
* The sympathetic nerves originate lower down the spine, between the 10th thoracic and 3rd lumbar segments
* Innervation from the lumbar spine T11- L2 via the hypogastric nerve, these Sympathetic nerves stimulates the rectum and anal canal to relax and the anal sphincter to contract, to maintain continence
Sensory and motor innervation of the bowel and pelvic floor is through the vagus nerve, the nervi erigentes (pelvic splanchnic nerves), direct sacral root branches and the pudendal nerve.
Mixed nerves supply the somatic voluntary muscles of the pelvic floor and the external anal sphincter.
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The enteric nervous system or intrinsic nervous system is the internal nervous system of the gut and is embedded in the wall of the gut, it begins at the oesophagus and extends to the anus.
The enteric nervous system coordinates gut secretion, blood flow and muscular activity.
The enteric nervous system normally communicates with the central nervous system though the parasympathetic (vagus nerve) and the sympathetic nervous systems but it is also able to function independently.
If the vagus nerve is severed the enteric nervous system can continue to function itself without input from the central nervous system. This can be seen in some individuals who have severe nerve damage but who still have some gut motility. (This is different to the nerve control for the bladder, which can become atonic).
What happens in the mouth?
- digestion begins in the mouth w/ chewing + salivation
- saliva (digestive component) ==> a-amylase => initiates carbohydrate digestion
Esophagus
- muscular tube
- transports food from mouth to stomach
- upper part is striated muscle
- lower part is smooth muscle
swallowing
- controlled via swallowing center in reticular formation (where brain stem is?) of brain
- two phases – oropharyngeal phase (voluntary swallowing) and esophageal phase (involuntary swallowing)
How does food move?
- peristalsis - waves of sequential relaxations and contractions of the muscularis
Lower esophageal sphincter
- opens to admit swallow food into stomach
- Then closes to prevent regurgitation of food back into esophagus
Stomach
- secretes digestive juices
- mixes/stores food
- propels partially digested food (chyme) into duodenum (1st section of small intestine)
- smooth muscles — outer logntiduinal, middle circular, and internal oblique
Vagus nerve
- stimulates gastric (stomach) secretion and motility
Gastrin and motilin
- hormones that stimulate gastric emptying
Secretin and cholecystokinin
- delay gastric emptying (hormones)
Gastric glands
- located in fundus (top of stomach) and body of stomach
- release intrisc factors needed for:
1) vitamin b12 absorption (fx of brain and nervous system, formation of RBC // prevent megaloblastic anemia – blood condition that makes people tired and weak)
2) Hydrocloric acid (inside gastric juice - dissolves food fibers, kills bacteria, and actives enzyme pepsin [breakdown proteins])
Chief cells
- inside stomach
- secretes pepsinogen, which is converted to pepsin in the acidic environment created by hydrochloric ac
- Digest proteins
Acid secretion
- stimulated by vagus nerve, gastrin (G-cells lining stomach and upper small intestine), and histamine
- inhibited by sympathetic stimulation and cholecystokinin (stimulates your gallbladder to contract and release bile into your small intestine // stimulates your pancreas to release pancreatic enzymes)
What is secreted thoroughout stomach?
- mucus
- protects stomach wall from aid and digestive enzymes
3 phases of acid secretion
1) cephalic phase (anticipation and swallowing)
2) gastric phase (food in the stomach)
3) intestinal phase (chyme in the intestine)
Small intestine
- 5 m long
- 3 segments (duodenum, jejunum, and ileum – descending direction until reach large intestine)
- digestion and absorption of all major nutrients and most ingested water ocur in the small intestine
Peritoneum
- double layer of membranous tissue
- visceral covers abdominal organ
- parietal layer extends along abdominal wall
- in between the two is the parietal cavity!
blood flow to the small intestine
- superior mesenteric artery (bottom main artery)
duodenum
- receives chyme from stomach through PYLORIC valve
- presence of chyme stimulates liver and gallbladder to deliver bile and pancreas to deliver digestive enzymes and alkaline secretions (alkaline secretion is important in buffering of acid chyme entering duodenum and for activation of enzymes; rich in digestive enzymes and bicarbonate)
- bile and enzymes flow through an opening guarded by sphinceter of Oddi (pancreatic juices, exiting from pancreas to small intestine)
bile
- produced by liver and needed for fat digestion and absorption
- bile’s alkalinity helps neutralize acidic chyme
- l/t pH that enables pancreatic enzymes to digest proteins, carbohydrates and sugars
Enzymes secreted by small intestine
- maltase (breaks down maltose into glucose), sucrase (breaks down sucrose [table sugar] into glucose and fructose), lactase (breaks down milk sugar into glucose and galactose)
pancreatic enzymes
- proteases (break down proteins via peptide bonds that join amino acids through hydrolysis)
- amylase (digest starch into smaller molecules => maltose (then clevaed by small intestine’s maltase)
- lipase (breakdown fats in food so they can be absorbed in the intestines)
bile salts
- bile (contains these salts, phopholipids, cholesterol, conjugated billrubin, electrolyes, and water) => liver => ducts => common hepatic dut
- act in small intestine to digest proteins, carbohydrates, fats
- specifically, they emlusify and hydrolyze fats and put them in water-soluble micelles (carry other substances) via unstirred water layer (next to intestinal membrane or brush border) then to the brush border (stria of microvilli on the plasma membrane of an epithelial cell (as in a kidney tubule) that is specialized for absorption, start carbohydrate digestion)
- Fat content inside the micelle readily diffuse through the epithelium into lacteals (lymphatic ducts) in the villi
- From there, fats travels from lympathics and into the systemic circulationh, which delivers them to the liver!
- also includes fat soluble vitamins (A, D, E, K)
What happen to digested substances
- Absorbed across intestinal wall and then transported to liver through hepatic portal vein (capillaries branch out from intestines then to this vein under the liver), where they are metabolized further
ileocecal valve
- connects small and large intestines
- prevents reflux into small intestine
Villi
- small finger like projections that extend from small intestinal mucosa
- increases absorptive surface area
Where are carbohydrates, amino acids, fats absorbed primarily?
- duodenum and jejunum
where are bile salts and vitamin b12 absorbed?
ileum (last part of small intestine)
* absorption of vit b12 requires presence of intrinsic factor
How are minerals and water soluble vitamins absorbed?
- active and passive transport via small intestine
How is chyme moved along the intestinal tract, plus mixed?
- Peristaltic movements created by longitudinal muscles push chyme along
- While circular muscles (segmentation) mix the chyme and promote digestion
Ileogastric reflex
- inhibits gastric motility when ileum is distended
intestinointestinal reflex
- inhibits intestinal motility when one intestinal segment is overdistended
gastroileal reflex
- increases intestinal motility when gastric motility increaes
large intestine structures
- cecum (site for cellulose digestion in our ancestors but now acts as a reservoir for chyme which it receives from ileum)
- appendix (branch off of 1st part of large intestine near cecum, possibly immune function)
- colon (asecnding, transverse, descending and sigmoid – dehydrate food, form stool, taking water and electrolyes out while bacteria feed on waste)
- rectum (holding area for stool) when full then push out!
- anal canal (defecation and maintaining fecal continence)
teniae coli
- 3 bands of longitudinal muscles that extend length of the colon
Haustra
- pouches of colon that are formed with alternating contraction and relaxation of the circular muscles
What is is inside the mucosa of large intestines? How about function?
- mucus-secreting cells
- mucosal folds
- NO VILLI
fx - large intestine massages the fecal mass and absorbs water and electrolytes
Gastrocolic reflex
- this happens when the ileum is distended l/t propulsion of feces to rectum
Defecation when?
- occurs when rectum is distended w/ feces
- the conelike contracted internal anal sphincter RELAXES
- and if voluntarily controlled external sphinceter relaxes
- this causes defecation
Where are the largest amounts of intestinal bacteria?
anaerobes
1. bacteroides (digest carbohydrates)
2. clostridia (regulate intestinal permeability, inflammation, and immune fx)
3. coliforms (metabolism){
4. lactobacilli (protects intestinal barrier by creating mucus and proteins)
Intestinal tract at birth?
- initially sterile
- colonized in 3-4 weeks
Endogenous infections of GI Tract
- occur by excessive proliferation of bacteria, perforation of the intestine, or contamination from neighbor structures
Liver
- largest organ in the body
- digestive, metaoblic, hematologic, vascular, and immune fx
- divided into left and right lobe
- supported by falciform, round, and coronary ligaments
Liver lobules consist of?
- plates of hepatocytes (functional cells of the liver)
Circulation of liver?
1) hepatic artery supplies blood
2) hepatic portal vein (before there are the mesenteric and splenic veins) receiving blood from inferior mesnteric, gastric and cystic veins
Hepatocytes
- synthesize 700-1200 cc of bile per dasy
- secrete into bile canaliculi (small channels between hepatocytes)
- then dump the bile canaliculi draine bile in the common bile duct and then into the duodenum through an opening called the major duodenal papilla (sphincter of oddi) <=== connects with pancreas
Sinusoids
- capillaries located between the plates of hepatocytes; blood from the hepatic portal vein and hepatic artery flows though these capillaries to a central vein in each lobule and then into hepatic vein and inferior vena cava
Kupffer cells
- mononuclear phagocyte system
- line the sinusoids (microvascular bed in hepatocytes) and destroy microorganisms in the sinosoidal blood
Primary bile acids are made?
- synthesized from cholesterol by hepatocytes
- primary acids are then conjugated into form bile salts
- SECONDARY bile acids are product of bile salt deconjugation by bacteria in the intestinal lumen
How are bile salts and acids recycled?
- absorption of bile salts and acids from the terminal ileum
- when returned to liver are known as enterohepatic circulation of bile
- portal vein
- 90%
billirubin
- pigment resulting from lysis of old RBCs in the liver and spleen (globin becomes amino acids, hemo or haem becomes iron and billverdin => unconjugated billrubin)
- unconjugated billrubin is fat solube and can cross cell membranes (bound to albumin then transported to liver to be converted)
- this unconjugated form is converted to water soluble conjuagted billrubin by hepatocytes and is secreted with bile
- go into small intestines (Once in the colon, colonic bacteria deconjugate bilirubin and convert it into urobilinogen. Around 80% of this urobilinogen is further oxidised by intestinal bacteria and converted to stercobilin and then excreted through faeces. It is stercobilin which gives faeces their colour.
Around 20% of the urobilinogen is reabsorbed into the bloodstream as part of the enterohepatic circulation. It is carried to the liver where some is recycled for bile production, while a small percentage reaches the kidneys. Here, it is oxidised further into urobilin and then excreted into the urine)
How is fats created by liver?
- Created by liver from protein and carbohydates, which include glycerol, free fatty acids, phospholipids, cholesterol (sugar into glycogen, if excess sugar into fats)
- fat absorbed by intestinal lacteal (villus => located in middle as lymphatic vessel) is mainly TRIGLYCERIDES – which are hydrolyzed into glycerol and free fatty acid
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glycerol — ATP production through gluconeogenesis (source of energy from adipose when in fasting state
free fattty acids — energy source
phospholipid (cell membrane… and other cellular processes, such as bile)
cholesterol (maintain the integrity and fluidity of cell membranes and to serve as a precursor for the synthesis of substances that are vital for the organism including steroid hormones, bile acids, and vitamin D)
How is protein synthesized?
- liver needs all essential amino acids
- liver synthesizes albumin, globulin, and other serum enzymes
- can convert amino acids to carbohydrates by removal of ammonia
- albumin — helps move many small molecules through the blood, including bilirubin, calcium, progesterone, and medicines. It plays an important role in keeping the fluid in the blood from leaking into the tissues.
- globulins – enzymes, transporter proteins, or immunoglobulins
carbohydrates and liver
- can be released as glucose, stored as glycogen, or converted as fat
Other important liver fx
- detoxification of exogenous and endogenous substances
gallbladder
- saclike organ @ inferior surface of the liver
- stores bile between meals and jects it when chyme enters duodenum
- stimulated by cholecystokinin (secreted by endocrine cells in proximal small intestine and released when food ingested)
- this hormone causes gallbladder to contract and force bile through cystic duct and into the common bile duct; sphincter of oddi relaxes allow it to flow thorugh major duodenal pailla into DUOdenam
Pancreas
- gland located behind stomach
- endocrine pancreas - release hormones (glucagon and insulin) that facilitate formation and cellular uptake of glucose [islets of langerhans]
- exocrine pacnreas — release alkaline solution and enzymes (trypsin, chymotrypsin, carboxypeptidase, a-amylase, lipase) [acinar and duct tissue] — 85% of mass of pancreas
1) trypsin function (digest protein; happens in small intestine)
2) chymotrypsin fx – proteolysis or breakdown of protein and polypeptides
3) a-amylase — hydrolyze starch molecules => glucose untis
4) lipase — fats (breaks down ester bonds of lipids and fats => fatty acids, glycerol, and other alcohols)
Secretin and pancreas?
- secretin stimulates pancreatic secretion of alkaline fluid (bicarbonate) and cholecystokinin (stimulates pancreas to release enzymes + bile from gallbladder); Ach stimulate secertion of enzymes
- These pancreatic seceretions start in acini (small saclike cavity in gland) then empty into the duodenum through the common bile duct or accessory duct into dudenum
- stimulated by gastric acid
Gastric and stool analysis
- digestion, absorption, secretion
Liver function
- LDH, AST, and ALT
Prothrombin times
- prolonged in hepatitis and chronic liver disease
Obstructive disease of the gall bladder
- evident with elevated serum billirubin levels, elevated urobilinogen levels (High levels of urobilinogen in urine may be a sign that: Your liver is making too much bilirubin because your body breaks down red blood cells faster than it can make them) and increased stool fats
Serum leukocyte levels
elevated with inflammation of the gallbladder
Pancreatic dysfunction
increased serum amylase and stool fat
advancing age and GI
loss of teeth, less taste/smell, lower salivary secretions l/t eating prob and appetite challenge
* lower motility and seceretions (especially HCL) => slower emptying and digestion
Efficiency of drug and alcohol metabolism with age
- decrease with age
- r/t decreased liver perfusion and decreased liver enzymes
