The gastrointestinal system Flashcards
diarrhoea
50% of deaths from diarrhoea are from acute watery diarrhoea where sufferer cannot get rehydrated. 35% are through persistent diarrhoea.
treatment of diarrhoea
oral rehydration solution or zinc and potassium supplements.
GI physiology and disease
malnutrition, infections by rotavirus and bacteria, infestations with parasites, constipation (often die to side effects of drugs)
5 stages of digestion
ingestion, peristalsis, digestion, absorption, defaecation.
the digestive organs
GI tract; mouth, pharynx, oesophagus, stomach, small intestine 9duodenum, jejunum, ileum) large intestine (colon, rectum)
accessory organs; teeth, tongue, salivary glands, liver, gallbladder, pancreas.
mucosa
epithelial layer and connective tissues, capillaries, enteric neurones, immune cells, surface area greatly increased by villi/glands
submucosa
loose connective tissues and large blood vessels, contains glands for secretion
muscle layer
2 layers- circular/longitudinal
enteric NS
serosa
connective tissue, squamous epithelia
layers of mucosa
epithelium, responsible for absorption o digested food and secretion of gastric or intestinal fluid
lamina propria, contains blood vessels, lymph tissues and lymph nodes
muscularis mucosa, smooth muscle cells which cause folds in mucous membranes to increase SA for absorption
layers of submucosa
lose connective tissue which binds mucosa to tunica muscularis. contains Meissner’s plexus which; is part of autonomic nerve supply to smooth muscle and partially controls secretions of GI tract
layers of muscularis
consists partly of skeletal muscle (to aid swallowing) in; mouth, pharynx, oesophagus. smooth muscle only found in rest of GI tract; circular layer, longitudinal layer, contraction mixes and propels food. contains myenteric plexus- receives input from SNS and PNS, mostly controls GIT motility.
layers of serosa
outermost layer of alimentary canal, composed of connective tissue and epithelium
the mouth
food mixed with saliva as it is chewed, three pairs of salivary glands (parotid, submaxillary, sublingual) around 1500ml saliva secreted each day.
what does saliva contain
mucous to help lubricate food, alpha-amylase to initiate breakdown of carbohydrate.
formation of saliva
isotonic fluid produced by acinar cells by secretion of electrolytes and water, fluid modified as it flows along salivary duct. final composition depends upon flow rate
regulation of salivary secretion
modulated by ANS;
p-symp- watery saliva, rich in amylase and mucous
symp- promotes output of amylase, reduces blood flow to glands, overall effects a reduction in secretion
stages of swallowing
voluntary stage; tongue pushes bolus backwards to ortho-pharynx
pharyngeal stage; reflex action controlled by deglutition centre in medulla and pons of CNS.
soft palate and uvula move up to seal off nasopharynx
larynx raised and epiglottis covers and seal glottis and breathing suspended for 1-2 seconds.
oesophageal phase- no absorption of food or secretion of digestive enzymes occurs but mucous produced to aid swallowing. food pushed along by peristalsis controlled by medulla of the brain and autonomic nerves. Lower oesophageal sphincter relaxes to allow food to enter stomach
Passage of bolus takes 4-8 seconds
oesophagus
tube connecting to stomach, muscularis layer s striated for first third to assist in swallowing. middle third is mixture of striated and smooth muscle. lower third is all smooth muscle.
peristalsis
contractile activity in GI tract. segmentation- occurs largely in small intestine to facilitate mixing of food
peristalsis- concerned mainly with propulsion of food along GI tract
innervation of the GI tract
enteric NS, found within the wall of the GI tract from oesophagus to anus. myenteric plexus largely motor function. submucosal plexus largely sensory, receiving signals from epithelium and stretch receptors. release range of substances; CCK, VIP, somatostatin, enkephalins
extrinsic innervation
nerve plexus linked to CNS via afferent fibres activated by; stretch, chemical stimulation
receive efferent innervation from ANS; p-symp input stimulates gut motility and secretory activity
symp nerves cause pre-synaptic inhibition of p-symp induced contraction.
hormonal regulation of GI tract
around 20 different regulatory peptides act through endocrine/pancrine pathways.
endocrine; (hormonal) peptides regulate secretions in stomach and pancreas
paracrine (local) agents regulate secretion in salivary glands and stomach.
functions of the stomach
temporary storage of food, chemical digestion of proteins, mechanical digestion by stomach movements, regulation of passage of chime into small intestine, secretion of intrinsic factor-essential for absorption of B12.
what is stomach mucosa composed of
epithelium containing many openings
what are the 4 types of cells that together produce gastric juices
cheif or peptic or zymogenic which produce pepsinogen
parietal or oxyntic cells secrete HCL and intrinsic factor, goblet cells secrete mucous]enteroendocrine (G-cells)
digestion in the stomach
mechanical- 3 layers of muscularis enable food to be churned, food mixed with gastric juice to produce chime, chime passes through pyloric sphincter to duodenum
chemical- proteins broken down by pepsin, converted from pepsinogen by HCL, mucous protects stomach cells from gastric juices, gastric lipase breaks down fat at higher pH
phases of gastric secretion
cephalic phase- stimulated by sight, smell, taste, thought of food or blood glucose. gastric phase- stimulated by stomach distension due to presence of food
intestinal phase- stimulated by digested proteins in the duodenum
gastrin and control of acid secretion
G-cells; release gastrin in pyloric antrum and duodenum (and pancreas) stimulated by AAs. gastrin transported vi local blood supply to stimulate parietal cells and ECL (entero chromaffin like cells) cells to enhance acid secretion. gastrin also stimulates CCK (cholecystokinin) receptors on D-cells (release 5_HT/somatostatin) which has a negative feedback effect on gastrin release.
why doesn’t the stomach digest itself
tight junctions between mucosal epithelial cells prevents leakage of gastric juice onto underlying tissue. mucous secreted by epithelial cells has higher pH, providing localised neutralisation and physical barrier to acid, prostaglandins increase mucosal thickness and stimulate bicarbonate production.
how long does it take to digest
mouth(1 min) oesophagus (4-8 secs) stomach (2-4 hrs) small intestine (3-5hrs) colon (10 hrs to several days)
inhibition of gastric secretion
occurs once food has left stomach
neuronal inhibition; partially digested protein in duodenum inhibits secretion by “enterogastric reflex”, reflex mediated by medulla and leads to parasympathetic stimulation.
hormonal inhibition; secretin and cholecystokinin released from duodenum, other inhibitors include motilin, GIP, gastrin, glucagon, VIP
gastric motility
results from contraction of the 3 layers of muscle in the stomach wall due to; grind, churn, knead, twist and propel contents
coordinated by myenteric plexus which received input from ANS.
parasympathetic stimulation increases motility
sympathetic stimulation decreases motility
gastric emptying
stimulated and inhibited in a similar manner to gastric secretion. emptying occurs at a rate proportional to gastric volume. physical and chemical nature of contents influence rate. stomach emptying involves; constriction of lower oesophageal spinchter, contration of gastric muscularis, relaxation of pyloric spinchter
absorption in the stomach
stomach wall impermeable to a range of compounds-through water, alcohol and some drugs and electrolytes can be absorbed
absorption usually only starts when contents reach small intestine where food is acted upon by products of; pancreas, liver, gallbladder.
vomiting
occurs due to reflex coordinated by reticular formation of medulla, activated by; stretch, chemical agents, bacteria and toxins, sight, smell. motion, balance, pain, emotions, cytotoxic drugs
prolonged vomiting can cause metabolic alkalosis through loss of gastric acid.
emesis (and nausea)
triggered by afferent impulses to the emetic centre (from chemoreceptor trigger zone, vestibular system, limbic system or periphery) afferent impulses integrated by the emetic centre. output to medullary control centres (vasomotor, respiratory) and subsequent somatic and visceral outputs to effector organs
the structure of the small intestine
mucosa, submucosa, luminal surfaces
mucosa in small intestine
pits lined with glandular epithelium, intestinal glands secrete enzymes of digestive juices.
submucosa in small intestine
contains Brunner’s glands which secrete alkaline mucous to; protect intestinal wall, neutralise acid chime.
luminal surfaces in small intestine
covered in microvilli to increase absorptive surface area. each villus has; arteriole, capillary bed, venule and lymphatic for efficient transport of nutrients.
folds of mucosa and submucosa further increase SA.
how big is small intestine
around 6-8meters.
intestinal juices
2-3L liquid (pH7.6) produced per day. contains water, mucous and enzymes. secretion regulated by reflex stimulated by presence of chime. CCK and secretin also stimulate juice secretion.
digestion in small intestine
mechanical
chemical; digestive process started by salivary amylase and stomach protein. digestive process complete in small intestine by combined actions of; pancreatic juice, bile, intestinal juice.
breakdown products in small intestine
monosaccharides (from carbohydrates)
peptides and AA ( from proteins)
monoglycerides and free fatty acids (from fats)
digestion of carbohydrate
starch converted to disaccharides by pancreatic amylase. disaccharides (sucrose and lactose) converted to monosaccharide by glycosidase. monosaccharide absorbed (glucose, fructose, galactose.
digestion of proteins
polypeptides arriving from stomach catabolised by pancreatic trypsin and chymotrypsin. digestion completed by peptidases released from glandular epithelium of the intestine. peptidases cleave amino acids; carboxypeptidases act as carboxyl end
aminopeptidases act at amino end
dipeptidases convert dipeptides to AA
digestion of fats
around 80g fat absorbed from intestine (jejunum) each day.
bile plays essential part
-fat globules in duodenum coated with bile salts to create emulsion and disperse fat molecules. facilitates breakdown of triglycerides by pancreatic lipases to form monoglycerides and free fatty acids.
digestion of nucleic acids
ribonuclease and deoxyribonuclease break down respective nucleic acid nucleotides to constituent pentose and nitrogen base subunits. subunits then absorbed through intestinal wall.
absorption in small intestine
process by which digestion products transported into epithelial cells and blood. almost all absorption occurs in small intestine. about 8-10 L of water and 1KG of nutrients pass through gut wall daily. absorption through intestinal mucosa via; active transport, diffusion.
absorption of monosaccharides
absorption occurs largely in duodenum and upper jejunum. glucose and galactose; taken into endothelium against concentration gradients by sodium dependent o-transport. gradient maintained by NA/K ATPase. leave epithelial cell by facilitated diffusion.
fructose- absorbed by NA independent facilitated diffusion.
absorption of amino acids and peptides
around 200g absorbed each day in the adult.
amino acids; absorbed as brush border of epithelial cells by sodium dependant co-transport. most absorbed in first part of small intestine, some metabolised in colon by bacteria.
peptides; carrier thought to be linked to H influx. broken down to constituent amino acids. both leave via AA carrier system.
absorption of fats
monoglycerides and FFAs associate with bile salts and lecithin to form micelles. micelle enters aqueous layer of microvilli
MGs, FFAs, cholesterol, fat soluble vitamins and lecithin diffuse passively into duodenal cells. bile salt portion remains in lumen of gut until terminal ileum and then recycled by enterohepatic circulation
lipids accumulate in vesicles of smooth ER of epithelial cell to form chylomicrons. chylomicrons leave intestine in lymph for delivery into venous circulation. lipids therefor bypass the liver in the short term. faeces contain around 5% fat.
absorption of water and electrolytes,
around 2L fluid ingested daily, 7-8L secreted in form of digestive juices, almost all is reabsorbed (around 500ml leaves the body in the faeces)
most electrolyte actively absorbed along intestine.
-sodium by osmosis or active transport (stimulated by aldosterone)
Cl, I, NO2 may follow sodium passively or be transported actively. potassium generally absorbed passively.
absorption of vitamins
fat soluble vitamins ( A D E K must be absorbed in micelles
water soluble vitamins (B and C0 must be absorbed by diffusion
vitamin B12 required the presence of intrinsic factor (produced by stomach)
structure of large intestine
similar structure to small intestine except; no villi or permanent folds present. simple epithelium contains only goblet cells. longitudinal muscle broken down into 3 flat bundles (taenia coil)
contraction of taenia coil form haustra
digestion in large intestine
mechanical; chime enters through ileocecal valve
haustral churning
peristalsis occurs slowly
chemical; only bacterial enzymes present which
ferment remaining carbs to produce flatus, break down remaining AA, produce vitamin K and some B vitamins
absorption In large intestine
500-1000ml water enter large intestine each day, all but 100ml reabsorbed
faeces formed consisting of; undigested food, inorganic salts, sloughed off epithelial cells, bacterial products, bacteria
defaecation
reflex initiated by distension of rectum
longitudinal muscle shortens
voluntary contractions of diaphragm and abdomen
anal sphincter opens
the pancreas
exocrine (99% of cells) - accessory organ to digestion, secretion of enzyme rich fluid into duodenum
endocrine (1% of cells) release of hormones into bloodstream.
-insulin (islet beta-cells)
-glucagon (islet alpha cells)
somatostatin
structure of pancreas
structure similar to salivary glands, acinar cells secrete enzymes and fluid into duct system
pancreatic duct fuses with bile duct on entry to duodenum
composition of pancreatic juice
aqueous component; 200-800ml secreted each day, rich in bicarbonate (pH 8), helps to neutralise acidic chime as it enters duodenum, secretion stimulated by secretin.
enzymatic component
- proteolytic enzymes (cleave peptides) secreted in inactive form, activated in duodenum e.g. trypsin
- pancreatic amylase (breaks down starch) responsible for majority of starch digestion, secreted in active form
- lipolytic enzyme e.g., cholesterol esterase, PLA2, activated by trypsin in duodenum
control of pancreatic secretion
neuronal and hormonal control, nervous reflex involves medulla and vagal innervation (cephalic phase)
gastrin released in response to stomach distension (gastric phase)
secretin and CCK secreted by mucosa in response to presence of chime in duodenum (intestinal phase)
secretin-aqueous juice
CCK0 enzymatic juice
the liver
lies in abdomen under diaphragm.
hepatic (liver) cells arranged in radial pattern around central vein
hepatocytes make up functional units called lobules
lobules from lobes of liver
hepatic blood supply
receives double blood supply, oxygenated blood from hepatic artery, de-oxygenated, nutrient rich blood from portal vein
hepatic cells; extract oxygen and most nutrients, detoxify or store poisons and drugs, secrete products (except bile) into hepatic vein.
bile
2 characteristics- excretory product of liver metabolism, a digestive secretion
bile salts emulsify fat into small droplets
bile cholesterol made soluble by bile salts and lecithin
bile pigments (bilirubin) give faeces their colour
secretion increased by vagal stimulation and secretin
absorption of fats in the liver
mono glycerides and FFAs associate with bile salts and lecithin to form micelles, micelle enters aqueous layer of microvilli, MGs, FFAs, cholesterol, fat soluble vitamins and lecithin diffuse passively into duodenal cells. bile salt portion remains in lumen of gut until terminal ileum and then recycled by enterohepatic circulation.
lipids accumulate in vesicles of smooth ER of epithelial cell to form chylomicrons, chylomicrons leave intestine in lymph for delivery into venous circulation. lipids therefore bypass the liver in the short term.
metabolic function of the liver
storage0 converts excess monosaccharides to glycogen
metabolism; breaks down stored glycogen, fat or protein to glucose (hormonal control)
metabolises drugs and breaks down poisons
liver- bile drainage
canaliculi (mesh between hepatocytes)> small terminal bile ducts > peri lobular ducts> interlobular bile ducts
gallbladder
stores ad concentrates bile by extracting water and ions, bile enters by cystic duct when small intestine is empty.
ejection of bile occurs when; proteins or fat rich chyme enters duodenum causing CCK release. gallbladder contracts and sphincter or oddi relaxes. bile ejects into duodenum
bile secretion
sphincter of oddi controls flow of bile (and pancreatic secretions)
basal contraction prevents reflux of duodenal contents into bile duct.
gallbladder emptying hormonal and cholinergic regulation
enterohepatic circulation of bile acids
12-36 g of bile acids secreted each day
liver basal rate of synthesis of bile secretion is 600mg/day
GI tract recycling bile acids; total pool 3g. must be recirculated 4-12 times a day, more for a fat rich meal.
recycling via enterohepatic circulation- terminal ileum and colon reabsorbs bile acids to return to liver via portal blood
