Physiology Flashcards
four layers of the GI tract
mucosa
submucosa
muscularis externa
serosa/adventitia
where is the muscularis mucosae located?
mucosa
where is the submucous plexus located?
submucosa
where is the myenteric plexus located
muscularis externa
which layer is made up of the circular and longitudinal muscle layers?
muscularis externa
function of the circular muscle layer?
makes the lumen longer and narrower
function of the longitudinal muscle layer
makes the lumen shorter and fatter
what initiates slow wave activity in the GI tract?
interstitial cells of Cajal (pacemaker cells between the two muscle layers)
major patterns of motility in the stomach and small intestine
- peristalsis
- segmentation
- colonic mass movement (faeces into rectum)
- migrating motor complex (stomach to terminal ileum)
- tonic contractions
gastric factors affecting stomach emptying
volume and consistency of chyme
patterns of motility in the large intestine
- haustration (slow wave)
- peristalsis propulsion
- defecation
two areas of secretion in the stomach
- oxyntic gland area
- pyloric gland area
cells in the oxyntic gland area
- chief cells (pepsinogen)
- parietal cells (HCl, intrinsic factor and gastroferrin)
- enterochromaffin (histamine)
cells in the pyloric gland area
- D cells (somatostatin)
- G cells (gastrin)
role of intrinsic factor and gastroferrin
bind B12 and Fe2+
role of somatostatin
inhibit HCl secretion
how does the mucosa protect itself from being digested?
secretes PGE2 and PGI2
two motility regions in the stomach
- orad: tonic contractions- vagus
- caudad: slow wave (retropulsion)
two types of secretions in the pancreas
- endocrine: islets of Langerhans
2. exocrine: acinar cells
five pancreatic enzymes (precursors)
- trypsinogen
- chymotrypsinogen
- carboxypeptidase
- pancreatic amylase
- pancreatic lipase
how is Cl- secreted
through CFTR on apical membranes (overstimulated causes secretory diarrhoea- loss of K+)
describe luminal digestion
pancreatic enzymes secreted into the duodenum
describe membrane digestion
mediated by enzymes at the brush border of epithelial cells
in carbohydrate digestion what is the role of alpha-amylase
breaks alpha-1,4-links
in carbohydrate digestion what is the role of isomaltase?
breaks alpha-1,6 links
what are oligosaccharidases
integral membrane proteins with a catalytic domain that faces the lumens
examples of oligosaccharidases
lactase, maltase, sucrase and isomaltase
substances involved in protein digestion
- HCl and pepsin in stomach
- pancreatic enzymes function as either endopeptidases or exopeptidases in the small intestine
describe lipid digestion
converted to an emulsion by lipases which is reduced by pancreatic lipase
what does failure to secrete bile salts result in?
lipid malabsorption (steatorrhoea and vitamin deficiency- A, D, E and K)
how are glucose and galactose absorbed?
secondary active transport mediated by SGLT1 (Na+)
how is fructose absorbed
facilitated diffusion mediated by GLUT5
how do all monosaccharides exit enterocytes
facilitated diffusion by GLUT2
how are peptides absorbed at the brush border?
by PepT1 (co-transport with H+)
how is cholesterol absorbed
NPC1L1 (ezetimibe)
what is the taeniae coli
longitudinal smooth muscle in the colon is divided into three strands to cause the production of haustra
how is the appendix connected to the caecum?
appendiceal orifice
three major functions of the large intestine
- secretion of K+, HCO3- and mucus
- absorption of Na+, Cl- and H2O (& short chain fatty acids)
- reservoir for periodic elimination
what enhances Na+ absorption and K+ secretion
aldosterone
is the absorption of water a passive process?
yes, it follows solutes (usually Na+)
three mechanisms of water absorption in the jejunum
- Na+/glucose co-transport
- Na+/amino acid co-transport
- Na+/H+ exchange
mechanism of water absorption in the ileum and proximal colon
Na+/H+ and Cl-/HCO3- exchange (reduced NaCl absorption causes diarrhoea)
mechanism of water absorption in the distal colon
epithelial Na+ channels (enhanced by aldosterone)
define assimilation
overall process of digestion and absorption
major functions of the liver
- metabolism
- deactivation of hormones (insulin, glucagon, ADH, steroids)
- storage of A, D, E, K vitamins, iron, copper and glycogen
- synthesis of protein (acute phase proteins, coagulation, albumin, etc.)
- protection: Kupffer cells
- detoxification
- secretion of bile
describe the secretion of bile by the liver
secreted by hepatocytes into canaliculi which drains into bile ducts, neutralising chyme an forms micelles
two phases of drug metabolism in the liver
Phase 1: oxidation (CYP450s), reduction and hydrolysis.
phase 2: conjugation e.g. glucuronidation
what is the enteric nervous system
it is the autonomic nervous system that supplies the GI tract
parasympathetic neurotransmitter in the ENS
both preganglionic and postganglionic synapse with ACh
role of the parasympathetic in the ENS
‘rest and digest’
- increases blood flow, secretion and smooth muscle contraction
- relaxation of sphincters
neurotransmitters in the sympathetic of the ENS
preganglionic release ACh and postganglionic release NA
role of the sympathetic in the ENS
- increased sphincter tone
- decreased motility, secretion and blood flow
define retching
rhythmic reverse peristalsis with forceful contraction of abdominal muscles and the diaphragm
define emesis
forceful expulsion of gastric-intestinal contents out of the mouth
what stimuli act directly on the VC
- pain, repulsive sights, smells and emotional factors
- motion (signals via the vestibular nucleus)
what stimuli act on the NTS
pharyngeal stimulation, gastric/duodenal distension or irritation
what stimuli act on the CTZ (no BBB)
- pharyngeal stimulation, gastric/duodenal distension or irritation
- toxins, drugs and vagal afferents
consequences of severe vomiting
- dehydration
- metabolic alkalosis
- hypokalaemia
- oesophageal damage e.g. Mallory-Weiss tear
which area of the brain ‘causes’ obesity
lesioning ventromedial hypothalamus (leptin and insulin report fat status)
define satiety
period of time between termination of one meal and the next (gherlin is the hunger signal)
