Physiology Flashcards
What nervous system is stimulatory to the enteric NS and what is inhibitory?
Sympathetic is inhibitory, and PNS is stimulatory
How does the sympathetic NS and the PNS have effects on the GI system?
SNS has effects on blood vessels, secretory cells and the enteric NS
PNS has only effects on the enteric NS
Three main salivary glands?
Sub-maxillary gland
Parotid
Sub-lingual
Main composition of saliva?
Enzymes: amylase, lipase, lysosymes
Inorganic: Ca++, Phosphate, K+, Cl-, Na+
Organic: Urea, citrate, AA’s, Steroid hormones
Functions of saliva?
Moistens food
Digests food
Buffer action
What are the effects of the SNS and PNS on salivary glands?
SNS causes vasoconstriction, and less secretion
PNS causes vasodilatation and more secretion
When both are activated the PNS overrides the action of the SNS
Two phases of saliva secretion?
Cephalic phase triggered by sight/smell of food
Reflex phase triggered by chewing
Actions of the two oesophageal sphincters?
UOS - prevents air swallowing
LOS - prevents acid reflux
Phases of swallowing?
- Close nasal opening
- Displace larynx superiorly and anteriorly
- Close epiglottis
- Relax UOS
- Contract pharynx
- UOS contracts
Phases of gastric acid secretion?
Cephalic phase - sight/smell of food
Gastric phase - presence of food in stomach causes gastrin secretion
intestinal phase - food in the duodenum first stimulates motility + acid secretion then inhibits it.
Three parts of the stomach?
Fundus (superior)
Body
Antrum (inferior)
What cells release H+ into the stomach lumen?
Parietal cells
What two compounds stimulate the release of H+ into the stomach lumen?
Histamine and gastrin
What things can induce vomiting?
Blood-borne stimuli affecting the CTZ
Intestinal blockage
Smells/tastes/sights affecting the VC in reticular formation
Labyrinthian system affecting the VC
Stages of vomiting?
Retching - rhythmic contractions of the diaphragm and abdominal muscles against a closed glottis
Stomach tone in fundus and peristaltic activity is decreased
Duodenal and proxial jejunal tone is increased, duodenal contents reflux back into stomach
Strong contractions LOS opens, mouth opens
Consequences of vomiting?
Metabolic alkalosis Hypovolaemia Hypokalaemia Hyponatraemia Mallory Weiss tear Fatigue Teeth damage
What is trypsinogen converted to and by what?
To trypsin by enteropeptidase
What three enzymes of pancreatic secretion are converted to their active forms by trypsin?
Chymotrypsinogen to Chymotrypsin
ProcarboxypeptidaseA,B to carboxypeptidase A,B
Proelastase to Elastase
Enzymes in pancreatic secretions?
Trypsin, Chymotrypsin, carboxypeptidase A,B, Elastase
Amylase, Lipase, Nuclease, Kallikrein
What four things control pancreatic secretion?
Secretin: Na+ and H2O secretion
CCK and gastrin: Stimulates enzyme secretion
Vagus nerve activity: increased enzyme and H2O and Na+ secretion
When is secretin released, where and it’s effects?
Cells in lieberkuhn crypts release it in low pH
Stimulates Na+ and H2O release in pancreas
Inhibits stomach acid secretion
When is CCK released and it’s effects?
Stimulus is food in duodenum/jejunum
Stimulates gall bladder contraction
Stimulates pancreatic enzyme secretion
Some gastrin-like activity
Phases of pancreatic secretion?
Cephalic phase by vagus
Gastrin phase by the distension of the stomach
Intestinal - secretin, CCK and vagus stimulates
Components of bile?
Bile acids
Phospholipids
Cholesterol
Bile pigments
Protein
Inorganic ions: Na+, K+ e.t.c.
How is bilirubin formed?
From the breakdown of haemoglobin
What is free bilirubin?
Unconjugated bilirubin, usually travels bound to plasma proteins
What happens to free bilirubin once it is taken up by the liver?
It is conjugated and then secreted into the bile through the bile duct
What can cause dark urine? Why?
Biliary obstruction: conjugated bilirubin will pass into the blood and then go to the urine
Process that conjugated bilirubin follows as it is converted to stercobilin?
Conjugated bilirubin is converted by bacteria to urobilinogen
Urobilinogen to urobilin
Urobilin to Stercobilinogen
Stercobilinogen to Stercobilin (brown colour in faeces)
What are the three signs of excessive haemolysis, relating to bilirubin? Why?
Jaundice (free and conjugated bilirubin in plasma)
Yellow urine (Urobilin in plasma)
Dark faeces (more stercobilin in the gut)
What are the three signs of Biliary obstruction, relating to bilirubin? Causes?
Jaundice (excessive plasma bilirubin in skin)
Dark Urine (excessive conjugated bilirubin in urine)
Pale faeces (bile pigments do not reach lower gut)
What are the three signs of liver failure in relation to bilirubin? Causes?
Jaundice (excessive free bilirubin in skin)
Pale urine (conjugated bilirubin not in plasma)
Pale faeces (stercobilinnot formed in gut)
Three things that stones can be made of?
Cholesterol
pigment
Mineral (Ca2+)
How does bile reach the duodenum?
Between meals the sphincter at the base of the bile duct is contracted to divert bile into the gall bladder
Food in the duodenum causes CCK to be released which causes the gall bladder to contract
The sphincter opens and bile passes into the duodenum as food passes through
What is the function of bile acids?
Promote choleresis (bile flow)
Stimulate phospholipid secretion
Help solubilise cholesterol in the bile duct and gall bladder
Emulsify lipids in the jejunum
Aid absorption of fat soluble vitamins A, D, E and K
Stimulate colonic motility
Why do bile acids not reach the colon?
Reabsorbed in terminal ileum
What are the two proteins that bind B12 in the human body, how do they function together to absorb B12 into the body?
R protein and Intrinsic Factor
R protein saturates B12 with a higher affinity than IF
In the stomach most B12 is bound to R protein, in the jejunum R protein is digested and B12 moves to IF instead
When bound to IF it binds to a cell surface receptor which is then internalised and B12 moves to the ECF
What does reduced B12 absorption cause?
What is it called when its due only to lack of intrinsic factor?
Anaemia
Pernicious anaemia
Why does a continuous bleed of only a few ml a day so easily lead to anaemia?
Only a small amount of iron is absorbed daily, and only a small amount of red cells with new iron are produced, most is recycled, so therefore a loss of more red cells than are made will cause anaemia
How is iron taken into the body?
DMT (Divalent metal transporter) uptakes iron into epithelial cells
In epithelial cells some iron binds to ferritin and some passes into blood and transported bound to transferrin
The ferritin iron stays in the cell and is not used
EPO decreases the production of ferritin and therefore increases the amount of iron in the bloodstream
How is Vitamin D made into calcitriol?
Vit D made in the skin under UV light
Hydroxylated at position 25 in the Liver
Hydroxylated at position 1 in the kidneys
Major action of calcitriol?
Promote Gi absorption of calcium
How is Copper taken into the body?
Taken up into epithelial cells by the DMT transporter
Then transported from epithelial cells to the ECF by an ATPase
Travels bound to plasma proteins and amino acids
Taken into cells by amino acids, then the excess secreted by menkes ATPase
What is Menke’s disease and Wilsons disease?
Menke’s disease - Low activity of Menke’s ATPase
Wilsons disease - inability to excrete copper into the bile
How long does the chyme take to transverse the small intestine?
3-5hrs
What is an MMC or migrating myoelectric complex?
A burst of intense activity in the intestine, repeated every 90mins or so in the fasting state to clear debris
What initiates an MMC?
The hormone motilin
What is somatostatin, two examples of what causes it’s release?
Inhibits gastrin secretion
Excess acid and CCK causes D cells to release it
Why is the amount of chyme that enters the duodenum through the pylorus regulated?
Because the chyme has a high osmotic activity and even more so when it is further broken down, if not regulated there would be fluid expansion from the epithelium, possibly causing syncope
What is in the ‘intestinal juice’?
Duodenal (from Brunner’s glands):
- Mucus
- Pepsinogen II
- EGF
- Bicarbonate
Intestinal glands
- Mucus
- Enteropeptidase
- Bicarbonate
What are the main protein digesting enzymes?
Pepsin, trypsin, chymotrypsin, carboxypeptidases, elastase
What converts pepsinogen to pepsin?
H+ and also pepsin itself
How efficient is fat absorption in the body?
Very, 95% absorbed.
What does lipase do to triglycerides? What happens to the products of this?
Turns them into a monoglyceride and two free fatty acids
These are taken up in micelles and then taken into epithelium, reconstituted to triglycerides and then transported in chylomicrons
What are the 4 fat-soluble vitamins, how are they transported into the body?
Vit A, D, E and K
Transported into the body via micelles
What is the total uptake of water from the GI system?
11 Litres
Differences in water absorption in duodenum, jejunum, ileum and colon?
The duodenum is very permeable and there tends to be a osmotic flow into the lumen
The jejunum has a rapid absorption of sodium, cholride, bicarbonate, sugars and amino acids
The ileum has a smaller surface area than the jejunum and transport is not as rapid
The colon powerfully reabsorbs sodium chloride and water, whilst potassium is actively secreted and bicarb is in exchange for chloride
What does each of these do to absorption and secretion in the intestine.
- Stimulation of the sympathetic nervous system
- Stimulation of the Parasympathetic nervous system
- Aldosterone and Angiotensin II
- Histamine
- Sympathetic NS: Decreases secretion and increases absorption
- PNS: Increases secretion, decreases absorption
- Promote absorption
- Increase secretion and decrease absorption
Main role of all the sodium that is absorbed in the GI tract?
Used to absorb the products of digestion and carbohydrates
Three main reasons for diarrhoea?
Increased rate of transit: e.g. IBS or laxatives
Failure to absorb e.g. Familial chloride diarrhoea, Damaged mucosa
Increased secretion e.g. ‘Pancreatic Cholera’, Cholera Disordered prostaglandin metabolism
How does cholera produce diarrhoea?
Cholera recognises ganglioside Gm1 (a glycolipid)
catalyses the transfer of ADP to the α subunit, adenylyl cyclase is constantly stimulated, causing constant secretion
Innate defence mechanisms of the GI system to infection?
Mucus forms physical barrier
Trefoil factors - aid barrier repair and wound healing
Defensins (antimicrobial proteins)
What cells synthesise defensins?
Paneth cells
What is the immunological lining of the gut called?
Gut associated lymphoid tissue (GALT)
What is the Waldeyer ring?
Tonsils and adenoids which form a ring at the entrance to the gut and airways
What are Peyers patches?
Patches in the small intestine that contain 30-40 lymphoid follicles
What are microfold cells (M cells)?
They are cells in the gut that are in the epithelial barrier over peyers patches, they transfer particles from the gut to antigen presenting cells at the basal layer
What is pocketing in M cells?
When a ‘pocket’ forms in the M cell to allow antigens to be presented to T memory cells, B cells and APC
What infectious organisms exploit M cells?
Salmonella, Cholera and polio
What is the antibody of the mucosal immune system, what are the two forms?
IgA - IgA1 and IgA2
Where/how is IgA made
Made in plasma cells of gut lamina propria
Transported across epithelial cells
Released into gut lumen, where it binds to mucus
What are intra-epithelial lymphocytes?
Unconventional T cells that were concentrated in the gut
Functions of the large intestine?
Absorption of salt and water
Secretion of potassium and bicarbonate
Manufacture of some vitamins by some bacteria
Is the ileo-caecal valve normally open or closed? Why?
Closed - don’t want colon contents e.g. bacteria refluxing into the ileum
What causes the ileo-caecal valve to open?
Material in ileum induces peristalsis, producing the gastro-ileal reflex
What happens in the proximal and distal colon?
Proximal is where water and salt absorption mostly occurs
Distal is where thick paste is moved anally by short range peristalsis
What is the gastro colonic reflex
Colonic movements: Acts by vagus and pelvic innervation, with gastrin and CCK playing a role, about 30 mind after a meal
Two types of colon movements?
Colono-colono reflexes
Gastro-colonic reflex
What hormone controls ion movements in the colon?
Aldosterone
Mechanism for ion movements in the colon?
Tight junctions have a very low permeability, causing a very high PD across the colon
How is the urge to defaecate produced?
Mass movements of distal colon fill the rectum
Distension of rectum:
Reduces internal anal sphincter tone
Increases external anal sphincter tone
How is continence usually maintained?
The rectum is usually fairly empty
The angle of the sigmoid colon in unfavourable
Puborectal muscle is contracted
Tone of anal sphincters is high (usually)
How do dietary fibres speed transit?
Provide bulk to distend the gut tube
What hormone controls Na+ absorption in the colon?
What enhances Na+ absorption?
Aldosterone
Short chain fatty acids
Where in the Gi tract is most of the water absorbed?
The colon
Metabolic functions of bacteria in the colon?
Hydrolyse urea (salvage nitrogen)
Salvage short chain fatty acids
Synthesise Vit K
What do the colonic bacteria use for energy? Where do they get it from?
CHO
Dietary residue and endogenous losses
Whats NSP?
Non-starch polysaccharide, and example of CHO used by colonic bacteria
