GI physiology Flashcards
What does EAR stand for?
Estimated average requirement
what does LRNI stand for?
Lower reference nutrient intake
what does URNI stand for?
Upper reference nutrient intake
what percentage of energy comes from fat store?
80-90%
how much protein do we require daily?
0.75g/kg/day
what are the 9 essential amino acids?
Methionine, valine, histidine, leucine, phenylalanine, tryptophan, isoleucine, lysine, threonine
how many calories does 1g of fat give you?
9
why are fats important other than energy?
precursors for important molecules eg hormones
why do you need omega 3+6?
prevent heart disease
what are the macronutrients in your diet?
protein, lipids, carbohydrate
why are vitamins important in a diet?
mostly coenzymes that are needed for normal metabolic function. Cannot be synthesised in the body
why are minerals in the diet important?
have important physiological functions.
eg iron in haem, calcium in signalling etc
what are the 3 main reasons nutritional deficiencies arise?
Inadequate intake
inadequate absorption
Excess loss/increased requirement
what is vitamin B1, what is its role and what does a deficiency cause?
Thiamin, cofactor in decarboxylation, Beri-Beri (muscle weakness + nerve damage)
what is vitamin B2, what is its role and what does a deficiency cause?
riboflavin, helps make FAD, stomatitis
what is vitamin B3, what is its role and what does a deficiency cause?
niacin, helps make NADH, Pellagra (diarrhoea, depression, dermatitis)
what is vitamin B5, what is its role and what does a deficiency cause?
pantothenic acid, helps make Coenzyme A, deficiency very rare.
what is the role of biotin?
carrier for CO2
what is vitamin B6, what is its role and what does a deficiency cause?
pyridoxal phosphate, cofactor for protein production, irritable + depression + confusion
what is vitamin B9, what is its role and what does a deficiency cause?
folic acid, carbon transfer in dna synthesis, neural tube defects
what is vitamin B12, what is its role and what does a deficiency cause?
cobalamin, collagen synthesis, macrocytic anaemia
what is vitamin C, what is its role and what does a deficiency cause?
ascorbic acid, collagen synthesis, scurvy
vitamin E, what is its role and what does a deficiency cause?
antioxidant in lipid membranes, rare - can cause haemolytic anaemia + thrombocytosis
vitamin K, what is its role and what does a deficiency cause?
cofactor for clotting cascade, causes haemorrhagic diseases
vitamin A, what is its role and what does a deficiency cause?
is a collection of retinoids - aloows you to see, can cause xerophthalmia (keratinisation of cornea) + blindness
vitamin D, what is its role and what does a deficiency cause?
used in bone formation, causes rickets in kids and osteomalacia in adults
how long is the gut tube?
8-9m
What are the 6 functions of the GI tract?
Ingestion mechanical processing digestion secretion absorption excretion
what is the peritoneum and what is its function?
lines the abdominal cavity (parietal) and covers organs (visceral). it forms mesenteries and secretes peritoneal fluid for lubrication.
what is the function of mesenteries?
suspend and support organs to prevent tangling
carries blood vessels, nerves, lymph to the GI tract
what are the 4 layers of the gut tube?
mucosa, submucosa, muscularis externa, serosa
what is the mucosa split up into?
epithelium and lamina propria
what features are found in the submucosa?
arteries, veins, lymph, submucosal plexus
what features are found in the muscularis externa?
circular muscle, longitudinal muscle layers, myenteric plexus
what is the serosa the same thing as?
visceral peritoneum
how must all active processes cross cells?
transcellularly
what are the 3 ways passive processes can occur?
non-coupled (down electrochemical gradient)
via pores or channels
facilitated diffusion via transporters
what is Primary active transport?
transport requires energy from ATP and goes against gradient
what is secondary active transport?
co-transport where the transport of 1 down electron gradient allows another at the same time
which system has neural control of the gut?
Autonomic
How does Parasympathetic effect the gut?
increases digestion (stimulatory)
which parasympathetic nerves control the foregut and midgut?
vagus
which parasympathetic nerves control the hindgut?
splanchnic nerves (S2-S4)
how does sympathetic effect the gut?
inhibitory effect
What plexuses make up the enteric system and what does this system do?
made up of myenteric and submucosal plexi
provides local regulation and control
what are slow waves?
regular oscillations in the resting potential
what are slow waves controlled by?
ICC cells and cells of cajal
what are ICC cells?
single layer in plexus regions - connect smooth muscle cells via gap junctions
what is the function of the hard palate in mastication?
provides an anvil for tongue to mash food against
what is the function of the soft palate in swallowing?
directs bolus of food into oesophagus
what are the 4 functions of saliva?
lubrication + cleaning
facilitation of taste
protection against acid + bacteria
digestion
how is saliva regulated?
neural control. Parasympathetic makes it watery, sympathetic makes it mucoid.
controlled by salivary centre in brain
where is saliva produced?
parotid, submandibular and sublingual glands
how is saliva produced?
starts as isotonic fluid (mainly NaCl, protein and mucus)
salt reabsorbed making it hypotonic and alkaline
rate of travel down duct changes the extent of this
where is the innervation of the parotid gland controlled from?
PS - CN IX (glossopharyngeal)
S - superior cervical ganglion
What is saliva from the parotid gland rich in?
lingual amylase and proline-rich proteins
where is the innervation of the submandibular gland controlled from?
PS - CN VII (facial)
S - superior cervical ganglion
What is saliva from the submandibular gland rich in?
lysozymes
where is the innervation of the sublingual gland controlled from?
PS - CN VII (facial)
S - superior cervical ganglion
What is saliva from the sublingual gland rich in?
lipase
which is the largest saliva gland?
parotid
which gland produces most of the saliva?
submandibular (70%)
What is the function of lingual lipase?
Cleave one chain to form diacyglycerol from triclyceride
What is the function of ptyalin A?
Breaks 1-4 bonds in polysaccharides leaving maltose and limit dextrins
Where are taste buds found?
Papillae of the tongue
What are the 3 forms of papillae?
Foliate, circumvallate, fungiform
How are salty tastes sensed?
Ion based channel sensor for Na+
How are sour tastes sensed?
Ion based channel sensor for H+
How are sweet, bitter and umami tastes sensed?
Activation of specific GPCRs
What are the 3 phases of swallowing?
Voluntary, pharyngeal, oesophageal
What is the epithelium in the oesophagus?
Stratified squamous
How is the oesophagus innervated?
Oesophageal plexus
What are the 3 functions of the LOS and cardia of the stomach?
Prevent reflux, regulate belching, secrete mucus and HCO¬3 -
What are the 3 functions of the fundus and body of the stomach?
Acts as reservoir of volume, tonic force during emptying, secretes – H+, intrinsic factor, mucus, HCO3- , pepsinogen and lipase.
What are the 3 functions of the antrum and Pylorus of the stomach?
Mixing, grinding and sieving, regulate emptying, secrete mucus and HCO3-.
How does the stomach accommodate food?
Signalling from stretch receptors via vagus causes dilation and relaxation of stomach
What cell types are found in gastric pits?
Mucous cells, parietal cells, chief cells, G cells
What is the function of mucous cells?
Produce mucous and HCO3-
What is the function of Parietal/oxyntic cells?
Secrete HCl and intrinsic factor
What is the function of chief cells?
Produce pepsinogen
What is the function of G cells?
Secrete gastrin into blood stream
What is the function of gastrin and how is its release controlled?
Function: stimulate acid secretion and promote mucosal growth. Release stimulated by luminal proteins and PS stimulation. Release inhibited by H+.
What is the function of Somatostatin and how is its release controlled?
Function: inhibit production of gastrin, CCK and secretin. Release stimulated by H+. Release inhibited by Ach.
How are parietal cells activated and what happens when they are?
Activated by protein kinases via gastrin on Gq proteins or histamine on H2 receptor causing production of cAMP. When activated tubulovesicles form canaliculi for larger surface area.
How does mucus and bicarb protect the epithelium?
Mucus traps bicarb which neutralises H+
How is pepsinogen release controlled?
Ach from vagal nerves acting on M3 receptors. Gastrin and CCK also produce mild stimulatory effect
What does acid secretion induce?
Secretion by release of secretin
How is pepsin activated?
Cleaves spontaneously below pH 5 and other pepsin cleaves it.
How do pepsins act?
Endoproteases – cut within peptide chain. Preferential for aromatic or large side chained AAs.
What are the 3 main enzymes for gastric digestion?
Pepsin, gastric lipase, Ptyalin A
How does the small intestine amplify its surface area?
Villi and microvilli
What do crypts secrete?
Bicarbonate rich fluid
What is peristalsis and how does it happen?
It is waves of muscular contraction. Circular muscles contract behind the bolus and longitudinal muscles contract ahead of the bolus shortening the segments.
What is segmentation and why does it happen?
Alternate contraction of neighbouring segments. This churns the bolus and fragments it.
What does MMC stand for?
Migrating motor complex
What is the purpose of an MMC?
Helps clean the gut, prevent reflux and reduce bacterial growth.
What are the 6 things the crypts of lierberkuhn secrete and which cells do they come from?
CCK from I cells, Secretin from S cells, Motilin from M cells, Gastrin from G cells, mucus from goblet cells, bicarb from pancreatic duct cells.
What can the intestine absorb without digestion?
Water, electrolytes, vitamins
Describe how Carbohydrates are broken down and absorbed in the small intestine.
Amylases break 1-4 bonds and then remaining are broken down by specific enzymes at the brush cell border. Glucose and galactose are absorbed by SGLT1 and Fructose is absorbed passively through GLUT5.
Which enzymes are involved in the breaking down of carbohydrates in the small intestine?
Sucrose, isomaltase, glucomylase, lactose
Describe how proteins are broken down and absorbed in the small intestine.
Pepsin begins breakdown in the stomach. Pepsin deactivated in duodenum. Enterokinase secreted by crypt cells along with pancreatic enzymes break protein down into small peptides. Diffuse to brush border. Single peptides are taken up by Na linked 20 active transporters. Dipeptides taken up by proton linked 20 active transporters.
Which proteases are endopeptides?
Trypsin, elastase, chymotrypsin.
Which proteases are exopeptides?
Carboxypeptidase A+B
Describe how fats are broken down and absorbed in the small intestine.
Bile salts break up lipid droplets. Pancreatic lipase cleaves second outside fatty acid leaving a monoglyceride and 2 free fatty acids. These complex with bile salts which solubilise the fats and form a micelle with cholesterol etc. the mixed micelles can be taken up by the enterocytes and inside they are resynthesized to chylomicrons which can be taken away in lacteal lymphatics.
Describe how iron is taken up in the small intestine.
Fe3+ reduced to Fe2+. Co-transported via DMT into the cell. Oxidised either by heame in cell or transported by mobilferrin and oxidised outside.
Describe how calcium is taken up in the small intestine.
Paracellularly or transcellularly regulated by vitamin D.
How is B12 absorbed in the small intestine?
Binds to intrinsic factor which can then be taken up by enterocytes.
Where do pancreatic secretions enter the duodenum?
The ampulla of vater
What controls the entering of secretions?
The sphincter of oddi – relaxes in response to CCK-PZ (cholecystokinin pancreozymin)
What does APUD stand for?
Amine Precursor Uptake and Decarboxylation
CCK – what is its effect and what is it stimulated by?
Causes pancreatic secretion and stimulates emptying of gall bladder. Release is stimulated by lipids and peptides in the small intestine. Binds to CCK a+b receptors. A better for CCK, b better for gastrin.
Secretin – what is its effect and what is it stimulated by?
Stimulates the production and secretion of bicarb from cholangiocytes, inhibits secretion of gastrin. Release stimulated by acid in the small intestine.
What is bile a mixture of?
Bile salts, phospholipids, cholesterol, bile pigments, inorganic ions.
Describe how bile salt synthesis occurs.
Hepatocytes synthesise primary bile salts from cholesterol. These are metabolised by intestinal bacteria to secondary bile salts. These are excreted into blood stream and absorbed by hepatocytes. Bile acids conjugate with glycine and taurine to form bile salts.
What are the 2 primary bile acids?
Cholic acid and chenodeoxycholic acid
What are the 2 secondary bile acids?
Deoxycholic acid and lithocholic acid
Describe how bile pigments are synthesised and handled.
Senescent RBCs broken down by kupffer cells. Haemoglobin broken down to haem and globin. Haem cleaved to give bilirubin and iron. Bilirubin complexes with albumin and is transported to liver. Binds to glucoronic acid in hepatocyte and forms bilirubin glucuronide. Excreted and is catabolised by bacteria to form urobilinigen leading to formation of urobilin and then urine.
Describe the secretion of bicarbonate.
Bile flows along duct and bicarb salt and water are added. Bicarb co transport with cl- from lumen. It is stimulated by secretin acting on the cholangiocytes. Negatively charged bicarb drags Na+ and therefore water into the lumen.
How does the gall bladder work?
Reabsorbs salt and water from bile so it can store 450g worth in 50g. bile forms micelles so it is still isotonic.
Where are pancreatic enzymes made?
Acinar cells
How are pancreatic enzymes released?
Packed into zymogen granules and migrate to apical membrane. Ca2+ induces exocytosis which is induced by Ach, gastrin and CCK
What are the 5 pancreatic secretions?
Proteases, amylase, lipases, nucleases, bicarbonate
List the parts of the large colon in order starting with the ileocaecal valve.
Ileocaecal valve, caecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum.
What epithelium is in the large intestine?
Simple columnar epithelium
What is haustration?
The slow contractions of the circular muscles – same as segmentation is small intestine.
What is mass movement?
Organised peristaltic wave that causes the haustra to disappear. It occurs a few times daily.
Where does the control of the colon come from?
Enteric nervous system, some PS control.
Which reflexes trigger mass movements?
Gastrocolic and orthocolic reflexes
What is the ileal brake?
If fat is detected in the distal ileum this triggers slowing of gastric emptying and peristalsis.
How is digestion performed in the colon?
By bacteria
What can be digested in the colon?
Fibre to fatty acids, urea and AAs to ammonia, bilirubin to urobilinigen, primary bile acid to secondary bile acid etc
What is absorbed in the colon and how?
Short chain fatty acids by Na+ linked transporters, salts both para and transcellularly – stimulated by aldosterone.
What is secreted by the colon and how?
Chloride by increased cAMP and Ca2+ (can cause secretory diarrhoea), potassium – both actively and passively – mainly passively. Balance regulated by aldosterone and cAMP
What epithelium is found in the rectum and anus?
Stratified squamous
What are the 2 sphincters made from in the rectum?
Internal – smooth muscle, external – skeletal muscle.
