Science of Medicines Week 8 Flashcards
define gastrointestinal system
a network of organs and specialised cells that enable you to transform food into the energy and nutrients required for life
Which organs is the GI tract made up of?
mouth, oesophagus, stomach, small intestine, large intestine, anus
Name the accessory organs
salivary glands, liver, gallbladder, pancreas
define accessory organ
an organ that plays a role in the function of the GI tract but is not part of it
define mesentery
a membranous tissue that surrounds the organs of the GI tract
What is the function of the mesentery?
- holds intestines in place
- has lots of blood vessels and lymphatics for absorption and immunity
What prevents food travelling to the lungs?
the pharynx and epiglottis
What is the function of the oesophagus?
to transport food bolus to the stomach
What are the main functions of the stomach?
- mix food
- digest protein
- emulsify fat
How many layers is the stomach wall made up of?
4
Which cells is the stomach acid produced by?
parietal cells
Which 3 sections is the small intestine divided into?
duodenum, jejunum, ileum
What is the function of the small intestine?
primary site of absorption
What increases the surface area of the small intestine?
finger-like projections called villi that contain microvilli with epithelial cells on surface
Which 3 sections is the small intestine divided into?
ascending colon, transverse colon, descending colon
What is the function of the large intestine?
- role in conservation of water and ions
- storage of faecal matter
- crypts that are involved in lysosome secretion
How many sets of salivary glands do humans have?
3
What is the function of saliva?
- contains some digestive enzymes
- lubricates food with mucus
What is the function of the liver in digestion?
- produces bile and involved in excretion
- bile is needed for emulsification of fats
What is the function of the gallbladder?
- collects secretions from liver
- delivers bile to duodenum
What is the function of the pancreas?
- makes enzymes to digest proteins, fats, carbohydrates
- produces insulin and glucagon from islets of Langerhans
What are the 4 functions of the GI system?
- ingestion
- secretion
- movement
- digestion
define ingestion
the process of taking in food or drink through the mouth
define secretion
the release of substances that contribute to digestion
What are the 2 main types of movement in the GI system?
segmentation and peristalsis
define segmentation
‘squishing’ of substances backwards and forwards to help move them
define peristalsis
coordinated movement with contraction and relaxation of intestine
define digestion
breakdown of food into smaller components that can be absorbed into the bloodstream
What are the 2 types of digestion?
mechanical and chemical
define absorption
the process through which nutrients, water and electrolytes enter the blood (through villi and microvilli)
define excretion
the process by which metabolic waste is eliminated from the body
define swallowing
the passage of a food bolus from the oral cavity to the stomach
What muscles does swallowing involve?
muscles in the oral cavity, pharynx, larynx and oesophagus
What are the 3 phases of swallowing?
oral, pharyngeal, oesophageal
What are the steps of swallowing?
- stimulated when a food bolus stimulates pressure receptors in the back of the throat and larynx
- signals to the swallowing centre in the brain
- swallowing centres trigger nerve impulses
- this stimulates skeletal muscles in the pharynx and upper oesophagus
- involuntary contraction in the pharyngeal muscles pushes material into the oesophagus
What are the 2 types of movement in the stomach?
churning, peristalsis
When food is mixed, what are the stomach contents referred to as?
chyme
What does peristalsis in the stomach cause?
opening of the pyloric sphincter
What does the rate of gastric emptying depend on?
viscosity of chyme
How does movement in the stomach affect drugs?
churning is involved in dissolution and gastric emptying rate influences drug absorption
What happens to a drug if there is delayed gastric emptying?
the drug stays in the stomach and affect its stability as it is in acid for longer
What are the two types of movement in the small intestine?
segmentation (causes mixing) and peristalsis
What is peristalsis in the small intestine coordinated by?
the enteric nervous system
define segmentation
when non-adjacent segments of the small intestine alternately contract and relax, moving the food forwards then backwards
What are the steps of peristalsis in the small intestine?
- contraction of circular muscles behind food mass
- contraction of longitudinal muscles ahead of food mass
- contraction of circular muscle again forces food mass forward
What does the enteric nervous system control?
intestinal and colonic motility
What are the 2 main branches of neurones in the ENS?
Auerbach’s (myenteric) plexus and Meissner’s (submuscosal) plexus
What is the ENS influenced by?
neurotransmitters and autonomic input
How does the sympathetic NS affect the ENS?
inhibits the ENS -> decreases peristalsis, blood flow, secretion and absorption
How does the parasympathetic NS affect the ENS?
stimulates ENS -> increases peristalsis, blood flow, secretion and absorption
Which hormones increase activity of the ENS?
gastrin, motilin, serotonin, insulin, cholecystokinin
Which hormones decrease the activity of the ENS?
secretin and glucagon
How does intestinal motility affect drug absorption?
- transit time -> influences absorption
- distribution -> contractions in GI tract increase SA of drug
- migrating mobility complex -> involved in regulating drug absorption
define migrating motility complex
a recurring motility pattern that is regulated by electrical activity
When does migrating motility complex occur?
between feeding
What is the function of the migrating motility complex?
used to get rid of undigested food -> cleansing mechanism
How can the migrating motility complex affect drug administration?
if people take drug with an empty stomach, then it helps it pass through the body quickly and be absorbed quicker
What are the 2 types of motility in the large intestine?
haustral shuttling and mass movement
How does haustral shuttling work?
done by localised contraction and relaxation of haustra (bands) and occurs all the time
What is mass movement?
coordinated contraction of the intestine that moves the contents of the colon towards the rectum
Where does mass movement occur?
ascending and transverse colon
What is mass movement triggered by?
the gastrocolic reflex that signals to your body that you need to defaecate
What causes constipation?
weak mass movement, increased transit time, too much water absorbed into the body
What causes diarrhoea?
hyperactive mass movement, decreased transit time, too little water absorbed into the body
define vomiting
forcible expulsion of gastric contents through the mouth
Where are signals relayed for vomiting to?
the vomiting centre in the brainstem
What are the steps of vomiting?
- increase salivation to protect teeth
- relaxation of the lower oesophagael sphincter to allow contents to move up
- contraction of diaphragm and abdominal muscles to increase pressure in stomach
- opening of the uppoer oesophageal sphincter
mouth and oesophagus role in motility
chewing, swallowing, peristalsis
stomach role in motility
churning, peristalsis, gastric emptying
small intestine role in motility
segmental contractions, peristalsis
large intestine role in motility
haustral shuttling, mass movement, defaecation
sphincters role in motility
regulation of movement
define digestion
the sequence by which food is broken down and chemically converted so that it can be absorbed by the cells of an organism
What are the 2 types of digestion?
mechanical and chemical
What are the 4 sites of digestion?
- mouth
- stomach
- small intestine
- large intestine
What are the secretions from the salivary glands?
amylase, ligual lipase, mucus
Which type of cell secretes amylase in the salivary glands/
serous cells
What happens to amylase in the stomach?
it is inactivated due to low pH
Which cell is ligual lipase secreted from?
serous cells
What does ligual lipase do?
breaks down lipids and is NOT degraded in the stomach
Why are mucous cells in salivary glands important?
they secrete mucus to lubricate food and protect the mouth from stomach acid
What are the 4 types of cells that secrete substances in the stomach?
- parietal cells
- chief cells
- mucous cells
- G cells
What do parietal cells do?
they secrete HCl and release pepsinogen, which is cleaved to pepsin
What do chief cells do?
secrete pepsinogen also, which is cleaved to pepsin to digest proteins
What do mucous cells do?
secrete mucous for lubrication and protection of food from acid
What do G cells do?
produce gastrin
What is gastrin?
a hormone involved in regulating the release of HCl
What are the 4 main secretions from the stomach?
pepsinogen, HCl, gastrin, mucus
What is gastric acid production stimulated by?
histamine, gastrin, acetylcholine
What cells does gastric acid production involve?
parietal cells, G cells, enterochromaffin cells
What does the gallbladder do?
collects bile from the liver and concentrates it -> bile then passes through bile duct into small intestine
What does the pancreas secrete?
- bicarbonate (neutralise stomach acid)
- lipase
- amylase
- RNAase and DNAase
- trypsinogen, chymotrypsinogen
How is the digestive process initiated and controlled?
- stretch receptors in the stomach trigger secretion of digestive enzymes and juices
- the stomach releases gastrin, which causes proudction of gastric acid
- intestines release secretin and cholecystokinin to stimulate the secretion of pancreatic juice and bile
How is the small intestine adapted for absorption?
cells have projections called villi, which contain microvilli, giving a large SA
What is the route of absorption for sugar and amino acids?
absorbed directly into the bloodstream
How is absorption of fats different?
they are absorbed into the lymph lacteal system
What are the 2 main mechanisms of absorption?
active transport and facilitated diffusion
Where are proteins digested and by which enzymes?
- stomach -> pepsin
- pancreas -> trypsin and chymotrypsin
- intestinal brush border -> peptidases
How are proteins absorbed?
by active transport into the gut cells, then by diffusion into the bloodstream
Where are carbohydrates digested and by which enzymes?
- mouth -> salivary amylase
- small intestine -> pancreatic amylase
- enzymes in the intestinal brush border
Where does carbohydrate digestion stop?
in the stomach as the stomach acid denatures amylase
How are carbohydrates absorbed?
by facilitated diffusion and active transport
Where is fat digested and by which substances/enzymes?
- small intestine -> secretions from liver, pancreas and gallbladder to emulsify
- gallbladder and pancreas -> lipase and bile
How are lipids absorbed?
- lipids are broken down into fatty acids
- fatty acids absorbed into epithelial cells of small intestine
- they are coated with cholesterol, phospholipids and protein to form chylomicron
- chylomicron is water soluble and can enter the lacteal system
- transported to bloodstream
What does an enteric coating do?
prevents degradation of drugs in the stomach by protecting them from gastric acid -> allows them to be release in the small intestine
What is an enteric coating made from?
a gastric resistant polymer which is insoluble at low pH
Anatomically, how many lobes does the liver have? What are they called?
4 = left, right, caudate, quadrate
define diaphragmatic surface
the upper surface of the liver that sits underneath the diaphragm
define visceral surface
the lower surface of the liver that faces the abdominal organs and gallbladder
How is bile removed from the liver?
the bile duct feeds into the gallbladder, which joins the pancreatic duct and empties into duodenum of small intestine
What are Kupffer cells?
immune cells in the liver
What are the 3 blood vessels connected to the liver?
hepatic artery (oxygenated blood from heart), hepatic portal vein (little oxygen from GI tract), hepatic vein
What are the 6 main functions of the liver?
- endocrine function
- exocrine function
- clotting
- cholesterol metabolism
- organic and drug metabolism
- secretion of plasma proteins
What is the exocrine function of the liver?
secretes bile salts and HCO3-
What is bile made up of?
bile salts
What are bile salts important for?
solubilising fats
What is bilirubin?
byproduct of breakdown of red blood cells
What do epithelial cells secrete?
bicarbonate
What do hepatocytes secrete?
bile, cholesterol, phospholipids
What happens to bile during a meal?
the body must prepare to digest fats, to the gallbladder contracts and releases bile into the duodenum via the sphincter of Oddi
What is contraction of the gallbladder stimulated by?
cholecysotkinin which is triggered by fatty acids
define enterohepatic circulation
when the liver extracts bile salts from the blood and secretes them back into the bile to prevent having to synthesise them constantly
Where are small amounts of bile lost to?
faecas and urine
What are new bile salts synthesised from?
cholesterol
What happens to bile salts during a meal?
- bile salts are absorbed into the small intestine by Na+ coupled transporters
- most is then returned to the liver through the hepatic portal vein via enterohepatic circulation
What does dietary fibre do to cholesterol?
it conceals the bile in the small intestine to prevent it returning to the liver and it is removed from the body with the cholesterol it contains
What are the functions of cholesterol?
- membrane synthesis
- bile synthesis
- precursor for steroid hormones
Where and how is cholesterol synthesised in the body?
in the liver by HMG-CoA
How are cholesterol levels maintained in a normal range?
increasing dietary cholesterol or suppressing HMG-CoA
What do saturated fatty acids do?
increase plasma cholesterol
What do poly/monounsaturated fatty acids do?
decrease plasma cholesterol
How does cholesterol circulate in the body?
in lipoprotein complexes
What do low density lipoproteins do (LDLs)?
deliver cholesterol to cells
What do high density lipoproteins (HDLs) do?
remove cholesterol from plasma and deliver it to the liver
What is atherosclerotic disease caused by?
deposition of cholesterol in the artery walls, increasing risk of heart attack
What decreases HDL?
smoking
What increases HDL?
weight loss, oestrogen (until menopause)
What are gallstones caused by?
cholesterol crystallising out of bile salts in the gallbladder
What do gallstones cause?
- they block the gallbladder and bile duct
- prevents fat digestion and causes fat build up in the gut -> diarrhoea
- decrease in absorption of fat-soluble vitamins, clotting issues and calcium deficiency
What are the endocrine functions of the liver?
- synthesis of hormones and peptides
- synthesis of plasma proteins
- synthesis of clotting factors
How is the liver important in clotting?
- bile absorbs fats, which absorb vitamin K
- vitamin K is needed for prothrombin production
- this is a precursor for thrombin
What is hepatitis caused by?
viruses or excessive alcohol intake
What is liver cirrhosis caused by?
drugs, alcohol, viruses
In liver function blood tests, what do raised levels of liver enzymes indicate?
liver damage -> most of the enzymes should be in hepatocytes, but are detected in blood
Which compounds are used in liver function tests?
- alanine transaminase (ALT)
- aspartate aminotransferase (AST)
- alkaline phosphatase (ALP)
- gamma-glutamyltransferase (GGT)
- bilirubin
- albumin
What does any raised hepatocyte enzyme indicate?
liver cell damage
What does an AST:ALT ration above 2:1 indicate?
alcoholic liver disease
What do raised levels of ALP indicate?
bone disease
What do raised levels of GGT indicate?
alcohol excess
What do raised levels of ALP and GGT indicate?
obstruction of the bile duct
