gastrointestinal system Flashcards
what are the functions of GI secretions?
- chemical digestion
- lubrication -> mucus secretions, help movement of food
- signalling
- protection -> stomach acids
- activation of enzymes
- excretion of waste
exocrine glands
- local action
- produce and secrete substances onto an epithelial surface by way of a duct
endocrine glands
- local or systematic (can be in the blood stream)
- secrete their products, hormones directly into the blood rather than through a duct
gastric secretions
- acid
- pepsin
- gastric lipase
liver / gallbladder
- secretion, storage and modification of bile
salivary glands
- lubricating fluid containing enzymes that break down carbohydrates
pancreas
- exocrine cells: secrete buffers and digestive enzymes
- endocrine cells: secrete hormones
small intestine
- digestive enzymes
large intestine main role
reabsorption of water
types of salivary glands
- major salivary glands
- minor salivary glands
major salivary glands
- 3 pairs
- exocrine -> secrete saliva via a duct
minor salivary glands
- 600-1000
- mucosal lining of the oral cavity, lips, cheeks and palate
- exocrine with own duct
what does saliva consist of?
- 99.4% water
- 0.6%: mucins, electrolytes, antibodies (immuniglobin), enzymes (amylase)
function of saliva
- buffer -> ions keep pH at 7
- mucosa stays moist
- protects against any mechanical damage
- needed for speech
- solvent -> dissolves chemicals in food to help taste receptors detect them
what is xerostomia?
- sensation of dryness in mouth due to prolem in saliva production
- symptoms include: acid erosion of teth, infection, cracked lips..
how is saliva production controlled?
by the automic nervous system
- both parasympathetic and sympathetic divisions
role of parasympathetic ns in saliva production
it controls the volume of fluid secreted
(cranial nerves 8 and 9)
role of sympathetic ns in saliva production
modulate the composition of saliva (e.g. amylase, IgA)
(somatic nerves 1-3?)
gastric secretions
- stomach: exocrine and endocrine secretions
- gastric juice helps stomach functions
what does gastric juice contain?
- water
- HCL
- pepsinogen
- intrinsic factor
- mucus
why does the stomach have 2 sphinctors at each end?
to prevent things leaving & entering the worng end of stomach
what is heartburn?
when stomach acid goes back up into the oesophagus (acid reflux)
gastric gland cells
- parietal cells
- chieff cells
- mucous cells
role of parietal cells?
secrete intrinsic factor
- needed for vitamin B12 absorption
- secrete HCL which kills microbes, denatures proteins and activates enzymes
- in stomach
role of chief cells?
secrete pesinogen
- converted to pepsin in its active form
- breaks certain peptide bonds
secrete gastric lipase
- splits short-chain trigylcerides into fatty acids and monoglycerides
- the first enzyme that starts breaking down lipids in the stomach
at what pH is pepsinogen activated to pepsin?
low pH due to highly acidic environment in the stomach
(pepsinogen found in stomach)
role of mucous cells?
secrete mucous
- forms a protective barrier
- has alkaline properties
superficial epithelial
mucous neck cells
exocrine pancreatic secretions
- water
- bicarbonate
- enzymes
pancreatic enzymes
- amylase, lipases and nucleases: secreted in active form -> need ions / bile for optimum activity
- proteases: secreted in inactive form -> activated in duodenum
why are proteases secreted in inactive form?
to prevent them from digesting the pancrease
endocrine pancreatic secretions
alpha cells:
- secrete glucagon in response to fall in blood glucose
- stimulates glycogenolysis and gluconeogenesis
beta cells:
- secrete insulin to respond to rising blood glucose
- inhibited by adrenaline in acute stress
- allows cells to utilise glucose
gamma cells:
- secrete somatostatin
- stmulated by cholinergic innervation
- inhibits gastrin release
bile
- produced by hepatocytes
- pH 7.6-8.6
- 800-1000mL per day
- excretory product
what does bile contain?
- bile salts
- bilirubin
- cholesterol
- neutral fats
- phospholipids
- electrolytes
bile functions
- lipid assimilation
- elimination
- neutralise gastric acid and provide optimum pH for pancreatic enzymes
gallbladder
- thin-walled, pear-shaped
- muscular sac on the ventral surface of the liver
- stores and concentrates bile by absorbing its water and ions
- releases bile via the cystic duct, which flows into the bile duct
phases in secretion regulation
- cephalic phase
- gastric phase
- intestinal phase
cephalic phase
- smell, sight, thought and taste of food activates CNS
- facial, glossopharyngeal and vagus nerves are activated
- salivary and gastric glands activated
- prepares mouth and stomach for food (moist mouth)
gastric phase
- food distends the stomach and stimulates stretch receptors
- chemoreceptors in stomach detect increase in pH
- peristalsis and gastric juice secretion
- chyme empties into duodenum
- low pH and low distension
- negative feedback loop
intestinal phase
- starts when food enters small intestine
- inihibits exit of chyme from stomach
- contraction of pyloric sphincter to prevent things entering stomach or small inestine
- promotes digestion of food in small intestine
- neural enterogastric reflex
- hormones: cholecystokinin and secretin
when are cholecystokinin and secretin hormones released?
during the intestinal phase of digestion
what effect do cholecystokinin and secretin have on gastric secretions?
inhibitory
absorption
when small molecules move through epithelial cells into underlying blood or lymphatic vessels
- in GI done to get nutrients back into the blood
where does the most absorption take place?
the small intestine
large intestine
- receives undigested / unabsorbed material
- has a large microbiota that can break down more food and nutrients to be able to be absorbed
how can absorption occur?
- simple diffusion
- facilitated diffusion (neded if charged)
- active transport (primary or secondary / coupled)
secondary active transport
- does not directly need energy
- use a cotransporter
- only let a molecule back in via diffusion that initially moved out of membrane via active transport if it couples with another molecule -> that other molecule indirectly uses active transport
job of absorption in GI tract?
- key to intestinal absorption: Na+/K+ ATPase on the basolateral membrane
- to build a Na+ gradient outside the cell so that Na+ will want to move back into the cell
what needs to be absorbed?
- carbohydrates
- lipids
- proteins
- nucleic acids
- vitamins
- minerals
- water
- drugs
carbohydrates
starch:
- polysaccharides that need to be broken down
- long chains of glucose units
- found in rice, pasta, potatoes..
sugars:
- shorter chains
- mono/disaccharides
-found in fruit, veg, diary…
in what form are carbohydrates absorbed?
as monosaccharides
lactose is broken down by enzyme lactase. what would happen if that enzyme is not present?
lactose will not be broken down
has an osmotic pull on water so water will not be absorbed -> diarrhea can occur
why do carbs need to be broken down to an absorbable size at brush border instead of in the small inestine lumen?
prevents bacteria in the lumen potentially using it as nutrients
which monsaccharides neeed energy for absorption?
-glucose
- galactose
proteins
- long amino acid chains -> polypeptides
- meat, fish, diary, eggs, pulses, cererals
- also absorbed from digestive juices and dead mucosal cells
amino acid absorption
each amino acid has a different cohort of transporters
examples of lipids
- triglycerides
- phospholipids
- cholesterol
- steriods
- fat-soluble vitamins
lipid absorption
- lipases break trigylcerides into monoglycerides and fatty acids (long or short chain)
- simple diffusion
problem of lipids in the watery GI tract environment
causes lipids to clump together
to be absorbed they need to be in small amounts instead of clumps
emulsification
- breaks large lipid droplets into smaller droplets
-> increases surface area - starts in stomach with churning
-> allows enzymes to break them down into even smaller parts - bile salts: have hydrophobic and hydrophilic regions
chylomicrons
- formed when lipids recombine in sER in SI epithelial cells
- proteins are added
-enter lymph instead of blood because they are too big to enter the blood wall & dont interact well with blood components
vitamins
organic substances that cant be made by the body
vitamin absoprtion in the small intestine
fat soluble:
- vitamins A,D, E and K
- carried by micelles -> then diffuse into absorptive cells
water soluble:
- vitamins C and B
- absorbed by passive or active transporters
vitamin B12:
- binds with intrinsic factors
- absorbed by endocytosis
vitamin absorption in large intestine
- vitamins K
- B vitamins from bacterial metabolism
how could a blockage of the bile duct lead to vitamin E deficiency?
- it is a fat soluble vitamin
- needs to be emulsified in the bile duct
- needs to be carried with a micelle
-> bile duct needed for formation of this
electrolyte absorption
- along lenth of small intestine
- active transport
- iron: enters via divalent metal ion transporter 1
- calcium: absorption regulated by vit D and PTH
- Na+: coupled with absorption of glucose and amino acids
- K+: diffuses due to osmotic gradients
water absorption
- around 9L absorbed per day (many are recycled secretions -> absorb water frpm that)
- 95% in small intestine by osmosis (osmotic gradient)
- disturbance in this mechanism = water loss
- net osmosis occurs when the concentration gradient is established by active transport of solutes
- water uptake coupled with solute uptake
- paracellular = between cells (with osmotic gradienr)
- well regulated in tight junctions (controls which substances water is coupled with)
causes of malabsorption
- interferences with delivery of bile or pancreatic juice
- damaged intestinal mucosa
- gluten-sensitive enteropathy (celiac)
-> gluten damages the intestinal villi and brush border
functions of the liver
- metabolism
- detoxification
- excretion
- storage
- synthesis
- phagocytosis
- heat production
anatomy of liver
- largest internal organ
- located in upper right quadrant of the abdomen
- sits under the diaphragm & protected by rib cage
- right kidney posteriorly located, stomach to right, colon inferior
lobes of the liver
- right lobe (large)
- left lobe (smaller)
- caudate lobe
- quadrate lobe
(inferiorly: right lobe also includes the caudate and quadrate lobes)
role & location of the falciform ligament
- between right and left anterior lobe
- attaches the liver to the anterior abdominal wall
porta hepatis
- main site where structures enter and leave the liver
- structures include: portal triad, nerves & lymphatics
3 structures of the portal traid
- hepatic artery
- hepatic portal vein (entering liver)
- bile duct (leaving liver)
liver blood supply
dual blood supply:
- hepatic artery -> from coelic trunk of aorta (supplies most organs), main blood supply, carries nutrients / toxins absorbed by GI tract
- hepatic portal vein -> convergence of veins draining the GI tract, smaller, provides oxygen rich blood, brings blood to liver before the heart to allow blood to be processed
venous drainage of the liver
after blood passes through liver -> drained into the inferior vena cava by 3 hepatic veins
hepatic portal system
- blood draining from GI tract, pancreas and spleen carried to the liver first
- key for delivery of nutrients / toxins for processing & hormone signalling
what does a portal system do?
(blood related) connects one organ to another without returning to the heart first
hepatocytes
- make up 75-80% of liver cells
- play role in metabolism, detoxification and amino acid synthesis
- seperated from sinusoids by space of disse
- membranes of hepatocytes facing the space of disse have microvilli for large s.a for better absorption
sinusoids
- specialised capillaries (leaky)
- large, low pressure vessels
- drain blood from hepatic artery & hepatic portal vein into the central vein
- consist of fenestrated epithelium: facilitate the transfer of metabolites between plasma and hepatocytes
- contain kupffer cells
stellate cells
- in space of disse
- inactive in healthy conditions
- cause fibrosis in disease states
- store vit A
- produce collagen when activated
kupffer cells
- in sinusoids
- specialised macrophages
- form part of the reticuloendothelial system (breaks down rbcs)
- help recycle rbcs
liver lobules
- each lobe in liver made up of thousands of lobules
- hepatocytes and vessels of liver arranged in hexagons with branches of the portal traid at each corner + a central vein
- bile canaliculi run in a network between hepatocytes -> receive bile secretions + join to form small bile ducts
- blood supply from hepatic artery and hepatic portal vein enter via sinusoids
phases of drug metabolism / detoxification
- 1: modification
- 2: conjugation
- 3: excretion
phase 1 - modification
- mainly done by cytochrome P450 enzymes (in smooth ER of hepatocytes)
- enzymatic incorporation of polar groups (O or OH)
- oxidation
- reduction
- hydrolysis
- act as a handle for conjution phase
- can produce metabolites that are very reactive and toxic (e.g. paracetamol)
phase 2 - conjugation
- addition of an ionised group (e.g. glutathione, sulfate, glycine, glcoronic acid)
- occurs in cytoplasm of hepatocytes
- makes metabolite water soluble for transport
- inactivates the metabolite (drug inactive = safe)
phase 3 - excretion
- smaller metabolites excreted by kindey
- larger ones in bile
bilirubin
- a yellowish substance made during your body’s normal process of breaking down old red blood cells
- broken down by macrophages to bilirubin
- free bilirubin can be toxic if crosses into blood brain barrier
- bound to albumin in plasma to be transported to the liver
- excess bilirubin = jaundice (yellowish skin + eyes)
bilirubin excretion
- when bilirubin is conjugated to glucuronic acid it forms bilirubin glucuronide
- conjugated bilirubin: more soluble + can be excreted by the hepatocyte into the biliry canaliculi
carbohydrate metabolism
- liver helps maintain a normal blood glc level
- necessary for brain, ns + energy
- hormones regulate the way the liver maintains proper blood glc:
-> insulin (anabolic)
-> glucagon (catabolic)
what happens when there is high blood glc?
- increased glc uptake via the GLUT2 receptors
- insulin is released from the pancreas
- more glc retained by hepatocytes
- glc -> glucose-6-phosphate (G6P) via glucokinase
- liver converts G6P -> glycogen via glycogen synthase (glycogenesis)
- increased triglyceride synthesis
what happens when there is low blood glc?
- glucagon released from pancreas
- liver converts glycogen -> glucose-1-phosphate via glycogen phosphorylase
- G1P -> glc via phosphoglucomutase
- gluconeogenesis: synthesis of glc from aa and triglycerides in an 11 enzyme catalysed reaction that needs ATP and GTP
lipid metabolism
synthesis of triglycerides:
- excess glc, acetyl CoA -> fatty acid synthesis
- storage in liver or transported to adipose tissue and muscle as VLDL
- phospholipids
synthesis of cholesterol:
- base for steriod hormone production (cortisol, testosterone)
- cell membranes
protein metabolism
- synthesis of all non-essential a.a
- synthesis of almost all plasma proteins (albumin, clotting factors)
- deamination -> removal of amino group from a.a -> energy production (ammonia produced)
- urea cycle: processing of toxic ammonia to urea for excretioon
liver regeneration
- mature hepatocytes can undergo cell division / mitosis -> maintain number of healthy cells
- severe injury -> activation of liver progenitor stem cells -> can differentaite into other cell types (e.g. that line the bile ducts)
- can lead to chronic diseases
- healthy liver is smooth
- sorosis = scarring
function of the kindeys
- HOMEOSTASIS
- disposal of waste from the body
- osmoregulation / balance of ions
- regulation of blood volume & pressure (bore blood vol = higher bp)
- regulation of blood pH
- hormone production
anatomy of renal system
- ureters
- bladder
- urethra
anatomy of kidneys
- posterior wall of abdomen (towards the back)
- retroperitoneal (behind peritoneal membr)
- either side of vertebral column
- around level of 12th rib
- left slightly higher
kidney blood supply
- directly from aorta (heart) via renal arteries
- returned to inferior vena cava via renal veins (back to heart)
- get 20-25% of restin caridac output
- blood supply needed as their role is to filter blood
nephrons
- makes up cortex and medulla
- functional units of the kidneys
- comprise a renal corpuscule and renal tubule
- needed for ultrafiltration of blood and reabsorption / excretion
- 1.3 million in each kidney
renal corpuscle
- each nephron consists of a renal corpuscle and a renal tubule
- where blood plasma is filtered
- lies within renal cortex
- consists of glomerulus and bowman´s capsule
- inner visceral layer: wraps around the endothelial cells of glomerular capsule
- outer parietal layer: forms outer wall of capsule
what is unusaul about the blood supply to the renal corpuscle?
- it is drained and supplied by arterioles
- efferent + afferent arterioles
- arterioles can control the blood flow & pressure in & out of the capillary beds
- lower bp = less blood filtered into bowman´s capsule
glomerulus
- specialized bundle of capillaries that are uniquely situated between two resistance vessels
- filtration
bowman´s capsule
- double walled cup of epithelial cells
- a part of the nephron that forms a cup-like sack surrounding the glomerulus
capuscular space
area between 2 layers of nephron
ultrafiltration
- glomerular fitration = first step of urine production
- water + most solutes in blood plasma pass from glomerular capillaries to glomerular capsule (glomerular filtrate)
- filters 180l/day
- elimate 2l of urine a day
- glomerular capsule drains into renal tubule
renal tubule
consists of:
- proximal convoluted tubule
- loop of henle (extends into renal medulla)
- distal convoluted tubule (within renal cortex)
needed for reabsorption / secretion
proximal convoluted tubule
- largely responsible for reabsorption of glc, sodim & other solutes
- where most of the nutrients are absorbed back into the body
loop of henle
- counter current multiplier
- creates osmotic gradiant + dilute urine
distal convoluted tubule
reabsorbes water from filtrate
counter current multiplier
- The loop of Henle utilizes the countercurrent multiplier system to increase the concentration of solute and ions within the interstitium of the medulla.
- This ultimately allows the nephron to reabsorb more water and concentrate the urine while at the same time using as little energy as possible.
- higher solute conc deeper in medula -> water moves out of filtrate via the osmotic gradient
regulation of the kindeys - ADH
anti-diuretic hormone (ADH)
- stimulates insertion fo aquaporin channels
- increases water permeability
- more water reabsorbed = urine more concentrated
- low ADH = water diuresis
- ADH is a vasocontrictor / vasopressin
- ADH increases blood pressure
regulation of kidneys - renin-angiotensin-aldosterone system
angiotensinogen -> angiotensin I (renin in kidney) -> angiotensin II (ACE) -> aldosterone + Na+/H2O retention + ADH in post pit + systematic vasoconstriction
- RAAS: controls blood pressure
- lead to increase in blood volume + pressure
- if RAAS overactive -> hypertension
regulation of kindeys - sympatheic NS
- can decrease Na+ and water excretion
- can increase angiotensin 2 formation
regulation of kindeys - parathyroid hormone
- increase reabsorption of calcium in DCT
regulation of kidneys - natriuretic peptides
- atrial natriuretic peptide (ANP)
- brian natriuretic peptide (BNP)
- c-type natriuretic peptide (CNP)
what regulates the kindeys?
- anti-diuretic hormone
- renin-angiotensin-aldosterone system
- sympathetic NS
- parathyroid hormone
- natriuretic peptides
how does the mucosa lining the oral cavity protect against friction?
- mucosa lining in oral cavity made from stratified squamous epithelium
- epithelium is thick -> protects underlaying layers from damage
- extra protection by keratinized stratified squamous epithelium og hard palate and gums
- keratin is tough & makes the epithelium stronger, reducing impact of friction
what is the rugae of the stomach?
-internal folds
- stretch to accomodate more food
muscles in the stomach
- three different oreintations
- allow food to be mixed more thoughroighly with the stomach acid
- allows for greater breakdown of food into molecules to then be absorbed as pass through small intestine
the muscularis externa
- made of two layers
inner layer = circular mucle fibres
outer layer = longitudinal fibres - in oesophagus and small intestine
what is bile?
- digestive secretion
- constains bile salts, bilirubin, cholesterol, neutral fats, phospholipids & electrolytes
- bile helps assimilate lipids
- gets rid of unwanted substances
- neutralises gastric acid
- provides optimum pH for pancreatic enzymes
which hormones are released during the intestinal phase of digestion?
- cholecystokinin (CCK) & secretin
- inhibit gastric secretions
where does the common bile duct pass fluid from?
- cystic duct (gallbladder)
- hapatic duct (liver)
how does bile enter the small intestine?
via the major duodenal papilla
function of pancreatic acini
secrete digestive enzymes
How do the plicae circulares, villi, and microvilli contribute to the function of the small intestine?
- increase surface area of mucosa avaible for digestion and absorption
- nutrients are then absorbed into the blood stream (via capillaries in villi) and lipids absorbed into lymph (via lacteals in villi)
why is the small intestine the longest organ of digestice tract?
longer = more s.a. = more absorption
what are the 3 main areas of lipid breakdown in GI tract? which 3 enzymes involved?
- mouth: lingual lipase
- stomach: gastric lipase
- small intestine: pancreatic lipase (main one in adults)
how is a hepatic lobe structured?
- stacked hepatocytes that radiate out from a central vein
- hepatocytes form “walls”
what processes are hepatocytes involved in?
- protein synthesis and storage
- detoxification
- synthesis of cholesterol, bile salts and phsopholipids
- transform and store carbs
function of the renal pelvis
receives urine from major calyces & carries it towards the ureter
effect of increas anti-diuretic hormone on urine production.
- stimulates insertion of aquaporin channels
- increases water permeability
- urine more concentrated
