D.2 DIGESTION Flashcards
ESSENTIAL IDEA
Digestion is controlled by nervous and hormonal mechanisms.
EXOCRINE CELLS SECRETE WHERE?
secrete to the surface of the body or the lumen of the gut
EXAMPLES OF EXOCRINE CELLS
salivary glands
gastric glands
pancreatic glands
intestinal glands
SALIVARY GLANDS
secrete saliva which contain amylase (breaks down starch)
GASTRIC GLANDS
secretes gastric juices which includes HCl acid and protease and amylase (breaks down proteins)
PANCREATIC GLANDS
secretes pancreatic juices which include lipase and protease (breaks down protein)
INTESTINAL GLANDS
secretes intestinal juices via crypts of Lieberkuhn in the intestinal wall
EXOCRINE GLANDS ARE COMPOSED OF
a cluster of secretory cells which collectively form an acinus secretory cell
SECRETORY CELLS POSSESS
a highly developed ER and golgi network for material secretion and are rich in mitochondria
WHERE ARE EXOCRINE CELLS RELEASED TO AND FROM?
released via secretory vesicles into a duct, which connects to an epithelial surface
EXOCRINE CELL IMAGE
http://ib.bioninja.com.au
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WHAT DO NERVOUS AND HORMONAL MECHANISMS CONTROL? x2
secretion of digestive juices
volume and content of gastric secretions
NERVOUS MECHANISM
STEP 1
the sight/smell of food triggers an immediate response by which gastric juice is secreted by the stomach pre-ingestion
NERVOUS MECHANISM
STEP 2
when food enters the stomach it causes distension, which is detected by stretch receptors in the stomach lining
NERVOUS MECHANISM STEP 3
signals are sent to the brain, which triggers the release of digestive hormones to achieve sustained gastric stimulation
HORMONAL MECHANISM
STEP 1
gastrin is secreted into the bloodstream from gastric pits of the stomach and stimulates the release of stomach acids
HORMONAL MECHANISM
STEP 2
if stomach pH drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin)
HORMONAL MECHANISM
STEP 3
when digested food passes into the small intestine, the duodenum also releases digestive hormones
DIGESTIVE HORMONES RELEASED BY DUODENUM
secretin and CCK stimulate the pancreas and liver to release digestive juices
WHAT DO PANCREATIC JUICES CONTAIN and what do they do?
bicarbonate ions which neutralise stomach acid, while the liver producers bile to emulsify fats
WHAT DOES THE LIVER PRODUCE AND WHY?
bile to emulsify fats
CONTROL OF GASTRIC SECRETIONS OVERVIEW
food stimulus
stretch receptors send signals to brain
triggers secretion of digestive juices
stomach releases gastrin
promotes production of stomach acids
intestine releases secretin and CCK
stimulating secretion of pancreatic juice and bile
CCK
cholecystokinin
WHAT DO ACID CONDITIONS IN THE STOMACH FAVOUR?
some hydrolysis reactions
help control pathogens in ingested foods
ACID AND DIGESTION
assist digestion by dissolving chemical bonds within food molecules
ACID AND PROTEASE
acid in stomach activates protease
ACID AND PATHOGENS
prevents pathogenic infection as stomach acid destroy micro-organisms in ingested foods
WHAT IS THE STOMACH WALL LINED WITH?
layer of mucus
WHAT DOES THE LAYER OF MUCUS THAT LINES THE STOMACH WALL DO?
protects stomach lining from being damaged by the acid conditions
PH AND PANCREAS
the pancreas releases bicarbonate ions into the duodenum which neutralises the stomach pH (7/8)
CERTAIN FOODS LIKE ANTACIDS MAY
also neutralise stomach acids
impairing digestion and increase chances of infection
WHAT DO PROTON PUMP INHIBITOR DRUGS DO?
reduce stomach acid secretion
WHAT IS THE LOW PH ENVIRONMENT OF THE STOMACH MAINTAINED BY?
proton pumps in parietal cells of the gastric pits
WHAT CAN LOWER THE PH IN THE STOMACH?
certain medication and disease conditions can increase the secretion of H+ ions
=lowering pH
PPIs
proton pumps inhibitors
drugs which irreversibly bind to the proton pumps
prevent secretion
PPIs RESULT
effectively raises the pH in the stomach to prevent gastric discomfort caused by high acidity- acid reflux
PPIs SIDE EFFECTS
may have increased susceptibility to gastric infections due to the reduction of acid secretion
ONCE DIGESTED FOOD HAS PASSED THROUGH THE STOMACH WHERE DOES IT GO?
it enters the small intestine for absorption
VILLI?
finger like projections highly folded in the inner epithelial lining of the small intestine
WHAT DO MANY VILLI PRODUCE?
the intestinal lumen
greatly increasing the available S.A for absorption
FEATURES OF VILLI MNEMONIC
MR
SLIM
VILLI FEATURES
microvilli
rich blood supply
single layer epithelium
lacteals
intestinal glands
membrane proteins
VILLI FEATURES
MICROVILLI
further increase S.A for absorption
VILLI FEATURES
RICH BLOOD SUPLLY
dense capillary network rapidly transports absorbed products
VILLI FEATURES
SINGLE LAYER EPITHELIUM
minimises diffusion distances between lumen and blood
VILLI FEATURES
LACTEALS
absorb lipids from the intestine into the lymphatic system
VILLI FEATURES
INTESTINAL GLANDS
exocrine pits release digestive juices
VILLI FEATURES
MEMBRANE PROTEINS
facilitates transport of digested materials into epithelial cells
EPITHELIAL LINING STRUCTURAL FEATURES THAT OPTIMISE ITS CAPACITY TO ABSORD DIGESTED MATERIAL ARE
Tight Junctions
Microvilli
Mitochondria
Pinocytotic Vesicles
SUMMARY OF EGESTED MATERIALS
BELCH
Bile pigments Epithelial cells Lignin Cellulose Human flora (bacteria)
EGESTION
materials that are not absorbed by the small and large intestine are ultimately egested from faeces
A LARGE PORTION OF HUMAN FAECES CONSIST OF?
dietary fibre
cellulose and lignin
DIETARY FIBRE
the indigestible portion of food derived principally from plants and fungi
- cellulose/chitin
WHAT DOES THE SMALL INTESTINE ABSORB?
usable food substances
nutrients
- monosaccharides
- AA
- fatty acids
- vitamins
WHAT DOES THE LARGE INTESTINE ABSORB?
water and dissolved minerals from the indigestible food residues
WHAT DO HUMAN LACK?
the necessary enzymes to break down certain plant matter (cellulose)
RATE OF TRANSIT OF MATERIALS THROUGH THE LARGE INTESTINE IS…
positively correlated with their fibre content
- provides bulk in the intestines to help keep materials moving through the gut
- absorbs water: keeps bowel movements soft and easy to pass
HEALTH BENEFITS OF DIETS RICH IN FIBRE
reduces the frequency of constipation and lowers risk of colon and rectal cancer
lowers blood cholesterol and regulates blood sugar levels (by slowing rate of absorption)
aids with weight management (contributes fewer calories despite consisting of a large volume of ingested material)
HELICOBACTER PYLORI
a bacterium that can survive the acid conditions of the stomach by penetrating the mucus lining
STOMACH ULCERS
Are inflammed and damaged areas in the stomach wall, typically caused by exposure to gastric acids
There is a strong positive correlation between Helicobacter pylori infection and the development of stomach ulcers
HELICOBACTER PYLORI
FORMATION STEP 1
H. pylori anchors to the epithelial lining of the stomach, underneath the mucus lining
HELICOBACTER PYLORI
FORMATION STEP 2
An inflammatory immune response damages the epithelial cells of the stomach – including the mucus-secreting goblet cells
HELICOBACTER PYLORI
FORMATION STEP 3
This results in the degradation of the protective mucus lining, exposing the stomach wall to gastric acids and causing ulcers
HELICOBACTER PYLORI
FORMATION STEP 4
The prolonged presence of stomach ulcers may lead to the development of stomach cancer over many years (20 – 30 years)
HELICOBACTER PYLORI
TREATMENT
H. pylori infections can be treated by antibiotics (previously, stomach ulcers were considered stress related and not treatable)
VIBRIO CHOLERAE
a bacterial pathogen that infects the intestines and causes acute diarrhoea and dehydration
CHOLERA
can kill within hours unless treated with oral rehydration therapies
VIBRIO CHOLERAE
eleases a toxin that binds to ganglioside receptors on the surface of intestinal epithelium cells
VIBRIO CHOLERAE TOXIN EFFECT 1
This toxin is internalised by endocytosis and triggers the production of cyclic AMP (a second messenger) within the cell
VIBRIO CHOLERAE TOXIN EFFECT 2
Cyclic AMP (cAMP) activates specific ion channels within the cell membrane, causing an efflux of ions from the cell
VIBRIO CHOLERAE TOXIN EFFECT 3
The build up of ions in the intestinal lumen draws water from cells and tissues via osmosis – causing acute diarrhoea
VIBRIO CHOLERAE TOXIN EFFECT 4
As water is being removed from body tissues, dehydration will result if left untreated
WHAT DO PROTON PUMPS SECRETE?
These proton pumps secrete H+ ions (via active transport), which combine with Cl– ions to form hydrochloric acid
