D2 Digestion Flashcards
WHat is the role of exocrine glands?
Exocrine glands produce and secrete substances via a duct onto an epithelial surface
What 2 places do exocrine glands secrete substances into/onto?
The surface of the body (e.g. sweat glands, sebaceous glands)
The lumen of the digestive tract / gut (e.g. digestive glands)
What are 4 examples of digestive glands?
salivary
gastric
pancreatic
intestinal glands
What is the role of salivary glands?
Salivary glands – secrete saliva which contains amylase (breaks down starch)
What is the role of gastric glands?
secretes gastric juices which includes hydrochloric acid and proteases (breaks down protein)
What is the role of pancreatic glands?
secretes pancreatic juices which include lipase, protease and amylase
What is the role of intestinal glands?
secretes intestinal juices via crypts of Lieberkuhn in the intestinal wall
What are exocrine glands composed of?
Exocrine glands are composed of a cluster of secretory cells which collectively form an acinus (plural = acini)
What are acini surrounded by?
The acini are surrounded by a basement membrane and are held together by tight junctions between secretory cells
WHat do the secretory cells possess ? (organelles)
The secretory cells possess a highly developed ER and golgi network for material secretion and are rich in mitochondria
How are exocrine products secreted?
Exocrine products are released (via secretory vesicles) into a duct, which connects to an epithelial surface
How may ducts arise? (from what are they formed)
These ducts may arise from a convergence of smaller ductules (each connected to an acinus) in order to enhance secretion
What is the secretion of digestive uices controlled by?
The secretion of digestive juices is controlled by both nervous and hormonal mechanisms
What do nervous and hormonal mechanisms control?
These mechanisms control both the volume of secretions produced and the specific content (e.g. enzymes, acids, etc.)
What does the sight and smell of food do?
The sight and smell of food triggers an immediate response by which gastric juice is secreted by the stomach pre-ingestion
What happens when food enters the stomach? (nervous mechanism)
When food enters the stomach it causes distension, which is detected by stretch receptors in the stomach lining
What does the stretching of the stomach lining cause?
Signals are sent to the brain, which triggers the release of digestive hormones to achieve sustained gastric stimulation
nervous mechanism
What secretes gastrin?
Gastrin is secreted into the bloodstream from the gastric pits of the stomach and stimulates the release of stomach acids
What happens to gastrin when stomach pH is too low?
If stomach pH drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin)
What stimulates the small intestine to secrete hormones?
When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones:
What is the role of secretin and CCK?
Secretin and cholecystokinin (CCK) stimulate the pancreas and liver to release digestive juices
What is the role of pancreatic juices?
Pancreatic juices contain bicarbonate ions which neutralise stomach acids, while the liver produces bile to emulsify fats
What do the gastric glands secrete apart from hormones?
The gastric glands that line the stomach wall secrete an acidic solution that creates a low pH environment within the stomach
What is the normal pH of the stomach and why?
The normal pH of the stomach is roughly 1.5 – 2.0, which is the optimum pH for hydrolysis reactions by stomach enzymes
What are the 3 functions of the acid conditions of the stomach?
Assists in the digestion of food
Activates stomach proteases
Prevents pathogenic infection
How do the acidic conditions in the stomach help the digestion of food?
by dissolving chemical bonds within food molecules
How do the acidic conditions of the stomach help activate stomach proteases?
e.g. pepsin is activated when pepsinogen is proteolytically cleaved in acid conditions
How do the acidic conditions of the stomach help prevent pathogenic infections?
stomach acids destroy microorganisms in ingested food
What is the stomach wall lined by and why?
The stomach wall is lined by a layer of mucus, which protects the stomach lining from being damaged by the acid conditions
What does the pancreas release in relation to pH?
The pancreas releases bicarbonate ions into the duodenum which neutralises the stomach pH (intestinal pH ~7.0 – 8.0)
What may also neutralise stomach pH?
Certain foods (e.g. antacids) may also neutralise stomach acids, impairing digestion and increasing chances of infection
What maintains low pH environment of stomach?
The low pH environment of the stomach is maintained by proton pumps in the parietal cells of the gastric pits
How do proton pumps maintain acidic conditions in stomach?
These proton pumps secrete H+ ions (via active transport), which combine with Cl– ions to form hydrochloric acid
What can decrease stomach acidity?
Certain medications and disease conditions can increase the secretion of H+ ions, lowering the pH in the stomach
What are proton pump inhibitors?
Proton pump inhibitors (PPIs) are drugs which irreversibly bind to the proton pumps and prevent H+ ion secretion
How do proton pump inhibitors affect pH?
This effectively raises the pH in the stomach to prevent gastric discomfort caused by high acidity (e.g. acid reflux)
What is a side effect of PPI’s?
Individuals taking PPIs may have increased susceptibility to gastric infections due to the reduction of acid secretion
What is the role of the small intestine apart from absorbing nutrients?
The small intestine also releases digestive enzymes to ensure the complete hydrolysis of food molecules
What is the structure of the small intestine?
The inner epithelial lining of the small intestine is highly folded into finger-like projections called villi (singular: villus)
What is the (basic) function of the villi?
Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption
What are 6 structural features of the villi?
Microvilli Rich blood supply Single layer epithelium Lacteals Intestinal glands Membrane proteins
MR SLIM
What is the role of microvilli?
Ruffling of epithelial membrane further increases surface area
What is the role of rich blood supply?
Dense capillary network rapidly transports absorbed products
What is the role of single layer epithelium?
Minimises diffusion distance between lumen and blood
What is the role of lacteals?
Absorbs lipids from the intestine into the lymphatic system
What is the role of intestinal glands?
Exocrine pits (crypts of Lieberkuhn) release digestive juices
What is the role of membrane proteins?
Facilitates transport of digested materials into epithelial cells
What 4 features does the epithelial lining of villi contain ?
The epithelial lining of villi contains several structural features which optimise its capacity to absorb digested materials:
tight junctions
microvilli
mitochondria
pinocytotic vesicles
What are tight junctions?
Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier
What i the role of tight junctions?
They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement
What is the role of microvilli?
Microvilli borders significantly increase surface area of the plasma membrane (>100×), allowing for more absorption to occur
what is the membrane of microvilli embedded with?
The membrane will be embedded with immobilised digestive enzymes and channel proteins to assist in material uptake
Why do epithelial cells have large volumes of mitochondria?
Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms
What may ATP be used for in villi?
ATP may be required for primary active transport (against gradient), secondary active transport (co-transport) or pinocytosis
What is pinocytosis?
Pinocytosis (‘cell-drinking’) is the non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
How are materials ingested via pinocytosis?
These materials will be ingested via the breaking and reforming of the membrane and hence contained within a vesicle
What is the role of human intestines?
The human intestines function to complete the process of digestion and absorb digested products into the bloodstream
What is the role of the small intestine?
The small intestine absorbs usable food substances (i.e. nutrients – monosaccharides, amino acids, fatty acids, vitamins, etc.)
What is the role of the large intestine?
The large intestine absorbs water and dissolved minerals (i.e. ions) from the indigestible food residues
What is dietary fibre?
Dietary fibre, or roughage, is the indigestible portion of food derived principally from plants and fungi (cellulose, chitin, etc.)
Why is cellulose indigestible for humans?
Humans lack the necessary enzymes to break down certain plant matter (e.g. lack cellulase required to digest cellulose)
Why can certain herbivores digest cellulose?
Certain herbivores (ruminants) possess helpful bacteria in the digestive tract that can break down indigestible plant matter
What is fibre content positively correlated with?
The rate of transit of materials through the large intestine is positively correlated with their fibre content:
How does roughage help things in intestines move?
Roughage provides bulk in the intestines to help keep materials moving through the gut
What does roughage also help with?
Roughage also absorbs water, which keeps bowel movements soft and easy to pass
What are 3 health benefits of dietary fibre?
It reduces the frequency of constipation and lowers the risk of colon and rectal cancer
It lowers blood cholesterol and regulates blood sugar levels (by slowing the rate of absorption)
It aids in weight management (contributes few calories despite consisting of a large volume of ingested material)
What happens to not absorbed materials?
Materials that are not absorbed by the small and large intestines are ultimately egested from the body as faecesv
What does a large proportion of human faces consist of?
A large portion of human faeces consists of dietary fibre, such as cellulose and lignin
Also present in faeces are the remains of intestinal epithelial cells, bile pigments and human flora (intestinal bacteria)
Mnemonic to help remember egested materials?
Bile pigments Epithelial cells Lignin Cellulose Human flora (bacteria)
Mnemonic: BELCH
What are stomach ulcers?
Stomach ulcers are inflammed and damaged areas in the stomach wall, typically caused by exposure to gastric acids
What is (most likely) the cause of stomach ulcers?
there is a strong positive correlation between Helicobacter pylori infection and the development of stomach ulcers
What is helicobacter pylori?
Helicobacter pylori is a bacterium that can survive the acid conditions of the stomach by penetrating the mucus lining
- Where do H. pylori attach to?
H. pylori anchors to the epithelial lining of the stomach, underneath the mucus lining
- How does H. pylori begin damage to stomach?
An inflammatory immune response damages the epithelial cells of the stomach – including the mucus-secreting goblet cells
- What does the prolonged inflammatory immune response lead to?
This results in the degradation of the protective mucus lining, exposing the stomach wall to gastric acids and causing ulcers
- What can prolonged stomach ulcers cause?
The prolonged presence of stomach ulcers may lead to the development of stomach cancer over many years (20 – 30 years)
How can H.Pylori be treated?
H. pylori infections can be treated by antibiotics (previously, stomach ulcers were considered stress related and not treatable)
What is Vibrio cholerae?
Vibrio cholerae is a bacterial pathogen that infects the intestines and causes acute diarrhoea and dehydration
What disease is caused by vibrio cholerae?
The associated disease – cholera – can kill within hours unless treated with oral rehydration therapies
What is the first step of a cholera infection?
V. cholerae releases a toxin that binds to ganglioside receptors on the surface of intestinal epithelium cells
- How does V. Cholerae enter the intestine?
This toxin is internalised by endocytosis and triggers the production of cyclic AMP (a second messenger) within the cell
- What does cAMP cause?
Cyclic AMP (cAMP) activates specific ion channels within the cell membrane, causing an efflux of ions from the cell
- How is an osmotic imbalance caused?
The build up of ions in the intestinal lumen draws water from cells and tissues via osmosis – causing acute diarrhoea
- What is the final result of cholera?
As water is being removed from body tissues, dehydration will result if left untreated
