D2 Flashcards
What controls the volume and content of gastric secretions ?
nervous and hormonal mechanisms
Explain the characteristics of the nervous mechanism
- Immediate response triggered by sight and smell of food
–> gastric juice secreted by stomach pre-ingestion - food enters stomach = distension
–> detected by stretch receptors in stomach lining - signals are sent to the brain
–> triggers release of digestive hormones
–> sustained gastric stimulation
Explain the characteristics of the hormonal mechanism
- Gastrin is secreted in the bloodstream from gastric pits in stomach
–> stimulates release of stomach acid - If stomach PH too low = gastrin secretion is inhibited by gut hormones (secretin/somatostatin)
- Digested food (chyme) passes into small intestine –> duodenum relases digestive hormones
–> Secretin & cholecystokinin (CCK): stimulate pancreas and liver to release digestive juices
–> pancreatic juices: contain bicarbonate ions to neutralise stomach acids
–> liver produces bile to emulsify fats
Explain the role of exocrine glands
Produce and secrete substances via a duct onto an epithelial surface. Either on the surface of the body (eg: sweat glands) or on the lumen of the digestive tract/gut (eg: digestive glands)
state examples and functions of 4 digestive glands
Salivary: secrete saliva containing amylase to break down starch
Gastric: secrete gastric juices (hydrocloric acid and proteases)
Pancreatic: secrete pancreatic juices (lipase, protease, amylase)
Intestinal: secrete intestinal juices via crypts of lieberkuhn in intestinal wall
How do you identify exocrine glands?
- composed of a cluster of secretory cells which form an acinus
- the acini are surrounded by a basement membrane and are held together by tight junctions between secretory cells
- secretory cells possess highly developed ER & Golgi network, and are rich in mitochondria
- exocrine products are released via secretory vesicles into a duct connecting to epithelial surface
How and why does the stomach have acidic conditions?
the gastric glands in the stomach lining secrete an acidic solution that lowers the pH (1.5-2)
why
- Assists in digestion of food (dissolves chemical bonds)
- Optimum pH for hydrolysis reactions by stomach enzymes
- Activates stomach proteases
- Prevents pathogenic infection
How is the structure of cells of the epithelium of the villi is adapted to the absorption of food?
- MICROVILLI: ruffling of epithelial membrane further increases surface area
- RICH BLOOD SUPPLY: dense capillary network rapidly transports absorbed products
- SINGLE LAYER EPITHELIUM: minimizes diffusion distance between lumen and blood
- LACTEALS: absorbs lipids from the intestine into lymphatic system
- INTESTINAL GLANDS: exocrine pits (cypts of Lieberkuhn) release digestive juices
- MEMBRANE PROTEINS: facilitates transport of digested materials into epithelial cells
What are the structural features of the epithelia lining which optimise absorption ?
Tight junctions
–> btw plasma membrane & 2 adjacent cells
–> create impermeable barrier
–> keep digested fluids separated from tissues
–> maintain concentration gradient, one way movement
Microvilli
–> increase SA of plasma membrane (>100x)
–> membrane embedded with immobilised enzymes and channel proteins to assist material uptake
Mitochondria
–> provide ATP for primary and secondary active transport
–> pinocytosis
Pinocyotic vesicles
–> pinocytosis (cell drinking), non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
–> materials ingested via the breaking and reforming of membrane
How do intestines complete the process of digestion?
small intestine absorbs usable food substances (nutrients, monosaccharides, amino acids, fatty acids, vitamins) and 90% water
Large intestine absorbs water and dissolved minerals from indegistible food residues
How is the rate of transit materials through large intestine correlated with their fibre content?
dietary fibre (roughage) provides bulk in the intestines to help keep materials moving through gut
it also helps absorb water, keeping bowel movement soft and easy to pass
What are the health benefits associated with diets rich in dietary fibre?
- reduces frequency of Constipation
- lowers risk of colon and rectal Cancer
- lowers blood Cholesterol
- regulates blood Sugar levels
- aids in Weight management (large volume, little calories)
What elements are not absorbed and egested as feces ?
Bile pigments
Epithelial cells
Lignin
Cellulose
Human flora (bacteria)
Outline the reduction of stomach acid secretion by proton pump inhibitor drugs
- Low pH environment in stomach is maintained by proton pumps (in parietal cells of gastric pits)
- PP secrete H+ ions via active transport
- H+ ions combine with CI- ions to form hydrochloric acid
- certain medications/disease can increase H+ secretion, lowering pH
- Proton pumps inhibitors are drugs that irreversibly bind to PP, preventing H+ ion secretion
- This raises pH, reducing high acidity gastric discomfort
- reduction of acid secretion = increased susceptibility to gastric infections
Outline the dehydration due to cholera toxin
- Vibrio cholera releases toxin that binds to ganglioside receptors on intestinal epithelium cells
- toxin is internalised by endocytosis
- triggers production of cyclic AMP within cell
- cAMP activates specific ion channels within cell membrane
- causes eflux of ions from the cell
- build up of ions in intestinal lumen draws water from cells via osmosis
–> accute diarrhea + dehydration
