d2 digestion

Question 1

what do exocrine glands do?

Answer 1

• secrete to the surface of body or lumen of gut
• exocrine glands are usually glands that secrete out of body with help of ducts
• e.g. sweat glands excrete water and some waste materials (e.g. urea) to surface of body (skin)
• sebaceous glands secrete oily materials that lubricates skin and hair
• 3 (4) main organs that secrete into lumen of gut are:
  • liver (bile salts)
  • pancreas (pancreatic juice)
  • small intestines (intestinal juice)
  • salivary glands (saliva)

Question 2

compare nervous and hormonal mechanisms

Answer 2

SIMILARITIES

• can accomplish same function through diff means
• coordination and control

DIFF
nerves:
- message conducted via impulse
- definite start and end [nervous pathway]
- has nerve structure
- point a and b are directly connected
- impulse is faster

hormonal:

• message carried by molecules
• goes via circulatory system and has potential to reach anywhere in your body
• slower but still speedy
• definite start but not definite end
• can illicit different responses in different parts of the body 誅

Question 3

how do nervous mechanisms control the secretion of digestive juices?

Answer 3

• upon sight and smell of foods, nerve impulses sent from receptors in eyes and nose to brain
• impulses can trigger secretions of digestive juices (in stomach, and saliva in mouth)
• brain processes signals from sensory organs, and hence memory and past experiences can alter responses to sensory inputs
• stretch receptors in stomach send nervous signals to brain as well, leading to secretions of digestive juices

Question 4

how do hormonal mechanisms control the secretion of digestive juices?

Answer 4

• gastrin (peptide hormone) released by gastric pits in stomach, in response to certain factors like stretching (distension) of stomach [stretch receptors stimulate other cells to release gastrin]
  • gastrin stimulates release of hydrochloric acid by parietal cells in stomach
  • gastrin operates via negative feedback as presence of acids in stomach inhibits release of gastrin
• hormones like CCK (cholecystokinin) released by duodenum that stimulates release of digestive juices from pancreas and bile from liver

Question 5

how are the volume and content of gastric secretions controlled?

Answer 5

• by nervous and hormonal mechanisms
• types of secretions dependent on various conditions in gastro-intestinal tract
• 1 main signal is stretching of stomach walls, which leads to both nervous and hormonal responses that leads to sustained production of gastric juices (enzymes and HCl) as long as stomach remains stretched (food is present): form of positive feedback
  • presence of proteins and partially digested proteins can also stimulate gastrin production
• some feedback mechanisms are seen in stomach, when low pH exerts negative feedback on HCl production via inhibition of gastrin production
• in duodenum, presence of certain amino acids and fatty acids in chyme (from stomach) triggers release of hormones that exert positive feedback on production of intestinal digestive juices, and this happens only when food particles are present

Question 6

what is the purpose of acid condition in the stomach?

Answer 6

• favour some hydrolysis reactions and help to control pathogens in ingested food
• presence of strong acids can help in breaking of bonds of polymers via hydrolysis reactions (acid hydrolysis)
• acids can also activate protease pepsinogen into active form of pepsin that digests proteins efficiently in stomach
• low pH conditions in stomach also denatures proteins, and help to expose peptide bonds in many proteins to proteases present
• denaturation effect on proteins can also kill some microorganisms and help to control amount of live pathogens entering gut in ingested food

Question 7

how is stomach acid secretion reduced?

Answer 7

• by proton pump inhibitor drugs
• to achieve low pH found in stomach, proton pumps used to pump H+ ions into stomach via active transport (chloride flows into stomach lumen via other types of membrane channels, together they form hydrochloric acid)
• proton pumps are found on the parietal cells on the stomach surface
• proton pump inhibitors are drugs prescribed to treat symptoms and diseases associated with release of HCl into stomach
• examples of such diseases include peptic ulcers and gastroesophageal reflux
• drugs work by binding irreversibly to proton pumps in parietal cells in stomach, preventing them from working and pumping H+ ions into stomach
• greatly lowers acid release in stomach

Question 8

how are the structure of cells of epithelium of villi adapted for absorption of food?

Answer 8

• villi itself well adapted for absorption of food
  • thin epithelium (one cell thick)
  • richly supplied with blood vessels (capillaries) [for absorption of amino acids and sugars)
  • presence of lacteal (lymphatic vessels) to help in the transport of fats [fatty acids and glycerol]
  • specially adapted epithelial cells
• epithelial cells in intestines contain micro villi that increases surface area available for absorption
• epithelial cells held together by tight junctions to form contiguous (adjacent) impermeable layer so that pathogens cannot pass through them
• presence of special protein channels and pumps in cell membrane of cells to aid in absorption, and cells contain higher amount of mitochondria to power active transport pumps

Question 9

what is the impact of fibre content of food on digestion process?

Answer 9

• rate of transit of materials through large intestine positively correlated with fibre content
• dietary fibre refers to any components of consumed food not digested and absorbed by body
• main form of fibre for humans would be cellulose from plants, as humans lack necessary enzymes to digest cellulose (other herbivorous animals including ruminants have symbiotic bacteria in their gut to digest cellulose)
• hence, such fibre able to help transit of materials through large intestines, as fibre adds bulk to contents, also absorbs water making stool soft and easier to move through large intestines
• besides helping with transit of materials, fibre content also helps person’s appetite be sated more easily with added non-digestible bulk

Question 10

what happens to materials not absorbed into body?

Answer 10

• egested
• egestion refers to movement of bowels where faeces is passed out of body
• faeces contains undigested materials including:
  • cellulose
  • lignin
  • bile pigments (which gives faeces its colour)
  • intestinal bacteria (which for pathogenic bacteria can lead to spread of diseases)

Question 11

what is the effect of cholera toxin on body?

Answer 11

• dehydration
• cholera: disease arising from infection of small intestines by bacteria Vibrio cholera
• bacteria releases cholera toxin (CTX) which binds to GM1 gangliosides on surface of intestinal epithelium cells
• toxin brought into cell via endocytosis, then causes constitutive (continuous) production of cyclic AMP in cell
• high levels of cyclic AMP in cell results in excess secretion of water, sodium, potassium and bicarbonates into intestinal lumen
• water also moves out of intestinal cells via osmosis, as salt ions are pumped out of cells into intestinal lumen, increasing the water potential in the epithelial cells
• results in massive diarrhoea, and will lead to dehydration in body and can lead to death if untreated

Question 12

what is the impact of helicobacter pylori infection and how does it come about?

Answer 12

• cause of stomach ulcers
• stomach ulcers: ulcers and wounds formed on inner surface of stomach
• open wounds exposed to stomach acids and enzymes released hence wound healing takes a longer time (in normal humans, stomach lining is protected from effects of acid and enzymes with a mucus layer secreted from mucous cells in stomach lining)
• helicobacter pylori: species of bacteria linked to formation of stomach ulcers
• bacteria capable of moving under mucus layer on stomach walls with aid of flagella, and attaches onto cells on stomach epithelial cells with help of adhesins
• can secrete compounds that damages epithelial cells, or disrupt gap junctions or even cause cell death
• this can also trigger immune response causing inflammation at site of infection
• due to inflammation and immune response, mucus lining of area is compromised, and epithelial cells are now exposed to stomach acid and enzymes
• will lead to ulceration of the stomach, and may even lead on to formation of stomach cancers