L3 - Food intake and Processing 1: Saliva

Question 1

how is the GIT organised in terms of amounts of secretions and how it affects food and water

Answer 1

taking up food and water
- only approx. 10% is finally excretes
- huge volumes of water are secreted and reasborbed to keep water homeostasis
- diarrhoea or vomiting can massively disturb water homeostasis
secretions by different organs prepare food for digestion by:
- diluting it to the osmolarity of the plasma
- altering pH for optimal digestion
- digesting food
- protection/lubrication

In:
1200mL per day ingested
1500mL salivary secretions
2000mL gastric secretions
500mL bile
1500mL pancreatic secretions
1500mL intestinal secretions
Out:
1400mL absorbed into blood (large intestine)
6700mL absorbed into blood (small intestine)
100mL water in faeces

Question 2

describe the representative values of plasma and saliva (Vol., osmolarity, pH, [Na], [K], [Cl], [HCO3]

Answer 2

Plasma:
3L per day, 300mOsm, pH7.4, [Na] 150mmolL, [K] 5mmolL, [Cl] 110 mmolL, [HCO3] 24
- plasma is relatively maintained

Saliva:
1.5L per day, 100mOsm, 7.5pH, [Na] 40, [K] 15, [Cl] 25, [HCO3] 30
- saliva is HYPOTONIC, slightly alkaline solution compared to plasma. it has a lower osmolarity than saliva
- bicarbonate neutralises acid in the mouth
- potassium higher bc stomach highly reliant on potassium to produce gastric acid

Question 3

what is the stricture of the salivary glands and what do they secrete?

Answer 3

3 main pairs of glands:
- parotid (serous - watery saliva)
- submandibular (mixed serous/mucous)
- sublingual (mucous - protection of epithelia)
- plus buccal glands in mouth, pharynx and esophagus

Question 4

what is the functional anatomy of the salivary glands?

Answer 4

acinus (little balls):
- serous acinar cells (zymogen granules - vesicles containing proteins)
- mucous acinar cells
- site of primary secretion (isotonic NaCl solution)

ducts:
- intercalated ducts
- striated ducts
- drain into mouth
- reabsorption from primary fluid (dilute solution, lumped further secretion)

Question 5

what are the functions of saliva?

Answer 5

oral hygiene:
- xerostomia (dry mouth, lack of saliva)
aids in:
- talking
- chewing and swallowing by moistening and dissolving food
assists in rendering food isosmotic (HCO3-)
amylase - starch digestion
saliva is ESSENTIAL

Question 6

what is the diagnostic value of saliva?

Answer 6

• window to our body
  saliva samples can be used to analyse:
• DNA (forensic, ancestry)
• cancer
• heart attack
• infections (HIV)
• homeopathy (neurotransmitter balance, burn-out syndrome, migraine)
• COVID-19 testing

Question 7

describe the volume and composition of saliva

Answer 7

1.5L per day: basal rate of 0.5 mL per min and when stimulated can increase 10-fold
composition:
- mucus (sublingual, submandibular and buccal glands)
- digestive enzymes (alpha-amylase = parotid gland and lingual lipase = sublingual gland)
- serous secretion:
- primarily parotid, lesser extent submandibular.
- hypo-osmotic solution of NaCl with slightly elevated K+ and HCO3-
- composition varies with the rate of secretion

Question 8

describe the variation in composition with the rate of secretion of saliva in terms of concentrations

Answer 8

critical points:
[Na+] and [Cl-] < in plasma
- [K+] higher then in plasma
- [HCO3-] higher than plasma
osmolarity increases as the rate of secretion increases BUT saliva is always hypo-osmotic/hypotonic !!

Question 9

describe the variation in composition with the rate of secretion in terms of cells

Answer 9

• rate of secretion by acini variable (leaky epithelium!!)
• rate of absorption by duct is constant (tight epithelium!)
• slow rates of secretion: primary fluid is in contact with duct epithelium for an extended period of time -> greater absorption of NaCl
• high rates of secretion: primary fluid passes through ducts rapidly -> limited absorption of NaCl

Question 10

describe the para/sypathetic control of salivary secretions

Answer 10

control of salivary secretions is mainly nervous - both branches ANS regulate secretion

sympathetic control: fight or flight response
- inhibits blood flow
- low amount of mucous/sticky saliva

parasympathetic control: rest and digest
- main system
- stimulates blood flow
- high amount of serous/water saliva
- comes mainly from parotid glands

Question 11

describe situations where there is a loss of control of salivary secretions

Answer 11

Too much (Sialorrhea or severe drooling):
- medication (antipsychotics, sedatives, poisons, toxins, some herbal preparations)
- nausea
- teething
- dental caries
- viral cavity infections
- cancer

Too little (Xerostomia):
- medication (diuretics, antidepressants, antihistamines, opioids, NSAIDs)
- sjogrens syndrome (autoimmune disease affecting secretory glands of the eyes and mouth)
- glandular pathology (cancer)

Question 12

describe the stimulation of saliva secretion (what reflexes)

Answer 12

saliva secreted in response to thought of food, approach of food, food in the mouth, parasympathetic nervous system
Two reflexes involved:
- conditioned reflex: due to the sight and small of food, or learned response - Pavlov bell ringing
- unconditioned reflex: due to the presence of food in the mouth and the response to taste receptors

Question 13

describe how the formation of saliva is a two step response

Answer 13

acini:
- produce primary fluid
- isotonic NaCl (amylase and mucus)
striated and excretory ducts:
- modification of fluid by reabsorption of Na+ and Cl- without water (limited amount of K+ and HCO3- secretion, and mucus)

Question 14

describe the actual process of secretion of water by acini cells

Answer 14

driven by CHLORIDE!
- chloride leaves apical side of cell by a chloride channel
- sodium potassium ATPase working normally
- negative charge on the apical side results in sodium following paracellularly
- sodium, potassium, chloride cotransporter pumps these ions into the cell and then potassium leaves through a channel into the interstitium
- osmotic gradient changes so water follows transcellularly and paracellularly

Question 15

describe the reabsorption by duct cells (how it actually works in the cell)

Answer 15

• isotonic primary saliva
• Na+ reabsorbed
• K+ secreted
• CFTR pump in there exchanging Cl- in a cycle with CL-/HCO3- pump on lumen side
• HCO3- secreted by chloride/HCO3- pump (but made first inside the cell by CO2 + H20), resulting H+ pumped back into unterstitium by H+/Na+ pump and then Na+/HCO3- pump on interstitium pumps both of these ions into the cell as well
• sodium potassium ATPase working as usual
• water cannot move bc tight epithelium
• resulting hypotonic saliva