What are the different constituents of saliva?
- water (hypotonic solution)
- sodium chloride (lower conc. than plasma)
- calcium, potassium (byproduct of saliva production), iodide (higher conc. than plasma)
- hydrogen carbonate (higher conc. than plasma)
- bacteriostats (iodide)
- mucus
- enzymes (e.g. salivary amylase)
Outline the different factors involved in mastication.
TEETH = incisors (cut) & molars (crush)
MUSCLES = masseter (trigeminal nerve)
SALIVA =
- keeps mucosa moist (dry mucosa -> necrosis -> infection)
- washes teeth
- maintains an alkaline environment (neutralises acid produced by bacteria; so calcium does not dissolve -> dental caries)
- high [calcium] (reduces diffusion gradient - saturated solution)
How is saliva secreted and modified?
SECRETION = active secretion of Cl-; water & other ions follow passively (fluid isotonic with ECF but [I-] is greater and [Cl-] is lower + enzymes)
MODIFICATION =
- Na+-K+-ATPase on basolateral membrane (contributes to resting conc.)
- NHE on basolateral membrane (H+ reacts with HCO3- so that CO2 & H2O diffuse into the duct cell)
- HCO3-/Cl- on apical membrane (removing Na+ & Cl- from saliva -> makes saliva more dilute - hypotonic compared to plasma)
Therefore, decrease in Na+, increase in [K+]; at rest: decrease in HCO3-, when stimulated: increase in HCO3- (eating stimulates acid so more hydrogen carbonate required to buffer)
How is the volume and composition of saliva determined?
Volume = acinar secretion
Composition = ductal modification
note: ductal cells have a maximum rate of modification, therefore the more rapidly saliva is produced, the less modified it is (excluding HCO3-)
How is salivary secretion controlled?
PARASYMPATHETIC: medulla - glossopharyngeal (9th) & otic ganglion
(taste & acid on tongue, nose, conditioned reflexes)
- acts on acinar cells to promote formation of primary secretion
- acts on duct cells to promote HCO3- secretion
- co-transmitters stimulate extra blood flow
SYMPATHETIC: superior cervical ganglion
Reduces blood flow -> dry mouth
note: muscarinic antagonists often have dry mouth as a side-effect
Describe the phases of swallowing.
- VOLUNTARY: separation of bolus -> moves into pharynx
- PHARYNGEAL: pressure receptors in palate & anterior pharynx -> brainstem -> inhibits respiration, raises larynx, closes the glottis, opens upper oesophageal “sphincter”
- OESOPHAGEAL: rapid peristaltic wave
- coordinated by extrinsic nerves (swallowing centre) & lower oesophageal “sphincter”
- transit time ~9s
- upper 1/3 of oesophagus = voluntary muscle, lower 2/3 of oesophagus = smooth muscle
note: peristalsis occurs in oesophagus & distal colon only
What are the immune components of saliva?
Lysozymes
Lactoperoxidase (effective against Gram-ve bacteria)
Complement
IgA
Washes toxins into stomach
What condition results from reduced salivary flow?
Xerostomia
Microbial overgrowth that results can cause parotitis or black hairy tongue
How long is the oesophagus? How far down the GI tract is the gastro-oesophageal junction?
~25cm long (from lower border of cricoid cartilage to cardiac orifice of stomach - level of 7th costal cartilage)
Incisor teeth —> GOJ = ~38-40cm
What mechanisms are present to prevent stomach contents refluxing into the oesophagus?
Lower oesophageal sphincter is a physiological sphincter
- acute angle of entry into stomach acts like a valve
- muscosal folds at GOJ act like a valve
- positive intra-abdominal pressure compresses walls of intra-abdominal oesophagus (abdominal pressure > thoracic pressure)
- right crus of diaphragm acts as a “pinch-cock”
What are the functions of saliva?
- washes teeth
- maintains moist & alkaline environment (neutralises acid produced by bacteria & prevents calcium dissolving)
- high [Ca2+] prevents teeth dissolving —> dental caries
- aids swallowing by forming a food bolus
- digestion of carbohydrates (salivary amylase)
