Physiology
Physiology: what is tested
This domain covers organ-system physiology with emphasis on oral and craniofacial function. Expect cause-and-effect reasoning and simple calculations rather than isolated definitions.
Core rules
- Resting membrane potential depends mainly on K+ permeability; the Na+/K+-ATPase maintains ionic gradients.
- Action-potential depolarization is Na+ influx; repolarization is K+ efflux.
- Renal glucose reabsorption is saturated at the transport maximum, so glucose appears in urine when the threshold is exceeded.
- Saliva is formed isotonic in acini and modified in ducts: Na+ and Cl- are reabsorbed, K+ and HCO3- secreted, giving hypotonic final saliva.
- With high flow, ductal contact time falls: saliva becomes less hypotonic, pH and bicarbonate rise, and amylase output increases.
- Salivary secretion is mainly parasympathetic; muscarinic blockade reduces watery flow and leaves viscous secretion.
- PTH raises serum calcium through bone resorption, renal calcium reabsorption, and activation of vitamin D; calcitonin lowers calcium.
- Respiratory regulation: ventilation removes CO2, so hypoventilation raises PCO2 and produces respiratory acidosis.
Worked example
A patient with dry mouth takes an antimuscarinic drug. Reasoning: acinar watery secretion is driven mainly by parasympathetic acetylcholine on M3 receptors. Blockade reduces flow and water content while protein-rich secretion persists. Expected findings are low flow, viscous saliva, reduced buffering, and higher caries and erosion risk. Because the gland is not destroyed, stopping the drug can reverse the effect.
Traps
- Claiming saliva is always hypotonic; at maximal flow it approaches isotonicity.
- Mixing respiratory acidosis (low pH, high PCO2) with metabolic acidosis (low pH, low HCO3-).
- Treating calcium balance as bone-only; kidney and intestine are essential.
- Assuming sympathetic stimulation abolishes salivary flow; it modifies protein composition.