General Physiology
General Physiology tests membrane potentials, nerve and muscle excitation, cardiovascular and respiratory function, renal clearance, gastrointestinal secretion, and endocrine control, including oral and salivary physiology. Expect calculations from simple formulas and interpretation of direction of change.
Core rules
- Resting membrane potential is set mainly by potassium permeability; the Na+/K+-ATPase maintains the gradients.
- Action potential order: threshold, depolarization by fast Na+ channels, repolarization by K+ efflux, hyperpolarization, return to rest.
- Cardiac output equals heart rate times stroke volume. Mean arterial pressure is approximately diastolic pressure plus one-third of pulse pressure.
- Renal clearance: C = (U x V)/P, where U is urine concentration, V is urine flow, and P is plasma concentration.
- Glomerular filtration rate depends on filtration pressure and permeability; filtration fraction is GFR divided by renal plasma flow.
- Oxygen carriage: most oxygen is bound to hemoglobin, so saturation matters more than dissolved oxygen.
Worked example
A substance has urine concentration 100 mg/dL, urine flow 1 mL/min, and plasma concentration 1 mg/dL. Clearance = (100 x 1)/1 = 100 mL/min. If the same substance were freely filtered and neither reabsorbed nor secreted, its clearance would equal GFR; a value below GFR indicates net reabsorption, and a value above GFR indicates net secretion.
Traps
- Match units before dividing in clearance calculations.
- Sympathetic stimulation raises heart rate and contractility but constricts most systemic arterioles.
- Do not confuse filtration fraction with filtration rate.