Physiology: numbers, curves and compensation
Physiology in this exam tests how systems defend a set point and how a change in one variable forces compensation elsewhere. The examiner prefers quantities and curves to descriptive lists: values you can calculate, graphs you can read, and reflex loops where you must name sensor, afferent, integrator and effector.
Core rules
- Fick principle: clearance of a substance = (urine concentration x urine flow) / plasma concentration.
- Renal: inulin clearance estimates GFR because it is filtered and neither secreted nor reabsorbed; PAH clearance estimates renal plasma flow at low plasma concentrations; glucose reabsorption saturates at the transport maximum.
- Cardiac: cardiac output = heart rate x stroke volume; mean arterial pressure = cardiac output x systemic vascular resistance; stroke volume follows preload, afterload and contractility.
- Respiratory: compliance = change in volume / change in pressure; V/Q falls from apex to base of lung.
- Endocrine: negative feedback, with a distinct set of hormones that use positive feedback or pulsatile release.
Worked example
Urine inulin is 30 mg/mL, urine flow 1.5 mL/min, plasma inulin 0.3 mg/mL. Clearance = (30 x 1.5) / 0.3 = 150 mL/min. Since inulin is freely filtered and untouched by the tubule, this equals GFR. If the same patient's PAH clearance were 600 mL/min and haematocrit 40%, renal blood flow = 600 / 0.6 = 1000 mL/min. Filtration fraction = 150/600 = 0.25.
Traps that cost marks
- Clearance is a virtual volume per unit time, not the mass excreted per minute.
- PAH clearance underestimates renal plasma flow if the concentration is high enough to saturate secretion.
- A right shift of the oxygen-haemoglobin curve (raised CO2, acidosis, heat, 2,3-BPG) means reduced affinity, not reduced carriage.
- Plasma reaches steady state after roughly four to five half-lives, not one.