Physiology: mechanisms, transport and integrated control
What the exam tests
Physiology is examined as mechanism, not vocabulary: how the membrane potential is generated, how cardiac output is regulated, how the kidney handles sodium and water, how gas exchange and acid–base control work, and how endocrine feedback loops behave.
Core rules
- Fick principle. Uptake of a substance equals blood flow multiplied by the arteriovenous concentration difference.
- Clearance. Cx = (Ux × V)/Px. Inulin clearance measures GFR; PAH clearance measures effective renal plasma flow; glucose clearance is zero below the renal threshold.
- Starling forces. Net filtration = (capillary hydrostatic − interstitial hydrostatic) − (capillary oncotic − interstitial oncotic).
- Feedback. Most hormones are controlled by negative feedback; the classic exception is oxytocin in labour, which is positive feedback.
Worked example
A subject consumes 250 mL of oxygen per minute. Arterial oxygen content is 200 mL/L and mixed venous content is 150 mL/L. Cardiac output = 250 ÷ (200 − 150) = 250 ÷ 50 = 5 L/min. If the same person has a GFR of 125 mL/min and a renal plasma flow of 600 mL/min, the filtration fraction is 125/600 ≈ 0.21, that is about 21%.
Traps that cost marks
- Confusing clearance with excretion rate; clearance is excretion rate divided by plasma concentration.
- Reversing the direction of the Starling equation and so predicting filtration where there is reabsorption.
- Treating oxygen saturation and PaO2 as interchangeable; saturation depends on the haemoglobin and the dissociation curve, not on dissolved oxygen alone.
- Forgetting that in a first-order process a constant fraction, not a constant amount, is removed per unit time.