Physiology: Integrated Control and Compensatory Limits
Physiology questions ask what the body will do next and how far compensation can go. Revise the major control systems as loops with sensors, integrating centres and effectors. For circulation, understand the determinants of cardiac output (preload, afterload, contractility, heart rate) and how the baroreceptor reflex responds to haemorrhage: sympathetic outflow rises, giving tachycardia, vasoconstriction and a narrowed pulse pressure before blood pressure falls. For respiration, understand the alveolar gas equation, oxygen delivery as the product of cardiac output and arterial oxygen content, and the difference between hypoxaemia from shunt, from ventilation-perfusion mismatch and from hypoventilation.
For acid-base, apply a stepwise method: check the pH, identify the primary disorder from the carbon dioxide and bicarbonate, calculate compensation, then calculate the anion gap. For renal physiology, know the segments of the nephron and the hormones that act on them. For endocrine physiology, know the axes and their negative feedback.
Traps include assuming a normal blood pressure means adequate perfusion, and misreading a mixed acid-base picture as a single disorder.
Rule of thumb
In any shock stem, ask which variable is being defended. Early haemorrhage defends blood pressure at the cost of tachycardia and vasoconstriction; decompensation is when the defended variable finally falls. In acid-base, if the measured compensation does not match the expected compensation, suspect a second disorder.