Physiology
Physiology tests how organ systems maintain homeostasis-especially fluid and electrolyte balance, acid–base regulation, cardiac output, gas exchange and renal function. Expect interpretation rather than pure recall.
Core rules and concepts
- Feedback: negative feedback reverses a change (insulin, ADH, baroreceptors); positive feedback amplifies it (labour contractions, clotting cascade).
- Cardiac output: CO equals heart rate multiplied by stroke volume; normal adult CO is about 4–8 L/min. Mean arterial pressure equals diastolic pressure plus one-third of pulse pressure.
- RAAS: low blood pressure or sodium triggers renin, then angiotensin II, causing vasoconstriction and aldosterone release with sodium and water retention.
- Acid–base: the lungs regulate CO2 (respiratory component) and the kidneys regulate bicarbonate (metabolic component). Normal pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3 22–26 mmol/L.
- Nephron: glomerular filtration, then mostly proximal tubular reabsorption, then concentration in the loop of Henle under ADH influence.
- Action potential: sodium enters the cell during depolarisation and potassium leaves during repolarisation.
Worked example
A patient has vomiting for two days. Hydrogen and chloride ions are lost, so pH rises above 7.45 and bicarbonate rises: this is metabolic alkalosis. The expected respiratory compensation is hypoventilation, which raises PaCO2. Potassium also falls because the kidneys conserve hydrogen in exchange for potassium, so monitor for hypokalaemia and cardiac dysrhythmia.
Traps that cost marks
- Reversing the direction of respiratory and metabolic compensation.
- Assuming potassium is unaffected in alkalosis.
- Thinking ADH increases urine output; it reduces it.
- Confusing cardiac output with stroke volume.