The short answer
Predict which way heart rate, stroke volume, resistance and pressure move when a circulation is disturbed, using CO = HR × SV and MAP ≈ CO × TPR as reasoning tools rather than formulae to recite.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Section III rarely asks what cardiac output is. It disturbs a circulation (a bleed, a drug, a tilt) and asks which way each variable moved.
Two relationships carry almost every question
CO = HR × SV, and MAP ≈ CO × TPR — Ohm's law for a circulation (strictly, minus right-atrial pressure, a few mmHg).
Combine: MAP ≈ HR × SV × TPR. Every disturbance acts through one of the three terms, so your first move is never recall — which term did this touch, and which did the body move back?
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- Pressure in (mmHg)
- Pressure out (mmHg)
- Drop across segment (mmHg)
| Segment | Pressure in (mmHg) | Pressure out (mmHg) | Drop across segment (mmHg) |
|---|---|---|---|
| Aorta and large arteries | 93 | 90 | 3 |
| Small arteries | 90 | 80 | 10 |
| Arterioles | 80 | 30 | 50 |
| Capillaries | 30 | 15 | 15 |
| Venules and veins | 15 | 4 | 11 |
A faster heart does not mean more output
After a bleed the rate climbs while output falls — SV fell further than HR rose, so the tachycardia is the correction, not the cause. That generalises: a reflex defends the variable it senses and lets the others take the strain, restoring it without undoing the disturbance. So sort the changes into what moved first and what moved in response, and read the most normal-looking number as the least informative.
Worked example
900 mL is withdrawn from a volunteer. Before: 120/80 mmHg, HR 70. Ten minutes later: 108/88, HR 105, hands cool. Has cardiac output risen or fallen, and what holds the mean pressure up?
Check yourself
In an anaesthetised animal preparation, a drug acting only on arteriolar smooth muscle lowers mean arterial pressure from 90 to 75 mmHg, while cardiac output rises from 5.0 to 6.0 L/min. What has happened to total peripheral resistance?
Key takeaways
- MAP ≈ HR × SV × TPR: every disturbance acts through one of the three terms.
- SV = EDV − ESV, set at the valve crossings.
- Over half of TPR is arteriolar, because R ∝ 1/r⁴.
- A reflex defends what it senses: near-normal pressure plus a fast heart can hide falling output.
Practise this with real GAMSAT-style questions
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