The short answer
Read a partial-pressure table or an oxygen dissociation curve and predict where oxygen loads, where it unloads, and what a shift does to tissue delivery.
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 hands you a partial-pressure table for an unfamiliar animal, or a curve for a mutant haemoglobin. Every number you need is on the page, not in your memory.
- PO₂
- PCO₂
| Site | PO₂ | PCO₂ |
|---|---|---|
| Inspired air, warmed and humidified | 150 | ~0 |
| Alveolar gas | 100 | 40 |
| Blood leaving the lung (arterial) | 100 | 40 |
| Resting tissue | 40 | 46 |
| Blood returning to the lung (mixed venous) | 40 | 46 |
Partial pressure is not content
PO₂ is the driving pressure; saturation is the fraction of sites occupied; content is what a litre carries — set by the haemoglobin. Halve the haemoglobin: PO₂ and saturation still read normal, each litre carries half.
- normal (P₅₀ 26.6)
- right-shifted (P₅₀ 36)
Reading any dissociation curve
Drop both PO₂ values onto the curve
The vertical distance between lung point and tissue point is the fraction unloaded — that alone answers most delivery questions.
Re-read both values on the shifted curve
A shift is not a separate fact. Hot, acidic, CO₂-rich blood shifts right: lower affinity, wider gap.
Scale it to the body
Per minute = flow × what each litre gives up. What arrives can be normal while what is released doubles.
Worked example
Muscle PO₂: 40 mmHg at rest, 20 in hard exercise, hot and acidic throughout (dashed curve at both ends). Alveolar PO₂ 100 mmHg. More oxygen from the PO₂ fall, or the shift?
Check yourself
A study reports that in one resting subject arterial blood leaves the lung 98% saturated and mixed venous blood returns 73% saturated. During steady exercise the same subject's arterial blood is 97% saturated, mixed venous blood returns 45% saturated, and cardiac output has tripled. Haemoglobin concentration is unchanged. By roughly what factor has the amount of oxygen the tissues actually take up each minute increased?
Key takeaways
- Flat above ~60 mmHg protects loading; steep at 20–40 mmHg delivers.
- Hot, acidic, CO₂-rich blood shifts right — more unloaded, little lost.
- Saturation is a percentage; content depends on haemoglobin. Normal PO₂ proves nothing.
- Oxygen taken up = flow × what each litre gives up.
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