The Diving Seal and the Limits of Diffusion
A research team studies a Weddell seal that dives to 500 m and remains submerged for 22 minutes. Muscle biopsies show myoglobin concentration roughly 10 times that of human muscle, and the seal's muscle fibres are unusually small in diameter. During the dive, blood is shunted away from the periphery and gut toward the brain and heart, while muscle metabolism becomes almost entirely anaerobic. Arterial oxygen falls from 95% to 30% saturation in the final minutes. A student argues that the seal's muscles must rely on diffusion of oxygen from blood alone, and that small fibre diameter is simply a consequence of the animal's size.
1. Explain, using Fick's law and the concept of surface-area-to-volume ratio, why small muscle fibre diameter is an advantage during a long dive, and evaluate the student's claim.
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Reasoning chain.
- Diffusion rate is proportional to surface area × concentration gradient ÷ distance (Fick's law). In tissue, the relevant distance is from capillary to the centre of the fibre.
- A large fibre has a low surface-area-to-volume ratio, so its centre is far from any capillary and oxygen cannot reach it fast enough - especially once arterial saturation collapses to 30%.
- Small-diameter fibres have a high surface-area-to-volume ratio and short diffusion distances, so oxygen (and myoglobin-bound oxygen) can be delivered and used even at low gradients.
Evaluating the claim. The student is partly right but misses the point. Small fibres are not merely a scaling consequence - they are a functional adaptation that shortens diffusion distance. More importantly, the claim that muscles rely on blood-borne oxygen alone is wrong: the 10× myoglobin store acts as an oxygen reservoir and facilitates intracellular transport, which is exactly why the seal can sustain anaerobic and low-oxygen metabolism for 22 minutes. High myoglobin plus small fibres plus blood shunting are complementary adaptations, not alternatives.
2. The team wants to compare oxygen delivery in seal muscle and human muscle. State one variable they must control and explain how an uncontrolled variable could produce a misleading conclusion.
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Valid controlled variables (any one): muscle temperature; fibre type composition (slow vs fast twitch); fixation/staining protocol; time between biopsy and measurement; body mass or age of the animal.
Why it matters. Suppose seal biopsies were kept warmer before analysis than human biopsies. Warmer tissue has higher metabolic rate and different myoglobin stability, so measured oxygen content could differ for reasons unrelated to the adaptation. The team might then attribute a difference to diving adaptation when it was actually a handling artefact - a confounding variable producing a false causal claim.
Marking note. Full marks require a named variable, a direction of effect, and an explicit link to a wrong conclusion.