Explain how the physiology of a diving mammal, such as a Weddell seal, allows it to sustain aerobic respiration in the muscles during a prolonged dive.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Weddell seals (Leptonychotes weddellii) can dive to depths of over 600 m and remain submerged for more than 80 minutes without surfacing to breathe.
Model answer (4 marks)
1. Weddell seals have a very high concentration of myoglobin in their skeletal muscle, which stores large amounts of oxygen for use during a dive.
2. Myoglobin binds oxygen with a higher affinity than haemoglobin, so it keeps oxygen bound at the low partial pressures that occur in active muscle and only releases it when the oxygen tension falls very low.
3. Their blood volume and haemoglobin concentration (haematocrit) are greater than in non‑diving mammals, giving a larger total oxygen store in the bloodstream.
4. During a dive the diving reflex causes bradycardia and peripheral vasoconstriction, reducing oxygen delivery to non‑essential tissues and conserving oxygen for the heart, brain and active muscles, thereby extending aerobic metabolism.
2. Myoglobin binds oxygen with a higher affinity than haemoglobin, so it keeps oxygen bound at the low partial pressures that occur in active muscle and only releases it when the oxygen tension falls very low.
3. Their blood volume and haemoglobin concentration (haematocrit) are greater than in non‑diving mammals, giving a larger total oxygen store in the bloodstream.
4. During a dive the diving reflex causes bradycardia and peripheral vasoconstriction, reducing oxygen delivery to non‑essential tissues and conserving oxygen for the heart, brain and active muscles, thereby extending aerobic metabolism.
Examiner tips
- Use the exact terms: myoglobin, haemoglobin, haematocrit, diving reflex, bradycardia, peripheral vasoconstriction.
- Show the causal chain – high myoglobin → oxygen storage → high affinity → release at low PO₂; high blood O₂ → more oxygen available; diving reflex → conservation of O₂ for essential organs.
- Keep each point brief and to the point, matching the four mark scheme items.
Common mistakes
- Mixing up the roles of myoglobin and haemoglobin; forgetting that myoglobin has higher affinity.
- Failing to mention the diving reflex or its cardiovascular effects.
- Using vague language such as “more oxygen” without specifying blood volume or haematocrit.
Mark scheme (4 marks)
- Diving mammals have a greatly elevated myoglobin concentration in skeletal muscle, which stores oxygen bound to myoglobin for use during the dive.
- Myoglobin has a higher affinity for oxygen than haemoglobin, so it retains oxygen at the low partial pressures found in respiring muscle, only releasing it when oxygen tension is very low.
- Diving mammals have a higher blood volume and/or elevated haemoglobin concentration / higher haematocrit compared with non-diving mammals, increasing the total oxygen carried in the blood.
- Cardiovascular adjustments during diving (diving reflex) include bradycardia and peripheral vasoconstriction, which reduces oxygen delivery to non-essential organs and conserves the oxygen supply for the heart and brain, extending the time aerobic metabolism can be sustained in active muscles.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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