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Splenic contraction

When a body holds its breath, the spleen squeezes and pushes the red cells it was storing into circulation — a blood bank sitting in the abdomen. In humans the spleen loses 15–40% of its volume within 30–60 seconds, lifting haematocrit from about 42% to 45–46% and adding roughly 50–100 mL of oxygen: enough for another fifteen to thirty seconds of breath-hold. The effect is real but modest, and that modesty is the thing worth remembering. In seals the spleen is enlarged enough to hold up to 60% of all their red cells, driving haematocrit to 65–70% on a dive — an oxygen-dense slurry, with none of the viscosity cost paid while resting at the surface.