The replacement of fluid following withdrawal of up to 40 % of the blood from Octopus vulgaris can be tracked over a period of days by measuring the dilution of haemocyanin, which is not simultaneously replaced. Haemocyanin concentration was measured from the copper content or the oxygen-carrying capacity of further small blood samples. Fluid lost was replaced within 1–2 h, provided that the digestive gland ducts were left intact. If these were ligated, the haemocyanin concentration remained the same as before withdrawal of the initial large blood sample and the animals died within a few hours. Evidence presented elsewhere has indicated that the site of the fluid uptake is the digestive gland appendages. Urine production would be continued or increased during the restoration of blood volume. When urine volume is added to the volume of fluid replaced, it appears that this fluid transport system must be capable of moving at least its own volume of fluid from the gut into the blood every 5 min. An immediate consequence of blood withdrawal is a fall in blood pressure and pulse amplitude, followed within minutes by a transient rise to high blood pressures, apparently as a result of an increase in peripheral resistance as circulation to the arms is restricted, conserving the blood for vital central organs. Following these transient swings, the diastolic blood pressure returns to normal values despite blood loss; pulse amplitude returns as blood volume is replaced. Duct-ligated animals continue to show a reduced pulse.

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