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Ronald et al. (1977) suggested that blood flow in the caudal/lumbar sections of the extradural intravertebral vein (EIV) of seals changes direction from running towards the head before diving, to the opposite during diving. The possible advantage would be that the oxygen-depleted venous effluent from the brain is routed via the EIV to the posterior parts of the hepatic sinuses and the inferior caval vein and, hence, is prevented from mixing with the more oxygen-rich venous blood in their anterior parts. We have re-examined this hypothesis by use of Doppler flowmetry. A catheter-tip flow probe was introduced into the EIV of two similar-sized juvenile harp seals, and flow direction and rate determined before, during and after simulated dives lasting for 5 min, at three positions (caudal, lumbar and thoracic) along the EIV. Regardless of probe position, blood was mainly flowing towards the head in 11 of 13 experiments prior to diving, in 8 of 13 experiments during diving and in 11 of 13 experiments during recovery after diving (and away from the head in the remaining experiments). Flow direction was most variable in the caudal position. Mean blood velocity in the EIV was substantially lower during diving (0.10 ± 0.22 cm s–1 (n=5) in thoracic position) than in the pre-dive (3.98 ± 3.32 cm s–1 [n=5]) and post-dive (5.75 ± 4.07 cm s–1 [n=5]) situations. Thus, the direction and rate of flow in the EIV was variable, particularly during diving, as is to be expected in a system of anastomosing, valveless veins. We conclude that the hypothesis of Ronald et al. (1977) most likely is false.
Department of Arctic Biology and Institute of Medical Biology, Medical Faculty, University of Tromsø, Tromsø, Norway 2:
Department of Physiology, University of Oslo, Blindern, Oslo, Norway