On the direction and velocity of blood flow in the extradural intravertebral vein of harp seals (Phoca groenlandica) during simulated diving

Ronald et al. (1977) suggested that blood flow in the caudal/lumbar section s of the extradural intravertebral vein (EIV) of seals changes direction fr om running towards the head before diving, to the opposite during diving. T he possible advantage would be that the oxygen-depleted venous effluent fro m the brain is routed via the EIV to the posterior parts of the hepatic sin uses 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-exami ned 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 f low direction and rate determined before, during and alter simulated dives lasting for 5 min, at three positions (caudal, lumbar and thoracic) along t he 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 variab le 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 EV 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.