CIRCULATION TO THE HEAD OF BAROSAURUS REVISITED - THEORETICAL CONSIDERATIONS

Various means of driving blood to the head of the Barosaurus with a lo ng neck have been postulated, The multiple hearts concept of Choy and Altman has not been received enthusiastically. We favor the siphon loo p concept based on the interaction between viscous flow pressure (=Q o ver dot R) and the gravitational pressure (rho gh). The actual pressur e in all vessels of the head of the Barosaurus is calculated to have b een negative. The notion that flow cannot occur under negative intrava scular pressures is unjustified hemodynamically. The intracranial vess els are prevented from collapsing by the cerebrospinal fluid, which is also subjected to a similar negative pressure. The extracranial vesse ls can be prevented from collapsing by several possible means. (1) A v ery high aortic viscous flow pressure (e.g. +740 mmHg) to counteract t he negative gravitational pressure, assuming the gravitational zero pr essure is at the level of the heart. (2) A relatively low aortic visco us flow pressure (e.g. +300 mmHg) would result in a marked negativity in all the vessels of the head and neck, assuming the gravitational ze ro is at heart level. Collapse of these vessels may be prevented by th ick vascular walls and dense connective tissue attached to a thick sem irigid skin. (3) A relatively low aortic viscous flow pressure (e.g. 300 mmHg) with gravitational zero at about the middle of the long neck would reduce the negativity of cranial vessels (as compared with the preceding model) and provide positive pressure for filling the thick-w alled right ventricle.