A recent article in the Journal of Experimental Biology revisits the siphon principle to explain how sauropods might have browsed with the head held high in trees (Hughes et al., 2016). Hughes et al. state that the principal objection to a vascular siphon is the creation of sub-atmospheric pressures that would collapse cranial veins and thus prevent flow of blood. Their physical models involve flow of water in rigid ‘arteries’ and deformable, but not collapsible, ‘veins’, or completely collapsible ‘veins’ inside of a rigid tube. These models demonstrate that sub-atmospheric pressures could exist at the top of an inverted U-shaped tube if all of the tubing were rigid or protected from collapse by external rigid structures. Cavitation from ‘boiling’ occurs if the siphon is high enough and the sub-atmospheric pressure is below the water vapour pressure. They postulate that the water would boil at a lesser height at higher altitudes, because the atmospheric pressure is lower. All of these demonstrations are true, because they are simply physics of water flow in single tubes that are essentially rigid or protected from collapse. However, there are at least three separate problems that make the siphon physiologically impossible in the upright neck of a tall sauropod dinosaur.
First, every single blood vessel in the neck and head must be prevented from collapse if the blood pressure at each location is sub-atmospheric. This includes not only the veins, but also the arteries and capillaries at every level. Conceivably, the vessels in the cranium and spinal column could be held open by balancing vascular pressure with sub-atmospheric pressure in the cerebral spinal fluid, as demonstrated in humans (Dilenge et al., 1975; Rushmer et al., 1948), or shunting of flow into a vertebral venous plexus (Zippel et al., 2001). However, there are many blood vessels in the neck and head that cannot be protected. Several arteries proceed up the neck of birds (Glenny, 1951), and it is reasonable to assume that the dinosaurs, which gave rise to birds, also had similar unprotected main arteries in the neck. Very simply, these arteries will collapse if the intravascular pressure falls below atmospheric (Lillywhite and Donald, 1994). Moreover, there must have been small vessels of the microcirculation associated with the sense organs (especially the eyes), mouth, oesophagus, etc., which could not be protected from collapse. If even a single blood vessel in the head and neck were unprotected, it would receive no blood flow if connected to an artery having sub-atmospheric pressure.
Second, if the blood went from the lungs at atmospheric pressure up to the head, oxygen in the blood would be progressively exposed to sub-atmospheric total pressure and would dissociate from the haemoglobin, coming out of solution to form bubbles, long before the blood would boil with water vapour. If the oxygen carrying capacity were similar to that of humans, every litre of blood would hold approximately 200 ml of oxygen. Thus, if this oxygen came out of 1 litre of blood at 760 mmHg (1 atm), the bubbles would equal 200 ml, but if exposed to a pressure of 260 mmHg (=−500 mmHg sub-atmospheric pressure) in the head, the volume of bubbles would be 585 ml, or over half of the volume of blood. Hughes et al. (2016) assume that the blood gets to the head quickly enough to avoid oxygen bubbles before the oxygen is consumed by the tissues. However, degassing of dissolved nitrogen in the blood is an unavoidable problem, because nitrogen is not consumed. Aviators can get the bends (nitrogen bubbles in the blood) at absolute pressures of approximately 510 mmHg (Hills, 1977), and goats flown to a barometric pressure of 440 mmHg also produce bubbles in the blood (Hill et al., 1994). These hypobaric experiments involved relatively slow aircraft ascents when some loss of nitrogen could occur through the lungs during the ascent. However, the speed of blood flow up the sauropod neck would have been quicker, and no loss of nitrogen is possible. Super-saturated dissolved nitrogen would be delivered continuously to the head, and bubbles would be inevitable at much lower head height than the limit suggested by Hughes et al. (2016).
Third, sub-atmospheric blood pressures would have several problems associated with capillary function and vascular damage. These include lack of ultrafiltration of fluid into tissues, dehydration of tissues including the eyes, lack of lymph flow, lack of a blood clotting mechanism in wounds that do not bleed, and aspiration of air into any wounds larger than the size of arterioles (Seymour and Lillywhite, 2000).
The proposition that sauropods had extravascular hydrostatic gradients throughout the interstitial fluids of the body, thus creating a ‘G-suit’ like those protecting aviators from the effects of high g-forces, has been shown to be impossible, because the high extravascular pressures would bear on the pulmonary vessels, raising pulmonary blood pressures greatly and certainly causing pulmonary oedema (Seymour and Lillywhite, 2000).
Hughes et al. (2016) acknowledge previous discussion of the siphon principle in regard to sauropod necks and assert that the subject is ‘controversial’. However, a scientific controversy should have valid arguments or at least recognition of the arguments on both sides. Proponents of the siphon principle have never responded to critical facts. Surprisingly, Hughes et al. (2016) cite papers with other models of cephalic circulation (Seymour, 2000) and arguments as to why sub-atmospheric pressures in sauropod heads are unsustainable (Seymour and Lillywhite, 2000), yet they do not criticise, or even mention, the relevant points made there. Unfortunately, the siphon principle continues to attract speculation related to postural behaviour of long-necked dinosaurs, despite the facts that make its relevance physiologically untenable.