Siphon mechanism in collapsible tubes: application to circulation of the giraffe head

Abstract

Controversy exists over the principles involved in determining blood flow to the head of a giraffe, specifically over the role of gravity pressure (pgh) in the collapsible jugular vein in facilitating uphill flow in arteries. This study investigated the pressures within vertically oriented models containing both rigid and collapsible tubes. An inverted U tube was constructed (height = 103 cm) of thick rubber tubing in the ascending limb and collapsible dialysis tubing in the descending limb. Water flow was induced by a variable speed pump maintained at the reservoir level such that the descending limb was partially collapsed. Pressure measurements were made at various levels within the U tube by two methods: 1) with the transducer at same level as the tip of the water-filled catheter and 2) with the transducer at the reservoir level. During flow, the pressure at any point was nearly atmospheric along the length of the descending limb. Such methods of obtaining pressure indicated that the pressure gradient within the partially collapsed descending limb was the sum of viscous flow pressure (P1-P2 of Poiseuille) and gravitational pressure (pgh). To study the facilitatory effect of a siphon, the descending limb was compared with a horizontally placed limb (length = 100 cm), and the flow was kept constant. Calculations of hydraulic "work" (pressure x flow) indicated that with a partially collapsed descending limb, work of the pump was reduced by 15% compared with uphill flow to the elevated horizontal position. It is concluded that the siphon mechanism operates in a partially collapsed descending limb of a siphon loop.

The origin of mean arterial and jugular venous blood pressures in giraffes

“…When all the tubes were rigid, pump flow rate at 4·l·min -1 , and the valve at P4 closed, it was obvious that a siphon can assist flow, can significantly reduce pressures throughout the system by the amount of the siphon pressure head (approximately 30·mmHg), and can reduce the work of the pump, as suggested by Hicks and Badeer (Hicks and Badeer, 1989) (Table·1; Fig.·4E). For example at the 'heart' level (P1), in the absence of a jugular extension the pressure at P1 was +22.8±0.5·mmHg.…”

“…Unlike other mechanical models of the giraffe cranial circulation (Seymour and Johansen, 1987;Hicks and Badeer, 1989;Pedley et al, 1996;Badeer, 1997) but like Seymour's model (Seymour, 2000), our model contained a 'brain' circulation that could be made rigid to simulate Goetz's 'deep, non-collapsible venous channels' (Goetz et al, 1960) or the effects of positive or negative cerebrospinal fluid pressure, or collapsible to represent a physiological microcirculation in which an internal pressure for achieving filtration exists, but which is subject to collapse if transmural pressure falls towards zero. Similarly the 'jugular' limb could be rigid to represent a jugular vein supported by extravascular connective tissue or filled by a high volume flow of blood, or one that was collapsible to represent its normal, observed, physiological state.…”

“…This lower than predicted average pressure may be because some of the animals were anaesthetized at the time of measurement, or were holding their heads at an average angle less than vertical, or did not have two meter long necks, but it is also possible that mechanisms exist that reduce the work of the heart. Thus, Badeer (Badeer, 1986;Badeer, 1988;Badeer, 1997) and Hicks and Badeer (Hicks and Badeer, 1989) have suggested that as the giraffe cranial circulation can be regarded as an inverted U-tube that functions as a siphon, gravitational effects are neutralized and the high pressure results from high Using a mechanical model of the giraffe neck and head circulation consisting of a rigid, ascending, 'carotid' limb, a 'cranial' circulation that could be rigid or collapsible, and a descending, 'jugular' limb that also could be rigid or collapsible, we have analyzed the origin of the high arterial and venous pressures in giraffe, and whether blood flow is assisted by a siphon. When the tubes were rigid and the 'jugular' limb exit was lower than the 'carotid' limb entrance a siphon operated, 'carotid' hydrostatic pressures became more negative, and flow was 3.3·l·min -1 but ceased when the 'cranial' and 'jugular' limbs were collapsible or when the 'jugular' limb was opened to the atmosphere.…”

“…The origin of these pressures is controversial. The controversy has been reviewed at least six times (Badeer, 1986;Seymour and Johansen, 1987;Pedley, 1987;Badeer, 1988;Badeer and Hicks, 1992;Seymour et al, 1993) and evaluated empirically five times using mechanical models (Holt, 1959;Hicks and Badeer, 1989;Pedley et al, 1996;Badeer, 1997;Seymour, 2000).…”

“…He also concluded that high blood pressure 'is simply to minimize the collapse of the vessels in the head and neck', thus allowing a siphon effect to exist (Badeer, 1997), and that its functional advantage was to reduce the work of the heart (Hicks and Badeer, 1987). When a pump was used to drive fluid through a vertical U-tube system, a siphon reduced 'heart' work by 12-15% (Hicks and Badeer, 1989), although Seymour et al could not repeat this result and concluded that this finding was an artefact (Seymour et al, 1993).…”