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The heart works against gravity
R S Seymour1, A R Hargens, T J Pedley
1Department of Zoology, University of Adelaide, South Australia.
Insights
The siphon principle does not aid blood flow in vertebrate circulatory systems when vessels can collapse. Gravity
Area of Science:
- Cardiovascular Physiology
- Biomechanics
- Vertebrate Anatomy
Background:
- Vertebrate circulatory systems are closed, with blood returning to the heart at the same level.
- It is commonly believed the heart works against viscous resistance, not gravity, even in superior vascular loops.
- The siphon principle is thought to assist blood flow in these superior loops.
Purpose of the Study:
- To investigate the role of the siphon principle in blood flow within superior vascular loops.
- To determine if vascular collapse affects the siphon principle's assistance.
- To challenge the established understanding of gravitational effects on circulation.
Main Methods:
- Theoretical analysis of fluid dynamics in collapsible vessels.
- Application of the Poiseuille equation to collapsible and non-collapsible vascular segments.
- Laboratory modeling using water flow through collapsible tubing.
Main Results:
- The siphon principle does not assist blood flow in superior loops if descending vessels are collapsible.
- Vascular collapse halts blood flow if central arterial pressure is insufficient to support the blood column.
- Potential energy is lost to friction in collapsed vessels, negating siphon assistance.
- Flow rate in partially collapsed vessels is independent of resistance.
Conclusions:
- The siphon principle does not facilitate blood flow in superior vascular loops of vertebrates.
- Vascular collapse, not gravity, is a critical factor limiting blood flow in these systems.
- Previous findings may be influenced by experimental artifacts related to vessel collapsibility.
Abstract:
The circulatory systems of vertebrate animals are closed, and blood leaves and returns to the heart at the same level. It is often concluded, therefore, that the heart works only against the viscous resistance of the system, not against gravity, even in vascular loops above the heart in which the siphon principle operates. However, we argue that the siphon principle does not assist blood flow in superior vascular loops if any of the descending vasculature is collapsible. If central arterial blood pressure is insufficient to support a blood column between the heart and the head, blood flow ceases because of vascular collapse. Furthermore, the siphon principle does not assist the heart even when a continuous stream of blood is flowing in a superior loop. The potential energy gained by blood as it is pumped to the head is lost to friction in partially collapsed descending vessels and thus is not regained. Application of the Poiseuille equation to flow in collapsible vessels is limited; resistance depends on flow rate in partially collapsed vessels with no transmural pressure difference, but flow rate is independent of resistance. Thus the pressure developed by the heart to establish a given flow rate is independent of the resistance occurring in the partially collapsed vessels. The pressure depends only on the height of the blood column and the resistance in the noncollapsed parts of the system. Simple laboratory models, involving water flow in collapsible tubing, dispel the idea that the siphon principle facilitates blood flow and suggest that previously published results may have been affected by experimental artifact.