Postmortem Blood Flow
That the heart is a pump and it is the sole driver of blood flow is so ingrained in our understanding of physiology that few would even consider questioning other possibilities. I know I had not, but recent experiences and some utterly fantastic research suggests that the pump paradigm may be incomplete. Apparently, in certain experimental models, blood can flow for a period of time without a beating heart to drive it. Ostensibly, this has been demonstrated in several animals including, mice, rats, dogs and amphibian larvae and chick embryos but I have only been able to confirm the work using embryos. Other articles cited were not available. Notwithstanding, this is utterly mind blowing research with huge implications for the living. Importantly, this means that there is an additional mechanism influencing circulation, one that perhaps offers unrecognized health benefits. Before we get into that, let us look at the research.
A Bit of History
Much of the physiological research on postmortem blood flow was conducted decades ago, suggesting there has been some awareness of postmortem circulation, at least in research circles. Indeed, commentary published in 1934 cites research from 1910 proposing a third mechanism, beyond the heart and the nervous system, controlling blood flow:
…failure of this third mechanical factor in the circulation, rather than that of either the heart or vasomotor system, that causes the weakened circulation of the blood following illness and the extreme depression of the circulation in surgical shock.
A study published in 1953, found that amphibian embryos, tadpoles, could live and continue develop up to 6 days after the heart was excised. The development was abnormal compared to controls, with significant morphological changes in the digestive tract, the liver, and brain, but neither circulation nor development ceased immediately after the heart removed. Other studies using a variety of amphibians, also published in the fifties, presented similar findings (see references listed in above article). That these tadpoles continued to develop at all is remarkable and just a little mind blowing.
In 1966, among the articles I was not able to access, researchers found that circulation continued in dogs for up to two hours after cardiac arrest. From a secondary source:
Dr. Manteuffel-Szoege repeated the experiments of S. A. Thompson, who had shown that in asphyxiated dogs, residual circulation continued for up to two hours after cardiac arrest.23,24 Other research has shown 20% to 40% increases in canine cardiac output after occlusion of the thoracic aorta.15
Fast forward many decades, bioengineers exploring fluid dynamics may have discovered why the circulation remains for a period of time after the heart stops. It has nothing to do with the heart itself or interactions between the brain and vasomotor system, but everything to do with fluid dynamics, and likely, metabolism in general.
Fluid Dynamics and the Exclusion Zone
Water rushing through a cylindrical tube placed in container of water appears to equalize quickly. To naked eye, once this occurs, flow ceases. On a microscopic level, however, there is much going on that cannot be recognized without additional tools. What looks like equilibrium and essentially an absence of energy exchange or movement, belies an active zone of self-driven flow and energy. The kinetics in this active zone are what may propel postmortem blood flow.
In the initial studies, researchers essentially placed cylinders horizontally in a tub of water and monitored macro and microscopic flow patterns. What they found was that in the zone closest to the inside perimeter of the cylinder, which was later deemed the exclusion zone (EZ) because of the absence of solutes, absorbed energy that allowed it to maintain flow.
The exclusion zone is built from energy absorbed by the water. We showed that absorption of radiant energy, particularly of infrared and some ultraviolet wavelengths, expand the exclusion zone [1]. Hence, energy from the environment continually ‘charges’ the exclusion zone, and thus maintains separated charge for an indefinite period. If this is the case, then the kind of instability mentioned above might be perpetuated, and it is conceivable that this energy could drive continuous flow.
This flow is recognized as being self-driven because it moves by drawing energy from the suspension of water itself and by creating its own pressure gradient between the space nearest the edges of the cylinder (negatively charged), the water flowing in the center of the cylinder (positively charged), and in the tub of water (no charge) in which the cylinder is placed.
In essence, while the water may cease to flow at the center of cylinders, it still flows at the edges. I know what you may be thinking – ‘So what? This is water in a tube, what does any of this have to do with real life circulation, especially if the flow is only generated at the perimeter?’ Well, it turns out, something similar occurs in blood vessels. There is a ‘surface-induced flow mechanism’ that can propel blood flow in the absence of a beating heart and in the absence of any sort of vascular contraction. This is cool in and of itself, but even more interesting is that flow is amplified by infrared energy (IR) – light at certain wavelengths.
IR and Postmortem Blood Flow
An article published in 2023 – On the driver of blood circulation beyond the heart – describes postmortem blood flow. For this study, the researchers utilized chick embryos, whose hearts were arrested using potassium chloride. There were three conditions and four groups, controls/living, native postmortem blood flow (e.g. no IR added), postmortem blood flow plus IR exposure, and physiological blood flow with IR deficiency (embryos were removed from incubators).
In all postmortem groups, both venous and arterial blood continued to flow, albeit with different patterns.
In the native postmortem group, venous flow sometimes stalled or reversed direction initially, but within 2 minutes accelerated, stabilized, and continued for up to 50 minutes postmortem. Capillary blood flow remained in about half of the capillary beds. In the arteries, the direction of the blood flow reversed, traveling from the thinner to larger arteries, for up to 10 minutes before changing directions to a normal flow. Flow continued until arteries emptied of red blood cells.
When IR was added to the mix, velocity increased by 30%.
In contrast, when IR was removed (the living embryos were removed from the incubators), heart rate declined significantly, capillary blood flow stopped, as did vasomotor synchronicity.
Wow. Blood may flow without a beating heart and if we apply IR, which is essentially light, velocity increases.
Is the Heart Secondary?
Clearly, there are enormous differences between species and developmental states that make it difficult to compare this research to human physiology. Notwithstanding, there is a body of research dating back decades (and speculation dating back hundreds of years) on human heart anomalies where cardiac output is impaired but circulation is maintained. The authors of this review contend that those circumstances refute the dominant ‘cardiocentric’ view of circulation e.g. the pump model, in which the heart is the primary driver of blood flow. Similarly, the same authors contend that the venous-centric view (e.g. the Starling response), which proposes blood volume changes initiate contraction and relaxation of the heart muscle, still conceptualizes the heart as essentially a pump and falls short of fully appreciating the complexity of blood flow. They argue that these models are unable to account for a variety of conditions where circulation is maintained in the face of cardiac anomalies, experimental cases of postmortem blood flow, changes in hemodynamics relative to exercise, where cardiac output is mismatched with tissue demand, or several other situations where there is a discrepancy between cardiac output and tissue usage and circulation. Instead of the primary driver of circulation, the authors argue that heart is integrative and
…plays a mediating role between the pulmonary and systemic circulations. Rather than being the power source for blood’s propulsion, it assumes an autonomous, integrative function between the oxygen supply in the lung and its consumption at the periphery.
This makes sense. As an integrative organ, the pumping action is simply part of the flow but not the sole driver for the flow. Here, “the heart is a mechanism inserted in the blood circuit…” rather than “…a pumping engine… [with] enough power to drive the mass of the blood through the whole vascular system.”
The authors argue that a more appropriate analogy might be the hydraulic ram. The difference between a pump and a ram, though seemingly innocuous, may yield important clues to how the heart interacts with circulation, and more broadly, with the environment in which it resides. A pump is used to move stagnant water from one place to another – let’s say, for example, from a flooded house to a sewer drain. The pump actively sucks up water from one place and essentially spits it out somewhere else. To do this requires an external power source. It will not run on its own. In contrast, a hydraulic ram is used where there is already moving water, such as in a stream or river, and although it transfers water from location to another like the pump, it is propelled by the energy drawn from the fluids running through it. If the heart is merely a pump, when that pump breaks or is removed, as of the case of the postmortem studies, circulation should cease immediately. Similarly, if it the heart is a pump and the electrical circuits powering the pump are removed, circulation should also stop. If we conceptualize the heart as a hydraulic ram, however, we open the discussion to the possibility that blood flow itself carries an inherent kinetic energy that in someway propels circulation through the heart.
…that the former [pump] treats the blood as an inert fluid transported by mechanical force, while the latter sees it as a dynamic fluid organ that gains movement through its essential mediating of metabolic processes via reciprocal relationships with the physiological life of the tissues it supplies with oxygen and nutrients.
What is this kinetic energy and how is it derived? Well, part of it has to do with interactions between the EZ and its environment e.g. heat and light energy transfer and part involves red blood cells (RBCs), oxygen (O2), and ATP signaling e.g. basic metabolism. With the former, light of varying wavelengths is converted in the EZ from radiant energy to kinetic energy. That energy drives flow. Among the mechanisms by which it does this, is by increasing nitric oxide production and utilization. Both infrared land ultraviolet light induce the vasodilator nitric oxide. With the latter, RBCs play a role. While the RBC – ATP aspects of flow are topic for another day, the pathway is interesting, and coincidentally, lands us in the same place as light energy – with an increase in nitric oxide. Research from 1995 showed that RBCs were not merely passive O2 carriers as has been conceptualized by the cardiocentric/pump model of cardiovascular function. Rather, they are active regulators of O2 availability. Red blood cells sense and respond to low oxygen and elevated carbon dioxide, hypoxia and hypercapnia, respectively, by releasing ATP. ATP then binds to its cognate purinergic receptors in the endothelium to release nitric oxide, so that more O2 carrying RBC can reach the area. Accordingly, some researchers have suggested that
…25% to 30% of basal human blood flow can be attributed to red blood cell induced production of nitric oxide by vascular endothelium.
While others contend that this significantly underestimates the contribution of RBCs. Whatever the true number, it is clear that many components of the current heart as pump/RBC as passive carriers of oxygen propelled by that pump, are incorrect. Importantly, there is a world of interactions at the perimeter of blood vessels that compel blood flow and those interactions are influenced by innumerous factors, including light. That light increases the velocity of postmortem blood flood leads us to the role of light in antemortem blood flow and that is what we will tackle in subsequent posts. For now though, let us be in awe of the fact that blood may flow absent a beating heart.
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