If you circulate a coolant through the circulatory system, the cooling speed is limited by the coolant heat capacity and mass flow rate. For a given maximum pressure difference, the maximum flow that you can achieve depends on the fluid density, viscosity, and the structure of the circulatory system. In the simplified case of laminar flow through a stiff circular straight pipe Hagen–Poiseuille equation applies: mass flow rate is proportional to fluid density and the square of cross-sectional area, and inversely proportional to fluid viscosity and pipe length. The circulatory system is mostly made by long and thin capillaries, with curves and branching that add further resistence compared to a straight pipe.
Blood has approximately the same density of water and five times its viscosity, but it is a non-Newtonian fluid optimized for flowing through thin capillaries. With any water-based coolant, you wouldn’t be able to achieve a much higher flow rate than normal circulatory flow rate, but you can use a water-based coolant since water would freeze. Anything more viscous, such as a cryoprotectant mixture, can be circulated at a much lower flow rate. That’s why cryoprotectant perfusion as practiced by cryonicists takes many hours. Forcing an higher flow would not only risk rupturing the blood vessels, but also heat them instead of cooling. If you were to use cryoprotectant as a coolant (which, AFAIK, no cryo company does), viscosity would also increase as temperature decreases. And I presume that the maximum allowable pressure in blood vessels decreases with temperature: much like rubber hoses, I expect them to become brittle as they approach glass transition temperature.
Add the fact that a typical cryonics “patient” won’t usually have an intact and highly functional circulatory system: hours of ischemia and pre-mortem conditions can usually result in stiff, obstructed, collapsed or outright ruptured blood vessels, making impossible to rely on circulatory function to perform cooling. In fact, it’s even unclear whether proper cryoprotectant perfusion could be achieved in most cases.
Kinda.
If you circulate a coolant through the circulatory system, the cooling speed is limited by the coolant heat capacity and mass flow rate. For a given maximum pressure difference, the maximum flow that you can achieve depends on the fluid density, viscosity, and the structure of the circulatory system. In the simplified case of laminar flow through a stiff circular straight pipe Hagen–Poiseuille equation applies: mass flow rate is proportional to fluid density and the square of cross-sectional area, and inversely proportional to fluid viscosity and pipe length. The circulatory system is mostly made by long and thin capillaries, with curves and branching that add further resistence compared to a straight pipe.
Blood has approximately the same density of water and five times its viscosity, but it is a non-Newtonian fluid optimized for flowing through thin capillaries. With any water-based coolant, you wouldn’t be able to achieve a much higher flow rate than normal circulatory flow rate, but you can use a water-based coolant since water would freeze. Anything more viscous, such as a cryoprotectant mixture, can be circulated at a much lower flow rate. That’s why cryoprotectant perfusion as practiced by cryonicists takes many hours. Forcing an higher flow would not only risk rupturing the blood vessels, but also heat them instead of cooling. If you were to use cryoprotectant as a coolant (which, AFAIK, no cryo company does), viscosity would also increase as temperature decreases. And I presume that the maximum allowable pressure in blood vessels decreases with temperature: much like rubber hoses, I expect them to become brittle as they approach glass transition temperature.
Add the fact that a typical cryonics “patient” won’t usually have an intact and highly functional circulatory system: hours of ischemia and pre-mortem conditions can usually result in stiff, obstructed, collapsed or outright ruptured blood vessels, making impossible to rely on circulatory function to perform cooling. In fact, it’s even unclear whether proper cryoprotectant perfusion could be achieved in most cases.