Bernoulli and the Prep Room

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There’s always a lot of discussion amongst embalmers about the chemicals they use, the cosmetics they like, and the techniques they deploy in restoration. When we think of embalming, we can easily call to mind that related sciences are chemistry, biology, anatomy, and microbiology, but what about physics? Occasionally you will see someone post a question in an online forum about what pressure people like to use during injection, and invariably the answer is “slow and low.” The “slow” is in reference to the rate of flow or speed of the embalming solution and the “low” is addressing the pressure of the injection. Let’s take a look and decide if this answer is a reasonable one.  

To understand the concept, we have to remind ourselves of Bernoulli’s Principal which explains that when pressure goes up, rate of flow goes down. Simply, the faster fluid (or gas) moves the less time it has to press on something. Makes sense so far, right? An example of this that you may be able to picture in your mind is letting the air out of a balloon. The air inside creates static pressure that decreases when you let the air out of it, thereby increasing the rate of flow. Remember, pressure is relieved by a change in rate of flow. Now, in embalming it is not an either-or situation because of the pump inside the machine. Many embalming machines make use of a centrifugal pump. The way that this works is that fluid is sucked into what is called the “volute.” The volute is a spinning wheel, that you may be able to recognize when thinking about the Gravitron ride at the carnival. This is the one that pins its victims to the wall due to the centrifugal force exerted on them as it spins at top speed. In this case however, instead of people, its liquid and it has an outlet for the fluid to flow. The dynamic between pressure and rate of flow in this case is disrupted by what is known as Euler’s Pump Formula, which is several letters and numbers long so we will not get into that here but what you need to know is that complicates the simplicity of Bernoulli’s “if one goes up, the other goes down” idea. The kinetic energy produced by the volute is used to increase or reduce rate of flow based on its speed.  

“if one goes up, the other goes down”

So how does this relate to embalming? Very early on in our training we are taught that using too high of a rate of low can cause swelling, and to understand this, we can once again return to the analogy of the balloon. This time however, we will start with a balloon with no air in it. If we blow it up slowly, the balloon takes longer to fill up because the rate of flow we are producing is low. However, if we give one good breath, the rate of flow we produce will blow the balloon up very fast. The same can be said of our embalming solution. If we rush the solution into the body too fast before the tissues can absorb it, or if they are already saturated, it will begin to swell. Think of an elevator where a crowd of people try to get on at once, and invariably some people are left standing and waiting for the next one.  

Besides avoiding swelling, using a low rate of flow means that there is more time for the tissues to absorb the solution. Unlike a balloon, the vascular system is an open system as it allows for the movement of gases and liquids from the arteries into the tissues via the capillary beds in a process called “diffusion.” A lower rate of flow in this case allows time for this to happen with better effectiveness. Examine if you will, the activity of your neighborhood paper boy on his route. If he rides by a customer’s house and whips the paper from his bike on the street, he may make the delivery but who knows if that paper will hit the ditch, the porch, or a hapless dog minding their own business. But if he rides up the driveway and purposely places the paper on the stoop, his delivery is more accurate, and he is likely to get better tips at Christmas. In this case, his rate of flow is slowed, and he is able to apply more pressure to each of the houses.  

But what about the “low” part? Well, this is kind of tricky considering what we just learned about pumps disrupting our buddy Bernoulli. Factor in that some embalming machines have back flow valves built in to allow for fluid pressure to be relieved in order to protect the machine. Also, how do we quantify “low pressure?” Some models have pressure that goes up to 160 psi (pounds per square inch) while others only go up to 30. The newest machine on the market has a safety feature that stops the rate of flow when it detects a pressure of 105 psi. There are also machines that automatically set their own pressure based on the rate of flow set by the operator and others that do the opposite. On top of that, pumps act like any other machine and wear out, thereby being inconsistent with the pressure they produce. We all know that embalmers famously pride themselves on having the oldest embalming machine they possibly can, so pressure and rate of flow is always questionable. 

So, is the adage of “slow and low” good advice? Well, it would seem that “slow” is good advice, but “low” remains to be seen. Perhaps a better way of thinking about it is remembering the relationship between rate of flow and pressure, and understanding what that means in the context of your own machine.  

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