My fluids final is over; however, I still have to complete my project for the course due this Monday. The project is a 10 page research paper on the bladder and prostate.
Background
In a healthy male, the prostate does not restrict the diameter of the urethra. This means the urethral diameter is more or less a constant along its length and has an average size of about 6mm. When the sphincter muscle relaxes, it acts like an open valve between the bladder and the urethra, shown in fig 1. Urine can then flow out through the hydrostatic
pressure of the urine inside the bladder. People who have trouble with their sphincter muscle usually suffer from incontinence. This is urinary leakage caused by the inability of the sphincter to tighten and fully restrict the flow of urine from the bladder.
Fig.1
In addition to relaxation of the sphincter muscle, two pressures are exerted to increase the flow-rate of urine through the urethra. The first pressure is called the detrusor pressure. This pressure is caused by the layers of muscles lining the bladder itself. The secondary pressure is caused by the external pressure of your body. This pressure occurs when you move your torso, cough etc. These pressures increase the internal pressure in your bladder, causing a larger flow of urine.
Fig 2.
When the prostate enlarges, it constricts the opening of the urethra. Because the prostate is enclosed within the body, it cannot expand radially outward; consequently, it expands inward and causes a restriction within the urethra, Fig 2.
Design Project
Our project is to design a working bladder where we can vary the detrusor and external pressures separately. We must then investigate the effects of the prostate and the required pressures to cause urinary flow. From there, we can scale our results from our larger bladder to make conclusions about a real patient’s bladder.
The Over-Engineer
Unfortunately, there is one guy in our group who I call the
Overengineer. The Overengineer is the guy who takes his right hand and goes all the way over his head to say look, I can touch my left ear.
He said to us two weeks ago he would have a working bladder system made by last weekend. He showed it to us last Friday, and it looked iffy. It was made from a balloon covered in layer upon layer of liquid latex. Not only that, the prostate was set up so that it was constricting the bladder and not the urethra. The prostate was a second balloon around the bottom neck of the first balloon. He told us that the first balloon they made failed because when they emptied it the latex touched itself in the interior and fused….. I knew this was going to be a bad idea. We tried to get it to work, but gave up.
If you first don’t succeed, try try to over engineer again!
Okay, so the next day, last Saturday, rolls around and we all meet up again. This time he comes with a new bladder system. Only this time, it's a
2.5 GALLON bladder bag. This monstrosity is covered in a plastic bag and hooked up to a vacuum supply. The goal was to have the vacuum suck the back and increase the flow rate (detrusor muscles). Weights were then be hung from sides of the bag to increase the pressure and cause the external body pressures……We hooked the bladder up to the shop vacuum supply, and guess what, it wont work! It's not enough vacuum to see any effects on this big bladder, and we had like 30-50mm of Hg!
Now, let's just take a step back and think about this for one second. Does a
2.5 GALLON bladder seem
REASONABLE to anyone? It better not!
Not only was the vacuum not working, the prostate was a bicycle tire wrapped around the bottom of this big bladder bag. The weight of the water above the bicycle tire was so great that it was literally pushing the tire off of the bladder bag. Because the bicycle tire was not restricted to expand outward like a real body, it expanded radially outward and did NOTHING to restrict the flow!
We wasted the ENTIRE last weekend fiddling with this set up, when we could have been studying for the final exam.
So after our final exam on Wednesday, we get together to talk about how we are going to get this thing working. He tells us, “I have a solution!” He shows us a one way threaded valve that only allows flow in one direction after a certain amount of pressure.
Mr. Over-engineer is hard at work! This
added complication has NOTHING to do with making the experiment work.
Say Hello to Mr. Practical!
So today we decide to make a new bladder system. This time, we designed it without Mr. Over-engineer. The three of us that designed it included me, an older student who is retired from the military, and another student who used to be in construction and mechanic school.
All you engineering students should take notes, right now. I call these guys, Mr. Practical.
Together, we decide to get a
small bladder. The smaller, the better. We go to REI, a sporting equipment store and buy a 1.5
Liter bladder bag, the smallest we could find. Next, we go to home depot. We find a nice clear plastic container, (3 for $10) and decide to use this as our pressure vessel. (LOOK, NO CONSTRUCTING A PRESSURE VESSLE!). We then try to find a sealant to make the lid air tight. The ex-military guy said we should use silicon sealant, but the mechanic said no, we should buy a hot glue gun and hot glue it because sealant takes too long to dry. Mr. Practical is hard at work. Since I don’t own a hot glue gun, I said sure I will buy one because I can use it at home after the project is over.
Things are looking promising!
We took the box and cut some simple round holes into it on a drill press. We then thread on a shop air nipple, a pressure gage, and an exit port for the bladder hose to exit and drain out so we can measure the flow rate.
We then pressurized the system and found it to have minimal leaks. In fact, because we are hooked up to a shop air supply, we can leave the valve partially open and not even worry about sealing the leaks!
We set it up and by god it worked the first time! With no pressure, the bladder had a flow rate of 23mL/s, with pressure, we doubled that rate to 43mL/s!
Success! Mr. Practical’s practicality paid off.
Make this your motto
As my mentor used to say:
"Good enough, is good enough."
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and my personal favorite,
Tomorrow we are going to modify the bladder system to include the detrusor pressure of the bladder muscle.
So get to the point already!
Point of this story,
DO NOT OVER ENGINEER STUFF. The simplest design that does the job is the BEST design, ALWAYS!
Remember Occam's razor:
entia non sunt multiplicanda praeter necessitatem
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In english:
entities should not be multiplied beyond necessity.
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Side: My professor works on fluid flow for biological systems at NIH. He told me he is looking for a small group of 5 students to do an actual design and research to construct a medical device for a prostate as part of a design competition among other colleges. Unfortunately, I don’t have any time with my upcoming schedule, too bad.
Attention Future Engineers!
At least he's in engineering school and not med school. 
