system
Given the following post, respond as if you're commenting on Reddit:
human
What would happen to my body if I were somehow physically prevented from urination/defecating?
gpt
I will cover the urination side of this question. 

There are two main categories of urinary obstruction. There is acute urinary retention (all of the sudden not being able to pee) and there is chronic obstruction (narrowing making it harder to pee for a long time).  

In the presence of acute urinary retention (such as with someone with an enlarged prostate that has cut off the flow of urine), you will see an acute swelling of the bladder such that it can be easily felt through the skin on a non-obese person. Eventually, this can result in a back-up of urine higher in the kidneys. This can be seen on imaging such as ultrasound or CT scan and is called hydronephrosis. A kidney in this setting can lose some of its ability to properly filter solutes such as sodium. This can lead to disturbances in the solutes in the body. The kidney can also become compressed by the fluid in its central portion.

Chronic obstruction (such as with benign prostatic hyperplasia or a urethral stricture) can result in a deformed bladder which has many folds. This occurs due to the bladder having to push against a higher resistance for long periods of times and is adaptive similar to how a heart changes when it pushes against a deformed valve. This can result in problems in the one-way valves that urine passes through to the bladder. Again we can see dilation of the kidney and the tube that carries urine from the kidney to the bladder (ureter). Kidney problems can again occur.  

Interestingly, a fetus can have urinary obstruction as well. The amniotic fluid that surrounds a fetus is formed from the fetus's urine. Without urine, this fluid is not present as much as you would want it to be (called oligo- or anhydramnios). Amniotic fluid helps with lung development, so babies with this problem can have poorly functioning lungs. Also it is a buffer against the uterus, so babies can have deformed faces. The back-up of urine can also permanently damage their kidneys. One such disease that leads to this is posterior urethral valves. 

Also interesting, is that severe constipation can lead to urinary obstruction.