Distinction between edema and blister

I have been vac hanging for about a month. Every time, pretty much no matter how low weight I use, I get edema in my glans and especially around frenulum.

Today, however, in addition to the general swelling (which is quite large as usual), I also have a blister. Could someone tell me what exactly the difference between edema and blister is? I know they are both lymph fluid built up to protect body. But why are blisters right on the surface of the skin whereas edema is not? And why do they happen in vac hanging when edema is already happening?

Lastly, how long is this thing likely to be around? It hasn’t popped, but it is a true blister and not mere/more common fluid buildup.

Well, blisters I have had tend to hurt more and eventually will leak/pop. The edema I have had usually will subside after a nice hot shower for about 30-45 min.

I know those differences. I mean, why does one happen sometimes and the other other times. Why does the body produce a blister sometimes and edema other times?

How long do unpopped blisters usually take to go away?

Edema is usually caused by fluid leaking from vessels within the tissue and is not really a protective measure. This can be due to blockage of the vessel (compression) or a backup in pressure, a disease or sometimes position (gravity). A blister is a localized reaction to injury and the bleb is a form of protection as you noted. I’m not familiar enough with the vacuum hangers to make any suggestions. I will say if you have a blister you need to let it heal before hanging again.

Ok - let me give it a try:

Edema is fluid accumulation in the tissue, very evenly distributed between the cells like water in a sponge. A blister is contiguous water accumulation forming a fluid compartment under the skin. Usually, while hanging, edema occurs because of the negative pressure surrounding the blood vessels which tends to pull water out of the vessels. The lymphatic system drains part of that excess water, what remains forms edema first and if the fluid loading capacity of the tissue is exceeded, a blister will form. Why this happens sometimes (while edema forms every time) depends on some factors like blood composition, blood pressure and mechanical factors in the hanger, all of which vary from day to day.