Most people that get massage or some other kind of body work hear that they need to drink lots of water afterward to help flush out the released "toxins." Well as a curious consumer, you want to know what those toxins are, and what the reasoning behind the extra water intake is. If you ask most massage therapists, you will likely get one of the following two answers:
In reality, most of the "toxins" that are released are just normal waste from regular cell activity. Each cell produces waste during its normal activity and metabolism, and excretes this outside its cell membrane. From there it is located in what is called the "interstitial space" which is the space located between the cells. Through the activity of your muscles, the cells and the interstitial spaces are squeezed (or pumped) and the fluid is moved out of the area. The fluid then is typically directed to the lymph system, where it gets collected, concentrated, and dealt with. (If you don't know, the lymph system is the second line of defense of our bodies for dealing with pathogens, viruses, bacteria, etc. since it houses a lot of white blood cells. The first line of defense is the skin, which prevents a lot of pathogens from entering the body.)
When you receive massage or bodywork, cell waste (which is already in your system) gets released at a more rapid rate than normal. Your body has to deal with the larger amount of material within the same amount of time, and that is what can cause you to feel tired, sick, or sore afterward.
Fascia, the connective tissue of the body, is one of the components that is responsible for this phenomena. Fascia's main role in the body is as a divider and connector of body parts, organs, cells, etc. On first thought, divide and connect seem opposites, and indeed they are. But fascia does both at the same time. For understanding this concept, you can represent fascia as a piece of double-sided tape, and our two muscles can be represented by two balloons. When you put the double-sided tape onto one balloon, you now have a small extra layer of protection (to the balloon) where the tape is located. This barrier separates the balloon from anything on the other side of the tape. Now add the second balloon to the free side of the tape (so that your model is balloon/tape/balloon). The tape is doing the job of protection/separation for each individual balloon from the other, and at the same time it is connecting the two balloons to each other. Fascia fills this role for individual cells, groups of cells, organs, muscles, etc. within our bodies.
Fascia, when acting as a protective barrier is not impermeable like a solid wall. A more descriptive analogy would be a net made of rope. When stretched beyond its normal dimensions, the holes of a rope net narrow in one direction and lengthen in the other. Thus the effective opening of the holes are made smaller, allowing smaller and smaller items to pass through. When the net is released back to its normal shape, the holes effectively open up to normal size, and allow larger items through. When you receive massage, and the fascia is being restored to its normal shape, it is akin to many small faucets being opened up simultaneously causing an on-rush of fluid and waste that our systems have to deal with immediately. Being properly hydrated allows the processing of the waste to happen most efficiently. And since most people are slightly dehydrated, it is a good idea to drink extra water after your massage to maintain a proper hydration level.