just a quick question. Is the tissue fluid the same water potential as the cells it surrounds? I am a bit confused because if it was lower it would cause water to leave the cell and if it was higher water would enter the cell and both aren't really good for the cell. I believe tissue fluid is isotonic, am I correct?
Yes, cell membranes are highly permeable for water. While diffusion through the membrane is possible (description, and some science), it is most effective through water channel proteins (aquaporins). The different expression levels of aquaporins in different cell types result in differences in the permeability.
And yes, the tissue fluid has to be isotonic to the cell interior. The water balance in the body is therefore highly regulated. If not enough water is available, the blood osmolarity (ratio between solubles and water) increases. This is sensed in the hypothalamus (an area in the brain which is a major player in hormone regulation) and results in production of the hormone vasopressin. This will not only increase your thirst (and therefore add more water to the system) but also regulate aquaporin expression, improving reabsorbtion of water in the kidney (nicely shown here, actual science here). Urine production is of course also important in the response to drinking too much, which can be harmful as well..
If tonicity is deregulated, this has serious consequences for the body (some more science). For example, mutations in vasopressin or kindney aquaporins can result in very similar phenotypes, characterized by dehydration because water levels cannot be properly regulated (even more science).
edited for more explanation, because people complain when I give short answers to short questions ;)
Not necessarily- cells are surrounded by a plasma membrane that creates a waterproof seal from their surroundings. This membrane contains embedded proteins, called transporters and channels, that enable the selective passage of water, ions, and solutes to maintain a membrane potential, or concentration difference between inside and outside. This enables cells to maintain homeostasis, despite changing external conditions.
You can learn more about these processes here: