What is the function/purpose of water in your body? What happens if your body doesn’t get sufficient water or gets too much water? What are the best sources of water?
Water plays so many roles in your body that I’ll have to choose which one to describe. The role that strikes me as most important is water’s capacity to dissolve chemicals and carry them about. Our bodies deal with thousands or perhaps millions of different molecules, each of which must migrate from place to place to perform its required tasks. For example, the molecules that are responsible for energy—carbohydrates, fats, oxygen, and carbon dioxide—must all move through us to keep us alive. Water acts as the carrier for all of these molecules. Many of the molecules dissolve easily in water—meaning that water molecules surround these molecules and carry them about one molecule at a time. Carbohydrates, carbon dioxide, salts, and many other organic compounds dissolve easily in water and are carried about by it. Other chemicals, particularly fats and oxygen, don’t dissolve easily in water and need help. Hemoglobin in red blood cells is the molecular structure that carries oxygen through our blood. So my answer to the first question is that water is the principal solvent for all the chemicals in our body.
If your body gets too much or too little water, the dissolved chemicals become either too dilute or too concentrated. When you have too much water in your body, the water tends to flow out of you through any membranes that are permeable to water. Among other things, this process tends to make you excrete extra water as urine. When you drink too much water, it goes right through you. When you have too little water in your body, water tends to flow into you through any permeable membranes, or at least tends not to flow out of you. When you eat lots of salt and are overconcentrated, you don’t excrete much water and feel extremely thirsty—your body is looking for more water.
Finally, the best sources of water are those that simply don’t have many dissolved chemicals; or at least none that cause trouble for your body. That means that your water shouldn’t have much lead or arsenic dissolved in it or any of a number of noxious organic chemicals. The purest waters are distilled water, rain water (assuming minimal air pollution), and water that has been chemically filtered (via ion exchange, reverse osmosis, and/or activated carbon). Spring and well waters tend to contain substantial amounts of dissolved calcium and magnesium salts, which make the water less pure but probably don’t affect its healthfulness. One special case to look out for is water that was very hard but that has been passed through a water softener. The dissolved minerals that made the water hard will have been replaced by sodium compounds during the softening process and excessive sodium consumption may be a problem for some people.