The movement of water in the body is dependent on many organic substances e.g. glucose. These substances vary the water potential of the area. Osmosis is the movement of water from high water potential to lower water potential through a partially permeable membrane. If there is an area of high glucose concentration it will have a low water potential, thus causing water to diffuse into the area via osmosis. This concept is how water moves in and out of cells. It is important that the water potential of cells is maintained at a suitable level, or the cells will become either swollen, (which may result in the cell bursting) or shrinks. Because cells are important for normal bodily function it is important that the water content of the body is maintained in order for humans to survive.
In digestion, all substances are broken down into smaller more soluble substance which can be carried by water in the blood and delivered to the parts of the body which require it; it acts as a transport medium. If blood was not made up largely of water then the required substances would not be transported to