The design of our bodies is such that the extent of the ocean of water outside the cells is expanded to have the extra water available for filtration and emergency injection into vital cells. The brain commands an increase in salt and water retention by the kidneys.
This is how we get an oedema when we don’t drink enough water.
Initially, the process of water filtration and its delivery into the cells is more efficient at night when the body is horizontal. The collected water, that mostly pools in the legs, does not have to fight the force of gravity to get into the blood circulation. If reliance of this process of emergency hydration of some cells continues for long, the lungs begin to get waterlogged at night, and breathing becomes difficult. The person needs more pillows to sit upright to sleep. This condition is the consequence of dehydration. Here is the detailed information about this life-threatening state, of dehydration symptoms.
However, you might overload the system by drinking too much water at the beginning. Increases in water intake must be slow and spread out until urine production begins to increase at the same rate that you drink water. When we drink enough water to pass clear urine, we also pass out a lot of the salt that was held back. This is how we can get rid of oedema fluid in the body; by drinking more water. Not diuretics, but more water!
In people who have an extensive oedema and show signs of their heart beginning to have irregular or very rapid beats with least effort, the increase in water intake should be gradual and spaced out, but not withheld from the body. Naturally, salt intake should be limited for two or three days because the body is still in an overdrive mode to retain it. Once the oedema has cleared up, salt should not be withheld from the body. Salt has many other functions than just regulating the water content of the body.