This is caused by an increase in the interstitial fluid in the feet, the fluid that is neither within the cells nor within the blood vessels. To understand why it occurs, we need to know how the fluid in our bodies stays where it is supposed to be in the first place. Fluid in our cells accounts for about 40 percent of body weight, that in our blood vessels about 5 percent, and interstitial fluid makes up an additional 15 percent. Fluid in the blood vessels and interstitial fluid always contains sodium, in about the same concentration. The fluid in the cells contains little sodium.
Sodium is regulated by our kidneys.
Within a day of so of eating extra sodium, our kidneys will excrete more, and if our sodium consumption drops, our kidneys will conserve it. Since sodium passes freely between the fluid in our blood vessels and the interstitial fluid, three-quarters of the sodium we eat will wind up in the interstitial fluid, at least until the kidneys are able to excrete it, and this extra sodium is always accompanied by extra fluid. Another factor involved in the distribution of water is the protein in our blood, which cannot pass freely into the interstitial fluid, and therefore tends to hold fluid in the vessels by osmotic pressure. Also hydrostatic pressure, the pressure produced by the weight of the column of fluid in our blood vessels, will tend to force fluid out in areas where the pressure is high, as in the feet or lower legs. Interstitial fluid is constantly being collected and returned to the blood vessels through lymphatic channels.