At all times under normal physiological conditions, fluid is continually passing from the blood into the tissues and from the tissues into the blood. Held in solution in this fluid is the food materials and waste of the body. The transudation of fluid is not a mere filtration; it is no mere process of osmosis. On the contrary, it is due to secretory activities on the part of the endothelium (lining of the blood vessels) which incorporates such substances in solution, both from the blood and from the tissues, and selectively passes them on to the other side. Sodium chloride has a paralyzing effect upon the secretory activities of the vascular endothelium, thus interfering with the exchange of nutritive substances and waste.