Functional necessity is the mother of structural innovation. The origin of the interstitial venous system, or “lymphatic system”, is as a structure to compensate for the increase in hydrostatic pressure in the circulatory system in vertebrates. This increase in blood pressure meant that an equal increase in osmotic pressure would have been necessary to maintain balance in the transcapillary circulation (that is necessary to transport oxygen and glucose to the tissues of the body), and failure to achieve that led to a secondary venous system to return extra-vascular plasma fluid to the heart. The interstitial venous system evolved as hydrostatic pressure in the vascular system increased.