This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Difference In The Absorbing Power Of The Lacteals And The Blood Vessels has probably a considerable importance, dependent on the different courses taken by the two sets of vessels. The lacteals, which are deeply placed in the villi, and fed by other means than mere endosmosis, send their contents, by the thoracic duct, into the circulation at a point where the blood is returning to the heart and has only to be subjected to the influence of respiration before being diffused throughout the body. The blood-vessels, on the other hand, take up everything according to its diffusibility; but they carry their stream into the portal vein, whence it is conveyed through the capillaries of the liver; and not until it has been subjected to the influence of that organ— which has, besides other functions, an arrestive power—is it allowed to reach the heart.
As in the case of intestinal absorption, so also throughout the body, fluids appear to pass into the blood-vessels easily by endosmosis; and the circumstances are not well known which call for the necessity of lymphatics as well as bloodvessels. Yet, when it is considered that lymphatics at their origin are less distinctly bounded than the capillary bloodvessels, and much more closely communicate with the fluids of the tissues, it seems not improbable that, as has been suggested, the crystalloids are principally taken into the blood, while the colloids left behind are carried away by the lymphatics.