In regions with only continuous capillaries, apart from protein which is normally lysed in the tissues and which may be up to about a third of that which leaves the blood vessels (vide infra), the remaining protein which leaves them is returned via the lymphatic system. If this does not function properly considerable oedema results. A relatively small amount of fluid is also returned, but this amount is still very important in avoiding tissue oedema 2, 3, 2a. In regions where there are many fenestrated capillaries, it is becoming increasingly probable that there is a considerable circulation of plasma proteins, through the tissues, which re-enter the blood via the fenestrae on the venous side of the circulation% 3, 4, 12. It appears, however, that the lymphatics in this situation still are vital for the avoidance of oedema 24. The amount of protein they remove may only amount to about 10-30%, but because this is in a concentrated form the mean concentration in the tissues is considerably reduced, and with it the colloidal osmotic pressures and tissue hydrostatic pressures. It appears that this reduction is essential for the avoidance of a mean positive tissue pressure and oedema.