This is a type of circulatory bypass used when substances in blood or fluid need to be kept out of the general flow. A portal system begins in capillaries and ends in capillaries, and nothing leaves it undocumented. The hypothalamus sends hormones into the portal system between it and the pituitary, and the pituitary responds to it by secreting its own hormones, but dissolving the hypothalamus ones. Blood that leaves the intestinal tract, spleen, and pancreas (partially) goes into the liver's portal system and does not leave that organ until it has been thoroughly screened and altered.