The evolution of multicellular organisms was predicated on their ability to provide their cells with a relatively non-fluctuating internal fluid environment similar to the relatively stable environment that the oceans provided for the original single-celled organisms. The process of maintaining a constant internal fluid environment, called homeostasis, begins in the liver. During digestion, the blood of the intestinal villi picks up simple sugars and amino acids and, thus, has a much higher concentration of these substances than normal blood. The blood is collected by the portal vein which breaks into a capillary bed in the liver.
