The liver has a vast array of metabolic responsibilities; as the largest organ of the digestive system, it (Marieb 2010):
- secretes bile to emulsify and absorb dietary fats (Nicolaou 2012)
- receives fatty acids and lipoproteins from circulation and repackages them for distribution throughout the body (McGillicuddy 2011)
- stores the bulk of excess glucose as glycogen to fuel the nervous system during periods of starvation
- processes most excess amino acids into glucose and detoxifies their byproduct (ammonia) into urea to be removed from the body (Kaloyianni 1990; Jungas 1992)
- stores vitamins (B12, A, D, K) and minerals (iron, copper) (Loew 1999; Higashi, Sato, 2005; Shearer 1996; Anderson 2005; Roberts 2008)
- synthesizes albumin, the principal protein of the blood and a factor responsible for maintaining blood volume for proper circulation and oxygen transfer to peripheral tissues (Busher 1990)
- synthesizes most of the clotting factors, which are necessary for proper hemostasis during vascular damage (Mammen 1992)
The liver also functions as a biological filter, detoxifying foreign molecules and endogenous metabolites. The liver receives blood from the intestines (via the portal vein) and from the systemic circulation (through the common hepatic artery). The liver consists of several types of cells; of these, hepatocytes occupy almost 80% of the total liver volume and participate in most functions of the liver (Kmiec 2001). These extremely metabolically active cells are responsible for most of the liver’s detoxification, synthetic, and storage activities. Blood and its contents that have been filtered and metabolized by the hepatocytes exit the liver through the hepatic vein and enter the systemic circulation. Waste (excess hormones, foreign toxins, metabolites from dead cells) is excreted from the liver through its bile canaliculi and ducts, and along with bile, is stored in the gallbladder then poured into the intestines for ultimate excretion. In this way, the liver acts to constantly surveil the contents of circulating blood, and clear it of waste and toxins, while also providing a first-pass filtration of ingested foodstuffs from the intestines before they enter the general circulation (Marieb 2010).
Given its diversity of functions, it becomes apparent how stress on the liver (such as from excessive detoxification demanded during chronic alcoholism, infection by hepatitis viruses, or during storage of excessive amounts of fat) has the potential to affect several major processes simultaneously (eg, circulation, clotting, digestion, energy production, detoxification, waste elimination) and presents a serious health threat (Marieb 2010).