Chapter 28: Liver Transport & Metabolic Functions

The liver is the body’s largest gland, acting as a crucial organ that manages a vast range of biochemical and metabolic processes necessary for life, including serving as a central filter for all venous blood received from the intestine via the portal circulation before it reaches the rest of the body. This function is facilitated by the functional anatomy of the liver lobule, where blood percolates through highly permeable sinusoids lined by fenestrated endothelium, allowing intimate contact with hepatocytes. Specialized cells include Kupffer cells, which are macrophages anchored in the sinusoids that perform vital phagocytic functions, filtering bacteria and particulates from the blood. The hepatic acinus describes the functional unit where blood oxygenation decreases from Zone 1 to Zone 3, making the peripheral zone most vulnerable to anoxic injury. The liver plays a dominant role in metabolism, functioning as the primary glucose buffer by storing glycogen and performing gluconeogenesis, thereby stabilizing blood sugar levels. It is essential for lipid metabolism, converting excess cholesterol into bile acids and synthesizing most lipoproteins. Furthermore, the liver synthesizes the majority of plasma proteins, including albumin, which is the main determinant of plasma oncotic pressure, as well as acute-phase proteins and clotting factors. A critical detoxification function involves converting external toxins and endogenous substances like steroid hormones into less lipophilic metabolites through a two-phase process (Phase I mediated largely by cytochrome P450 enzymes, and Phase II esterification), preparing them for excretion. The organ is indispensable for whole-body ammonia homeostasis, converting toxic ammonia, primarily sourced from the colon, into harmless urea via the complete urea cycle, which is then excreted by the kidneys. Failure of this process leads to dangerously elevated ammonia levels and subsequent hepatic encephalopathy. Bile is synthesized by hepatocytes, consisting of an alkaline solution of bile acids, cholesterol, and bile pigments. A key pigment is bilirubin, the end product of heme breakdown, which hepatocytes conjugate with glucuronic acid using glucuronyl transferase to create water-soluble bilirubin diglucuronide, actively secreting it into the bile canaliculi. Abnormal accumulation of bilirubin in the blood causes jaundice. Bile is stored and concentrated in the gallbladder between meals. Upon ingestion of fatty meals, the hormone cholecystokinin (CCK) triggers gallbladder contraction and the relaxation of the sphincter of Oddi, allowing bile to enter the duodenum for fat digestion. Gallstones, often formed from cholesterol, occur when bile becomes supersaturated, a risk increased by bile stasis and an imbalance in the necessary ratio of bile acids to phosphatidylcholine.