After studying this chapter, you should be able to:
Describe liver functions of metabolism, detoxification, and excretion.
Understand the functional anatomy of the liver.
Define the characteristics of the hepatic circulation and its role.
Identify plasma proteins synthesized by the liver.
Describe the formation of bile and its role in the excretion of cholesterol and bilirubin.
Outline hepatic contributions to whole-body ammonia homeostasis.
Describe normal functioning of the gallbladder and the basis of gallstone disease.
The liver is the largest gland in the body. It is essential for life because it conducts a vast array of biochemical and metabolic functions. It is also the first port of call for most absorbed nutrients, supplies most of the plasma proteins, and synthesizes bile.
The liver serves as a filter between blood coming from the gastrointestinal tract and the blood in the rest of the body. Blood from the intestines reaches the liver via the portal vein. It percolates in sinusoids between plates of hepatic cells (hepatocytes) and eventually drains into the hepatic veins. During its passage through the hepatic plates, it is modified chemically. Bile is formed on the other side of each plate. Bile passes to the intestine via the hepatic duct (Figure 28–1).
Schematic anatomy of the liver. Hepatocytes are arranged radially in plates surrounding a central vein. Blood is supplied to the liver by branches of the portal vein (PV) and hepatic artery (HA), which empty into sinusoids (S) surrounding the hepatocytes. The direction of blood flow is indicated with black arrows. The endothelial cells that line the sinusoids are fenestrated and thus provide little hindrance to the transfer of substances from the sinusoids to the space of Disse, which abuts the basolateral membrane of the hepatocytes. The apical membranes of adjacent hepatocytes form bile canaliculi, which transfer bile to the bile ducts lined by cholangiocytes. Bile flows in the opposite direction to blood (green arrows). The bile duct, portal vein and hepatic artery comprise the “portal triad.” (Adapted with permission from Paulsen DF: Histology and Cell Biology: Examination and Board Review. 5th ed. McGraw-Hill, 2010.)
The plates of hepatocytes are usually only one cell thick. Large gaps occur between the endothelial cells, allowing plasma to contact the hepatocytes. Hepatic artery blood also enters the sinusoids. The central veins coalesce to form the hepatic veins. The average transit time for blood from the portal venule to the central hepatic vein is about 8.4 s. Kupffer cells are anchored to the endothelium of the sinusoids and project into the lumen.
Each hepatocyte is also apposed to bile canaliculi. The canaliculi drain into bile ducts, and these coalesce to form ...