- Understand the role of the liver in whole-body homeostasis and the structural features that subserve its functions
- Understand the functions of bile secretion and the anatomy of the biliary system
- Describe the unusual circulatory features of the liver and the relationship of blood flow to bile flow
- Identify the parenchymal and nonparenchymal cell types of the liver, their anatomic relationships, and their respective functions
- Understand pathological conditions where structure and function of the liver and biliary system is compromised
- Describe the role of liver transplantation in treating end-stage liver disease
- Describe the physiological rationale for commonly used tests of liver function, liver injury, and disease prognosis
The liver is the largest gland in the body, and conducts a myriad of vital metabolic and excretory functions. In addition, by virtue of its circulatory relationship to the absorptive surface of the gastrointestinal tract, the liver is the initial site where ingested nutrients, and other substances entering via the gastrointestinal tract, such as drugs and bacterial metabolites, are processed by the body. Thus, the liver is a gate-keeper that can process useful substances while detoxifying orally absorbed substances that are potentially harmful, such as toxic xenobiotics.
Metabolism and Detoxification
The liver contributes in a pivotal way to the biochemical status of the body as a whole. Indeed, a major portion of the discipline of biochemistry is concerned with the chemical reactions that take place in the cell type that makes up the majority of the hepatic mass, the hepatocyte. For this reason, it is beyond the scope of this text to provide a comprehensive analysis of all of the metabolic functions of the liver. Instead, we will focus our discussion on broad categories of metabolic functions of the liver that are relevant to the function of the gastrointestinal system, or are of particular importance to whole-body homeostasis.
First, the liver is an important repository of carbohydrate metabolism. The liver performs four specific functions in this regard: glycogen storage, conversion of galactose and fructose to glucose, gluconeogenesis, and the formation of many important biochemical compounds from the intermediate products of carbohydrate metabolism. Many of the substrates for these reactions derive from the products of carbohydrate digestion and absorption that travel directly to the liver from the gut, as will be described in more detail in Chapter 15. As a consequence, the liver plays a major role in maintaining blood glucose concentrations within normal limits, particularly in the postprandial period. The liver removes excess glucose from the blood and returns it as needed, in a process referred to as the glucose buffer function of the liver. This function is markedly impaired in an individual whose hepatic function has been reduced by disease, resulting in abnormally high postprandial glucose concentrations. However, because the liver also regulates the other aspects of glucose homeostasis described, in liver failure, hypoglycemia is seen. Indeed, hepatectomized animals rapidly develop fatal hypoglycemia.