Functions of the Liver

We have seen that the liver plays a central role in carbohydrate metabolism. Additional liver functions (table 26.7) were described in previous chapters and will be described later in this chapter. Except for phagocytosis, all of these are performed by the cuboidal hepatocytes described in chapter 25. Such functional diversity is remarkable in light of the uniform structure of these cells. Because of the wide range of functions performed by the liver, degenerative liver diseases such as hepatitis, cirrhosis, and liver cancer are especially life-threatening.

Saladin: Anatomy & I 26. Nutrition and I Text I © The McGraw-Hill

Physiology: The Unity of Metabolism Companies, 2003 Form and Function, Third Edition

1004 Part Four Regulation and Maintenance

Table 26.7 Functions of the Liver

Carbohydrate Metabolism

Converts dietary fructose and galactose to glucose. Stabilizes blood glucose concentration by storing excess glucose as glycogen (glycogenesis), releasing glucose from glycogen when needed (glycogenolysis), and synthesizing glucose from fats and amino acids (gluconeogenesis) when glucose demand exceeds glycogen reserves. Receives lactic acid generated by anaerobic fermentation in skeletal muscle and other tissues and converts it back to pyruvic acid or glucose 6-phosphate.

Lipid Metabolism

Degrades chylomicron remnants. Carries out most of the body's lipogenesis (fat synthesis) and synthesizes cholesterol and phospholipids; produces VLDLs to transport lipids to adipose tissue and other tissues for storage or use; and stores fat in its own cells. Carries out most p-oxidation of fatty acids; produces ketone bodies from excess acetyl-CoA. Produces HDL shells, which pick up excess cholesterol from other tissues and return it to the liver; excretes the excess cholesterol in bile.

Protein and Amino Acid Metabolism

Carries out most deamination and transamination of amino acids. Removes -NH2 from glutamic acid and converts the resulting ammonia to urea by means of the ornithine cycle. Synthesizes nonessential amino acids by transamination reactions.

Synthesis of Plasma Proteins

Synthesizes nearly all the proteins of blood plasma, including albumin, a and p globulins, fibrinogen, prothrombin, and several other clotting factors. (Does not synthesize plasma enzymes, peptide hormones, or y globulins.)

Vitamin and Mineral Metabolism

Converts vitamin D3 to calcidiol, a step in the synthesis of calcitriol; stores a 3- to 4-month supply of vitamin D. Stores a 10-month supply of vitamin A and enough vitamin B12 to last one to several years. Stores iron in ferritin and releases it as needed. Excretes excess calcium by way of the bile.

Digestion

Synthesizes bile acids and lecithin, which emulsify fat and promote its digestion.

Disposal of Drugs, Toxins, and Hormones

Detoxifies alcohol, antibiotics, and many other drugs. Metabolizes bilirubin from RBC breakdown and excretes it as bile pigments. Deactivates thyroxine and steroid hormones and excretes them or converts them to a form more easily excreted by the kidneys.

Phagocytosis

Macrophages cleanse blood of bacteria and other foreign matter.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

6. Identify the reaction steps in figures 26.3 and 26.4 at which vitamins are essential to glucose catabolism.

7. In the laboratory, glucose can be oxidized in a single step to CO2 and H2O. Why is it done in so many little steps in cells?

8. Explain the origin of the word glycolysis and why this is an appropriate name for the function of that reaction pathway.

9. What are two advantages of aerobic respiration over anaerobic fermentation?

10. What important enzyme is found in the inner mitochondrial membrane other than those of the electron-transport chain? Explain how its function depends on the electron-transport chain.

11. Describe how the liver responds to (a) an excess and (b) a deficiency of blood glucose.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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