Page 952 - Fundamentals of anatomy physiology
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hepatocytes break down glycogen reserves and release glu- Chapter 24 The Digestive System 939
cose into the bloodstream. They also synthesize glucose
from other carbohydrates or from available amino acids. cellular debris, and pathogens, removing them from the
The synthesis of glucose from other compounds is called bloodstream. Kupffer cells are antigen-presenting cells
gluconeogenesis. If blood glucose levels climb, hepatocytes (APCs) that can stimulate an immune response. p. 832
remove glucose from the bloodstream. They either store it
as glycogen or use it to synthesize lipids that can be stored Synthesis of Plasma Proteins. Hepatocytes synthesize and
in the liver or other tissues. Circulating hormones, such as
insulin and glucagon, regulate these metabolic activities. release most of the plasma proteins. These proteins include
the albumins (which contribute to the osmotic concentra-
pp. 662–663 tion of the blood), the various types of transport proteins,
clotting proteins, and complement proteins.
Lipid Metabolism. The liver regulates circulating levels of
Removal of Circulating Hormones. The liver is the primary
triglycerides, fatty acids, and cholesterol. When those levels
decline, the liver breaks down its lipid reserves and releases site for the absorption and recycling of epinephrine,
the breakdown products into the bloodstream. When the norepinephrine, insulin, thyroid hormones, and steroid
levels are high, the lipids are removed for storage. However, hormones, such as the sex hormones (estrogens and an-
this regulation takes place only after lipid levels have risen drogens) and corticosteroids. The liver also absorbs cho-
within the general circulation, because most lipids absorbed lecalciferol (vitamin D3) from the blood. Liver cells then
by the digestive tract bypass the hepatic portal circulation. convert the cholecalciferol, which may be synthesized
in the skin or absorbed in the diet, into an intermediary
Amino Acid Metabolism. The liver removes excess amino product, 25-hydroxy-D3, that is released back into the
bloodstream. The kidneys absorb this intermediary and
acids from the bloodstream. These amino acids can be used use it to generate calcitriol, a hormone important to Ca2+
to synthesize proteins or can be converted to lipids or glu- metabolism. p. 665
cose for energy storage.
Removal of Antibodies. The liver absorbs and breaks down
Waste Product Removal. When converting amino acids to
antibodies, releasing amino acids for recycling.
lipids or carbohydrates, or when breaking down amino
acids to get energy, the liver strips off the amino groups. Removal or Storage of Toxins. The liver absorbs lipid-soluble
This process is called deamination. Ammonia, a toxic waste
product, is formed. The liver neutralizes ammonia by con- toxins in the diet, such as the insecticide DDT (banned in
verting it to urea, a fairly harmless compound excreted by the United States since 1972, but still found in the environ-
the kidneys. The liver also removes other waste products, ment), and stores them in lipid deposits, where they do
circulating toxins, and drugs from the blood for inactiva- not disrupt cellular functions. The liver removes other tox-
tion, storage, or excretion. ins from the bloodstream and either breaks them down or
excretes them in the bile.
Vitamin Storage. Fat-soluble vitamins (A, D, E, and K) and
The Synthesis and Secretion of Bile. The liver synthesizes
vitamin B12 are absorbed from the blood and stored in the
liver. These reserves are used when your diet contains inad- bile and excretes it into the lumen of the duodenum. Hor-
equate amounts of those vitamins. monal and neural mechanisms regulate bile secretion. Bile
consists mostly of water, with minor amounts of ions, bili-
Mineral Storage. The liver converts iron reserves to ferritin rubin (a pigment derived from hemoglobin), cholesterol,
and an assortment of lipids collectively known as bile
and stores this protein–iron complex. p. 687
24salts. (Bile salts play a role in the digestion of lipids, as we
Drug Inactivation. The liver removes and breaks down cir-
discuss in the next section.) The water and ions help dilute
culating drugs, limiting the duration of their effects. When and buffer acids in chyme as it enters the small intestine.
physicians prescribe a particular drug, they must take into Bile salts are synthesized from cholesterol in the liver. Sev-
account the rate at which the liver removes that drug from eral related compounds are involved. The most abundant
the bloodstream. For example, a drug that is absorbed are derivatives of the steroids cholate and chenodeoxycholate.
relatively quickly must be administered every few hours to
keep blood concentrations at therapeutic levels. The Functions of Bile. Most dietary lipids are not water solu-
ble. Mechanical processing in the stomach creates large drops
Hematological Regulation. The liver receives about 25 percent containing a variety of lipids. Pancreatic lipase is not lipid
of cardiac output. It is also the largest blood reservoir in your soluble, so the enzymes can interact with lipids only at the
body. As blood passes through it, the liver performs the follow- surface of a lipid droplet. The larger the droplet, the more lip-
ing functions: ids are inside, isolated and protected from these enzymes. Bile
salts break the droplets apart in a process called emulsification
Phagocytosis and Antigen Presentation. Kupffer cells in the (e. -mul-si-fi-KA. -shun), which dramatically increases the surface
area accessible to enzymes.
liver sinusoids engulf old or damaged red blood cells,
