HUMAN BIOLOGY

Digestion anD nutrition 203

What are the accessory organs of digestion,

and What do they do?

• Accessory organs of digestion are the pancreas, liver, and
  gallbladder.


• The pancreas produces digestive enzymes that are released
  into the small intestine.


• The liver produces bile salts that are stored in the gallbladder
  (as bile).


• Beyond digestion, the liver’s many functions include removing
  toxins from the blood and helping to manage blood levels of
  glucose and other substances.

Take-Home message

bile Fluid that contains bile

salts; it forms in the liver.

gallbladder Organ that

stores bile from the liver.

hepatic portal system

System of blood vessels

that divert blood from the

small intestine to the liver

for processing, then return it

to the bloodstream.

liver Organ that produces

bile salts used in fat diges-

tion. Its other roles include

storing excess glucose

in blood and detoxifying

waste ammonia from protein

digestion.

pancreas Source of

enzymes that dismantle

complex food molecules;

it also produces hormones

that regulate blood sugar.

empties bile into the small intestine where it aids in the
digestion and absorption of fats. When no food is in the
small intestine, a sphincter closes off the main bile duct,
and bile backs up into the gallbladder.

The liver is a multipurpose organ

Besides its digestive functions, the liver processes incom-
ing nutrients into substances the body requires. A system
of blood vessels called the hepatic portal system diverts
blood from the small intestine to the liver. As you can see in
Figure 11.10, blood entering the liver in this system arrives
in the hepatic portal vein and returns to the general circu-
lation via the hepatic vein. After a meal, when blood from
the small intestine enters the system loaded with nutrients,
liver cells manage this bonanza in various ways. For exam-
ple, if the blood contains more of the sugar glucose than
body cells can take up at the time, the liver removes some
of the excess and stores it as glycogen. The liver also stores
several vitamins and minerals and forms the active form
of vitamin D, which is essential for the uptake of calcium
from digested food. Liver cells use arriving amino acids to
synthesize proteins such as the albumin in blood plasma,
or process and reship them in a form cells throughout the
body can use to make ATP.
As you will read in Section 11.6, digested lipids don’t
enter the hepatic portal system, but they do reach the liver
in the general circulation. Liver cells may use some of these
lipids to make lipoproteins, including the HDLs and LDLs
that carry cholesterol.
The chart in Figure 11.9 lists some other major liver
functions. For instance, the liver removes alcohol and other
potential toxins, such as ammonia produced by the break-
down of amino acids. The ammonia is converted to urea,
a much less toxic waste product that is excreted in urine.
Liver cells also take up bilirubin, a pigment that forms as

aging or damaged red blood cells are

broken down and the hemoglobin in

them is recycled. Bilirubin is added

to bile and eventually is excreted in

feces. In addition, the liver also inac-

tivates many hormones, which move

via the blood to the kidneys and are

excreted in urine.

Bile often contains cholesterol

apart from that used to synthesize

bile salts. This excess may form a

gallstone in the gallbladder (Fig-

ure 11.10, right). If the gallbladder is

surgically removed—usually due to

the painful presence of gallstones—

the duct that connects it to the small

intestine enlarges and takes on the

role of bile storage. This is why mil-

lions of people are walking around

minus their gallbladder, with few or

no ill effects.

inferior
vena cava

hepatic vein

liver capillary
beds
liver stomach
gallbladder
spleen

pancreas

descending

colon of large

intestine

hepatic portal vein

ascending
colon of large
intestine

small intestine

appendix

Figure 11.10 The hepatic portal system diverts nutrient-rich blood to the liver. Arrows show the direction of blood flow.
The photograph at right shows a large gallstone that has been sliced in half. Cholesterol gives it a pale color. (© Cengage Learning)

C. James Webb/Phototake

A gallstone, sliced open

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