Human Physiology, 14th edition (2016)

The Digestive System 639

micelles, and the dual nature of the bile salts (part polar, part

nonpolar) allows them to emulsify fat in the chyme. Emulsifica-

tion refers to the conversion of larger fat globules by bile acids

into a finer suspension of smaller globules, which provide a

greater surface area for the fat to be digested by lipase enzymes

(see fig. 18.35 ).

The liver’s production of bile acids from cholesterol is

the major pathway of cholesterol breakdown in the body. This

amounts to about half a gram of cholesterol converted into bile

acids per day. No more than this is required, because approxi-

mately 95% of the bile acids released into the duodenum are

absorbed in the ileum by means of specific carriers, and so

have an enterohepatic circulation. Bile salts recirculate 6 to

10 times per day, with only about 0.5 g excreted in the feces.

not very water-soluble, and thus most is carried in the blood
attached to albumin proteins. This protein-bound bilirubin can
neither be filtered by the kidneys into the urine nor directly
excreted by the liver into the bile.
The liver can take some of the free bilirubin out of the
blood and conjugate (combine) it with glucuronic acid. This
conjugated bilirubin is water-soluble and can be secreted into
the bile. Once in the bile, the conjugated bilirubin can enter
the intestine where it is converted by bacteria into another
pigment— urobilinogen. Derivatives of urobilinogen impart a
brown color to the feces. About 30% to 50% of the urobilinogen,
however, is absorbed by the intestine and enters the hepatic por-
tal vein. Of the urobilinogen that enters the liver sinusoids, some
is secreted into the bile and is thus returned to the intestine in an
enterohepatic circulation; the rest enters the general circulation
( fig. 18.23 ). The urobilinogen in plasma, unlike free bilirubin, is
not attached to albumin. Urobilinogen is therefore easily filtered
by the kidneys into the urine where its derivatives produce an
amber color.
Bile acids are derivatives of cholesterol that have two to
four polar groups on each molecule. The principal bile acids
in humans are cholic acid ( fig.  18.24 ) and chenodeoxycholic
acid, conjugated with the amino acids glycine or taurine to form
the bile salts. In aqueous solutions these molecules “huddle”
together to form aggregates known as micelles ( fig. 18.24 ). The
nonpolar parts are located in the central region of the micelle
(away from water), whereas the polar groups face water around
the periphery of the micelle (chapter 2; see fig. 2.23). Lecithin,
cholesterol, and other lipids in the small intestine enter these

Figure 18.22 Simplified pathway for the metabolism

of heme and bilirubin. Heme can be formed from the

hemoglobin in red blood cells. The iron from the heme group

is recycled back to the bone marrow when the heme is

converted into biliverdin. Notice that carbon monoxide is

produced in this process and because it is toxic, must be

eliminated from the body.

Liver

Hemolysis in spleen,

liver, bone marrow

Red Blood Cells

Hemoglobin

Heme

Biliverdin

NADPH

NADP

O 2

Oxyhemoglobin

Hemoglobin

Carbon

monoxide (CO)

Recycled to

bone marrow

Carbon monoxide

eliminated in

exhaled breath

Bilirubin Carboxyhemoglobin

Conjugated bilirubin

(bilirubin glucuronide)

Bilirubin eliminated

in bile excreted in feces

Fe2+

Functional

Category Actions

Detoxication

of Blood

Phagocytosis by Kupffer cells

Chemical alteration of biologically active

molecules (hormones and drugs)

Production of urea, uric acid, and other molecules

that are less toxic than parent compounds

Excretion of molecules in bile

Carbohydrate

Metabolism

Conversion of blood glucose to glycogen and fat

Production of glucose from liver glycogen and

from other molecules (amino acids, lactic

acid) by gluconeogenesis

Secretion of glucose into the blood

Lipid

Metabolism

Synthesis of triglycerides and cholesterol

Excretion of cholesterol in bile

Production of ketone bodies from fatty acids

Protein

Synthesis

Production of albumin

Production of plasma transport proteins

Production of clotting factors (fibrinogen,

prothrombin, and others)

Secretion

of Bile

Synthesis of bile salts

Conjugation and excretion of bile pigment (bilirubin)

Table 18.3 | Major Categories of Liver
Function