Human Physiology, 14th edition (2016)

638 Chapter 18

eliminated in the feces; this is analogous to renal clearance of

blood through excretion in the urine (chapter 17, section 17.4).

Many compounds that are released with the bile into the

intestine are not eliminated with the feces, however. Some of

these can be absorbed through the small intestine and enter the

hepatic portal blood. These molecules are thus carried back to

the liver, where they can again be secreted by hepatocytes into

the bile ducts. Compounds that recirculate between the liver

and intestine in this way are said to have an enterohepatic

circulation ( fig.  18.21 ). For example, a few grams of bile

salts (discussed shortly) released into the intestine recirculate

6 to 10 times a day, with only about 0.5 g of bile salts per day

excreted in the feces.

Functions of the Liver

As a result of its large and diverse enzymatic content and its

unique structure, and because it receives venous blood from

the intestine, the liver has a greater variety of functions than

any other organ in the body. The major categories of liver func-

tion are summarized in table 18.3.

Bile Production and Secretion

The liver produces and secretes 250 to 1,500 ml of bile per day.

The major constituents of bile are bile pigment (bilirubin), bile

salts, phospholipids (mainly lecithin), cholesterol, and inorganic

ions.

Bile pigment, or bilirubin, is produced in the spleen, liver,

and bone marrow as a derivative of the heme groups (minus

the iron) from hemoglobin ( fig.  18.22 ). The free bilirubin is

away from the liver. Because blood travels in the sinusoids and bile
travels in the opposite direction in bile canaliculi within the hepatic
plates, blood and bile do not mix in the liver lobules.

Enterohepatic Circulation

In addition to the normal constituents of bile, a wide variety
of exogenous compounds (drugs) are secreted by the liver into
the bile ducts ( table 18.2 ). The liver can thus “clear” the blood
of particular compounds by removing them from the blood
and excreting them into the intestine with the bile. Molecules
that are cleared from the blood by secretion into the bile are

Figure 18.20 The flow of blood and bile in a liver
lobule. Blood flows within sinusoids from a portal vein to the
central vein (from the periphery to the center of a lobule). Bile
flows within hepatic plates from the center to bile ductules at the
periphery of a lobule.

Branch of hepatic artery Hepatic plate Central vein

Sinusoids

Bile

canaliculus

Bile

ductule

Branch of

portal vein

Liver

Common

Small bile duct

intestine

Gallbladder

Hepatic

portal vein

Figure 18.21 The enterohepatic

circulation. Substances secreted in the bile may be absorbed

by the intestinal epithelium and recycled to the liver via the

hepatic portal vein.

Table 18.2 | Compounds Excreted by the
Liver into the Bile Ducts

Category Compound Comments

Endogenous

(Naturally

occurring)

Bile salts,

urobilinogen,

cholesterol

High percentage reabsorbed

and has an enterohepatic

circulation*

Lecithin Small percentage reabsorbed

and has an enterohepatic

circulation

Bilirubin No enterohepatic circulation

Exogenous

(Drugs)

Ampicillin,

streptomycin,

tetracyline

High percentage reabsorbed

and has an enterohepatic

circulation

Sulfonamides,

penicillin

Small percentage reabsorbed

and has an enterohepatic

circulation

*Compounds with an enterohepatic circulation are absorbed to some degree by
the intestine and are returned to the liver in the hepatic portal vein.