R

Biliverdin

NADPH -NADP

Bilirubin

Conjugated bilirubin (bilirubin glucuronide)

Conjugated bilirubin (bilirubin glucuronide)

Recycled to bone marrow

Carbon monoxide (CO)

Hemoglobin

Carboxyhemoglobin

■ Figure 18.24 The enterohepatic circulation of urobilinogen. Bacteria in the intestine convert bilirubin (bile pigment) into urobilinogen. Some of this pigment leaves the body in the feces; some is absorbed by the intestine and is recycled through the liver. A portion of the urobilinogen that is absorbed enters the general circulation and is filtered by the kidneys into the urine.

Gallbladder

Gallbladder

Urobilinogen in feces

The Digestive System

CH3 I

■ Figure 18.25 The two major bile acids in humans. These more polar derivatives of cholesterol form the bile salts.

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 and chenodeoxycholic acid (fig. 18.25), conjugated with glycine or taurine to form the bile salts. In aqueous solutions these molecules "huddle" together to form aggregates known as micelles. As described in chapter 2, 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 (see fig. 2.21). Lecithin, cholesterol, and other lipids in the small intestine enter these micelles, and the dual nature of the bile salts (part polar, part nonpolar) allows them to emulsify fat in the chyme.

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 approximately 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.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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