93 CHAPTER 10 Bile Secretion and Gallbladder Function
and gallbladder, bile is normally present as a micellar solution.
The fourth major group of organic compounds found in bile comprises the bile pigments. These constitute only 2% of the total solids, and bilirubin is the most important.
Chemically, bile pigments are tetrapyrroles and are related to the porphyrins, from which they are derived. In their free form, bile pigments are insoluble in water. Normally, however, they are conjugated with glucuronic acid and are rendered soluble. Unlike the other organic compounds just mentioned, bile pigments do not take part in micellar for- mation. As their name implies, they are highly colored sub- stances. Other than being responsible for the normal color of bile and feces, the pigment properties of these com- pounds are used to assess the level of function of the liver.
In addition to the organic compounds just discussed, many inorganic ions are found in bile. The predominant cation is Na+, accompanied by smaller amounts of potas- sium (K+) and calcium (Ca2+). The predominant inorganic anions are chloride (Cl−) and HCO3−. Normally, the total number of inorganic cations exceeds the total number
of inorganic anions. No anion deficit occurs, however, because the bile acids, which possess a net negative charge at the pH values found in bile, account for the difference.
Bile is isosmotic even though the number of cations pres- ent is larger than expected. Because they are highly charged molecules, the bile acids attract a layer of cations that serve as counterions. These counterions are tightly associated with the micelles and thus exert little osmotic activity.
94 CHAPTER 10 Bile Secretion and Gallbladder Function
plates ensure that the blood is exposed to a large surface area. The hepatocytes remove substances from the blood and secrete them into the biliary canaliculi lying between the adjacent hepatocytes. The bile flows toward the periph- ery, countercurrent to the flow of the blood, and drains into bile ducts. This countercurrent relationship minimizes the concentration differences between substances in the blood and in the bile and contributes to the liver’s efficiency in extracting substances from the blood.
Bile Acids and the Enterohepatic Circulation The secretion of bile depends heavily on the secretion of bile acids by the liver. Once secreted, bile acids undergo an interesting journey (see Fig. 10.1). First, they may be stored in the gallbladder. Then they are propelled into and through the small intestine, where they take part in the digestion and absorption of lipids. Most of the bile acids themselves are absorbed from the intestine and travel via the portal blood to the liver, where they are taken up by the hepatocytes and resecreted. This process is termed the enterohepatic circulation.
Bile acids are secreted continuously by the liver. The rate of secretion, however, varies widely. Early experiments demonstrated that the rate of secretion depended on the amount of bile acids delivered to the liver via the blood; the more acids in the portal blood, the greater the secretion of bile. The amount of bile acids in the portal blood depends on the amount absorbed from the small intestine. The amount of bile acids in the intestine, in turn, depends on
the digestive state of the individual. Between meals, most bile secreted by the liver is stored in the gallbladder, with only small amounts delivered intermittently to the small intestine. During a meal, the gallbladder empties its con- tents into the duodenum in a more continuous pattern.
The ejected bile acids are then resorbed from the intestine and are secreted again by the liver.
Although many different bile acids are found in the bile, only cholic acid and chenodeoxycholic acid appear to be synthesized from cholesterol in significant amounts by human hepatocytes. For this reason, they are called “pri- mary” bile acids. Their synthesis by the liver is a continuous but regulated process. The amount synthesized depends on the amount of bile acids returned to the liver in the entero- hepatic circulation. When most of the bile acids secreted by the liver are returned, synthesis is low, but when the secreted acids are lost from the enterohepatic circulation, the rate of synthesis is high. Bile acids extracted from the portal blood act to feedback inhibit 7α-hydroxylase, the rate-limiting enzyme for the synthesis of bile acids from cholesterol. Normally, there are 2.5 g of bile salts in the enterohepatic circulation. If the absorptive processes in the intestine and liver are functioning properly, only approx- imately 0.5 g is lost daily. Synthesis is regulated to replen- ish this loss. If the enterohepatic circulation is interrupted (e.g., by a fistula draining bile to the outside), the rate of synthesis becomes maximal. In humans, this amounts to 3 to 5 g per day. In patients incapable of reabsorbing bile acids, the maximal rate of synthesis equals the total amount of bile acids secreted by the liver. Deoxycholic acid, litho- cholic acid, and other “secondary” bile acids are produced in the intestine through the action of microorganisms on primary bile acids. These acids then are absorbed along with the primary bile acids, taken up by the hepatocytes, conjugated with taurine and glycine, and secreted in the bile. The bile acid pool may circulate through the entero- hepatic circulation several times during the digestion of a meal, so that 15 to 30 g bile acids may enter the duodenum during a 24-hour period.
The enterohepatic circulation of bile acids is carried out by both active and passive transport processes (Fig. 10.5).
The more hydrophobic bile acids, which are those with fewer hydroxyl groups and those that have been decon- jugated, are absorbed passively throughout the intestine.
The more hydrophilic acids and the remaining hydropho- bic acids are absorbed by an active process in the ileum.
Uptake across the apical membrane of the enterocytes is mediated by a specific Na+-dependent transport protein.
Cytoplasmic binding proteins transport the acids through the cell to the basolateral membrane. Transport across the basolateral membrane out of the cell is mediated by an Na+-independent anion exchange process that involves Liver sinusoid
Central vein
Bile canaliculi Branch of
portal vein
Branch of hepatic artery Bile duct
Fig. 10.4 Schematic diagram of the relationship between blood vessels, hepatocytes, and bile canaliculi in the liver. Each hepatocyte is exposed to blood at one membrane surface and a bile canaliculus at the other. (From Johnson LR: Essential Med- ical Physiology, 3rd ed. Philadelphia, Academic Press, 2003, p 524.)
95 CHAPTER 10 Bile Secretion and Gallbladder Function
a carrier different from the one on the apical membrane.
As stated previously, the ileal transport process is highly efficient, delivering more than 90% of the bile acids to the portal blood.
In the liver, additional transport processes remove bile acids from the portal blood. Uptake across the basolateral or sinusoidal membrane of the enterocytes is mediated pri- marily by two types of systems. One group includes a spe- cific Na+-coupled transporter protein, the Na+ taurocholate cotransporting polypeptide (NTCP), which can transfer both conjugated and unconjugated bile acids. A group of Na+-independent transporters includes the organic anion transport proteins (OATPs), which can take up both bile acids and other organic anions. At the canaliculus, bile acids and other organic anions appear to be secreted by at least two adenosine triphosphate (ATP)-dependent pro- cesses. One of these is termed the bile salt excretory pump (bsep), and the other is the multidrug resistance protein 2 (mrp2). The hepatic transport of bile acids also is highly efficient. Practically all bile acids contained in the portal blood are removed during one passage through the liver.
The process does have a transport maximum, but this is seldom reached.
Cholesterol and Phospholipids
Cholesterol and phospholipids, primarily lecithins, also are secreted by the hepatocytes. The exact mecha- nisms of secretion are not known, but secretion appears to depend, in part, on the secretion of bile acids. The higher the rate of bile acid secretion is, the higher the rate of cholesterol and phospholipid secretion will be.
Once secreted into the intestine along with the other components of bile, cholesterol and lecithin are mixed with and handled as ingested cholesterol and lecithin (see Chapter 11).
Bilirubin
The primary bile pigment in humans, bilirubin, is derived largely from the metabolic breakdown of hemo- globin (Fig. 10.6). Most of the hemoglobin comes from aged red blood cells (RBCs) that are disposed of by cells of the reticuloendothelial (RE) system. In the RE cells, Colon
Gallbladder
Sphincter of Oddi Cholesterol Bile acids
Portalblood
Liver
Jejunum-Ileum
Ileum Duodenum
Fig. 10.5 Enterohepatic circulation of bile acids. Bile acids are actively secreted by the liver. Once in the intestine they participate in the digestion and absorption of lipids. As they are propelled toward the distal small bowel, some of the “primary” acids are altered and become “secondary” acids. The more hydropho- bic bile acids are absorbed passively throughout the intestine. The more hydrophilic acids are absorbed by a sodium-coupled active transport process that is localized to the ileum. A minor fraction of bile acids is not absorbed but is instead propelled into the colon. The absorbed bile acids are transported via the portal circula- tion to the liver, where they are extracted actively from the blood (at a rate of almost 100%) and resecreted.
Synthesis of new primary acids from cholesterol occurs at a rate to compensate for the acids lost from the bowel. Solid arrows denote active absorption, secretion, and synthesis; open arrows denote passive absorp- tion and propulsion of contents by contractions of the intestine.
96 CHAPTER 10 Bile Secretion and Gallbladder Function
hemoglobin is split into hemin and globin. The hemin ring is opened and oxidized, and the iron is removed to form bilirubin, which is then transported via the blood from the cells of the RE system to the hepatocytes. In tran- sit, bilirubin is tightly bound to plasma albumin; very lit- tle is free in the plasma. Hepatocytes can extract bilirubin from blood, conjugate it with glucuronic acid, and secrete the conjugated product into the bile. Bilirubin secretion into the bile by the hepatocytes is mediated by an active anion transport system. This system is different from the one for active transport of bile acids, but it is shared by certain other organic anions (e.g., sulfobromophthalein [Bromsulphalein, or BSP], various radiopaque dyes).
Some evidence indicates that one of the OATPs partici- pates in the transport of bilirubin.
Bilirubin is not absorbed from the intestine in any appreciable amount. Some of the product, however, is altered in the bowel. Bacteria, primarily in the distal small bowel and colon, reduce bilirubin to urobilinogen, which is unconjugated. Some urobilinogen is converted to ster- cobilin and is excreted in the feces. Urobilinogen also is absorbed into the portal blood and returned to the liver.
There most is extracted, conjugated, and secreted into the bile; however, some passes into the systemic circulation and is excreted by the kidneys. The urobilinogen is oxidized
in the urine to form urobilin. Stercobilin and urobilin are pigments that are in large part responsible for the color of the feces and urine, respectively. Following damage to the liver, sufficient bilirubin may not be extracted, and the skin takes on a yellow tinge. This condition, termed jaundice, is usually especially noticeable in the eyes. Bilirubin is also responsible for the yellow color that bruises develop after several days.
Water and Electrolytes
Two components of bile water and electrolyte secretion have been identified. One is called bile acid–dependent secretion. Bile acids, regardless of whether they are newly synthesized or extracted from the portal blood, are the major component actively secreted by the hepatocytes.
Because they are anions, their secretion is accompanied by the passive movement of cations into the canaliculus, which in turn sets up an osmotic gradient down which water moves (Fig. 10.7). Canalicular bile is thus primar- ily an ultrafiltrate of plasma as far as the concentrations of water are concerned. In some species, although not proven in humans, there is evidence for the active transport of Na+ by the hepatocytes. The higher the rate of return of bile acids is to the liver, the faster they are secreted and the greater is the volume of bile.
Bilirubin glucuronide Bilirubin
Glucuronic acid+ Liver
Urobilinogen
Bilirubin glucuronide Bilirubin
Urobilinogen
Intestine Feces
Bacteria H2
Bilirubin Plasma albumin
Systemic blood Portal blood
Bilirubin
Hemoglobin RBCs
Kidneys Urobilinogen Urobilinogen
O2
Urine RE System
Urobilin
Fig. 10.6 Excretion of bile pigments. Bilirubin is produced by cells of the reticuloendothelial (RE) system from aged red blood cells (RBCs). The unconjugated pigment is then carried, tightly bound to plasma albumin, to the liver. There it is actively taken up, conjugated with glucuronic acid, and secreted into the bile. The water-soluble conjugates are propelled along the intestine. In the distal small bowel and colon a portion of the conjugated pigment is acted on by bacteria and becomes unconjugated bilirubin and other pigments. Some of these pigments are absorbed passively into the blood and either returned to the liver and resecreted or passed through the liver and excreted by the kidneys. Most, however, pass through the colon and are excret- ed. Bold arrows indicate active absorption. H2, Hydrogen.
97 CHAPTER 10 Bile Secretion and Gallbladder Function
The contribution of the bile ducts and ductules to bile production is identical to that of the pancreatic ducts to pan- creatic juice. Secretin stimulates the secretion of HCO3−and water from the ductile cells, thereby resulting in a significant increase in bile volume, HCO3−concentration, and pH and a decrease in the concentration of bile salts. The mechanism of HCO3−secretion by the ducts of the liver involves active transport and is similar to the mechanism employed by the pancreas. When stimulated by secretin, the HCO3−concen- tration of the bile may increase two- or threefold over that of plasma. This fraction of secretion is called the bile acid–
independent or the secretin-dependent portion.