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4 The Liver, Gallbladder, and Pancreas

4 The Liver, Gallbladder, and Pancreas

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CHAPTER 25



The Digestive System



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Sternum

5th rib

Liver



(a) Location



Inferior vena cava

Caudate lobe



Bare area

Posterior



Right lobe



Left lobe

Falciform

ligament



Round ligament

Porta hepatis:

Hepatic portal vein

Hepatic artery proper

Bile duct



Anterior



Quadrate lobe

Right lobe



Gallbladder

(b) Anterior view



(c) Inferior view



FIGURE 25.19 Gross Anatomy of the Liver.

bile pigments, and drugs. At the same time, they secrete

albumin, lipoproteins, clotting factors, angiotensinogen,

and other products into the blood. Between meals, they

break down stored glycogen and release glucose into the

circulation. The sinusoids also contain phagocytic cells

called hepatic macrophages (Kupffer19 cells), which remove

bacteria and debris from the blood.

The liver secretes bile into narrow channels, the bile

canaliculi, between the back-to-back layers of hepatocytes

within each plate. Bile passes from there into small bile

ductules between the lobules, and these converge to ultimately form right and left hepatic ducts. They converge

on the inferior side of the liver to form the common

hepatic duct. A short distance farther on, this is joined by

the cystic duct coming from the gallbladder (fig. 25.21).

Their union forms the bile duct, which descends through

the lesser omentum toward the duodenum. Near the duodenum, the bile duct joins the duct of the pancreas and

forms an expanded chamber called the hepatopancreatic



ampulla. The ampulla terminates at a fold of tissue, the

major duodenal papilla, on the duodenal wall. This papilla

contains a muscular hepatopancreatic sphincter (sphincter of Oddi20), which regulates the passage of bile and

pancreatic juice into the duodenum. Between meals, this

sphincter is closed and prevents the release of bile into

the intestine.

The hepatic lobules are separated by a sparse connective tissue stroma. In cross sections, the stroma is especially visible in the triangular areas where three or more

lobules meet. Here there is often a hepatic triad consisting

of a bile ductule and two blood vessels—branches of the

hepatic artery proper and the hepatic portal vein.



19



20



Karl W. von Kupffer (1829–1902), German anatomist



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Circulation

The liver receives blood from two sources: about 70%

from the hepatic portal vein and 30% from the hepatic

arteries. The hepatic portal vein receives blood from the

Ruggero Oddi (1864–1913), Italian physician



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Hepatocytes



Bile

canaliculi

Stroma

Central vein

Hepatic triad:



Hepatic

sinusoid



Branch of

hepatic

portal vein

Branch of

hepatic artery

proper



Stroma



Bile ductule



(a)



Stroma

Central vein

Hepatic

lobule

Branch of

hepatic

portal vein



Hepatic

macrophage



Hepatocyte



Bile ductule

Lymphatic

vessel

Branch

of hepatic

artery proper

(b)



0.5 mm



Erythrocytes

in sinusoid

Endothelial

cells



Fenestration

Sinusoid

(c)



FIGURE 25.20 Microscopic Anatomy of the Liver. (a) The hepatic lobules and their relationship to the blood vessels and bile tributaries.

(b) Histological section of the liver. (c) A hepatic sinusoid.

● Identify two blood vessels in chapter 20 that supply blood to the hepatic sinusoids.



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CHAPTER 25



Gallbladder:

Neck

Body

Head



The Digestive System



977



Hepatic ducts



Common hepatic duct

Cystic duct

Bile duct

Accessory

pancreatic duct



Pancreatic duct



Duodenum

Minor duodenal

papilla

Circular folds

Hepatopancreatic

sphincter



Pancreas:

Tail

Body

Head

Duodenojejunal

flexure



Major duodenal

papilla

Hepatopancreatic

ampulla



Jejunum



FIGURE 25.21 Gross Anatomy of the Gallbladder, Pancreas, and Bile Passages. The liver is omitted to show more clearly the gallbladder,

which adheres to its inferior surface, and the hepatic ducts, which emerge from the liver tissue.



stomach, intestines, pancreas, and spleen, and carries it

into the liver at the porta hepatis; see the hepatic portal

system in table 20.8 (p. 790). All nutrients absorbed by

the small intestine reach the liver by this route except

for lipids (transported in the lymphatic system). Arterial

blood bound for the liver exits the aorta at the celiac trunk

and follows the route shown in figure 20.30 (p. 786):

celiac trunk → common hepatic artery → hepatic artery

proper → right and left hepatic arteries, which enter the

liver at the porta. These arteries deliver oxygen and other

materials to the liver.

Branches of the hepatic portal vein and hepatic arteries

meet each other in the spaces between the liver lobules, and

both drain into the liver sinusoids. Hence, there is an unusual mixing of venous and arterial blood in the sinusoids.

After processing by the hepatocytes, the blood collects in

the central vein at the core of the lobule. Blood from the

central veins ultimately converges in the right and left hepatic veins, which exit the superior surface of the liver and

empty into the nearby inferior vena cava.



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The Gallbladder and Bile

The gallbladder is a pear-shaped sac on the underside of

the liver that serves to store and concentrate bile. It is about

10 cm long and internally lined by a highly folded mucosa

with a simple columnar epithelium. Its head (fundus)

usually projects slightly beyond the inferior margin of the

liver. Its neck (cervix) leads into the cystic duct, which

leads in turn to the bile duct.

Bile is a green fluid containing minerals, cholesterol,

neutral fats, phospholipids, bile pigments, and bile acids.

The principal pigment is bilirubin, derived from the

decomposition of hemoglobin. Bacteria of the large intestine metabolize bilirubin to urobilinogen, which is responsible for the brown color of feces. In the absence of

bile secretion, the feces are grayish white and marked

with streaks of undigested fat (acholic feces). Bile acids

(bile salts) are steroids synthesized from cholesterol. Bile

acids and lecithin, a phospholipid, aid in fat digestion

and absorption, as discussed later. All other components



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DEEPER INSIGHT 25.3



Clinical Application



Gallstones

Gallstones (biliary calculi) are hard masses in the gallbladder or bile

duct, usually composed of cholesterol, calcium carbonate, and bilirubin. Cholelithiasis, the formation of gallstones, is most common in

obese women over the age of 40 and usually results from excess

cholesterol. The gallbladder may contain several gallstones, some

over 1 cm in diameter. Gallstones cause excruciating pain when they

obstruct the bile ducts or when the gallbladder or bile ducts contract.

When they block the flow of bile into the duodenum, they cause jaundice (yellowing of the skin due to bile pigment accumulation), poor

fat digestion, and impaired absorption of fat-soluble vitamins. Once

treated only by surgical removal, gallstones are now often treated with

stone-dissolving drugs or by lithotripsy, the use of ultrasonic vibration

to pulverize them without surgery. Reobstruction can be prevented by

inserting a stent (tube) into the bile duct, which keeps it distended and

allows gallstones to pass while they are still small.



the pancreas is exocrine tissue, which secretes 1,200 to

1,500 mL of pancreatic juice per day.

The cells of the secretory acini exhibit a high density

of rough ER and secretory vesicles (zymogen granules)

(fig. 25.22). The acini open into a system of branched

ducts that eventually converge on the main pancreatic

duct. This duct runs lengthwise through the middle of

the gland and joins the bile duct at the hepatopancreatic

ampulla. The hepatopancreatic sphincter thus controls

the release of both bile and pancreatic juice into the duodenum. Usually, however, there is a smaller accessory

pancreatic duct that branches from the main pancreatic

duct and opens independently into the duodenum at the



Acinar cells



Zymogen

granules



of the bile are wastes destined for excretion in the feces.

When these waste products become excessively concentrated, they may form gallstones (see Deeper Insight 25.3).

Bile gets into the gallbladder by first filling the bile

duct, then overflowing into the gallbladder. Between

meals, the gallbladder absorbs water and electrolytes from

the bile and concentrates it by a factor of 5 to 20 times.

The liver secretes about 500 to 1,000 mL of bile per day.

About 80% of the bile acids are reabsorbed in the

ileum, the last portion of the small intestine, and returned

to the liver, where the hepatocytes absorb and resecrete

them. This route of secretion, reabsorption, and resecretion,

called the enterohepatic circulation, reuses the bile acids

two or more times during the digestion of an average meal.

The 20% of the bile that is not reabsorbed is excreted in

the feces. This is the body’s only way of eliminating excess

cholesterol. The liver synthesizes new bile acids from cholesterol to replace the quantity lost in the feces.



Apply What You Know

Certain drugs designed to reduce blood cholesterol work by

blocking the reabsorption of bile acids in the ileum. Explain

why they would have this cholesterol-lowering effect.



(a)



Stroma



Ducts



Exocrine

acinar cells



The Pancreas

The pancreas (fig. 25.21) is a spongy retroperitoneal

gland posterior to the greater curvature of the stomach. It

measures 12 to 15 cm long and about 2.5 cm thick. It has

a globose head encircled by the duodenum, a midportion called the body, and a blunt, tapered tail on the left.

The pancreas is both an endocrine and exocrine gland.

Its endocrine part is the pancreatic islets, which secrete

insulin and glucagon (see chapter 17). About 99% of



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Vein



(b)



50 µm



FIGURE 25.22 Microscopic Anatomy of the Pancreas. (a) An

acinus. (b) Histological section of the exocrine tissue and some of the

connective tissue stroma.



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CHAPTER 25



Trypsinogen



Chymotrypsinogen

Procarboxypeptidase



Enterokinase



Chymotrypsin

Carboxypeptidase



The Digestive System



979



TABLE 25.2



Exocrine Secretions of the

Pancreas



Secretion



Function



Sodium bicarbonate



Neutralizes HCl



Zymogens



Converted to active digestive enzymes after

secretion



Trypsinogen



Becomes trypsin, which digests protein



Chymotrypsinogen



Becomes chymotrypsin, which digests protein



Procarboxypeptidase



Becomes carboxypeptidase, which hydrolyzes

the terminal amino acid from the carboxyl

(–COOH) end of small peptides



Enzymes



Trypsin



Pancreatic amylase



Digests starch



Pancreatic lipase



Digests fat



Ribonuclease



Digests RNA



Deoxyribonuclease



Digests DNA



FIGURE 25.23 The Activation of Pancreatic Enzymes in the

Small Intestine. The pancreas secretes trypsinogen, and enterokinase

secreted by the duodenum converts it to trypsin. Trypsin not only

digests dietary protein but also catalyzes the production of more trypsin

and activates two other pancreatic zymogens—chymotrypsinogen and

procarboxypeptidase.



minor duodenal papilla, proximal to the major papilla.

The accessory duct bypasses the sphincter and allows

pancreatic juice to be released into the duodenum even

when bile is not.

Pancreatic juice is an alkaline mixture of water,

enzymes, zymogens, sodium bicarbonate, and other electrolytes. The acini secrete the enzymes and zymogens,

whereas the ducts secrete the sodium bicarbonate. The

bicarbonate buffers HCl arriving from the stomach.

The pancreatic zymogens are trypsinogen (trip-SINoh-jen), chymotrypsinogen (KY-mo-trip-SIN-o-jen), and

procarboxypeptidase (PRO-car-BOC-see-PEP-tih-dase).

When trypsinogen is secreted into the intestinal lumen,

it is converted to trypsin by enterokinase, an enzyme

secreted by the mucosa of the small intestine (fig. 25.23).

Trypsin is autocatalytic—it converts trypsinogen into still

more trypsin. It also converts the other two zymogens into

chymotrypsin and carboxypeptidase, in addition to its

primary role of digesting dietary protein.

Other pancreatic enzymes include pancreatic amylase,

which digests starch; pancreatic lipase, which digests fat;

and ribonuclease and deoxyribonuclease, which digest

RNA and DNA, respectively. Unlike the zymogens, these

enzymes are not altered after secretion. They become fully

active, however, only upon exposure to bile or ions in the

intestinal lumen.

The exocrine secretions of the pancreas are summarized in table 25.2. Their specific digestive functions are

explained later in more detail.



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Regulation of Secretion

Three stimuli are chiefly responsible for the release of

pancreatic juice and bile.

1. Acetylcholine (ACh), coming from the vagus and

enteric nerves. ACh stimulates the pancreatic acini

to secrete their enzymes even during the cephalic

phase of gastric control, before food is swallowed.

The enzymes remain stored in the pancreatic acini

and ducts, however, in preparation for release later

when chyme enters the duodenum.

2. Cholecystokinin21 (CCK), secreted by the mucosa of

the duodenum and proximal jejunum (the next segment of the small intestine), primarily in response to

fats in the small intestine. CCK also stimulates the

pancreatic acini to secrete enzymes, but it is named

for its strongly stimulatory effect on the gallbladder.

It induces contractions of the gallbladder and relaxation of the hepatopancreatic sphincter, discharging

bile into the duodenum.

3. Secretin, produced by the same regions of the small

intestine, mainly in response to the acidity of chyme

from the stomach. Secretin stimulates the ducts of

both the liver and pancreas to secrete an abundant

sodium bicarbonate solution. In the pancreas, this

flushes the enzymes into the duodenum. Sodium

bicarbonate buffers the hydrochloric acid arriving

from the stomach, with the reaction

HCl + NaHCO3 → NaCl + H2CO3 (carbonic acid)

The carbonic acid then breaks down to carbon dioxide and water. CO2 is absorbed into the blood and

21



chole = bile; cysto = bladder (gallbladder); kin = action



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PART FOUR



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ultimately exhaled. What is left in the small intestine, therefore, is saltwater—NaCl and H2O. Sodium

bicarbonate is therefore important in protecting the

intestinal mucosa from HCl as well as raising the

intestinal pH to the level needed for activity of the

pancreatic and intestinal digestive enzymes.



Stomach

Duodenum

Duodenojejunal

flexure



Apply What You Know

Draw a negative feedback loop showing how secretin

influences duodenal pH.



Jejunum

Ascending

colon

Mesentery



Before You Go On



Ileocecal

junction



Answer the following questions to test your understanding of the

preceding section:



Cecum

Appendix



14. What does the liver contribute to digestion?



Ileum



15. Trace the pathway taken by bile acids from the liver and

back. What is this pathway called?

16. Name two hormones, four enzymes, and one buffer secreted

by the pancreas, and state the function of each.

17. What stimulates cholecystokinin (CCK) secretion, and how

does CCK affect other parts of the digestive system?



25.5 The Small Intestine

Expected Learning Outcomes

When you have completed this section, you should be able to

a. describe the gross and microscopic anatomy of the small

intestine;

b. state how the mucosa of the small intestine differs from

that of the stomach, and explain the functional significance

of the differences;

c. define contact digestion and describe where it occurs; and

d. describe the types of movement that occur in the small

intestine.

Nearly all chemical digestion and nutrient absorption

occur in the small intestine. To perform these roles efficiently and thoroughly, the small intestine is the longest

part of the digestive tract—about 2.7 to 4.5 m long in a

living person; in the cadaver, where there is no muscle

tone, it is 4 to 8 m long. The term small intestine refers

not to its length but to its diameter—about 2.5 cm (1 in.).



Gross Anatomy

The small intestine is a coiled mass filling most of the

abdominal cavity inferior to the stomach and liver. It is

divided into three regions (fig. 25.24): the duodenum,

jejunum, and ileum.

The duodenum (dew-ODD-eh-num, DEW-oh-DEEnum) constitutes the first 25 cm (10 in.). Its name refers



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FIGURE 25.24 Gross Anatomy of the Small Intestine.

The intestine is pulled aside to expose the mesentery and

ileocecal junction.

to its length, about equal to the width of 12 fingers.22 It

begins at the pyloric valve, arcs around the head of the

pancreas and passes to the left, and ends at a sharp bend

called the duodenojejunal flexure. Slightly distal to the

pyloric valve, it exhibits the previously described wrinkles called the major and minor duodenal papillae, where

it receives the pancreatic duct and accessory pancreatic

duct, respectively. Along with the pancreas, most of the

duodenum is retroperitoneal. It receives the stomach

contents, pancreatic juice, and bile. Stomach acid is neutralized here, fats are physically broken up (emulsified)

by the bile acids, pepsin is inactivated by the elevated

pH, and pancreatic enzymes take over the job of chemical

digestion.

The jejunum (jeh-JOO-num), by definition, is the first

40% of the small intestine beyond the duodenum—about

1.0 to 1.7 m in a living person. Its name refers to the fact

that early anatomists typically found it to be empty.23 The

jejunum begins in the upper left quadrant of the abdomen

but lies mostly within the umbilical region (see fig. A.6,

p. 33). It has large, tall, closely spaced circular folds. Its

wall is thick and muscular, and it has an especially rich

blood supply, which gives it a relatively red color. Most

digestion and nutrient absorption occur here.

The ileum24 forms the last 60% of the postduodenal

small intestine (about 1.6 to 2.7 m). It occupies mainly

the hypogastric region and part of the pelvic cavity.

duoden = 12

jejun = empty, dry

24

from eilos = twisted

22

23



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CHAPTER 25



Compared with the jejunum, its wall is thinner, less muscular, less vascular, and has a paler pink color. On the

side opposite from its mesenteric attachment, the ileum

has prominent lymphatic nodules in clusters called

Peyer25 patches (see chapter 21), which are readily visible to the naked eye and become progressively larger

approaching the large intestine.

The end of the small intestine is the ileocecal (ILL-eeoh-SEE-cul) junction, where the ileum joins the cecum of

the large intestine. The muscularis of the ileum is thickened at this point to form a sphincter, the ileocecal (ILLee-oh-SEE-cul) valve, which protrudes into the cecum. It

regulates the passage of food residue into the large intestine and prevents feces from backing up into the ileum.



Microscopic Anatomy

The tissue layers of the small intestine are reminiscent of

those in the esophagus and stomach with modifications

appropriate for nutrient digestion and absorption. The

lumen is lined with simple columnar epithelium. The

muscularis externa is notable for a thick inner circular

layer and a thinner outer longitudinal layer. The jejunum

and ileum are intraperitoneal and thus covered with a

serosa, which is continuous with the complex, folded

mesentery that suspends the small intestine from the

posterior abdominal wall. Most of the duodenum, being

retroperitoneal, has a serosa only on its anterior surface;

its other surfaces are covered by adventitia.

Effective digestion and absorption require the small

intestine to have a large internal surface area. This is provided by its relatively great length and by three kinds of

internal folds or projections: the circular folds, villi, and

microvilli. If the mucosa were smooth, like the inside of a

hose, it would have a surface area of about 0.3 to 0.5 m2,

but with these surface elaborations, its actual surface area

is about 200 m2—clearly a great advantage for nutrient

absorption. The circular folds increase the surface area

by a factor of 2 to 3; the villi by a factor of 10; and the

microvilli by a factor of 20.



Apply What You Know

The small intestine exhibits some of the same structural

adaptations as the proximal convoluted tubule of the kidney, and for the same reasons. Discuss what they have in

common, the reasons for it, and how this relates to this

book’s theme of the unity of form and function.



The largest folds of the intestinal wall are transverse

to spiral ridges, up to 10 mm high, called circular folds

(plicae circulares) (see fig. 25.21). These involve only

the mucosa and submucosa; they are not visible on the

external surface, which is smooth. They cause the chyme

to flow on a spiral path along the intestine, which slows



The Digestive System



981



its progress, causes more contact with the mucosa, and

promotes more thorough mixing and nutrient absorption.

Circular folds begin in the duodenum, attain their greatest height in the jejunum, and become smaller and more

sparse in the ileum. They are absent from the distal half

of the ileum, but most nutrient absorption is completed

by that point.

If the mucosa is examined closely it appears fuzzy,

like a terry cloth towel. This is due to projections called

villi (VIL-eye; singular, villus), about 0.5 to 1.0 mm high,

with tongue- to fingerlike shapes (fig. 25.25). Villi are largest in the duodenum and become progressively smaller in

more distal regions of the intestine. A villus is covered

with two kinds of epithelial cells: columnar enterocytes

(absorptive cells) and mucus-secreting goblet cells. Like

epithelial cells of the stomach, those of the small intestine are joined by tight junctions that prevent digestive

enzymes from seeping between them.

The core of a villus is filled with areolar tissue of the

lamina propria. Embedded in this tissue are an arteriole,

a capillary network, a venule, and a lymphatic capillary

called a lacteal (LAC-tee-ul). The blood capillaries absorb

most nutrients, but the lacteal absorbs most lipids. They

give its contents a milky appearance for which the lacteal

is named.26 The core of the villus also has a few smooth

muscle cells that contract periodically. This enhances

mixing of the chyme in the intestinal lumen and milks

lymph down the lacteal to the larger lymphatics of the

submucosa.

Enterocytes have a fuzzy brush border of microvilli

about 1 μm high. They increase the absorptive surface

area of the small intestine and contain brush border

enzymes in the plasma membrane. These enzymes carry

out some of the final stages of chemical digestion. They

are not secreted into the lumen; instead, the chyme must

contact the brush border for digestion to occur. This process, called contact digestion, is one reason that it is so

important that intestinal contractions churn the chyme

and ensure that it all contacts the mucosa.

On the floor of the small intestine, between the bases

of the villi, there are numerous pores that open into tubular glands called intestinal crypts (crypts of Lieberkühn;27

LEE-ber-koohn). These crypts, similar to the gastric

glands, extend as far as the muscularis mucosae. In the

upper half, they consist of enterocytes and goblet cells

like those of the villi. The lower half is dominated by

dividing stem cells. In its life span of 3 to 6 days, an epithelial cell migrates up the crypt to the tip of the villus,

where it is sloughed off and digested. A few Paneth28 cells

are clustered at the base of each crypt. They secrete lysozyme, phospholipase, and defensins—defensive proteins

that resist bacterial invasion of the mucosa.

lact = milk

Johann N. Lieberkühn (1711–56), German anatomist

28

Josef Paneth (1857–90), Austrian physician

26

27



25



Johann K. Peyer (1653–1712), Swiss anatomist



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Villi

Absorptive cell

Brush border

of microvilli

Capillary network

Goblet cell



Lacteal



Intestinal crypts

(a)



Venule

Arteriole

Lymphatic vessel



Paneth cell

Villi

(c)

Intestinal

crypts

Muscularis

mucosae

Duodenal

glands



Muscularis

externa



Serosa

(b)



0.5 mm



FIGURE 25.25 Intestinal Villi. (a) Villi (SEM). Each villus is about 1 mm high. (b) Histological section of the duodenum showing villi, intestinal

crypts, and duodenal glands. (c) Structure of a villus.



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CHAPTER 25



(a) Segmentation



The Digestive System



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(b) Peristalsis



FIGURE 25.26 Contractions of the Small Intestine. (a) Segmentation, in which circular constrictions of the intestine cut into the contents,

churning and mixing them. (b) The migrating motor complex of peristalsis, in which successive waves of peristalsis overlap each other. Each wave

travels partway down the intestine and milks the contents toward the colon.



The duodenum has prominent duodenal (Brunner29)

glands in the submucosa. They secrete an abundance of

bicarbonate-rich mucus, which neutralizes stomach acid

and shields the mucosa from its erosive effects. Throughout

the small intestine, the lamina propria and submucosa

have a large population of lymphocytes that intercept

pathogens before they can invade the bloodstream. In some

places, these are aggregated into conspicuous lymphatic

nodules such as the Peyer patches of the ileum.



Intestinal Secretion

The intestinal crypts secrete 1 to 2 L of intestinal juice

per day, especially in response to acid, hypertonic chyme,

and distension of the intestine. This fluid has a pH of 7.4

to 7.8. It contains water and mucus but relatively little

enzyme. Most enzymes that function in the small intestine are found in the brush border and pancreatic juice.



Intestinal Motility

Contractions of the small intestine serve three functions:

(1) to mix chyme with intestinal juice, bile, and pancreatic juice, allowing these fluids to neutralize acid and

digest nutrients more effectively; (2) to churn chyme and

bring it into contact with the mucosa for contact digestion

and nutrient absorption; and (3) to move residue toward

the large intestine.

Segmentation is a movement in which stationary

ringlike constrictions appear at several places along

the intestine and then relax as new constrictions form

elsewhere (fig. 25.26a). This is the most common type

of intestinal contraction. Its effect is to knead or churn

29



Johann C. Brunner (1653–1727), Swiss anatomist



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the contents. Pacemaker cells of the muscularis externa

set the rhythm of segmentation, with contractions about

12 times per minute in the duodenum and 8 to 9 times

per minute in the ileum. Since the contractions are less

frequent distally, segmentation causes slow progression

of the chyme toward the colon. The intensity (but not

frequency) of contractions is modified by nervous and

hormonal influences.

When most nutrients have been absorbed and little

remains but undigested residue, segmentation declines

and peristalsis begins. A peristaltic wave begins in the

duodenum, travels 10 to 70 cm, and dies out, only to

be followed by another wave that begins a little farther

down the tract than the first one did (fig. 25.26b). These

successive, overlapping waves of contraction are called a

migrating motor complex. They milk the chyme toward

the colon over a period of about 2 hours. A second complex then expels residue and bacteria from the small

intestine, thereby helping to limit bacterial colonization.

Refilling of the stomach at the next meal suppresses peristalsis and reactivates segmentation.

The ileocecal valve is usually closed. Food in the

stomach, however, triggers both the release of gastrin and

the gastroileal reflex, both of which enhance segmentation in the ileum and relax the valve. As the cecum fills

with residue, the pressure pinches the valve shut and

prevents the reflux of cecal contents into the ileum.



Before You Go On

Answer the following questions to test your understanding of the

preceding section:

18. What three structures increase the absorptive surface area of

the small intestine?



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19. Sketch a villus and label its epithelium, brush border, lamina

propria, blood capillaries, and lacteal.



Starch



20. Distinguish between segmentation and the migrating motor

complex of the small intestine. How do these differ in

function?



25.6 Chemical Digestion and

Absorption



Pancreatic

amylase



Expected Learning Outcomes

When you have completed this section, you should be able to

a. describe how each major class of nutrients is chemically

digested, name the enzymes involved, and discuss the

functional differences among these enzymes; and

b. describe how each type of nutrient is absorbed by the

small intestine.

Chemical digestion and nutrient absorption are essentially finished by the time food residue leaves the small

intestine and enters the cecum. But before going on to the

functions of the large intestine, we trace each major class

of nutrients—especially carbohydrates, proteins, and

fats—from the mouth through the small intestine to see

how it is chemically degraded and absorbed.



Carbohydrates

Most digestible dietary carbohydrate is starch. Cellulose is

indigestible and is not considered here, although its importance as dietary fiber is discussed in chapter 26. The amount

of glycogen in the diet is negligible, but it is digested in the

same manner as starch.

Starch is digested first to oligosaccharides up to eight

glucose residues long, then into the disaccharide maltose,

and finally to glucose, which is absorbed by the small

intestine. The process begins in the mouth, where salivary

amylase hydrolyzes starch into oligosaccharides. Salivary

amylase functions best at pH 6.8 to 7.0, typical of the oral

cavity. It is quickly denatured upon contact with stomach

acid, but it can digest starch for as long as 1 to 2 hours in

the stomach as long as it is in the middle of a food mass

and escapes contact with the acid. Amylase therefore

works longer when the meal is larger, especially in the

fundus, where gastric motility is weakest and a food bolus

takes longer to break up. As acid, pepsin, and the churning

contractions of the stomach break up the bolus, amylase is

denatured; it does not function at a pH any lower than 4.5.

Being a protein, amylase is then digested by pepsin along

with the dietary proteins.

About 50% of the dietary starch is digested before

it reaches the small intestine. Its digestion resumes in

the small intestine when the chyme mixes with pancreatic amylase (fig. 25.27). Starch is entirely converted



sal78259_ch25_953-999.indd 984



Maltose and

oligosaccharides



Contact digestion

Maltase, dextrinase,

and glucoamylase



Glucose



Absorption



FIGURE 25.27 Starch Digestion in the Small Intestine.

Pancreatic amylase digests starch into maltose and small

oligosaccharides. Brush border enzymes (maltase, dextrinase, and

glucoamylase) digest these to glucose, which is absorbed by the

epithelial cells.



to oligosaccharides and maltose within 10 minutes. Its

digestion is completed as the chyme contacts the brush

border of the enterocytes. Two brush border enzymes,

dextrinase and glucoamylase, hydrolyze oligosaccharides that are three or more residues long. The third,

maltase, hydrolyzes maltose to glucose.

Maltose is also present in some foods, but the major

dietary disaccharides are sucrose (cane sugar) and lactose (milk sugar). They are digested by the brush border

enzymes sucrase and lactase, respectively, and the resulting monosaccharides are immediately absorbed (glucose and fructose from the former; glucose and galactose

from the latter). In most of the world population, however, lactase production ceases or declines to a low level

after age 4 and lactose becomes indigestible (see Deeper

Insight 25.4).

The plasma membrane of the enterocytes has transport proteins that absorb monosaccharides as soon as the

brush border enzymes release them (fig. 25.28). About

80% of the absorbed sugar is glucose, which is taken up

by a sodium–glucose transporter (SGLT) like that of the

kidney tubules (see p. 911). The glucose is subsequently

transported out the base of the cell into the extracellular

fluid (ECF). Sugar entering the ECF increases its osmolarity, and this draws water osmotically from the lumen

of the intestine, through the now leaky tight junctions

between the epithelial cells. Water carries more glucose

and other nutrients with it by solvent drag, much as

it does in the kidney. After a high-carbohydrate meal,

solvent drag absorbs two to three times as much glucose

as the SGLT.



11/16/10 9:02 AM



CHAPTER 25



DEEPER INSIGHT 25.4



Clinical Application



The Digestive System



Epithelial cell of

small intestine



Core of villus



Lumen of small intestine



Lactose Intolerance

Humans are a strange species. Unique among mammals, we go on

drinking milk in adulthood, and moreover, the milk of other species!

This odd habit is largely limited, however, to people of western and

northern Europe, Mongolia, a few pastoral tribes of Africa, and their

descendants in the Americas and elsewhere. They have an ancestral

history of milking domestic animals, a practice that goes back about

10,000 years and has coincided with the continued production of

lactase into adulthood.

People without lactase have lactose intolerance. If they consume

milk, lactose passes undigested into the large intestine, increases

the osmolarity of the intestinal contents, and causes colonic water

retention and diarrhea. In addition, lactose fermentation by intestinal

bacteria produces gas, resulting in painful cramps and flatulence.

Lactose intolerance occurs in about 15% of American whites; 90%

of American blacks, who are predominantly descended from nonpastoral African tribes; 70% or more of Mediterraneans; and nearly all

people of Asian descent, including those of us descended from the

native migrants into North, Central, and South America. People with

lactose intolerance can consume products such as yogurt and cheese,

in which bacteria have broken down the lactose, and they can digest

milk and ice cream with the aid of lactase drops or tablets.



The SGLT also absorbs galactose, whereas fructose is

absorbed by facilitated diffusion using a separate carrier

that does not depend on Na+. Inside the enterocyte, most

fructose is converted to glucose. Glucose, galactose, and

the small amount of remaining fructose are then transported out the base of the cell by facilitated diffusion

and absorbed by the blood capillaries of the villus. The

hepatic portal system delivers them to the liver; chapter 26

follows the fate of these sugars from there.



Proteins

The amino acids absorbed by the small intestine come

from three sources: (1) dietary proteins, (2) digestive

enzymes digested by each other, and (3) sloughed epithelial cells digested by these enzymes. The endogenous

amino acids from the last two sources total about 30 g/day,

compared with about 44 to 60 g/day from the diet.

Enzymes that digest proteins are called proteases

(peptidases). They are absent from the saliva but begin

work in the stomach. Here, pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine, thereby

digesting 10% to 15% of the dietary protein into shorter

polypeptides and a small amount of free amino acids

(fig. 25.29). Pepsin has an optimal pH of 1.5 to 3.5, so it is

inactivated when it passes into the duodenum and mixes

with the alkaline pancreatic juice (pH 8).



sal78259_ch25_953-999.indd 985



985



Tight junction

Na+



Na+

K+

Glucose

Fructose

Galactose



Glucose

Fructose

Galactose

Na+



Fructose



H2O, glucose



Key

Leakage through

tight junction

Solvent

drag



Facilitated diffusion

Symports (SGLT)

Antiport



FIGURE 25.28 Monosaccharide Absorption by the Small

Intestine. Glucose and galactose are absorbed by the SGLT symport

in the apical membrane of the absorptive cell (right). Glucose is also

absorbed along with water through the paracellular route (between

cells) by solvent drag. Fructose is absorbed separately by facilitated

diffusion. Most fructose is converted to glucose within the epithelial

cell. The monosaccharides pass through the basal membrane of the

cell by facilitated diffusion (left) and are then absorbed by the blood

capillaries of the villus.

In the small intestine, the pancreatic enzymes trypsin and chymotrypsin take over protein digestion by

hydrolyzing polypeptides into even shorter oligopeptides. Finally, these are taken apart one amino acid at

a time by three more enzymes: (1) carboxypeptidase

removes amino acids from the –COOH end of the chain;

(2) aminopeptidase removes them from the –NH2 end;

and (3) dipeptidase splits dipeptides in the middle and

releases the last two free amino acids. The last two of

these are brush border enzymes, whereas carboxypeptidase is a pancreatic secretion.

Amino acid absorption is similar to that of monosaccharides. Enterocytes have several sodium-dependent

amino acid cotransporters for different classes of amino

acids. Dipeptides and tripeptides can also be absorbed, but

they are hydrolyzed within the enterocytes before their

amino acids are released to the bloodstream. At the basal

surfaces of the cells, amino acids behave like the monosaccharides discussed previously—they leave the cell by

facilitated diffusion, enter the capillaries of the villus, and

are carried away in the hepatic portal circulation.

The absorptive cells of infants can take up intact proteins by pinocytosis and release them to the blood by exocytosis. This allows IgA from breast milk to pass into an

infant’s bloodstream and confer passive immunity from

mother to infant. It has the disadvantage, however, that

intact proteins entering the infant’s blood are detected



11/16/10 9:02 AM



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