Digestion and Absorption in the GIT pdf - GIT

stituent amino acids.

hydrolysis. That is, the proteolytic enzymes return hydrogen and hydroxyl ions

bound together by condensation, and digestion occurs by the reverse effect:

succeeding one; thus, the successive amino acids in the protein chain are also

. At each linkage, a hydroxyl ion has been

and thereby split the fatty acid molecules away from the glycerol. Here again,

Digestion of the triglycerides consists of the reverse process: the fat-

molecule. During condensation, three molecules

erides (neutral fats), which are combinations of three

hydrolysis,

from each other. This process, called

bohydrates are converted into monosaccharides. Specific enzymes in the diges-

When carbohydrates are digested, the above process is reversed and the car-

combine with each other at these sites of removal, and the hydrogen and

OH) has been removed from the next one. The two monosaccharides then

) has been removed from one of the monosaccharides, and a hydroxyl ion

. This means that a hydrogen ion

monosaccha-

, which are combinations of

disaccharides

polysaccharides

Digestion of the Various Foods by Hydrolysis

products as well as water, electrolytes, and other substances are absorbed.

pounds for absorption and, second, the mechanisms by which the digestive end

which carbohydrates, fats, and proteins are digested into small enough com-

fore, this chapter discusses, first, the processes by

as nutrients without preliminary digestion. There-

trointestinal mucosa and, for this reason, are useless

. They generally cannot be

, and

The major foods on which the body lives (with the

Gastrointestinal Tract

Digestion and Absorption in the

808

exception of small quantities of substances such as
vitamins and minerals) can be classified as carbo-
hydrates, fats

proteins

absorbed in their natural forms through the gas-

Hydrolysis of Carbohydrates.

Almost all the carbohydrates of the diet are either

large

rides bound to one another by condensation
(H

(

hydroxyl ions combine to form water (H

O).

tive juices of the gastrointestinal tract return the hydrogen and hydroxyl ions
from water to the polysaccharides and thereby separate the monosaccharides

is the following (in which

≤-R¢ is a disaccharide):

Hydrolysis of Fats.

Almost the entire fat portion of the diet consists of triglyc-

fatty acid molecules con-

densed with a single glycerol
of water are removed.

digesting enzymes return three molecules of water to the triglyceride molecule

the digestive process is one of hydrolysis.

Hydrolysis of Proteins.

Proteins are formed from multiple amino acids that are

bound together by peptide linkages
removed from one amino acid and a hydrogen ion has been removed from the

from water molecules to the protein molecules to split them into their con-

R -R

H O

R OH R H

¢¢

+ ¢

¢¢ ¢ +

ææ

digestive

enzyme

marized in Figure 65

The major steps in carbohydrate digestion are sum-

products of carbohydrate digestion, and galactose and

In the ordinary diet, which contains far more

digestion are all monosaccharides. They are all water

. Thus, the

multiple molecules

glucose.

. Sucrose splits into a molecule of

these enterocytes.

, so that the

the intestinal microvilli brush border

rides. These enzymes are located

glucose polymers, into their constituent monosaccha-

charides lactose, sucrose, and maltose, plus other small

), which are capable of splitting the disac-

, and

enterocytes lining the villi of the small intestine

The

In general, the carbohydrates are almost totally con-

mixes with pancreatic juice, virtually all the carbohy-

Therefore, within 15 to 30 minutes after the chyme

tion, like saliva, contains a large quantity of

Digestion by Pancreatic Amylase.

secretions, as much as 30 to 40 per cent of the starches

on the average, before food and its accompanying

pH of the medium falls below about 4.0. Nevertheless,

secretions. Then activity of the salivary amylase is

However, starch digestion sometimes continues in

remains in the mouth only a short time, so that prob-

1. However, the food

molecules, as shown in Figure 65

mainly by the parotid glands. This enzyme hydrolyzes

food is chewed, it is mixed with saliva, which contains

When

be considered a food for humans.

human digestive tract. Consequently, cellulose cannot

which is a carbohydrate. However, no enzymes

The diet also contains a large amount of cellulose,

, and minor quantities of

pyruvic acid

potatoes and the different types of grains. Other car-

present in almost all nonanimal foods, particularly in

, which are large polysaccharides

starches

in milk; and

, which is a disaccharide found

, which is the disaccharide known popularly

of carbohydrates exist in the normal human diet. They

Digestion of Carbohydrates

All the digestive enzymes are proteins. Their secre-

is involved. The

because, in the case of all three major types of food,

Therefore, the chemistry of digestion is simple

Digestion and Absorption in the Gastrointestinal Tract

Chapter 65

809

the same basic process of hydrolysis
only difference lies in the types of enzymes required
to promote the hydrolysis reactions for each type of
food.

tion by the different gastrointestinal glands was dis-
cussed in Chapter 64.

Carbohydrate Foods of the Diet.

Only three major sources

are sucrose
as cane sugar; lactose

bohydrates ingested to a slight extent are amylose,
glycogen, alcohol, lactic acid,

, pectins, dex-

trins

carbohydrate derivatives

in meats.

capable of hydrolyzing cellulose are secreted in the

Digestion of Carbohydrates in the Mouth and Stomach.

the digestive enzyme ptyalin (an

a-amylase) secreted

starch into the disaccharide maltose and other small
polymers of glucose that contain three to nine glucose

–

ably not more than 5 per cent of all the starches will
have become hydrolyzed by the time the food is
swallowed.

the body and fundus of the stomach for as long as 1
hour before the food becomes mixed with the stomach

blocked by acid of the gastric secretions because the
amylase is essentially nonactive as an enzyme once the

saliva do become completely mixed with the gastric

will have been hydrolyzed mainly to form maltose.

Digestion of Carbohydrates in the Small Intestine

Pancreatic secre-

a-amylase

that is almost identical in its function with the
a-amylase of saliva but is several times as powerful.

empties from the stomach into the duodenum and

drates will have become digested.

verted into maltose and/or other very small glucose
polymers before passing beyond the duodenum or
upper jejunum.

Hydrolysis of Disaccharides and Small Glucose Polymers
into Monosaccharides by Intestinal Epithelial Enzymes.

contain four enzymes (lactase, sucrase, maltase
a-dextrinase

in the enterocytes cov-

ering
disaccharides are digested as they come in contact with

Lactose splits into a molecule of galactose and a

molecule of glucose
fructose and a molecule of

Maltose and other

small glucose polymers all split into
of glucose

final products of carbohydrate

soluble and are absorbed immediately into the portal
blood.

starches than all other carbohydrates combined,
glucose represents more than 80 per cent of the final

fructose each seldom more than 10 per cent.

–1.

Maltose and 3 to 9 glucose polymers

Lactose

Sucrose

Ptyalin (saliva)–20–40%

Starches

Glucose

Fructose

Galactose

Sucrase
(intestine)

Lactase
(intestine)

Pancreatic amylase–50–80%

Maltase and

-dextrinase

(intestine)

Figure 65–1

Digestion of carbohydrates.

minutes, virtually all the last dipeptides and tripeptides

ing types of linkages between amino acids. Within

Finally, inside the cytosol of the enterocyte are mul-

enterocyte.

into amino acids. Both the amino acids plus the dipep-

. They succeed in splitting the remaining larger

Two types of peptidase enzymes are especially

uids.

exterior, where they come in contact with the intestinal

projecting from the surface of each cell. In the mem-

microvilli

brush border

mainly in the duodenum and jejunum. These cells have

enterocytes that line the villi of the small intestine,

The last digestive stage of

tripeptides.

pancreatic juices. Most remain as dipeptides and

that partially hold meats together.

, which then digests elastin

, in turn, is con-

ends of the polypeptides.

cules into small polypeptides; carboxypolypeptidase

, as shown in Figure

, and

carboxy-

chymotrypsin

stomach, the partial breakdown products of the

in the duodenum and jejunum, under the in

protein digestion occurs in the upper small intestine,

linkages.

The dietary proteins are chemically

Digestion of Proteins

810

Unit XII

Gastrointestinal Physiology

Proteins of the Diet.

long chains of amino acids bound together by peptide

A typical linkage is the following:

Digestion of Proteins by Pancreatic Secretions.

Most

fluence

of proteolytic enzymes from pancreatic secretion.
Immediately on entering the small intestine from the

protein foods are attacked by major proteolytic
pancreatic enzymes: trypsin,

polypeptidase

proelastase

65–2.

Both trypsin and chymotrypsin split protein mole-

then cleaves individual amino acids from the carboxyl

Proelastase

verted into elastase

fibers

Only a small percentage of the proteins are digested

all the way to their constituent amino acids by the

Digestion of Peptides by Peptidases in the Enterocytes That
Line the Small Intestinal Villi.

the proteins in the intestinal lumen is achieved by the

that consists of hundreds of

brane of each of these microvilli are multiple pepti-
dases that protrude through the membranes to the

important, aminopolypeptidase and several dipepti-
dases
polypeptides into tripeptides and dipeptides and a few

tides and tripeptides are easily transported through
the microvillar membrane to the interior of the

tiple other peptidases that are specific for the remain-

Trypsin, chymotrypsin, carboxypolypeptidase,

Pepsin

Peptidases

Amino acids

Polypeptides

Amino acids

Proteoses
Peptones
Polypeptides

Proteins

proelastase

Digestion of proteins.

Figure 65–2

COOH

+ H

acids.

tides. This splitting of proteins occurs as a result of

the protein to proteoses, peptones, and a few polypep-

process of protein digestion, usually providing only 10

2, pepsin only initiates the

As shown in Figure 65

the other digestive enzymes and, therefore, may be

juices, the ingested meats are less well penetrated by

quently, in persons who lack pepsin in the stomach

bers be digested. Conse-

meats and digest the other meat proteins, it is

cellular connective tissue of meats; therefore, for the

enzymes. Collagen is a major constituent of the inter-

, an albuminoid

range of acidity for pepsin activity.

then averages around 2.0 to 3.0, a highly favorable

the nonoxyntic glandular cells of the stomach, the pH

glands at a pH of about 0.8, but by the time it is mixed

a large quantity of hydrochloric acid. This hydrochlo-

As explained in Chapter 64, the gastric glands secrete

action on protein, the stomach juices must be acidic.

Consequently, for this enzyme to cause digestive

of 2.0 to 3.0 and is inactive at a pH above about 5.0.

peptic enzyme of the stomach, is most active at a pH

, the important

Pepsin

acids. The physical and chemical characteristics of dif-

The characteristics of each protein are determined

by the types of amino acids in the protein molecule
and by the sequential arrangements of these amino

ferent proteins important in human tissues are dis-
cussed in Chapter 69.

Digestion of Proteins in the Stomach.

ric acid is secreted by the parietal (oxyntic) cells in the

with the stomach contents and with secretions from

One of the important features of pepsin digestion is

its ability to digest the protein collagen
type of protein that is affected little by other digestive

digestive enzymes of the digestive tract to penetrate

first nec-

essary that the collagen fi

poorly digested.

–

to 20 per cent of the total protein digestion to convert

hydrolysis at the peptide linkages between amino

End Products of Fat Digestion.

, but this is usually not

lipase, known as

the enterocytes of the small intestine contain still more

minute all triglycerides that it can reach. In addition,

tities in pancreatic juice, enough to digest within 1

, present in enormous quan-

Digestion of Triglycerides by Pancreatic Lipase.

for digestion of fats.

Consequently, it can be readily understood how impor-

and can attack the fat globules only on their surfaces.

The lipase enzymes are water-soluble compounds

cation process.

occurred is less than 1 micrometer, this represents an

manyfold. Because the average diameter of the fat

intestine, the total surface area of the fat increases

used in household cleaners for removing grease.

tation with the water in the small bowel. This action is

salts and lecithin, especially the lecithin, in the bile is

is great. Consequently, a major function of the bile

tion, can be broken up into many very minute particles

uid, on agita-

uid is low, this nonmiscible

When the interfacial tension of a globule of non-

uids, which greatly decreases the

jecting. The polar projections, in turn, are soluble in the

layer of the fat globules, with the polar portions pro-

highly soluble in fat. Therefore, the fat-soluble por-

lecithin molecules are highly soluble in water, whereas

cation of the fat. The polar parts (the points

, are extremely important for

. Both of these,

However, bile does contain a large quantity of

the liver that does not contain any digestive enzymes.

, the secretion from

Then, most of the emulsi

, and it begins

digestive enzymes can act on the globule surfaces. This

globules into very small sizes so that the water-soluble

The

Emulsification of Fat by Bile Acids and Lecithin.

intestine as follows.

Instead, essentially all fat digestion occurs in the small

swallowed with the saliva. This amount of digestion is

in the stomach

cholesterol is considered, from a dietary point of view,

fats and is metabolized similarly to fats. Therefore,

chemical characteristics of fats; plus, it is derived from

fatty acid, but it does exhibit some of the physical and

terol, however, is a sterol compound that contains no

therefore can be considered fats themselves. Choles-

pholipids, cholesterol, and cholesterol esters. The phos-

of animal origin but much, much less so in food of

3. Neutral fat is a major constituent in food

Figure 65

nucleus and three fatty acid side chains, as shown in

diet are the neutral fats, also known as

disturbances, as discussed in Chapter 34.

rare absorption of whole protein molecules. Even

with only rare absorption of peptides and very, very

products that are absorbed are individual amino acids,

the enterocyte and thence into the blood.

acids; these then pass on through to the other side of

Digestion and Absorption in the Gastrointestinal Tract

Chapter 65

811

are digested to the final stage to form single amino

More than 99 per cent of the final protein digestive

these very few absorbed molecules of whole protein
can sometimes cause serious allergic or immunologic

Digestion of Fats

Fats of the Diet.

By far the most abundant fats of the

triglycerides,

each molecule of which is composed of a glycerol

–

plant origin.

In the usual diet are also small quantities of phos-

pholipids and cholesterol esters contain fatty acid and

a fat.

Digestion of Fats in the Intestine.

A small amount of

triglycerides is digested

by lingual lipase

that is secreted by lingual glands in the mouth and

less than 10 per cent and generally unimportant.

first step in fat digestion is physically to break the fat

process is called emulsification of the fat
by agitation in the stomach to mix the fat with the
products of stomach digestion.

fication occurs in the duo-

denum under the influence of bile

bile salts

as well as the phospholipid lecithin
but especially the lecithin
emulsifi
where ionization occurs in water) of the bile salts and

most of the remaining portions of their molecules are

tions of these liver secretions dissolve in the surface

surrounding watery fl
interfacial tension of the fat and makes it soluble as
well.

miscible fl

far more easily than it can when the interfacial tension

to make the fat globules readily fragmentable by agi-

the same as that of many detergents that are widely

Each time the diameters of the fat globules are sig-

nificantly decreased as a result of agitation in the small

particles in the intestine after emulsification has

increase of as much as 1000-fold in total surface areas
of the fats caused by the emulsifi

tant this detergent function of bile salts and lecithin is

By far the

most important enzyme for digestion of the triglyc-
erides is pancreatic lipase

enteric lipase

needed.

Most of the triglyc-

erides of the diet are split by pancreatic lipase into

CH + 2H

(2-Monoglyceride)

(Stearic acid)

(Tristearin)

(CH

)

(CH

)

(CH

)

Lipase

(CH

)

(CH

)

CH + 2CH

Hydrolysis of neutral fat catalyzed by lipase.

Figure 65–3

small intestinal mucosa, showing many folds called

Figure 65

absorbed in small quantities.

such as alcohol and some drugs like aspirin, can be

tions. Only a few highly lipid-soluble substances,

type of absorptive membrane, and also because the

The stomach is a poor absorptive area of the gas-

day.

absorbed in the small intestine, leaving only 1.5 liters

a total of 8 to 9 liters. All but about 1.5 liters of this is

trointestinal secretions (about 7 liters). This comes to

The total quantity of

Anatomical Basis of Absorption

branes discussed in detail in Chapter 4. The following

Gastrointestinal Absorption

absorbed without this function of the micelles.

free fatty acids. Indeed, essentially no cholesterol is

The bile salt micelles play the same role in

hydrolyze the cholesterol ester, and

cholesterol ester hydrolase

acid within their molecules. Both the cholesterol esters

molecule of fatty acid. Phospholipids also contain fatty

lesterol in the diet is in the form of cholesterol esters,

process.

absorbed into the blood, as discussed later, but the bile

There the monoglycerides and free fatty acids are

the brush borders of the intestinal epithelial cells.

of which would otherwise be relatively insoluble, to

to carry the monoglycerides and free fatty acids, both

The bile salt micelles also act as a transport medium

groups are negatively charged, they allow the entire

cover the surface of the micelle. Because these polar

a small fat globule in the middle of a resulting micelle,

sterol nucleus encompasses the fat digestate, forming

and a polar group that is highly water-soluble. The

salt. These develop because each bile salt molecule is

small spherical, cylindrical globules 3 to 6 nanometers

, which are

water, have the propensity to form

Bile salts, when in high enough concentration in

occurs in the following way.

these end products of digestion are formed. This

ing fats quickly blocks further digestion. But the bile

reversible process; therefore, accumulation of mono-

The hydrolysis of triglycerides is a highly

Figure 65

, as shown in

812

Unit XII

Gastrointestinal Physiology

free fatty acids and 2-monoglycerides

–4.

Role of Bile Salts to Accelerate Fat Digestion—Formation of
Micelles.

glycerides and free fatty acids in the vicinity of digest-

salts play the additional important role of removing
the monoglycerides and free fatty acids from the vicin-
ity of the digesting fat globules almost as rapidly as

micelles

in diameter composed of 20 to 40 molecules of bile

composed of a sterol nucleus that is highly fat-soluble

with polar groups of bile salts projecting outward to

micelle globule to dissolve in the water of the diges-
tive fluids and to remain in stable solution until the fat
is absorbed into the blood.

salts themselves are released back into the chyme to
be used again and again for this “ferrying”

Digestion of Cholesterol Esters and Phospholipids.

Most cho-

which are combinations of free cholesterol and one

and the phospholipids are hydrolyzed by two other
lipases in the pancreatic secretion that free the fatty
acids—the enzyme

phospholipase A

to hydrolyze the phospholipid.

“ferry-

ing” free cholesterol and phospholipid molecule diges-
tates that they play in “ferrying” monoglycerides and

Basic Principles of

It is suggested that the reader review the basic princi-
ples of transport of substances through cell mem-

paragraphs present specialized applications of these
transport processes during gastrointestinal absorption.

fluid that must be absorbed each

day by the intestines is equal to the ingested fluid
(about 1.5 liters) plus that secreted in the various gas-

to pass through the ileocecal valve into the colon each

trointestinal tract because it lacks the typical villus

junctions between the epithelial cells are tight junc-

Absorptive Surface of the Small Intestinal Mucosa Villi.

–5 demonstrates the absorptive surface of the

(Bile + Agitation)

Pancreatic lipase

Emulsified fat

Fatty acids and
2-monoglycerides

Fat

Emulsified fat

Figure 65–4

Digestion of fats.

Villi

Food

movement

Food

movement

Valvulae
conniventes

conniventes covered by villi.

Figure 65–5

Longitudinal section of the small intestine, showing the valvulae

of several hundred grams of carbohydrates, 100 or

movement of the microvilli, keeping them constantly

been entrapped. Small amounts of substances are

are pinched-off portions of infolded enterocyte mem-

, which

pinocytic vesicles

Figure 65

shows a cross section of the villus, and

Figure 65

general organization of the villus, emphasizing (1) the

Figure 65

area of the mucosa perhaps 1000-fold, making a

villi, and the microvilli increases the total absorptive

Thus, the combination of the folds of Kerckring, the

7. This increases the surface

micrograph in Figure 65

chyme; these microvilli are shown in the electron

microvilli

brush border,

Finally, each intestinal epithelial cell on each villus

less profuse in the distal small intestine. The presence

that they touch in most areas, but their distribution is

6. The villi

5 and in individual detail in Figure 65

on the surfaces of the valvulae conniventes in Figure

millimeter from the surface of the mucosa, as shown

. These project about 1

developed in the duodenum and jejunum, where they

about threefold. These folds extend circularly most of

), which

folds of Kerckring

Digestion and Absorption in the Gastrointestinal Tract

Chapter 65

813

valvulae conniventes (or
increase the surface area of the absorptive mucosa

the way around the intestine and are especially well

often protrude as much as 8 millimeters into the
lumen.

Also located on the epithelial surface of the small

intestine all the way down to the ileocecal valve are
literally millions of small villi

65–

–

lie so close to one another in the upper small intestine

of villi on the mucosal surface enhances the total
absorptive area another 10-fold.

is characterized by a

consisting of as

many as 1000

1 micrometer in length and 0.1

micrometer in diameter protruding into the intestinal

–

area exposed to the intestinal materials at least
another 20-fold.

tremendous total area of 250 or more square meters
for the entire small intestine—about the surface area
of a tennis court.

–6A shows in longitudinal section the

advantageous arrangement of the vascular system for
absorption of fluid and dissolved material into the
portal blood and (2) the arrangement of the “central
lacteal” lymph vessel for absorption into the lymph.

–6B

–7 shows many small

brane forming vesicles of absorbed fluids that have

absorbed by this physical process of pinocytosis.

Extending from the epithelial cell body into each

microvillus of the brush border are multiple actin fila-
ments that contract rhythmically to cause continual

exposed to new quantities of intestinal fluid.

Absorption in the
Small Intestine

Absorption from the small intestine each day consists

Central
lacteal

Venules

Basement

membrane

Brush

border

Arteriole

Capillaries

Vein

Artery

Blood
capillaries

border at the other ends of these

Cross section showing a base-

Figure 65–6

Functional organization of the
villus. A, Longitudinal section.
B,
ment membrane beneath the
epithelial cells and a brush

cells.

Dr. William Lockwood.)

reticulum lying immediately beneath the brush border. (Courtesy

Brush border of a gastrointestinal epithelial cell, showing also

Figure 65–7

absorbed pinocytic vesicles, mitochondria, and endoplasmic

chloride in the feces and also little water loss. Thus, the

This effect of aldosterone is especially important in

substances.

absorption of chloride ions, water, and some other

by the intestinal epithelium. And the increased sodium

the adrenal glands. Within 1 to 3 hours this aldosterone

person becomes dehydrated, large amounts of aldos-

When a

villus.

nally, into the circulating blood of the

cells themselves. And osmotic movement of water

of the epithelial cells, but much also occurs through the

in the paracellular space. Much of this osmosis occurs

spaces. This occurs because a large osmotic gradient

The next step in the transport

Osmosis of the Water.

cells into the paracellular spaces.

into the epithelial cell cytoplasm. This provides still

(that is, about equal to that in plasma), sodium moves

8. Because the sodium con-

as also shown in Figure 65

tion inside the cell to a low value (about 50 mEq/L),

of the sodium ions.

fact, the negatively charged chloride ions are mainly

the sodium is absorbed along with chloride ions; in

enzymes in the cell membrane (see Chapter 4). Part of

port: it requires energy, and the energy process is cat-

8. This

strated by the heavy red arrows in Figure 65

these cells into paracellular spaces. This is demon-

The motive power for sodium absorption is pro-

mechanism, discussed in Chapter 4, are also essentially

8. The principles of this

intestine is shown in Figure 65

The basic mechanism of sodium absorption from the

discussions.

and amino acids, as we shall see in subsequent

absorbed through the intestinal mucosa. Sodium also

however, less than 0.5 per cent of the intestinal sodium

depleted to lethal levels within hours. Normally,

tions are lost to the exterior, as in extreme diarrhea,

Whenever signi

one seventh of all the sodium present in the body.

35 grams of sodium each day, which is equal to about

sodium into the feces, the intestines must absorb 25 to

sodium each day. Therefore, to prevent net loss of

In addition, the average person eats 5 to 8 grams of

are secreted in the intestinal secretions each day.

Twenty to 30 grams of sodium

Active Transport of Sodium.

Absorption of Ions

minutes, suf

charged from the stomach into the duodenum. Within

from plasma into the chyme. This

Conversely, water can also be transported in the

of the villi almost entirely by osmosis.

Therefore, when the chyme is dilute enough, water is

more, this diffusion obeys the usual laws of osmosis.

. Further-

Water is transported through the

Absorption of Water

tional water and ions, although very few nutrients.

water per day. The

700 grams of proteins per day, and 20 or more liters of

bohydrates per day, 500 grams of fat per day, 500 to

greater than this: as much as several kilograms of car-

to 100 grams of ions, and 7 to 8 liters of water. The

more grams of fat, 50 to 100 grams of amino acids, 50

814

Unit XII

Gastrointestinal Physiology

absorptive capacity of the normal small intestine is far

large intestine can absorb still addi-

Isosmotic Absorption.

intestinal membrane entirely by diffusion

absorbed through the intestinal mucosa into the blood

opposite direction—
occurs especially when hyperosmotic solutions are dis-

ficient water usually will be transferred by

osmosis to make the chyme isosmotic with the plasma.

ficant amounts of intestinal secre-

the sodium reserves of the body can sometimes be

is lost in the feces each day because it is rapidly

plays an important role in helping to absorb sugars

–

the same as for absorption of sodium from the
gallbladder and renal tubules as discussed in Chapter
27.

vided by active transport of sodium from inside the
epithelial cells through the basal and side walls of

–

active transport obeys the usual laws of active trans-

alyzed by appropriate adenosine triphosphatase

passively “dragged” by the positive electrical charges

Active transport of sodium through the basolateral

membranes of the cell reduces the sodium concentra-

–

centration in the chyme is normally about 142 mEq/L

down this steep electrochemical gradient from the
chyme through the brush border of the epithelial cell

more sodium ions to be transported by the epithelial

process is osmosis of water into the paracellular

has been created by the elevated concentration of ions

through the tight junctions between the apical borders

creates flow of fluid into and through the paracellular
spaces and, fi

Aldosterone Greatly Enhances Sodium Absorption.

terone almost always are secreted by the cortices of

causes increased activation of the enzyme and trans-
port mechanisms for all aspects of sodium absorption

absorption in turn causes secondary increases in

the colon because it allows virtually no loss of sodium

Active transport

Diffusion

Osmosis

Na
(50 mEq/L)

Na (142 mEq/L)

through the epithelial membrane.

Absorption of sodium through the intestinal epithelium. Note also

Figure 65–8

osmotic absorption of water—that is, water “follows” sodium

sugar.

, the galactose derived from milk and the fructose

most abundant carbohydrate food, the starches. The

of carbohydrate calories absorbed. The reason for

, usually accounting for more than 80 per cent

none as larger carbohydrate compounds. By far

absorbed in the form of monosaccharides; only a small

Absorption of Nutrients

body.

tion of sodium ions. Fortunately, only small quantities

amounts; for example, maximum absorption of

absorbed with ease and in great quantities. Conversely,

intestinal mucosa. In general, the monovalent ions are

phosphate,

Potassium

globin, are discussed in Chapter 32.

need for iron, especially for the formation of hemo-

intestine. The principles of iron absorption and regu-

Chapter 79.

calcium absorption. These effects are discussed in

Parathyroid hormone activates vitamin D, and the

by the parathyroid glands, and another is

the body for calcium. One important factor controlling

num, and the amount of calcium ion absorption is very

make up for the loss.

whole body that might ensue. In most instances, the life

in combating the disease, but too much of a good thing

uid along with the salt. All this excess

extreme osmosis of water from the blood, thus provid-

ride ions. Finally, all this extra sodium chloride causes

from inside the cell into the intestinal crypts. In turn, this

chloride channels, allowing chloride ions to

monophosphate, which opens tremendous numbers of

This stimulates formation of excess cyclic adenosine

a subunit of cholera toxin into the epithelial cells.

uid alone.

Within 1 to 5 days, many severely affected patients die

each day.

be reabsorbed, thus sometimes causing loss of 5 to 10

The toxins of cholera and of some other types of diar-

intestinal digestates.

the folds, thus providing

and water into the intestinal lumen. This secretion in

deep folds, the epithelial cells secrete sodium chloride

the luminal surfaces of the intestines. While still in the

new epithelial cells. These in turn spread outward over

the Large Intestine Epithelium in Some Types of Diarrhea.

Extreme Secretion of Chloride Ions, Sodium Ions, and Water from

in the large intestine.

in exchange for absorption of chloride ions. This is

The epithelial cells on the surfaces of the villi in the

and Large Intestine—Simultaneous Absorption

Secretion of Bicarbonate Ions in the Ileum

kidneys.

active absorption of bicarbonate ions.

subsequently expired through the lungs. Thus, this is

remains as part of the chyme in the intestines, but the

ciates to form water and carbon dioxide. The water

), which then disso-

of the gut in exchange for some of the sodium. These

as follows: When sodium ions are absorbed, moderate

bile.The bicarbonate ion is absorbed in an indirect way

the sodium ions.

the epithelial cells. Then chloride ions move along this

that is, absorption of sodium ions through the epithe-

the upper part of the small intestine, chloride ion

tubules, which also serves to conserve sodium chloride

Digestion and Absorption in the Gastrointestinal Tract

Chapter 65

815

function of aldosterone in the intestinal tract is the
same as that achieved by aldosterone in the renal

and water in the body when a person becomes
dehydrated.

Absorption of Chloride Ions in the Duodenum and Jejunum.

absorption is rapid and occurs mainly by diffusion—

lium creates electronegativity in the chyme and
electropositivity in the paracellular spaces between

electrical gradient to “follow”

Absorption of Bicarbonate Ions in the Duodenum and Jejunum.

Often large quantities of bicarbonate ions must be
reabsorbed from the upper small intestine because
large amounts of bicarbonate ions have been secreted
into the duodenum in both pancreatic secretion and

amounts of hydrogen ions are secreted into the lumen

hydrogen ions in turn combine with the bicarbonate
ions to form carbonic acid (H

carbon dioxide is readily absorbed into the blood and

so-called “

” It is

the same mechanism that occurs in the tubules of the

of Chloride Ions

ileum as well as on all surfaces of the large intestine
have a special capability of secreting bicarbonate ions

important because it provides alkaline bicarbonate
ions that neutralize acid products formed by bacteria

Deep

in the spaces between the intestinal epithelial folds are
immature epithelial cells that continually divide to form

turn is reabsorbed by the older epithelial cells outside

flow of water for absorbing

rheal bacteria can stimulate the fold secretion so greatly
that this secretion often becomes much greater than can

liters of water and sodium chloride as diarrhea

from this loss of fl

Extreme diarrheal secretion is initiated by entry of

flow rapidly

is believed to activate a sodium pump that pumps
sodium ions into the crypts to go along with the chlo-

ing rapid flow of fl
fluid washes away most of the bacteria and is of value

can be lethal because of serious dehydration of the

of a cholera victim can be saved by administration of
tremendous amounts of sodium chloride solution to

Absorption of Other Ions.

Calcium ions are actively

absorbed into the blood especially from the duode-

exactly controlled to supply exactly the daily need of

calcium absorption is parathyroid hormone secreted

vitamin D.

activated vitamin D in turn greatly enhances

Iron ions are also actively absorbed from the small

lation of its absorption in proportion to the body’s

, magnesium,

and probably still

other ions can also be actively absorbed through the

bivalent ions are normally absorbed in only small

calcium ions is only 1/50 as great as the normal absorp-

of the bivalent ions are normally required daily by the

Absorption of Carbohydrates

Essentially all the carbohydrates in the food are

fraction are absorbed as disaccharides and almost

the most abundant of the absorbed monosaccharides
is glucose

this is that glucose is the final digestion product of our

remaining 20 per cent of absorbed monosaccharides
are composed almost entirely of galactose and fruc-
tose
as one of the monosaccharides digested from cane

This allows direct diffusion of these short-chain fatty

are more water-soluble and mostly are not recon-

erides and absorbed by way of the lymphatics. The

as those from butterfat, are absorbed directly into the

quantities of short- and medium-chain fatty acids, such

epithelial cell, to

chylomicrons

endoplasmic reticulum; here, they are mainly used to

After entering the epithelial cell, the fatty acids and

only 40 to 50 per cent can be absorbed.

the fat is absorbed; in the absence of the bile micelles,

of an abundance of bile micelles, about 97 per cent of

is highly important for fat absorption. In the presence

function that

Thus, the micelles perform a

monoglycerides and fatty acids.

leaves the bile micelles still in the chyme, where they

are also soluble in the epithelial cell membrane. This

the epithelial cells, which is possible because the lipids

recesses among the moving, agitating microvilli. Here,

chyme. In this form, the monoglycerides and free fatty

of their highly charged exterior, they are soluble in

are only 3 to 6 nanometers in diameter, and because

bile micelles

acids, both of these digestive end products

Earlier in this chapter, it was pointed out that when

amino acids and peptides.

This multiplicity of transport proteins is required

the luminal membranes of intestinal epithelial cells.

ported, by facilitated diffusion.

of the amino acids and peptides.

and pulls the amino acid or peptide along with it. This

occur. After binding, the sodium ion then moves down

sodium co-transport of glucose occurs. That is, most

The energy for most of this transport is supplied by a

of dipeptides, tripeptides, and a few free amino acids.

after digestion, are absorbed through the luminal

As explained earlier in the chapter, most proteins,

Absorption of Proteins

one half that of glucose or galactose.

with sodium, its overall rate of transport is only about

into the blood. Because fructose is not co-transported

phosphorylated, then converted to glucose, and

Much of the fructose, on entering the cell, becomes

by the sodium co-transport mechanism. Instead, fruc-

glucose. Conversely, fructose transport does not occur

membranes.

To summarize, it is the initial active transport of

epithelial cell, other transport proteins and enzymes

with it the glucose at the same time. Once inside the

transported together to the interior of the cell. Thus,

simultaneously with the same transport protein, and

glucose. Fortunately, intestinal glucose also combines

, but the trans-

. That is, a sodium

Second, decrease of sodium inside the cells causes

thereby depleting sodium inside the epithelial cells.

through the intestinal membrane. First is active trans-

There are two stages in the transport of

The reason is that glucose absorption occurs in a co-

tinal membrane, virtually no glucose can be absorbed.

Glucose Is Transported by a Sodium Co-Transport Mechanism.

absorption of glucose.

an active transport process. Let us

Virtually all the monosaccharides are absorbed by

816

Unit XII

Gastrointestinal Physiology

first discuss the

In the absence of sodium transport through the intes-

transport mode with active transport of sodium.

sodium

port of sodium ions through the basolateral mem-
branes of the intestinal epithelial cells into the blood,

sodium from the intestinal lumen to move through the
brush border of the epithelial cells to the cell interiors
by a process of facilitated diffusion
ion combines with a transport protein
port protein will not transport the sodium to the inte-
rior of the cell until the protein itself also combines
with some other appropriate substance such as

then both the sodium ion and glucose molecule are

the low concentration of sodium inside the cell liter-
ally “drags” sodium to the interior of the cell and along

cause facilitated diffusion of the glucose through the
cell’s basolateral membrane into the paracellular
space and from there into the blood.

sodium through the basolateral membranes of the
intestinal epithelial cells that provides the eventual
motive force for moving glucose also through the

Absorption of Other Monosaccharides.

Galactose is trans-

ported by almost exactly the same mechanism as

tose is transported by facilitated diffusion all the way
through the intestinal epithelium but not coupled with
sodium transport.

finally

transported in the form of glucose the rest of the way

membranes of the intestinal epithelial cells in the form

sodium co-transport mechanism in the same way that

peptide or amino acid molecules bind in the cell’s

microvillus membrane with a specific transport protein
that requires sodium binding before transport can

its electrochemical gradient to the interior of the cell

is called co-transport (or secondary active transport)

A few amino acids do

not require this sodium co-transport mechanism but
instead are transported by special membrane trans-
port proteins in the same way that fructose is trans-

At least five types of transport proteins for trans-

porting amino acids and peptides have been found in

because of the diverse binding properties of different

Absorption of Fats

fats are digested to form monoglycerides and free fatty

first become

dissolved in the central lipid portions of

Because the molecular dimensions of these micelles

acids are carried to the surfaces of the microvilli of the
intestinal cell brush border and then penetrate into the

both the monoglycerides and fatty acids diffuse imme-
diately out of the micelles and into the interior of

function again and again to help absorb still more

“ferrying”

monoglycerides are taken up by the cell’s smooth

form new triglycerides that are subsequently released
in the form of

through the base of the

flow upward through the thoracic

lymph duct and empty into the circulating blood.

Direct Absorption of Fatty Acids into the Portal Blood.

Small

portal blood rather than being converted into triglyc-

cause of this difference between short- and long-chain
fatty acid absorption is that the short-chain fatty acids

verted into triglycerides by the endoplasmic reticulum.

acids from the intestinal epithelial cells directly into
the capillary blood of the intestinal villi.

Membr Biol 184:233, 2001.

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Pappenheimer JR: Intestinal absorption of hexoses and

mammals. Physiol Rev 80:1633, 2000.

Pacha J: Development of intestinal transport function in

64:635, 2002.

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Physiol Rev 82:245, 2002.

malian colon: mechanisms and implications for disease.

Kunzelmann K, Mall M: Electrolyte transport in the mam-

Gastroenterology 126:322, 2004.

salt transporters in normal physiology and liver disease.

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antigen processing and Toll-like receptor signaling in

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of altered colonic sodium transport. Ann N Y Acad Sci

Greig E, Sandle GI: Diarrhea in ulcerative colitis. The role

diarrhea. J Clin Invest 111:931, 2003.

Field M: Intestinal ion transport and the pathophysiology of

port. Physiol Rev 77:257, 1997.

Ferraris RP, Diamond J: Regulation of intestinal sugar trans-

troenterol Hepatol 18:479, 2003.

barrier: an interface between health and disease. J Gas-

Farhadi A, Banan A, Fields J, Keshavarzian A: Intestinal

peptide transport. Annu Rev Physiol 66:361, 2004.

Daniel H: Molecular and integrative physiology of intestinal

Rev Physiol 62:535, 2000.

epithelium: molecular basis and regulatory aspects. Annu

Barrett KE, Keely SJ: Chloride secretion by the intestinal

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oligopeptide transporter (Pept-1) in health and disease.

Adibi SA: Regulation of expression of the intestinal

hydrogen sulfide.

, and

food eaten. The actual odoriferous products include

ucts vary from one person to another, depending on

principally by products of bacterial action; these prod-

, derivatives of bilirubin. The odor is caused

cells. The brown color of feces is caused by

tive juices, such as bile pigment and sloughed epithelial

, and 30 per cent

, 10 to 20 per cent

The feces normally are about

The bacteria-formed vitamin K is especially important

cially carbon dioxide, hydrogen gas, and methane.

in the colon, espe-

avin, and

, thiamine, ribo

ity are vitamin K, vitamin B

ble importance in human beings.

cant, although it is of negligi-

nutrition for the body. In herbivorous animals, this

cellulose, in this way providing a few calories of extra

They are capable of digesting small amounts of

colon bacilli, are present even normally in the absorbing

Numerous bacteria, especially

uid each day,

rhea. As noted earlier in the chapter, toxins from

this amount, the excess appears in the feces as diar-

uid and electrolytes each day. When the total quan-

The

which in turn causes absorption of water.

end products of bacterial action in the large intestine.

described. The bicarbonate helps neutralize the acidic

small intestine, the mucosa of the large intestine

In addition, as occurs in the distal portion of the

port capability.

can occur in the small intestine. This is especially true

that is,

junctions, thus allowing the large intestinal mucosa to

than those of the small intestine. This prevents signif-

tion as well. The tight junctions between the epithelial

sodium, and the electrical potential gradient created

intestine, has a high capability for active absorption of

mucosa of the large intestine, like that of the small

The

Absorption and Secretion of Electrolytes and Water.

, whereas the distal

in the proximal one half of the colon, giving this

be lost in the feces.

essentially all the ions are absorbed, leaving only 1 to

uid to be excreted in the feces. Also,

absorbed in the colon, usually leaving less than 100

the ileocecal valve into the large intestine each day.

Intestine: Formation of Feces

Absorption in the Large

Digestion and Absorption in the Gastrointestinal Tract

Chapter 65

817

About 1500 milliliters of chyme normally pass through

Most of the water and electrolytes in this chyme are

milliliters of fl

5 milliequivalents each of sodium and chloride ions to

Most of the absorption in the large intestine occurs

portion the name absorbing colon
colon functions principally for feces storage until a
propitious time for feces excretion and is therefore
called the storage colon.

by absorption of the sodium causes chloride absorp-

cells of the large intestinal epithelium are much tighter

icant amounts of back-diffusion of ions through these

absorb sodium ions far more completely—
against a much higher concentration gradient—than

when large quantities of aldosterone are available
because aldosterone greatly enhances sodium trans-

secretes bicarbonate ions while it simultaneously
absorbs an equal number of chloride ions in an
exchange transport process that has already been

Absorption of sodium and chloride ions creates an

osmotic gradient across the large intestinal mucosa,

Maximum Absorption Capacity of the Large Intestine.

large intestine can absorb a maximum of 5 to 8 liters
of fl
tity entering the large intestine through the ileocecal
valve or by way of large intestine secretion exceeds

cholera or certain other bacterial infections often
cause the crypts in the terminal ileum and in the large
intestine to secrete 10 or more liters of fl
leading to severe and sometimes lethal diarrhea.

Bacterial Action in the Colon.

colon.

source of energy is signifi

Other substances formed as a result of bacterial activ-

various gases that contribute to flatus

because the amount of this vitamin in the daily ingested
foods is normally insufficient to maintain adequate
blood coagulation.

Composition of the Feces.

three-fourths water and one-fourth solid matter that
itself is composed of about 30 per cent dead bacteria, 10
to 20 per cent fat

inorganic matter, 2

to 3 per cent protein

undigested

roughage from the food and dried constituents of diges-

stercobilin

and urobilin

each person’s colonic bacterial flora and on the type of

indole, skatole, mercaptans

References

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Unit XII

Gastrointestinal Physiology

2003.

2004;66:419-46.

brate gastrointestinal tract to production and conservation