1. Animals digest food via intracellular and extracellular mechanisms. In intracellular digestion, food is engulfed by cells and digested within vacuoles. In extracellular digestion, food is broken down in compartments outside cells by enzymes secreted into cavities or tubes. 2. Many simple animals digest food extracellularly within a gastrovascular cavity with one opening. More complex animals digest food in multi-chambered alimentary canals containing organs for ingestion, mechanical and chemical breakdown, nutrient absorption, and waste elimination. 3. Digestion involves the mechanical and chemical breakdown of food. Mechanical digestion breaks food into smaller pieces using mastication or muscular grinding. Chemical digestion further breaks food down using enzymes secreted by

1. Introducción a las formas y funcionamiento de los
animales
Digestión y nutrición
Circulación e intercambio gaseoso
Sistema inmunitario
Osmoregulación y excresión
Hormonos y sistema endócrino
Reproducción
Desarrollo (embriología)
Sistema nervioso (mecanismos sensitivos y motores)
Comportamiento
1. Formas y funcionamiento de los Animales

2. Filter Feeding
Baleen
Mecanismos alimenticios
Bulk Feeding
Substrate Feeding
Caterpillar Feces
Fluid Feeding
s, some

3. Animals require 20 am
Figure 5.14). Most an
synthesize about half
diet includes sulfur
amino acids m
ricated fo
amino
hum
diet
ni
ing from DNA replication and cell division to vision and
flight. To meet the continuous requirement for ATP, ani-
mals ingest and digest nutrients, including carbohydrates,
Linoleic acid γ-Linoleic acid
Fatty acid desaturase
Phospholipids
(cell membrane components)
Prostaglandins
(used in cell signaling)
NADH
ESSENTIAL
AMINO ACIDS
(monomers of
polypeptide)
VITAMIN
(coenzyme)
ESSENTIAL
FATTY ACID
(substrate of
enzyme)
Vitamin B3
MINERAL
(cofactor)
Iron
Tyr
Glu
Gly
Phe
Phe
Leu
Ile
▲ Figure 41.2 Roles of essential nutrients. Linoleic acid is converted by the enzyme fatty acid
desaturase to γ-linoleic acid, a precursor for phospholipids and prostaglandins. This biosynthetic reac-
tion illustrates common functions of the four classes of essential nutrients, labeled in blue. Note that
Animals require 20 am
Figure 5.14). Most an
synthesize about half
diet includes sulfur
amino acids m
ricated fo
amino
hum
diet
ni
ing from DNA replication and cell division to vision and
flight. To meet the continuous requirement for ATP, ani-
mals ingest and digest nutrients, including carbohydrates,
Linoleic acid γ-Linoleic acid
Fatty acid desaturase
Phospholipids
(cell membrane components)
Prostaglandins
(used in cell signaling)
NADH
ESSENTIAL
AMINO ACIDS
(monomers of
polypeptide)
VITAMIN
(coenzyme)
ESSENTIAL
FATTY ACID
(substrate of
enzyme)
Vitamin B3
MINERAL
(cofactor)
Iron
Tyr
Glu
Gly
Phe
Phe
Leu
Ile
▲ Figure 41.2 Roles of essential nutrients. Linoleic acid is converted by the enzyme fatty acid
desaturase to γ-linoleic acid, a precursor for phospholipids and prostaglandins. This biosynthetic reac-
tion illustrates common functions of the four classes of essential nutrients, labeled in blue. Note that
Rol de los nutrientes esenciales

4. Blood glucose
level rises
(such as after eating).
Blood glucose
level falls
(such as after fasting).
Beta cells of the
pancreas secrete
the hormone
insulin into the
blood.
Glucagon promotes
the breakdown of
glycogen in the liver
and the release of
glucose into the blood.
Alpha cells of the
pancreas secrete
the hormone
glucagon
into the blood.
Insulin enhances
the transport of
glucose into
body cells and
stimulates the liver
to store glucose
as glycogen.
Insulin
Glucagon
Blood glucose
level falls.
Blood glucose
level rises.
NORMAL BLOOD GLUCOSE
(70–110 mg glucose/
100 mL)
nd storage of glucose,
Homeostasis de la glucosa

5. Indice de masa
corporal (IMC)=
masa/estatura2
(kg/m2)
Desequilibrio calórico
Blanco = Bajo peso (IMC <18,5)
Amarillo = Rango normal (IMC
= 18,5-24,99)
Naranja = Sobrepeso (IMC =
25-29,99)
Rojo = Obesidad (IMC ≥30)

6. lin-dependent diabetes, or
rized by a failure of tar-
lly to insulin. Insulin is
ail to take up glucose
glucose levels remain
y can play a role in type
ight and lack of exercise
isk. This form of diabetes
40, but even children can
ularly if they are overweight
Ghrelin
Satiety
center
+
Insulin
–
Leptin
–
PYY
–
Secreted by the stomach wall,
ghrelin is one of the signals
that triggers feelings of
hunger as mealtimes
approach. In dieters who lose
weight, ghrelin levels
increase, which may be one
reason it’s so hard to stay on
a diet.
A rise in blood sugar level
after a meal stimulates the
pancreas to secrete insulin. In
addition to its other functions,
insulin suppresses appetite by
acting on the brain.
Produced by adipose (fat)
tissue, leptin suppresses
appetite. When the amount of
body fat decreases, leptin
levels fall, and appetite
increases.
The hormone PYY, secreted
by the small intestine after
meals, acts as an appetite
suppressant that counters
the appetite stimulant
ghrelin.
© 2003 AAAS
▲ Figure 41.22 A few of the appetite-regulating hormones.
Secreted by various organs and tissues, the hormones reach the brain
90% of people with diabetes have
heir blood glucose levels with reg-
diet; some require medications.
tes is the seventh most common
d States and a growing public
.
n signaling in type 2 diabetes is
c defect in the insulin receptor
hway. In many cases, however,
ess activity of an otherwise func-
One source of this suppression
insulin. Treatment consists of
given several times daily. In the
from animal pancreases, but now
ined from genetically engineered
ensive source (see Figure 20.2).
meday provide a cure for type 1
lacement beta cells that restore
pancreas.
Hormonas en la
regulación del
apetito

7. Mechanical
digestion
Chemical
digestion
(enzymatic
hydrolysis)
Nutrient
molecules
enter
body cells
Undigested
material
INGESTION1
DIGESTION2
ABSORPTION3
ELIMINATION4
▲ Figure 41.5 The stages of food processing.
Etapas del procesamiento de
alimentos

8. Digestión intracelulas y extracelular
brings food in contact with the enzymes, allowing digestion to
occur safely within a compartment enclosed by a protective
membrane. A few animals, such as sponges, digest their food
entirely by this intracellular mechanism (see Figure 33.4).
Extracellular Digestion
In most animal species, hydrolysis occurs largely by extra-
cellular digestion, the breakdown of food in compartments
that are continuous with the outside of the animal’s body.
Having one or more extracellular compartments for diges-
tion enables an animal to devour much larger pieces of food
than can be ingested by phagocytosis.
Many animals with relatively simple body plans have a
digestive compartment with a single opening (Figure 41.7).
This pouch, called a gastrovascular cavity, functions in
digestion as well as in the distribution of nutrients through-
out the body (hence the vascular part of the term). The
cnidarians called hydras provide a good example of how a
gastrovascular cavity works. A carnivore, the hydra uses its
tentacles to stuff captured prey through its mouth into its
gastrovascular cavity. Specialized gland cells of the hydra’s
gastrodermis, the tissue layer that lines the cavity, then se-
crete digestive enzymes that break the soft tissues of the prey
into tiny pieces. Other cells of the gastrodermis engulf these
mouth and an anus (Figure 41.8). Such a tube is called a
complete digestive tract or, more commonly, an alimentary
canal. Because food moves along the alimentary canal in a
Mouth
Esophagus
Pharynx
Crop
Gizzard
Intestine
Anus
Gizzard
Stomach
(a)
Mouth
Esophagus
Foregut
Crop
Anus
Rectum
Gastric cecae
Midgut Hindgut
(b) Grasshopper. A grasshopper has several digestive chambers
grouped into three main regions: a foregut, with an esophagus and
crop; a midgut; and a hindgut. Food is moistened and stored in the
crop, but most digestion occurs in the midgut. Pouches called
gastric cecae (singular, ceca) extend from the beginning of the
midgut and function in digestion and absorption.
Earthworm. The alimentary canal of an earthworm includes a
muscular pharynx that sucks food in through the mouth. Food
passes through the esophagus and is stored and moistened in the
crop. Mechanical digestion occurs in the muscular gizzard, which
pulverizes food with the aid of small bits of sand and gravel.
Further digestion and absorption occur in the intestine.
Mouth
Food (Daphnia,
a water flea)
Tentacles
1 Digestive
enzymes are
released from a
gland cell.
Extracellular Digestion
In most animal species, hydrolysis occurs largely by extra-
cellular digestion, the breakdown of food in compartments
that are continuous with the outside of the animal’s body.
Having one or more extracellular compartments for diges-
tion enables an animal to devour much larger pieces of food
than can be ingested by phagocytosis.
Many animals with relatively simple body plans have a
digestive compartment with a single opening (Figure 41.7).
This pouch, called a gastrovascular cavity, functions in
digestion as well as in the distribution of nutrients through-
out the body (hence the vascular part of the term). The
cnidarians called hydras provide a good example of how a
gastrovascular cavity works. A carnivore, the hydra uses its
tentacles to stuff captured prey through its mouth into its
gastrovascular cavity. Specialized gland cells of the hydra’s
gastrodermis, the tissue layer that lines the cavity, then se-
crete digestive enzymes that break the soft tissues of the prey
into tiny pieces. Other cells of the gastrodermis engulf these
Mouth
Esophagus
Pharynx
Crop
Gizzard
Intestine
Anus
Mouth
Esophagus
Crop
Gizzard
Intestine
Anus
Stomach
(a)
Mouth
Esophagus
Foregut
Crop
Anus
Rectum
Gastric cecae
Midgut Hindgut
(b) Grasshopper. A grasshopper has several digestive chambers
grouped into three main regions: a foregut, with an esophagus and
crop; a midgut; and a hindgut. Food is moistened and stored in the
crop, but most digestion occurs in the midgut. Pouches called
gastric cecae (singular, ceca) extend from the beginning of the
midgut and function in digestion and absorption.
Earthworm. The alimentary canal of an earthworm includes a
muscular pharynx that sucks food in through the mouth. Food
passes through the esophagus and is stored and moistened in the
crop. Mechanical digestion occurs in the muscular gizzard, which
pulverizes food with the aid of small bits of sand and gravel.
Further digestion and absorption occur in the intestine.
3 Food
2 Enzymes
break food
down into
small particles.
Mouth
Food (Daphnia,
a water flea)
Tentacles
1 Digestive
enzymes are
released from a
gland cell.
3
2
that are continuous with the outside of the animal’s body.
Having one or more extracellular compartments for diges-
tion enables an animal to devour much larger pieces of food
than can be ingested by phagocytosis.
Many animals with relatively simple body plans have a
digestive compartment with a single opening (Figure 41.7).
This pouch, called a gastrovascular cavity, functions in
digestion as well as in the distribution of nutrients through-
out the body (hence the vascular part of the term). The
cnidarians called hydras provide a good example of how a
gastrovascular cavity works. A carnivore, the hydra uses its
tentacles to stuff captured prey through its mouth into its
gastrovascular cavity. Specialized gland cells of the hydra’s
gastrodermis, the tissue layer that lines the cavity, then se-
crete digestive enzymes that break the soft tissues of the prey
into tiny pieces. Other cells of the gastrodermis engulf these
Mouth
Anus
Mouth
Esophagus
Crop
Gizzard
Intestine
Anus
Stomach
(a)
Mouth
Esophagus
Foregut
Crop
Anus
Rectum
Gastric cecae
Midgut Hindgut
(b) Grasshopper. A grasshopper has several digestive chambers
grouped into three main regions: a foregut, with an esophagus and
crop; a midgut; and a hindgut. Food is moistened and stored in the
crop, but most digestion occurs in the midgut. Pouches called
gastric cecae (singular, ceca) extend from the beginning of the
midgut and function in digestion and absorption.
Earthworm. The alimentary canal of an earthworm includes a
muscular pharynx that sucks food in through the mouth. Food
passes through the esophagus and is stored and moistened in the
crop. Mechanical digestion occurs in the muscular gizzard, which
pulverizes food with the aid of small bits of sand and gravel.
Further digestion and absorption occur in the intestine.
(c) Bird. Many birds have a crop for storing food and a stomach and
gizzard for mechanically digesting it. Chemical digestion and
absorption of nutrients occur in the intestine.
3 Food
particles are
engulfed and
digested in food
vacuoles.
2 Enzymes
break food
down into
small particles.
Mouth
GastrodermisEpidermis
Food (Daphnia,
a water flea)
Tentacles
1 Digestive
enzymes are
released from a
gland cell.
3
2
membrane. A few animals, such as sponges, digest their food
entirely by this intracellular mechanism (see Figure 33.4).
Extracellular Digestion
In most animal species, hydrolysis occurs largely by extra-
cellular digestion, the breakdown of food in compartments
that are continuous with the outside of the animal’s body.
Having one or more extracellular compartments for diges-
tion enables an animal to devour much larger pieces of food
than can be ingested by phagocytosis.
Many animals with relatively simple body plans have a
digestive compartment with a single opening (Figure 41.7).
This pouch, called a gastrovascular cavity, functions in
digestion as well as in the distribution of nutrients through-
out the body (hence the vascular part of the term). The
cnidarians called hydras provide a good example of how a
gastrovascular cavity works. A carnivore, the hydra uses its
tentacles to stuff captured prey through its mouth into its
gastrovascular cavity. Specialized gland cells of the hydra’s
gastrodermis, the tissue layer that lines the cavity, then se-
crete digestive enzymes that break the soft tissues of the prey
into tiny pieces. Other cells of the gastrodermis engulf these
Mouth
Esophagus
Pharynx
Crop
Gizzard
Intestine
A
Mouth
Esophagus
Crop
Gizzard
Intestine
Anus
Stomach
(a)
Mouth
Esophagus
Foregut
Crop
A
Rectum
Gastric cecae
Midgut Hindgut
(b) Grasshopper. A grasshopper has several digestive chambers
grouped into three main regions: a foregut, with an esophag
crop; a midgut; and a hindgut. Food is moistened and stored in
crop, but most digestion occurs in the midgut. Pouches called
gastric cecae (singular, ceca) extend from the beginning of th
midgut and function in digestion and absorption.
Earthworm. The alimentary canal of an earthworm includes
muscular pharynx that sucks food in through the mouth. Food
passes through the esophagus and is stored and moistened in
crop. Mechanical digestion occurs in the muscular gizzard, wh
pulverizes food with the aid of small bits of sand and gravel.
Further digestion and absorption occur in the intestine.
(c) Bird. Many birds have a crop for storing food and a stomach
gizzard for mechanically digesting it. Chemical digestion and
absorption of nutrients occur in the intestine.
▲ Figure 41.8 Alimentary canals. These examples illustrate v
3 Food
particles are
engulfed and
digested in food
vacuoles.
2 Enzymes
break food
down into
small particles.
Mouth
GastrodermisEpidermis
Food (Daphnia,
a water flea)
Tentacles
1 Digestive
enzymes are
released from a
gland cell.
3
2
▲ Figure 41.7 Digestion in a hydra. Digestion begins in the gas-
teínas.Ladigestióncontinúaintracelularmentedentrodelas
vacuolasdigestivasylosnutrientesdigeridossedifundenha-
cia otras células. Las contracciones del cuerpo promueven la
expulsión de las partículas de alimento no digeridas a través
de la boca.
Los platelmintos de vida libre comienzan a digerir a su
presa incluso antes de ingerirla. Extienden la faringe a tra-
vés de su boca y secretan enzimas digestivas sobre su presa
(FIGURA 47-2b). Luego de ser ingerido, el alimento entra a
la cavidad gastrovascular ramificada, donde las enzimas con-
tinúan digiriéndola. Las células que revisten la cavidad gas-
trovascular fagocitan fragmentos de alimento parcialmente
digeridos, y la digestión se completa dentro de las vacuolas
digestivas. Así como en los cnidarios, la cavidad digestiva de
los platelmintos tiene una sola abertura, por lo que los dese-
chos no digeridos son expulsados por la boca.
Los sistemas digestivos de la mayoría de
los animales tienen dos aberturas
La mayoría de los invertebrados y todos los vertebrados
poseen un plano corporal de “tubo dentro de un tubo”. La
pared del cuerpo forma el tubo exterior. El tubo interior es
un sistema digestivo con dos aberturas, el cual algunas ve-
ces se conoce como sistema digestivo completo (FIGURA
47-3). El alimento entra por la boca y la comida no digerida
es eliminada por el ano. Los movimientos de mezcla y pro-
pulsión del sistema digestivo se denominan motilidad. La
actividad de propulsión característica de la mayoría de las
regiones del sistema digestivo es la peristalsis, ondas de
contracciónmuscularqueempujanelalimentoenunadirec-
ción. Es posible tomar más alimentos mientras los alimentos
previamente ingeridos están en proceso de digestión y son
absorbidos más abajo en el sistema digestivo. En un sistema
Algunos invertebrados tienen una cavidad
digestiva con una sola abertura
Tentáculo
Boca
Alimento
Cavidad
gastro-
vascular
Gastrodermis
Alimento
absorbido
Enzimas
secretadas por
la gastrodermis
(capa interna)
Alimento
Desechos
(a) Sistema digestivo de una hidra.
Epidermis
(b) Sistema digestivo de un platelminto.
Enzimas
Revestimiento
del intestino
Cavidad
gastrovascular
Partículas de
alimento
Faringe
Boca
Desechos
Alimento
Alimento
absorbido
BucheMolleja
FIGURA 47-2 Animada Sistemas digestivos de
invertebrados simples
Los (a) hidras y los (b) gusanos planos (planarios) tienen una cavi-
dadad gastrovascular, un tracto digestivo con una abertura simple
que sirve como boca y como ano.
▲

9. Oral cavity
Pharynx
Salivary
glands
Mouth
Stomach
Small
intestine
Pancreas
Liver
Large
intestine
Rectum
Anus
Esophagus
A schematic diagram of the
human digestive system
(accessory glands in purple)
Gall-
bladder
Esophagus
Sphincter
Stomach
Duodenum of
small intestine
Sphincter
Órganos y
funciones en el
sistema
digestivo

10. CHAPTER 41 Animal Nutrition 901
cells use an ATP-driven pump to expel hydrogen ions into
the lumen. At the same time, chloride ions diffuse into the
resulting lack of airflow
terial is not dislodged by
Larynx
Trachea
To lungs To stomach
Bolus of
food
Epiglottis
up
Glottis up
and closed
Esophageal
sphincter
contracted
Esophageal
sphincter
relaxed
Esophagus
Tongue
Pharynx
Glottis
Epiglottis
down
(a) Trachea open (b) Esophagus open
hu-
a and
acted
he
olus
ex is
pper
ed
m
he
e
chea
of
mach.
Faringe y esófago

11. Forma y función del estómago
Epithelium
Stomach
pepsinogen. HCl
converts pepsinogen to active pepsin by
molecule and exposing its active site.
and pepsin form in the lumen of the
Mucous cells secrete mucus,
which lubricates and protects
the cells lining the stomach.
Chief cells secrete pepsinogen,
an inactive form of the
digestive enzyme pepsin.
Parietal cells produce
the components of
hydrochloric acid (HCl).
Interior surface of stomach.
The interior surface of the
stomach wall is highly folded
and dotted with pits leading
into tubular gastric glands.
Gastric gland. The gastric
glands have three types of
cells that secrete different
components of the gastric
juice: mucous cells, chief cells,
and parietal cells.
Pepsinogen and HCl
are introduced into the
lumen of the stomach.
The production of
gastric juice
Pepsin then activates
more pepsinogen,
starting a chain
reaction. Pepsin
begins the chemical
digestion of proteins.
HCl converts
pepsinogen to pepsin.
1
3
21
2
3
Pepsin
(active enzyme)
Chief
cell
Parietal
cell
Pepsinogen
HCl
H+
Cl–

12. o garganta,
ionacomo
espiratorio
gar, un pe-
epiglotis,
piratoria.
ren el bolo
cia el estó-
as muscu-
del esófago
parte supe-
acia abajo.
ulos longi-
rte inferior
o contraen,
erguido, la
a mover el
unque esta
stronautas
ia, e inclu-
ada de ca-
stómago.
do
camente
del diente
Esófago
Bolo
alimenticio
Los músculos
longitudinales
se contraen,
acortando el
pasadizo adelante
del bolo
Capa de músculo
relajado
Esfínter
cerrado
Estómago Estómago
Capa de
músculo
relajado
Los músculos
circulares
se contraen,
reduciendo
el pasadizo
y empujando el
bolo hacia delante
Esfínter
abierto
1 2Un bolo es movido por el
esófago por contracciones
peristálticas.
Cuando el esfínter (anillo de
músculo) en la entrada del
estómago se abre, el alimento
entra en él.
aaaaaa
Peristalsis, movimiento del volo alimenticio por medio de
musculos circulares y longitudinales

13. CCK
CCK
Secretin
+
+
+
1 As food arrives at the stomach, it stretches the stomach walls,
triggering release of the hormone gastrin. Gastrin circulates via
the bloodstream back to the stomach, where it stimulates
production of gastric juices.
Gallbladder
Chyme
Liver Food
Stomach
Pancreas
Gastrin
Duodenum of
small intestine
Gastric
juices
Bile
HCO3
–, enzymes
+
2 Chyme—an acidic mixture of partially digested food—eventually
passes from the stomach to the duodenum. The duodenum
responds to amino acids or fatty acids in the chyme by releasing
the digestive hormones cholecystokinin and secretin.
Cholecystokinin (CCK) stimulates the release of digestive enzymes
from the pancreas and of bile from the gallbladder. Secretin
stimulates the pancreas to release bicarbonate (HCO3
–), which
neutralizes chyme.
Secretin
and CCK
CCK
CCK
Secretin
+
+
+
–
1 As food arrives at the stomach, it stretches the stomach walls,
triggering release of the hormone gastrin. Gastrin circulates via
the bloodstream back to the stomach, where it stimulates
production of gastric juices.
Chyme
Pancreas
Gastrin
Duodenum of
small intestine
Gastric
juices
Bile
HCO3
–, enzymes
Gastric
+
2 Chyme—an acidic mixture of partially digested food—eventually
passes from the stomach to the duodenum. The duodenum
responds to amino acids or fatty acids in the chyme by releasing
the digestive hormones cholecystokinin and secretin.
Cholecystokinin (CCK) stimulates the release of digestive enzymes
from the pancreas and of bile from the gallbladder. Secretin
stimulates the pancreas to release bicarbonate (HCO3
–), which
neutralizes chyme.
-
.
t
Secretin
and CCK
CCK
CCK
Secretin
+
+
+
–
1 As food arrives at the stomach, it stretches the stomach walls,
triggering release of the hormone gastrin. Gastrin circulates via
the bloodstream back to the stomach, where it stimulates
production of gastric juices.
3 If the chyme is rich in fats, the high levels of secretin and CCK
released act on the stomach to inhibit peristalsis and secretion of
gastric juices, thereby slowing digestion.
Chyme
small intestine
Bile
HCO3
–, enzymes
Gastric
juices
2 Chyme—an acidic mixture of partially digested food—eventually
passes from the stomach to the duodenum. The duodenum
responds to amino acids or fatty acids in the chyme by releasing
the digestive hormones cholecystokinin and secretin.
Cholecystokinin (CCK) stimulates the release of digestive enzymes
from the pancreas and of bile from the gallbladder. Secretin
stimulates the pancreas to release bicarbonate (HCO3
–), which
neutralizes chyme.
s
-
-
e
m.
e
-
is
ts
st
Regulación de la digestión
Secretin
and CCK
CCK
CCK
Secretin
+
+
+
–
3 If the chyme is rich in fats, the high levels of secretin and CCK
released act on the stomach to inhibit peristalsis and secretion of
gastric juices, thereby slowing digestion.
HCO3
–, enzymes
Gastric
juices
Key Stimulation Inhibition+ –
2 Chyme—an acidic mixture of partially digested food—eventually
passes from the stomach to the duodenum. The duodenum
responds to amino acids or fatty acids in the chyme by releasing
the digestive hormones cholecystokinin and secretin.
Cholecystokinin (CCK) stimulates the release of digestive enzymes
from the pancreas and of bile from the gallbladder. Secretin
stimulates the pancreas to release bicarbonate (HCO3
–), which
neutralizes chyme.
s
s
st
ar
Estimulación de las Gastrina a
la liberación de jugos gástricos
Liberación de bilis (sales
biliares) desde el higado
(=emulsificadores) y HCO3-
(muco) y proteasas desde el
pancreas.
Colecistoquinina (CCK),
supresor del apetito

14. 904 UNIT SEVEN Animal Form and Function
are absorbed by epithelial cells and recombined into triglyc-
erides. They are then coated with phospholipids, cholesterol,
fructose, for example, moves by facilitated diffusion down
its concentration gradient from the lumen of the small
Muscle layers
Intestinal
wall
Villi
Epithelial
cells
Villi Large
circular
folds
Vein carrying
blood to liver
Blood
capillaries
(toward
capillary)
Epithelial
cells
Lymph
vessel
Basal
surface
Microvilli (brush
border) at apical
(lumenal) surface Lumen
Lacteal
Nutrient
absorption
Key
▲ Figure 41.13 Nutrient absorption in the small intestine.
? Tapeworms sometimes infect the human alimentary canal, anchoring themselves to the wall of the
small intestine. Based on how digestion is compartmentalized along the mammalian alimentary canal,
what digestive functions would you expect these parasites to have?
Visit the Study Area
in MasteringBiology for the
BioFlix®
3-D Animation on Mem-
brane Transport.
A N I M AT I O N
Absorción
intestino delgado para la digestión y absorción de nutrientes. La superfi-
cie interna del intestino es incrementada aún más por microvellosida-
des, proyecciones de la membrana plasmática de las células epiteliales
el yeyuno y el íleo. La mayor parte de la digestión química se lleva a
cabo en el duodeno, la primera porción del intestino delgado, y no en el
estómago. Bilis del hígado y enzimas del páncreas se liberan hacia el duo-
El área superficial digestiva y absorbente del intestino delgado aumenta bastante por los millones de
vellosidades semejantes a dedos; el área superficial es incrementada aún más por microvellosidades
sobre las células epiteliales de las vellosidades.
(a) Sección transversal
microscópica de la porción
del yeyuno de la pared del
intestino delgado mostrando
vellosidades
Capa
muscular
Células epiteliales que
revisten las vellosidades
Red
capilar
Fibra nerviosa
Vaso lactífero
Vellosidades
Aberturas de
las glándulas
intestinales
Células de
Goblet
Glándulas
intestinales
Vaso linfático
Mucosa
Submucosa
1 μm
©JubalHarshaur
Cortesía de J. D. Hoskings, W. G. Henk, y Y. Z. Abdelbaki, American Journal
of Veterinary Research, 1982, Vol. 43, No. 10, pp. 1715–1720
PUNTO CLAVE
deno, las cuales actúan sobre el quimo. Luego, enzimas producidas por
las células epiteliales que revisten el duodeno catalizan los pasos finales
en la digestión de los tipos de nutrientes más importantes.
El revestimiento del intestino delgado se ve aterciopelado debido a
sus millones de diminutas proyecciones semejantes a dedos, las vellosi-
dadesintestinales(FIGURA 47-10).Éstasaumentaneláreasuperficialdel
intestino delgado para la digestión y absorción de nutrientes. La superfi-
cie interna del intestino es incrementada aún más por microvellosida-
des, proyecciones de la membrana plasmática de las células epiteliales
e de la digestión enzimática
o en el intestino delgado
mentos es completada en el intestino delgado y los
orbidos a través de su pared. El intestino delgado, que
m de longitud, consta de tres regiones: el duodeno,
o. La mayor parte de la digestión química se lleva a
o, la primera porción del intestino delgado, y no en el
hígado y enzimas del páncreas se liberan hacia el duo-
El área superficial digestiva y absorbente del intestino delgado aumenta bastante por los millones de
vellosidades semejantes a dedos; el área superficial es incrementada aún más por microvellosidades
sobre las células epiteliales de las vellosidades.
(a) Sección transversal
microscópica de la porción
del yeyuno de la pared del
intestino delgado mostrando
vellosidades
©JubalHarshaur
Cortesía de J. D. Hoskings, W. G. Henk, y Y. Z. Abdelbaki, American Journal
LAVE

15. The alimentary canal ends
with the large intestine,
which includes the colon,
cecum, and rectum. The
small intestine connects to
the large intestine at a
T-shaped junction
(Figure 41.15). One arm
of the T is the 1.5-m-long
colon, which leads to the
rectum and anus. The
other arm is a pouch called
the cecum. The cecum is
important for fermenting
ingested material, especially
in animals that eat large
amounts of plant material.
Compared with many other mammals, humans have a small
cecum. The appendix, a finger-like extension of the human
cecum, has a minor and dispensable role in immunity.
The colon completes the reabsorption of water that
began in the small intestine. What remain are the feces, the
wastes of the digestive system, which become increasingly
solid as they are moved along the colon by peristalsis. It
takes approximately 12–24 hours for material to travel the
length of the colon. If the lining of the colon is irritated—by
a viral or bacterial infection, for instance—less water than
normal may be reabsorbed, resulting in diarrhea. The op-
posite problem, constipation, occurs when the feces move
along the colon too slowly. Too much water is reabsorbed,
and the feces become compacted.
The undigested material in feces includes cellulose fiber.
Although it provides no caloric value (energy) to humans,
fiber helps move food along the alimentary canal.
A rich community of mostly harmless bacteria lives on
ming globules called chylomicrons. Being
lomicrons can dissolve in the blood and
ulatory system.
g the bloodstream, chylomicrons are first
an epithelial cell in the intestine into a
t the core of each villus (see Figures 41.13
n
glycerides
down to fatty acids
and monoglycerides.
2 After diffusing into
epithelial cells,
monoglycerides and
fatty acids are
re-formed into triglyc-
erides. (Some glycerol
and fatty acids pass
directly into capillaries.)
3 Triglycerides are
incorporated into
water-soluble globules
called chylomicrons.
4 Chylomicrons leave
epithelial cells by
exocytosis and enter
lacteals, where they
are carried away by
the lymph and later
pass into large veins.
bsorption of fats. Because fats are insoluble in
e needed to digest and absorb them. Bile salts (not
e fat droplets and maintain a small droplet size in
exposing more of the fat at the surface to enzymatic
cids and monoglycerides released by hydrolysis can
cells, where fats are reassembled and incorporated
ylomicrons that enter the lymphatic system.
Ascending
portion
of colon
Appendix
Cecum
Small
intestine
▲ Figure 41.15 Junction of
the small and large intestines.
recovery occurring in the small intestine. There is no mech-
anism for active transport of water. Instead, water is reab-
sorbed by osmosis when sodium and other ions are pumped
out of the lumen of the intestine.
Processing in the Large Intestine
The alimentary canal ends
with the large intestine,
which includes the colon,
cecum, and rectum. The
small intestine connects to
the large intestine at a
T-shaped junction
(Figure 41.15). One arm
of the T is the 1.5-m-long
colon, which leads to the
rectum and anus. The
other arm is a pouch called
the cecum. The cecum is
important for fermenting
ingested material, especially
in animals that eat large
amounts of plant material.
Compared with many other mammals, humans have a small
cecum. The appendix, a finger-like extension of the human
cecum, has a minor and dispensable role in immunity.
The colon completes the reabsorption of water that
began in the small intestine. What remain are the feces, the
LUMEN
OF SMALL
INTESTINE
Epithelial
cell
Lacteal
Phospholipids,
cholesterol,
and proteins
Chylomicron
Fatty acids
Triglycerides
Monoglycerides
Triglycerides 1 In the lumen,
triglycerides (fat
molecules) exposed on
the surface of fat
droplets (not shown)
are subject to
enzymatic hydrolysis.
The enzyme lipase
breaks the triglycerides
down to fatty acids
and monoglycerides.
2 After diffusing into
epithelial cells,
monoglycerides and
fatty acids are
re-formed into triglyc-
erides. (Some glycerol
and fatty acids pass
directly into capillaries.)
3 Triglycerides are
incorporated into
water-soluble globules
called chylomicrons.
4 Chylomicrons leave
epithelial cells by
exocytosis and enter
lacteals, where they
are carried away by
the lymph and later
pass into large veins.
Ascending
portion
of colon
Appendix
Cecum
Small
intestine
▲ Figure 41.15 Junction of
the small and large intestines.
Acción Lipasa (ruptura de los triglicéridos)
Difusión de los monoglicéridos y acidos grasos
Formación de los Quilomicrones
Transporte

16. Resumendeladigestión
Carbohidratos Proteínas Lípidos
Boca Polisacáridos
¡
Amilasasalival
Maltosaypolisacáridospequeños
Estómago SuaccióncontinúahastaqueelpH Proteína
ácidoinactivalaamilasasalival
¡
Pepsina
Polipéptidoscortos
Intestinodelgado Polisacáridosnodigeridos Polipéptidos Gotitadegrasa
¡
Amilasapancreática
¡
Tripsina,quimiotripsina
¡
Salesbiliares
Maltosayotrosdisacáridos Polipéptidosypéptidoscortos Gotitasdegrasaemulsionadas
Maltasa,sacarasa, Carboxipeptidasa, Lipasa
¡
lactasa
¡
peptidasas,dipeptidasas
¡
pancreática
Monosacáridos Aminoácidos Ácidosgrasosyglicerol
TABLA47-1

17. Adpataciones evolutivas del sistema digestivo en vertebrados
906 U N I T S E V E N Animal Form and Function
Dentition, an animal’s assortment of teeth, is one example
of structural variation reflecting diet (Figure 41.16). The
Carnivore
Carnivores, such as members of the dog
and cat families, generally have large,
pointed incisors and canines that can be
used to kill prey and rip or cut away pieces
of flesh. The jagged premolars and molars
crush and shred food.
Incisors MolarsCanines PremolarsKey
Herbivore
Herbivores, such as horses and deer, usually
have premolars and molars with broad,
ridged surfaces that grind tough plant
material. The incisors and canines are
generally modified for biting off pieces of
vegetation. In some herbivores, canines
are absent.
As omnivores, humans are adapted to
eating both plants and meat. Adults have
32 teeth. From front to back along either
side of the mouth are four bladelike incisors
for biting, a pair of pointed canines for
tearing, four premolars for grinding, and six
molars for crushing (see inset, top view).
Omnivore
▼ Figure 41.16 Dentition and diet.
much longer, enhancing the processing of fibrous, protein-
poor eucalyptus leaves from which the koala obtains nearly all
of its nutrients and water.
Stomach
Small intestine
Small
intestine
Cecum
Colon
(large
intestine)Carnivore
Herbivore
Dentadura
Paso particulas de la
serpientes
(b) Un oso herbívoro. Los grandes dientes planos y las bien
desarrolladas mandíbulas y músculos mandibulares del oso
panda gigante (Ailuropoda melanoleuca) son adaptaciones
para moler alimentos vegetales con alto contenido de fibras.
TomMcHugh/PhotoResearchersInc.
(d) Las serpientes son carnívoras. Esta serpiente
(Dromicus sp.) está estrangulando una lagartija de la lava
(Tropidurus sp.). Una serpiente puede engullir presas bastante
grandes debido a la estructura de sus mandíbulas. La muy
FransLanting/MindenPictures
Longitud del intestino

18. Adataciones simbióticas
Boca, dientes incisivos
cortar los pastos
Rumiantes, relaciones
simbióticas con
microorganismos
Redecilla, acción
bacteriana
Libro, acción bacteriana
Cuajar,
alimento
regurgitado,
digestión
Termitas, tracto digestivo
asociado a microbios
Panza
Trychonympha sp
Las heces que son expulsadas por primera vez, blandas (fermentadas por
bacterias en el ciego), las heces expulsadas por segunda vez son secas.
Capibara: Hydrochoerus hydrochaeris