Chapter 23
The Digestive System
899
23
rooting reflex
helps the infant find the nipple, and the
sucking
reflex
helps the baby hold onto the nipple and swallow.
Newborn babies tend to double their birth weight within six
months, and their caloric intake and food processing ability are
extraordinary. For example, a 6-week-old infant weighing about
4 kg (less than 9 lb) drinks about 600 ml of milk daily. A 65-kg
adult (143 lb) would have to drink 10 L of milk to ingest a cor-
responding volume of fluid! However, the stomach of a newborn
infant is very small, so feeding must be frequent (every 3–4 hours).
Peristalsis is inefficient, and vomiting is not unusual. As the teeth
break through the gums, the infant progresses to solid foods and is
usually eating an adult diet by the age of 2 years.
As a rule, the digestive system operates throughout childhood
and adulthood with relatively few problems. However, contami-
nated food or extremely spicy or irritating foods sometimes cause
gastroenteritis
(gas
0
tro-en
0
tĕ-ri
9
tis), inflammation of the GI tract.
Ulcers and gallbladder problems—inflammation or
cholecystitis
(ko
0
le-sis-ti
9
tis), and gallstones—are problems of middle age.
Aging and the Digestive System
During old age, GI tract activity declines. Fewer digestive juices
are produced, absorption is less efficient, and peristalsis slows.
Te result is less frequent bowel movements and, o±en, consti-
pation. ²aste and smell are less acute, and periodontal disease
o±en develops. Many elderly people live alone or on a reduced
income. Tese factors, along with increasing physical disability,
tend to make eating less appealing, and many of our elderly
citizens are poorly nourished.
Diverticulosis, fecal incontinence, and cancer of the GI
tract are fairly common problems of the aged. Stomach and
colon cancers rarely have early signs, and o±en metastasize
in which the palatine bones or palatine processes of the maxillae
(or both) fail to fuse, and
clef lip
, which o±en occur together.
Of the two, cle± palate is far more serious because the child is
unable to suck properly.
Another common defect is
tracheoesophageal fistula
, in
which there is an opening between the esophagus and the tra-
chea, and the esophagus o±en lacks a connection to the stom-
ach. Te baby chokes and becomes cyanotic during feedings
because food enters the respiratory passageways. Surgery can
usually correct these defects.
Cystic fibrosis (described in more detail in Chapter 22,
p. 842) primarily affects the lungs, but it also impairs the activ-
ity of the pancreas. In this genetic disease, the mucous glands
produce abnormally thick mucus, which blocks ducts and pas-
sageways of organs. Blockage of the pancreatic duct prevents
pancreatic juice from reaching the small intestine. Tis impairs
the digestion of chyme, and most fats and fat-soluble vitamins
are not digested or absorbed. Consequently the stools are bulky
and fat laden. Te pancreatic problems can be handled by ad-
ministering pancreatic enzymes with meals.
During fetal life, the developing infant receives all of its nu-
trients through the placenta. Nonetheless, the fetal GI tract is
“trained” in utero for future food digestion as the fetus natu-
rally swallows some of the surrounding amniotic fluid. Tis
fluid contains several chemicals that stimulate GI maturation,
including gastrin and epidermal growth factor.
Digestive System After Birth
Feeding is a newborn baby’s most important activity, and sev-
eral reflexes enhance the infant’s ability to obtain food: Te
Stomodeum
Foregut
Site of liver
development
Midgut
Spinal cord
Hindgut
Proctodeum
Endoderm
Brain
Oral membrane
Heart
Yolk sac
Cloacal
membrane
Body stalk
(a)
Lung bud
Liver
Gall-
bladder
Cystic duct
Ventral pancreatic bud
Dorsal
pancreatic
bud
Duodenum
Stomach
(b)
Bile
duct
Figure 23.36
Embryonic development of the digestive system.
(a)
Three-week-old
embryo. The endoderm has folded, and the foregut and hindgut have formed. (The midgut is
still open and continuous with the yolk sac.)
(b)
By five weeks of development, the accessory
organs are budding out from the endodermal layer, as shown in the enlargement.
previous page 933 Human Anatomy and Physiology (9th ed ) 2012 read online next page 935 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off