Maintenance of the Body
gradient for its absorption. Anything that interferes with water
absorption (resulting in diarrhea) not only reduces potassium
absorption but also “pulls” K
from the interstitial space into
the intestinal lumen.
For most nutrients, the amount
the intestine is the
amount absorbed, regardless of the nutritional state of the body.
In contrast, absorption of iron and calcium is intimately related
to the body’s need for them at the time.
Ionic iron, essential for hemoglobin production, is actively
transported into the mucosal cells, where it binds to the protein
ĭ-tin). Tis phenomenon is called the
. Te intracellular iron-ferritin complexes then serve as lo-
cal storehouses for iron. When body reserves of iron are adequate,
little (only 10–20%) is allowed to pass into the portal blood, and
most of the stored iron is lost as the epithelial cells later slough
oﬀ. However, when iron reserves are depleted (as during acute
or chronic hemorrhage), iron uptake from the intestine and its
release to the blood accelerate. In the blood, iron binds to
, a plasma protein that transports it in the circulation.
Menstrual bleeding is a major route of iron loss in females, and
premenopausal women require about 50% more iron in their di-
ets. Additionally, the intestinal epithelial cells of women have about
four times as many iron transport proteins as do those of men and
little iron is lost from the body other than that lost in menses.
Calcium absorption is closely related to blood levels of ionic
calcium. Te active form of
promotes active calcium
absorption. Decreased blood levels of ionic calcium prompt
) release from the parathyroid glands. Be-
sides facilitating the release of calcium ions from bone matrix and
enhancing the reabsorption of calcium by the kidneys, P±H stim-
ulates activation of vitamin D to calcitriol by the kidneys, which
in turn accelerates calcium ion absorption in the small intestine.
Approximately 9 L of water, mostly derived from GI tract secre-
tions, enter the small intestine daily. Water is the most abun-
dant substance in chyme, and 95% of it is absorbed in the small
intestine by osmosis. Most of the rest is absorbed in the large
intestine, leaving only about 0.1 L to so²en the feces.
Te normal rate of water absorption is 300 to 400 ml per
hour. Water moves freely in both directions across the intesti-
nal mucosa, but
occurs whenever a concentration
gradient is established by the active transport of solutes (par-
) into the mucosal cells. In this way, water uptake
is eﬀectively coupled to solute uptake and, in turn, aﬀects the
absorption of substances that normally pass by diﬀusion. As
water moves into mucosal cells, these substances follow along
their concentration gradients.
Malabsorption of Nutrients
, or impaired nutrient absorption, has many and
varied causes. It can result from anything that interferes with the
delivery of bile or pancreatic juice to the small intestine. Factors
that damage the intestinal mucosa (severe bacterial infections
and some antibiotics) or reduce its absorptive surface area are
also common causes.
A common malabsorption syndrome is
, which aﬀects one in 100 people. Tis
chronic genetic condition is caused by an immune reaction to
gluten, a protein plentiful in all grains but corn and rice. Break-
down products of gluten interact with molecules of the immune
system in the GI tract, forming complexes. Tese complexes ac-
tivate ± cells, which then attack the intestinal lining, damaging
intestinal villi and reducing the surface area of the brush border.
Bloating, diarrhea, pain, and malnutrition result.
Te usual treatment is to eliminate gluten-containing grains
from the diet. Not many children can envision a world without
cookies or pizza, but these are just two of the foods people with
celiac disease must avoid.
Check Your Understanding
What type of chemical reaction is the basis of all enzymatic
Fill in the blank: Amylase is to starch as ___ is to fats.
What is the role of bile salts in the digestive process? In
For answers, see Appendix H.
of the Digestive System
Describe embryonic development of the digestive system.
Describe abnormalities of the gastrointestinal tract at
different stages of life.
As we have described many times before, the very young
embryo is ﬂat and consists of three germ layers. From top to
bottom, they are ectoderm, mesoderm, and endoderm. Tis
ﬂattened cell mass soon folds to form a cylindrical body, and
its internal cavity becomes the cavity of the alimentary canal,
which is initially closed at both ends.
Te epithelial lining of the developing alimentary canal, or
, forms from endoderm
. Te rest of the
wall arises from mesoderm. Te anteriormost endoderm (that of
the foregut) touches a depressed area of the surface ectoderm called
um; “on the way to becoming the
mouth”). Te two membranes fuse, forming the
which soon breaks through to form the opening of the mouth. Simi-
larly, the end of the hindgut fuses with an ectodermal depression,
anus), to form the
sewer), which then breaks through to form the anus.
By week 5, the alimentary canal is a continuous “tube” extend-
ing from mouth to anus and opens to the external environment at
each end. Shortly a²er, the glandular organs (salivary glands, liver
with gallbladder, and pancreas) bud out from the mucosa at vari-
ous points (Figure 23.36b). Tese glands retain their connections,
which become ducts leading into the digestive tract.
Te digestive system is susceptible to many congenital defects
that interfere with feeding. Te most common are