Biology of Disease

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are bound by transport proteins, such as transferrin for iron (Figure 13.4) and
ceruloplasmin for copper. Table 11.4 summarizes the mechanisms of absorp-
tion of the major nutrients.


Approximately 9 dm^3 of fluid pass through the GIT each day. Reabsorption of
water from the GIT is essential to prevent dehydration. Most, about 95%, is
absorbed by the small intestine, 4% by the large intestine and only 1% is lost
from the GIT.


THE SMALL INTESTINE AND HOMEOSTASIS

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Product of digestion Uptake is by

Monosaccharides Na+-dependent mechanism

Dipeptides H+-dependent mechanism

Amino acids Na+-dependent mechanism

Monoacylglycerols,
monoacylphospholipids,
free fatty acids and cholesterol

lipid soluble and absorbed across enterocyte membrane from
micelles formed with bile salts

Nucleosides Na+-dependent mechanism

Water-soluble vitamins Na+-dependent mechanism

Fat-soluble vitamins lipid soluble and absorbed across enterocyte membrane from
micelles formed with bile salts

Table 11.4Overview of the absorption of nutrients by the small intestine


11.5 Activities of the Large Intestine


The large intestine is so named because its diameter is greater than that of the
small intestine though it is, in fact, much the shorter of the two. Fluid, con-
taining the unabsorbed products of digestion, directly enters from the small
intestine at a junction that is also the site of the vestigial cecum and appen-
dix (Figure 11.22). Absorption of Na+ and water occurs over the surface of the
large intestine, which also acts as a reservoir for material resistant to diges-
tion by GIT enzymes. However, bacterial action on this material releases some
nutrients from food, for example certain vitamins as well as about 200–2000
cm^3 of gas in 10–14 episodes per day. The final waste together with bacteria
forms the feces, which passes to the last section of the GIT, the rectum, and is
eliminated through the anus. Two sphincter muscles control elimination: the
first of smooth muscle opens involuntarily in response to pressure within the
rectum; the second is controlled voluntarily and allows for a conscious deci-
sion to defecate.


11.6 The Small Intestine and Homeostasis


Within enterocytes a portion of the monosaccharides absorbed are converted
to lactate by glycolysis. Excess nonessential amino acids, especially glutamine,
are used to synthesize alanine and ammonia (Figure 11.23). These products are
then delivered to the liver in the hepatic portal vein. Converting some of the
absorbed nutrients to lactate and alanine reduces the metabolic load on the
liver because it can easily regenerate pyruvate from them. Pyruvate is a versa-
tile liver metabolite; it is a substrate for the TCA cycle, allowing the formation
of ATP during oxidative phosphorylation but it can be used for the biosynthe-
sis of glucose and glycogen, ketone bodies, fatty acids and all but two of the
nonessential fatty acids and cholesterol. The GIT is a significant contributor
to nutrient homeostasis both during and after nutrient absorption because
the formation of lactate and alanine continues even when absorption ceases.


Large intestine

Ileocecal valve

Small intestine

Appendix

Cecum

Figure 11.22The junction of the small and large
intestines.
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