The Digestion and Absorption of Food
533
tion of the material entering the lumen of the gastrointestinal
tract. An additional 7000 ml of fl
uid from salivary glands, gas-
tric glands, pancreas, liver, and intestinal glands are secreted
into the tract each day (
Figure 15–5
). Of the approximately
8 L of fl uid entering the tract, 99 percent is absorbed; only
about 100 ml are normally lost in the feces. This small amount
of fl uid loss represents only 4 percent of the total fl
uids lost by
the body each day. Most fl uid loss is via the kidneys and respi-
ratory system. Almost all the salts in the secreted fl
uids are
also reabsorbed into the blood. Moreover, the secreted diges-
tive enzymes are themselves digested, and the resulting amino
acids are absorbed into the blood.
Finally, a critical component is the role of the central ner-
vous system in the control of gastrointestinal functions. The
Table 15–2
Summary of Liver Functions
A.
Exocrine (digestive) functions (Chapter 15)
1. Synthesizes and secretes bile salts, which are necessary for adequate digestion and absorption of fats.
2. Secretes into the bile a bicarbonate-rich solution, that helps neutralize acid in the duodenum.
B.
Endocrine functions
1. In response to growth hormone, secretes insulin-like growth factor I (IGF-I), which promotes growth by stimulating cell division in
various tissues, including bone (Chapter 11).
2. Contributes to the activation of vitamin D (Chapter 11).
3. Forms triiodothyronine (T
3
) from thyroxine (T
4
) (Chapter 11).
4. Secretes angiotensinogen, which renin acts upon to form angiotensin I (Chapter 14).
5. Metabolizes hormones (Chapter 11).
6. Secretes cytokines involved in immune defenses (Chapter 18).
C.
Clotting functions
1. Produces many of the plasma clotting factors, including prothrombin and fi brinogen (Chapter 12).
2. Produces bile salts, which are essential for the gastrointestinal absorption of vitamin K, which is, in turn, needed for the production of
the clotting factors (Chapter 12).
D.
Plasma proteins
1. Synthesizes and secretes plasma albumin (Chapter 12), acute phase proteins (Chapter 18), binding proteins for various hormones
(Chapter 11) and trace elements (Chapter 12), lipoproteins (Chapter 16), and other proteins mentioned elsewhere in this table.
E.
Organic metabolism (Chapter 16)
1. Converts plasma glucose into glycogen and triglycerides during absorptive period.
2. Converts plasma amino acids to fatty acids, which can be incorporated into triglycerides during absorptive period.
3. Synthesizes triglycerides and secretes them as lipoproteins during absorptive period.
4. Produces glucose from glycogen (glycogenolysis) and other sources (gluconeogenesis) during postabsorptive period and releases the
glucose into the blood.
5. Converts fatty acids into ketones during fasting.
6. Produces urea, the major end product of amino acid (protein) catabolism, and releases it into the blood.
F.
Cholesterol metabolism (Chapter 16)
1. Synthesizes cholesterol and releases it into the blood.
2. Secretes plasma cholesterol into the bile.
3. Converts plasma cholesterol into bile salts.
G.
Excretory and degradative functions
1. Secretes bilirubin and other bile pigments into the bile (Chapter 15).
2. Excretes, via the bile, many endogenous and foreign organic molecules as well as trace metals (Chapter 18).
3. Biotransforms many endogenous and foreign organic molecules (Chapter 18).
4. Destroys old erythrocytes (Chapter 12).
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