Page 315 - Color Atlas of Biochemistry
P. 315
306 Tissues and organs
Liver: functions the form of the polysaccharide glycogen or
converted into fatty acids. When there is a
Weighing 1.5 kg, the liver is one of the largest drop in the blood glucose level, the liver re-
organs in the human body. Although it only leases glucose again by breaking down glyco-
represents 2–3% of the body’s mass, it ac- gen. If the glycogen store is exhausted, glu-
counts for 25–30% of oxygen consumption. cose can also be synthesized by gluconeogen-
esis from lactate, glycerol, or the carbon skel-
eton of amino acids (see p. 310).
A. Diagram of a hepatocyte
Lipid metabolism. The liver synthesizes
The 3 10 11 cells in the liver—particularly the fatty acids from acetate units. The fatty acids
hepatocytes, which makeup90% of thecell formed arethen usedto synthesizefats and
mass—are the central location for the body’s phospholipids, which are released into the
intermediary metabolism. They are in close blood in the form of lipoproteins. The liver’s
contact with the blood, which enters the liver special ability to convert fatty acids into ke-
from the portal vein and the hepatic arteries, tone bodies and to release these again is also
flows through capillary vessels known as si- important (see p. 312).
nusoids, and is collected again in the central Like other organs, the liver also synthesizes
veins of the hepatic lobes. Hepatocytes are cholesterol, which is transported to other tis-
particularly rich in endoplasmic reticulum, sues as a component of lipoproteins. Excess
as they carry out intensive protein and lipid cholesterol is converted into bile acids in the
synthesis. The cytoplasm contains granules of liverordirectly excretedwith the bile (see
insoluble glycogen. Between the hepatocytes, p. 314).
there are bile capillaries through which bile Amino acid and protein metabolism. The
components are excreted. liver controls the plasma levels of the amino
acids. Excessamino acidsare broken down.
With the help of the urea cycle (see p. 182),
B. Functions of the liver
the nitrogen from the amino acids is con-
The most important functions of the liver are: verted into urea and excreted via the kidneys.
1. Uptake of nutrients supplied by the in- The carbonskeletonofthe aminoacids enters
testines via the portal vein. the intermediary metabolism and serves for
2. Biosynthesis of endogenous compounds glucose synthesis or energy production. In
and storage, conversion, and degradation of addition, most of the plasma proteins are syn-
them into excretable molecules (metabolism). thesized or broken down in the liver (see
In particular, the liver is responsible for the p. 276).
biosynthesis and degradation of almost all Biotransformation. Steroid hormones and
plasma proteins. bilirubin, as well as drugs, ethanol, and other
3. Supply of the body with metabolites and xenobiotics are taken up by the liver and in-
nutrients. activated and converted into highly polar me-
4. Detoxification of toxic compounds by tabolites by conversion reactions (see p. 316).
biotransformation. Storage. The liver not only stores energy
5. Excretion of substances with the bile. reserves and nutrients for the body, but also
certain mineral substances, trace elements,
and vitamins, including iron, retinol, and vi-
C. Hepatic metabolism
tamins A, D, K, folic acid, and B 12 .
The liver is involved in the metabolism of
practically all groups of metabolites. Its func-
tions primarily serve to cushion fluctuations
in the concentration of these substances in
the blood, in order to ensure a constant sup-
ply to the peripheral tissues (homeostasis).
Carbohydrate metabolism. The liver takes
up glucose and other monosaccharides from
theplasma. Glucoseisthen eitherstoredin
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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