section 22.2
Metabolic Roles of Organs
487
oxidation for energy. The rate-limiting enzymes of glu-
coneogenesis (pyruvate carboxylase, phosphoenolpyru-
vate carboxykinase, and fructose bisphosphatase) show
high activities in the liver and kidney, but very low gly-
colytic activities reflect the fact that the liver and kid-
ney are glucose producers whereas the other tissues are
glucose users. The overall metabolic patterns in the var-
ious tissues and organs of the body are illustrated in
Figure 22-1.
22.2 Metabolic Roles of Organs
Liver
The liver (see also Chapter 12) in a normal, 70-kg human
adult weighs approximately 1.5 kg, or about 2% of body
weight. (Metabolic fluxes quoted throughout this chapter
refer to the typical 70-kg human.) Its major cell type,
the hepatocyte, is of epithelial origin. Interposed between
the intestinal tract and the general circulation, the liver
is uniquely related to the endocrine pancreas; insulin and
glucagon, released from the pancreas, play a prominent
role in metabolic homeostasis. The liver receives a large
volume of blood from the intestinal tract via the portal vein
and a small volume of blood via the hepatic artery, and it
drains via the hepatic veins into the inferior vena cava. It
is the first organ to meet nutrients delivered from the intes-
tine, with the exception of lipid, and to meet the secreted
insulin and glucagon. By delivering bile (which contains
bile acids and cholesterol) into the intestine, the liver pre-
dominates in cholesterol homeostasis. It has the great-
est metabolic flexibility of any organ and shows tremen-
dous adaptability to fluxes of metabolites and nutrients.
It rapidly undergoes changes in size and glycogen and
protein content In the fed state, 5-10% of its wet weight
consists of glycogen but after a 24-hour fast the glyco-
gen disappears almost entirely. After a day or two on a
high-protein diet, the liver shows a large increase in activ-
ity of enzymes involved in amino acid metabolism and in
gluconeogenesis. A high-carbohydrate diet produces the
opposite effect. The liver is the primary site of glycogen
deposition and blood glucose maintenance. It also plays
a central role in lipid, protein, and nitrogen homeosta-
sis. In the typical adult, the liver exports daily 180 g of
glucose, 100 g of triacylglycerol, and 14 g of albumin.
Its metabolic energy is derived primarily from fatty acid
oxidation.
Adipose Tissue
Cosmetically, adipose tissue is viewed as an enemy; how-
ever its importance in energy homeostasis is second only
C y to p la s m ic p e r ip h e r a l rim
N u c le u s
F I G U R E 2 2 - 2
Schematic representation of a white adipocyte.
to that of the liver. The typical adult has 13 kg of adipose
tissue located extensively under the skin, in the abdomi-
nal cavity, around internal blood vessels, in skeletal
muscle, and in mammary glands. Obesity results when
this amount increases. White and brown adipocytes,
the two types of fat cell, are schematically depicted in
Figures 22-2 and 22-3. White adipocytes have a charac-
teristic spherical shape with a large central droplet of tri-
acylglycerol surrounded by a thin rim of cytoplasm, and
nucleus, mitochondria, and endoplasmic reticulum situ-
ated peripherally. Brown adipocytes have multiple lipid
droplets and numerous mitochondria, which impart a
F a t
F I G U R E 2 2 -3
Schematic representation of a brown adipocyte.
