section
22.2
Metabolic Roles of Organs
489
demand is used to maintain the transmembrane poten-
tial by action of membrane enzymes such as Na+,K+-
ATPase. In the well-fed human, the brain uses glucose
exclusively for energy (with little or no formation of
lactate) and depends on its continual supply, since it stores
little glycogen. A decrease in the blood glucose level be-
low ~ 50 mg/L can cause symptoms of dizziness and light-
headedness. More marked drops lead to coma and death if
uncorrected.
Heart
Although the molecular design of the contractile appa-
ratus of heart muscle is very similar to that of skele-
tal muscle, the two tissues differ metabolically. Under
essentially all circumstances, metabolism in heart mus-
cle is aerobic and uses fatty acids or ketone bodies, lac-
tate, and pyruvate. The heart stores some glycogen but
appears to utilize it only when occlusion of a coronary
artery prohibits aerobic metabolism. In starvation, car-
diac muscle increases its endogenous glycogen stores
approximately twofold. The heart can function well on
glucose, oxidizing it anaerobically or aerobically, but if
presented with glucose and fatty acids, it preferentially
uses the fatty acids for its energy needs. Cardiac mus-
cle does not contribute significantly to fuel homeostasis,
but its ability to utilize any metabolic fuel makes it a
“scavenger.”
Kidneys
The kidneys (see also Chapter 39) eliminate noxious ma-
terial while preserving important metabolites; normally,
very little glucose, ketone bodies, or amino acids is wasted.
Renal tissue is important in amino acid homeostasis (see
below) and has the same gluconeogenic capacity per gram
of tissue as does liver, although the liver provides a larger
amount because of its larger size. About 80% of the total
energy produced by the kidneys is utilized in the active
transport processes involved in urine formation. It readily
oxidizes fatty acids, ketone bodies, glucose, and amino
acids.
Gastrointestinal System
The principal nutrients of the body are of the same type
as those that it uses as stores, namely, complex carbohy-
drates, triacylglycerol, and protein. These are hydrolyzed
within the intestinal tract to produce primarily mono- and
disaccharides, amino acids and small peptides, glycerol,
and free fatty acids (Chapter 12).
Blood and Other Body Fluids
Blood distributes metabolic fuels among tissues. Approxi-
mately 40-50% of the body fluid of an adult is intracellular
and separated from other fluid compartments by the cell
membranes whose properties and transport systems de-
termine which metabolites pass across them. Figure 22-5
shows the relationships and volumes of the fluid compart-
ments of the body. If homeostasis is well maintained, the
plasma concentration of a metabolite does not change be-
cause its utilization by one organ is matched by its release
from another; thus, a low plasma concentration does not
indicate a low flux.
Simple monosaccharides (principally glucose) are the
means for transporting carbohydrates. Glycogen and phos-
phorylated sugars do not occur to any significant extent
in plasma. All 20 amino acids used in protein synthe-
sis are present in plasma but at ratios that do not re-
flect those found in proteins. Particularly high are the
levels of alanine and glutamine, which are important
carriers of carbon and nitrogen between muscle, liver,
and other tissues (Chapter 17). Organic acids present
as anions include lactate and pyruvate (from anaerobic
glycolysis in muscle and red blood cells), acetoacetate,
/1
-hydroxy butyrate (produced by oxidation of fatty acids),
and several intermediates of the TCA cycle, although lit-
tle importance has been attached to their interorgan trans-
port. Other constituents of plasma reflect the metabolism,
function, and metabolic processes of particular organs.
Thus, urea, synthesized in liver, is the major vehicle for
eliminating excess nitrogen; bilirubin is a reticuloendothe-
lial cell product of porphyrin degradation; creatinine and
its metabolite creatine, which is the primary elimina-
tion product, are derived almost entirely from muscle,
where phosphocreatine serves as an immediate source
of energy for contraction (Chapter 21). These specific
plasma constituents can be assayed to evaluate disease
processes.
Macromolecules found in plasma include simple pro-
teins, metalloproteins, glycoproteins, and lipoproteins. Al-
bumin and the lipoproteins play a special role in metabolic
I n te s tin a l
a b s o r p tio n
F I G U R E 2 2 -5
Blood-fluid relationships. Volumes are expressed as a percentage of body
weight.
