Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
927
24
Homeostatic Imbalance 24.2
When ketone bodies accumulate in the blood,
ketosis
results
and large amounts of ketone bodies are excreted in the urine.
Ketosis is a common consequence of starvation, unwise dieting
(too little carbohydrate), and diabetes mellitus.
Because most ketone bodies are organic acids, ketosis leads
to
metabolic acidosis
. Te body’s buffer systems cannot tie up
the acids (ketones) fast enough, and blood pH drops to danger-
ously low levels. Te person’s breath smells fruity as acetone
vaporizes from the lungs, and breathing becomes more rapid
as the respiratory system tries to reduce blood carbonic acid by
blowing off CO
2
to force the blood pH up. In severe untreated
cases, the person may become comatose or even die as the acid
pH depresses the nervous system.
Synthesis of Structural Materials
All body cells use phospholipids and cholesterol to build their
membranes. Phospholipids are important components of the my-
elin sheaths of neurons, and the ovaries, testes, and adrenal cortex
use cholesterol to synthesize their steroid hormones. In addition,
the liver:
Synthesizes lipoproteins to transport cholesterol, fats, and
other substances in the blood
Synthesizes cholesterol from acetyl CoA
Uses cholesterol to form bile salts
sugar is high, lipogenesis is the major activity in adipose tis-
sues and is also an important liver function.
Lipolysis
Lipolysis
(lĭ-pol
9
ĭ-sis; “fat splitting”), the breakdown of stored
fats into glycerol and fatty acids, is essentially lipogenesis in re-
verse (Figure 24.15, blue arrows). Te fatty acids and glycerol
are released to the blood, helping to ensure that body organs
have continuous access to fat fuels for aerobic respiration. (Te
liver, cardiac muscle, and resting skeletal muscles actually prefer
fatty acids as an energy fuel.)
Te adage “Fats burn in the flame of carbohydrates” becomes
clear when carbohydrate intake is inadequate. Under such con-
ditions, lipolysis accelerates as the body attempts to fill the fuel
gap with fats. However, the ability of acetyl CoA to enter the
Krebs cycle depends on the availability of oxaloacetic acid to act
as the pickup molecule (see Figure 24.7). When carbohydrates
are deficient, oxaloacetic acid is converted to glucose (to fuel the
brain). Without oxaloacetic acid, fat oxidation is incomplete,
and acetyl CoA accumulates.
Via a process called
ketogenesis
, the liver converts acetyl
CoA molecules to
ketone bodies
, or
ketones
, which are re-
leased into the blood. Ketone bodies include acetoacetic acid,
β-hydroxybutyric acid, and acetone. (Te
keto acids
cycling
through the Krebs cycle and the
ketone bodies
resulting from
fat metabolism are quite different and should not be confused.)
Lipolysis
Electron
transport
chain
Cholesterol
Stored fats in
adipose tissue
Dietary fats
Glycerol
Glycolysis
Glyceraldehyde 3-phosphate
Glucose
Pyruvic acid
Acetyl CoA
CO
2
+ H
2
O
+
Steroids
Bile salts
Fatty acids
Ketone
bodies
Lipolysis
β
Oxidation
Triglycerides
(neutral fats)
ATP
Certain
amino
acids
Ketogenesis (in liver)
Catabolic reactions
Anabolic reactions
Lipogenesis
Krebs
cycle
Figure 24.15
Metabolism of
triglycerides.
When needed for energy, fats
enter catabolic pathways. Glycerol enters the
glycolytic pathway and beta oxidation breaks
down the fatty acids to acetyl CoA, which
enters the Krebs cycle. When fats are to be
synthesized (lipogenesis) and stored, the
intermediates are drawn from glycolysis and
the Krebs cycle in a reversal of the processes
noted above. Excess dietary fats are stored
in adipose tissue. When triglycerides are the
primary energy source, the liver releases their
breakdown products in the form of ketone
bodies. Excessive amounts of carbohydrates
and amino acids are also converted to
triglycerides (lipogenesis).
previous page 961 Human Anatomy and Physiology (9th ed ) 2012 read online next page 963 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off