Page 471 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid, Electrolyte, and Acid-Base Disturbances in Liver Disease 459

Urea diffusion from
systemic blood to alimentary canal
Urease
Urea NH 3

Amines Nucleic acids
Gut generation
via bacteria Amino acids
Glutaminase Glutamate
Dehydrogenase
Glutamine Glutamine NH Glutamate
Synthetase 3
Oxaloacetate
1 Transaminase
HCO 3 NH 4
2
-ketoglutarate
Carbomyl phosphate
Aspartate
Citrulline Aspartate
Ornithine
5 Urea 3 TCA
Carbomyl cycle cycle
aspartate
Arginine Argininosuccinate
4
Urea
Orotic acid
Fumarate
Pyrimidines
1: Carbomyl phosphate synthetase 1
2: Ornithine transcarbamylase
3: Argininosuccinate synthetase
4: Argininosuccincase
5: Arginase
Figure 19-2 Diagram showing the biochemical reactions involved with nitrogenous waste production,
detoxification, and elimination in the liver. See text for explanations.

already protonated. 55 An alternative view is that urinary Hepatic albumin production fluctuates depending on
þ
excretion of NH 4 represents a mechanism by which physiologic conditions and requirements (Figure 19-3).
the liver is deprived of substrates for urea synthesis, The most important variables are nutrition and interstitial
resulting in less bicarbonate neutralization and mitigation osmoticpressureassensedbythehepatocyte. 178 Theinflu-
of acidosis. According to this hypothesis, the kidneys ence of nutrition on albumin production can be dramatic.
determine the route of nitrogen disposal, whereas the Albuminsynthesisdecreasesby50%within24hoursaftera
liver plays a more active role in systemic acid-base balance. fast or with consumption of a protein-deficient diet.
Serum albumin concentration reflects this change only
SERUM PROTEINS: ALBUMIN AND after a lag period ranging from days to weeks as a new bal-
GLOBULINS ance is achieved between exchangeable albumin pools.
Albumin Feeding excessive calories in a protein-restricted ration
Albumin accounts for 25% of the proteins synthesized by augments development of hypoalbuminemia, as does die-
115,132,179
the liver. 151 Serum albumin concentration reflects the net tary depletion of branched-chain amino acids.
result of synthesis by hepatocytes, systemic distribution, Hypoalbuminemia, caused in part by reduced albumin
and degradation. Being relatively small in size (66,000 synthesis, also can be a consequence of changes in serum
Da), albumin can be lost from the circulation through oncotic pressure related to hyperglobulinemia and treat-
66,178
pathologically altered vessels (e.g., vasculitis), gut wall ment with synthetic colloids (e.g., dextran). Synthe-
(e.g., lymphangiectasia), or glomeruli (e.g., glomerulo- sis of albumin also decreases, sometimes dramatically,
nephritis, amyloidosis) or into the peritoneal cavity as a during critical illness as part of a negative acute-phase
32,38
result of hepatic sinusoidal hypertension. Impaired or response.
down-regulated hepatic albumin synthesis or losses Hepatocellular synthesis of albumin is affected by a
exceeding synthetic capability result in hypoalbuminemia number of factors, the most important of which is the
179
of variable severity. The liver has a tremendous reserve COP of the hepatic interstitial matrix. A decrease in
capacity for albumin synthesis. 179 Normally, only 20% COP stimulates albumin production whereas an increase
to 30% of the hepatocytes produce albumin, and synthesis in COP results in decreased albumin synthesis. After syn-
can be increased as needed by a factor of 200% to 300%. 75 thesis in the hepatocyte, albumin is released into the space