Introduction The Nitrogens compound present in highest concentration in the blood is urea Urea is the major excretory product of protein metabolism It is formed in the liver from amino groups NH2 ID: 912850
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Slide1
UREA
Muthana
A. Al-
Shemeri
Slide2Introduction
The
Nitrogens
compound present in highest concentration in the blood is urea.
Urea is the major excretory product of protein metabolism. It is formed in the liver from amino groups (-NH2)
and/or
free ammonia generated during protein catabolism.
Since historic assays for urea were based on measurement of nitrogen, the term
blood urea nitrogen (BUN)
has been used to refer to urea determination.
Urea nitrogen (urea N)
is a more appropriate term.
Slide3Physiology
Protein metabolism produces amino acids that can be oxidized to produce energy or stored as fat and glycogen. These processes release nitrogen, which is converted to urea and excreted as a waste product. Following synthesis in the liver, urea is carried in the blood to the kidney, where it is readily filtered from the plasma by the
glomerulus
. Most of the urea in the glomerular filtrate is excreted in the urine,
The concentration of urea in the plasma is determined by renal function and perfusion, the protein content of the diet, and the rate of protein catabolism
Slide4In the International System of Units (SI), urea is reported in units of
millimoles
per liter.
Urea
nitrogen concentration expressed in milligrams per deciliter
(mg/
dL
) may
be converted to urea concentration in
millimoles
per liter by multiplying by 0.357
Slide5Specimen Requirements and Interfering Substances
Urea concentration may be measured in plasma, serum, or urine.
Although
the protein content of the diet influences urea concentration, the effect of a single protein-containing meal is minimal and a fasting sample is not required usually.
A
nonhemolyzed
sample is recommended. Urea is susceptible to bacterial decomposition, so samples (particularly urine) that cannot be analyzed within a few hours should be refrigerated.
Timed
urine samples should be refrigerated during the collection period.
Slide6Reference Intervals
ADULT
Plasma or
serum
15
– 45
mg/
dL
(2.1-7.1 mmol urea/day)
Urine, 24-h
12-20 g/day
(0.43-0.71 mol urea/day)
Slide7Pathophysiology
An elevated concentration of urea in the blood is called
azotemia
. Very high plasma urea concentration accompanied by renal failure is called
uremia
, or
the uremic syndrome
. This condition is eventually fatal if not treated by dialysis or transplantation. Conditions causing increased plasma urea are classified according to cause into three main categories:
prerenal
, renal, and
postrenal
.
Slide8Prerenal
azotemia:
is caused by reduced renal blood flow. Less blood is delivered to the kidney; consequently, less urea is filtered.
Causative
factors
include congestive heart failure, shock, hemorrhage, dehydration, and other factors resulting in a significant decrease in blood volume.
The
amount of protein metabolism also induces prerenal changes in blood urea concentration.
Slide9Renal:
Decreased
renal function causes an increase in plasma urea concentration as a result of compromised urea excretion. Renal causes of elevated urea include acute and chronic renal failure, glomerular nephritis, tubular necrosis, and other intrinsic renal disease.
Postrenal
azotemia:
can be due to obstruction of urine flow anywhere in the urinary tract by renal calculi, tumors of the bladder or prostate, or severe infection.
Slide10conditions affecting plasma urea concentration are summarized below:
INCREASED
CONCENTRATION
Prerenal
Congestive heart failure
Shock, hemorrhage
Dehydration
Increased protein catabolism
High-protein diet
Renal
Acute and chronic renal failure
Renal disease, including glomerular nephritis, tubular necrosis
Postrenal
Urinary tract obstruction
DECREASED CONCENTRATION
Low protein intake Severe vomiting and diarrhea Liver disease Pregnancy