UREA Muthana A. Al- Shemeri - PowerPoint Presentation

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UREA

Muthana

A. Al-

Shemeri

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)

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.

Physiology

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

In 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

Specimen 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.

Reference 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)

Pathophysiology

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

.

Prerenal

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.

Renal:

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.

conditions 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