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Concept
Azotemia
Résumé
Azotemia (azot, "nitrogen" + -emia, "blood condition") is a medical condition characterized by abnormally high levels of nitrogen-containing compounds (such as urea, creatinine, various body waste compounds, and other nitrogen-rich compounds) in the blood. It is largely related to insufficient or dysfunctional filtering of blood by the kidneys. It can lead to uremia and acute kidney injury (kidney failure) if not controlled.
Azotemia has three classifications, depending on its causative origin: prerenal azotemia, renal azotemia, and postrenal azotemia.
Measurements of urea and creatinine (Cr) in the blood are used to assess renal function. For historical reasons, the lab test measuring urea is known as "blood urea nitrogen" (BUN). The BUN:Cr ratio is a useful measure in determining the type of azotemia and will be discussed in each section below. A normal BUN:Cr is equal to 15.
Prerenal azotemia is caused by a decrease in blood flow (hypoperfusion) to the kidneys. However, there is no inherent kidney disease. It can occur following hemorrhage, shock, volume depletion, congestive heart failure, adrenal insufficiency, and narrowing of the renal artery among other things.
The BUN:Cr in prerenal azotemia is greater than 20. The reason for this lies in the mechanism of filtration of urea and creatinine. Renal Plasma Flow (RPF) is decreased due to hypoperfusion which results in a proportional decrease in Glomerular Filtration Rate (GFR). In turn, the decreased flow and pressure to the kidney will be sensed by baroreceptors in the Juxtaglomerular (JG) Cells of the afferent arteriole. If the decrease in blood pressure is systemic (rather than occlusion of the renal artery) baroreceptors in the carotid sinus and aortic arch will be stimulated. This leads to sympathetic nerve activation, resulting in renin secretion through β 1 -receptors. Constriction of the afferent arterioles causes a decrease in the intraglomerular pressure, reducing GFR proportionally. Renin is the main effector of the juxtaglomerular baroreceptors.
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