Azotemia occurs when-
Correct Answer: GFR 20% - 50% of normal
Description: 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 hea failure, adrenal insufficiency, and narrowing of the renal aery among other things. The BUN:Cr in prerenal azotemia is greater than 20. The reason for this lies in the mechanism of filtration of BUN and creatinine. Renal Plasma Flow (RPF) is decreased due to hypoperfusion which results in a propoional decrease in GFR. In turn, the decreased flow and pressure to the kidney will be sensed by baroreceptors in the Juxtaglomerular (JG) Cells of the afferent aeriole. If the decrease in blood pressure is systemic (rather than occlusion of the renal aery) baroreceptors in the carotid sinus and aoic arch will be stimulated. This leads to sympathetic nerve activation, resulting in renin secretion through b 1 -receptors. Constriction of the afferent aerioles causes a decrease in the intraglomerular pressure, reducing GFR propoionally. Renin is the main effector of the juxtaglomerular baroreceptors. Renin is secreted from granules in the JG cells, and once in the blood stream, it acts as a protease to conve angiotensinogen to angiotensin I, which is conveed by angiotensin conveing enzyme, to angiotensin II, which, in turn, stimulates aldosterone release. Increased aldosterone levels results in salt and water absorption in the distal collecting tubule. GFR = 20-50 of normal GFR A decrease in volume or pressure is a nonosmotic stimulus for antidiuretic hormone production in the hypothalamus, which exes its effect in the medullary collecting duct for water reabsorption. Through unknown mechanisms, activation of the sympathetic nervous system leads to enhanced proximal tubular reabsorption of salt and water, as well as urea (BUN), calcium, uric acid, and bicarbonate. The net result of these 4 mechanisms of salt and water retention is decreased output and decreased urinary excretion of sodium (< 20 mEq/L). The increased reabsorption of Na leads to increased water and urea reabsorption from the proximal tubules of the kidney back into the blood. In contrast, creatinine is actually secreted in the proximal tubule. This generally leads to a BUN:Cr ratio > 20 and a fractional excretion of Na of < 1% and an elevated urine osmolarity Ref Harrison20th edition pg 277
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