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The formation of urine is a process that begins with glomerular filtration and is greatly influenced by changes in renal hemodynamics. Selective filtration of the blood is possible because of the unique characteristics of the glomerulus and renal circulation. Many factors interact to maintain a consistent blood flow allowing filtration and urine formation to continue despite systemic changes in blood pressure. Factors that impact on renal hemodynamics include the autoregulatory mechanism, the renin-angiotensin mechanism, eicosanoids, kinins, the sympathetic nervous system (SNS), catecholamines, antidiuretic hormone, endothelin, nitric oxide, atrial natriuretic peptide, and dopamine. Knowledge of the effects of these factors will allow the nephrology nurse to predict, identify, and assist in the treatment of clinical conditions that can alter renal hemodynamics and glomerular filtration.
Editor's Note: This article continues a renal physiology continuing education series to run in the Nephrology Nursing Journal. The articles, which are updates of manuscripts that previously appeared in the journal, are written by experts in nephrology and contain the most up-to-date information and research available.
Glomerular filtration is the first step in the complex process of urine formation. For filtration to occur, a rapid renal blood flow (RBF) at a consistent pressure is essential. There are many factors that can alter RBF and, thus, the rate of glomerular filtrate generation. At any given time, especially under the condition of stress, multiple factors act and counteract to maintain a normal glomerular filtration rate (GFR) despite changes in RBF. This article will examine the unique characteristics of the renal circulation, describe the physiology of glomerular filtration, review the extrinsic and intrinsic factors that can alter renal hemodynamics, and discuss a clinical situation in which multiple factors are interacting in an effort to maintain the RBF and GFR despite systemic pressure changes.
Renal Circulation
Blood flows into the kidneys at a rate of about 1,000-1,200 ml per minute, representing approximately 20%-25% of the cardiac output. This rapid blood flow rate exceeds the metabolic and oxygen needs of the kidneys but facilitates efficient clearance of metabolic waste products.
To understand glomerular filtration, it is essential to consider the special characteristics of the renal circulation. Figure 1 illustrates the gross renal circulatory anatomy.
The renal artery pressure is approximately 100 mmHg. This high pressure is...