Renal efferent glomerular arteriolar constriction, mediated by sympathetic stimulation and angiotensin II, helps maintain glomerular filtration in the face of reduced cardiac output and renal blood flow. Higher oncotic and lower hydrostatic pressures develop in the peritubular capillaries, enhancing the reabsorption of tubular fluid and sodium. Angiotensin II - mediated aldosterone release further promotes sodium and water retention. Continued activation of these mechanisms leads to clinical edema and effusions.

Afferent arteriolar vasodilatation mediated by endogenous prostaglandins and natriuretic peptides can partially offset the effects of efferent vasoconstriction, but progressive impairment of renal blood flow leads to renal insufficiency. Diuretics not iaconly can magnify azotemia and electrolyte loss, they also can further reduce cardiac output and activate the neurohormonal mechanisms.

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The kidney is a remarkable organ charged with a diverse set of responsabilities in maintaining the homeostasis of the body. In mammals, the two kidneys receive approximately 25% of the cardiac output. Not only must the kidney filter this blood in order to excrete metabolic waste, but it must also retrieve those filtered materials that are needed by the body, including low-molecular-weight proteins, water and a variety of electrolytes. It must recognize when water and specific electrolytes are present in excess and respond by failing to reabsorb or by secreting these substances.

It contributes substantially to the maintenance of acid-base homeostasis. In addition, the production and release of hormones by the kidney play a vital role in the control of systemic blood pressure and red blood cell production. In order to accomplish these tasks, the kidney is composed of an extensive variety of cell types, each endowed with an individual set of functions and design to respond as needed to a complex battery of direct and indirect signals. These cells are arranged in a particular pattern to form the functional unit of the kidney, the nephron. The nephron is composed of the glomerulus, where the blood is filtered, and various distinct segments of the renal tubule, where filtered substances are absorbed from and plasma components are secreted into the tubular fluid. In the cortex the nephrons join with the collecting duct system, which traverses the kidney and ends with the inner medullary collecting duct, where the final alterations of the tubular fluid take place in the formation of urine.

The specific functions of the kidney and the methods by which they are accomplished are quite complex. Most of the experimental evidence for what we believe about renal physiology has been obtained in the rat or the rabbit. Much of what is currently believed about renal physiology remains subject to revision and reinterpretation as more information is gathered.

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