Medullary washout

Describe the phenomenon of medullary washout caused by loop diuretics. 

By extracting water from intracellular compartments, osmotic diuretics expand the extracellular fluid volume, decrease blood viscosity, and inhibit renin release. These effects increase RBF, and the increase in renal medullary blood flow removes NaCl and urea from the renal medulla, thus reducing medullary tonicity. Under some circumstances, prostaglandins may contribute to the renal vasodilation and medullary washout induced by osmotic diuretics. A reduction in medullary tonicity causes a decrease in the extraction of water from the DTL, which limits the concentration of NaCl in the tubular fluid entering the ATL. This latter effect diminishes the passive reabsorption of NaCl in the ATL. In addition, osmotic diuretics may also interfere with transport processes in the TAL. 

Because of the potential for ANF to be involved in blood pressure control, several investigators have studied its effects on the cardiovascular system of normal and hypertensive animals. In anesthetized normotensive and spontaneously hypertensive rats, atrial extract decreased arterial blood pressure in association with a decrease in both total peripheral resistance and cardiac contractility.il Also, a negative chronotropic effect has been reported in rats.H Dahl salt-sensitive rats appear to have greater amounts of ANF in their atria, but are hyporesponsive when exogenous material is injected.90 Results from this study also suggest an increase in renal papillary plasma flow and a washout of the medullary osmotic gradient, factors which could contribute significantly to the associated natriuresis and diuresis. Increased medullary and inner cortical blood flow also were reported in normotensive rats.118... 

---