Glomerular filtration, Autoregulation

Subsequent to the ingestion of iodine in various forms, I is absorbed by the small intestine and enters the blood. Two competing pathways are involved in the clearance of I from the blood renal filtration into urine and thyroidal uptake. The renal clearance rate for I (30-50 mL/minute) varies only with the glomerular filtration rate. However, the thyroidal 1 clearance rate is autoregulated to maintain an absolute thyroidal I uptake rate of approximately 100 jig I each day. To accomphsh this, the thyroidal I clearance rate may vary (3 to 100 mL/minute) depending on the concentration of I in the blood. 

Depending on the concentration, volatile anesthetics decrease the glomerular filtration rate and renal blood flow, and increase the filtration fraction. Since renal blood flow decreases during general anesthesia in spite of well-maintained or even increased perfusion pressures (due to increased renal vascular resistance), autoregulation of renal flow may be impaired by these drugs. 

FIGURE 46-2. Normal glomerular autoregulation serves to maintain intraglomerular capillary hydrostatic pressure, glomerular filtration rate (GFR), and ultimately, urine output. This is accomplished by modulation of afferent and efferent arterioles. Afferent and efferent arteriolar vasoconstriction are primarily mediated by angiotensin II, whereas afferent vasodilation is primarily mediated by prostaglandins. 

Filtration is regulated by glomerular blood pressure, which is autoregulated, affected by sympathetic control and hormonal regulation. 

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