Regulation of GFR: autoregulation via tubuloglomerular feedback, neural & hormonal regulation

• When blood pressure is above normal, rapid filtrate flow reduces ion retention so filtrate in tubule has more Na+, C1-, and water. • It is believed that vasoconstricting chemicals from the juxtaglomerular cells are released when the macula densa cells detect higher water and ion levels in the tubule. • These chemicals cause vasoconstriction of the afferent arteriole, thereby reducing blood flow to the glomerular capillaries. • GFR decreases. • Slow filtrate flow increases ion retention so filtrate in the tubule has less Na+, Cl-, and water. • Macula densa cells detect lower water and ion levels in the tubules, inhibiting the release of vasoconstricting chemicals from the juxtaglomerular cells. • The afferent arteriole vasodilates, increasing blood flow to the glomerular capillaries. • GFR increases. • At rest, sympathetic stimulation is weak and renal autoregulation is the dominant mechanism for controlling GFR. • Sympathetic stimulation is most important during extreme rises or falls in blood pressure. • In the case of an extreme drop in blood pressure, sympathetic output increases. • The afferent arterioles vasoconstrict. • The juxtaglomerular cells secrete renin, a chemical that promotes formation of angiotensin II, a potent vasoconstrictor. • The net result of increased sympathetic stimulation is a reduction of blood flow to the glomerular capillaries - a decrease of GFR. • This mechanism eventually reduces urine output, conserving water. • If both blood volume and pressure drop, the walls of the renal arterioles collapse. This causes the juxtaglomerular cells to produce renin. • Renin promotes the formation of angiotensin H. • Increased angiotensin II promotes vasoconstriction of afferent arterioles, reducing blood flow to the glomerular capillaries. • GFR decreases. • A sudden large increase in blood pressure will cause the atria of the heart to stretch. • ANP is released into the bloodstream. • ANP causes the mesangial cells of the glomerulus to relax, increasing the surface area of the capillaries. • Increased filtering area increases OFR.

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