Glomerular Filtration Rate and its Regulation

Glomerular filtration rate (GFR) is the volume of filtrate formed per minute by the glomeruli, averaging 125 mL/min in males and 105 mL/min in females. Maintaining a constant GFR is essential for the kidneys to regulate body fluid homeostasis and maintain extracellular stability, enabling effective kidney function.

The myogenic mechanism responds to blood pressure changes by adjusting afferent arteriole smooth muscle contraction. When systemic blood pressure rises, the afferent arterioles constrict, reducing glomerular blood flow and GFR. Conversely, decreased blood pressure causes arteriole dilation and increased glomerular hydrostatic pressure to maintain optimal filtration.

Macula densa cells control the tubuloglomerular feedback mechanism by detecting high sodium chloride levels in the filtrate. These cells release vasoconstrictor chemicals that constrict the afferent arteriole, reducing blood flow into the glomerulus and decreasing net filtration pressure and GFR to maintain homeostasis.

Under very low systemic blood pressure, the sympathetic nervous system releases norepinephrine, which constricts both efferent and afferent arterioles. This extrinsic control mechanism decreases GFR and helps restore normal blood volume and blood pressure when intrinsic autoregulation mechanisms cannot manage the extreme condition.

Angiotensin II reduces GFR by narrowing arterioles and decreasing renal blood flow, while also stimulating aldosterone release to promote sodium and water retention. In contrast, atrial natriuretic peptide increases GFR by dilating afferent arterioles and constricting efferent arterioles, enhancing sodium excretion and water loss to reduce blood pressure.

Intrinsic controls like the myogenic mechanism and tubuloglomerular feedback maintain GFR during normal blood pressure fluctuations. However, these mechanisms cannot manage extremely low systemic blood pressure, so extrinsic controls involving the sympathetic nervous system and hormones like angiotensin II and atrial natriuretic peptide take over to restore homeostasis.

Decreased systemic blood pressure triggers the myogenic mechanism, causing afferent arterioles to dilate and increasing glomerular hydrostatic pressure to maintain GFR. If blood pressure drops severely, extrinsic controls activate, with the sympathetic nervous system constricting arterioles to decrease GFR and help restore blood volume and pressure.