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Glomerulotubular Balance

  • Glomerulotubular Balance refers to the phenomenon whereby a constant fraction of the filtered load of the nephron is resorbed across a range of Glomerular Filtration Rates (GFR). In other words, if the GFR spontaneously increases, the rate of water and solute resorption in the tubule proportionally increases, thus maintaining the same fraction the filtered load being resorbed.
  • The proximal tubule appears to be the principal segment in which glomerulotubular balance operates and approximately 67% of the filtered load of solutes and water is resorbed in the proximal tubule regardless of the value of the GFR. The mechanistic basis of glomerulotubular balance is poorly understood but appears to act completely independently of neuroendocrine regulatory mechanisms and is likely an intrinsic property of the nephron itself. It seems likely that changes in GFR result in modification of the starling forces in the peritubular capillaries resulting in proportionally increased or decreased total nephronic resorption.
  • Glomerulotubular balance is a critical mechanism which protects distal segments of the nephron from being overloaded in contexts of short-term increases in GFR. Distal segments of the nephron have a very limited capacity to increase tubular resorption of water and solutes; consequently, a large increase in distal flow rates would result in a catastrophic loss of fluid in the urine. Glomerulotubular balance thus guarantees that any momentarily increased tubular flow is resorbed by proximal segments of the nephron which have significantly greater capacity for resorbing large fluid volumes. Glomerulotubular balance can be thought of as an additional layer of protection if mechanisms of tubuloglomerular feedback, that normalize rates of GFR, momentarily fail or are slow to be triggered.