I know that if blood pressure goes up that GFR will go up. Will this only increase the amount of filtrate, or will this increase the concentration and osmolarity of the filtrate as well?
Blood Pressure and GFR
- Thread starter kfcman289
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This leads to some confusion. GFR will actually remain constant within a set range (80 mmHg to 180 mmHg). Between this range, regardless if we have slight increases or decreases in BP, renal autoregulation occurs to increase or decrease blood flow to the glomerulus to maintain constant GFR. Above 180 mmHg or below 80 mmHg, renal autoregulation fails and we shift to other mechanisms to bring it back within that range. During these extremes, the priority shifts to maintain a homeostatic blood pressure by altering/changing GFR. So in your example, if pressure goes up beyond a set point (180 mmHg), our body will work to eliminate blood plasma by increasing GFR.
If we increase GFR, we are simply increasing the amount of plasma being pushed out of the glomerulus (ie. filtrate). That filtrate flows through the tubule (becomes tubular fluid); as this tubular fluid flows through the DCT and CD at that point we become concerned with the concentration and osmolarity (increasing or decreasing it to help maintain constant homeostatic blood pressure).
Ok, so just to clarify, the concentration of the immediate filtrate does not change? I'm asking this because I read a question where it said that increasing the GFR would increase the renal clearance of a compound.
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I'm not so sure I would say the concentration is changing, but rather more plasma being filtered would carry more solute with it simply because of the relative amount flowing through. For that reason, I would imagine that increasing GFR would increase renal plasma clearance as well.
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Sometimes I give a little too much information, but I thought I would throw the math at you to help clarify things a bit more. We use a plant sugar called "inulin" to measure GFR and there is an equation for it: GFR = UV/P, where U = concentration of inulin in urine, V = volume of urine produced per minute, and P = concentration of inulin in plasma. Normal GFR is around 125 mL/min and fluctuations from that value indicate renal dysfunction. Something to note about inulin is that it's neither reabsorbed nor secreted but rather just filtered out, which makes it a great reference compound for this measurement. Any substance that is neither reabsorbed or secreted but simply filtered would have a renal plasma clearance equal to GFR. To avoid the burden of calculating normal GFR (through inulin) we sometimes measure the renal plasma clearance of creatinin (a metabolite produced from muscle tissue) as a reference compound (it's not only filtered but secreted too), so it's renal plasma clearance is actually slightly higher than normal GFR (140 mL/min) but relatively equal. Obviously most substances are filtered or reabsorbed, so we define an independent measurement specific for that substance. The equation we use is the same (UxVx/Px), but the measured values ("x") are specific for the substance we are measuring; this is renal clearance.
My point in bringing this up is so you can understand and begin to compare the similarities of renal clearance and GFR, and when they differ.
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