The Filtration Barrier

As discussed earlier, the process of glomerular filtration produces an ultrafiltrate of plasma, that is, a fluid that has approximately the same composition as the blood plasma but virtually no protein. It is estimated that in humans much less than 1% of the plasma albumin that enters the kidney via the renal artery is filtered at the glomerulus, and this small amount of filtered protein is reabsorbed by the proximal nephron. Only trace amounts of the larger globulins are filtered and reabsorbed. On the other hand, substantial amounts of smaller proteins such as myoglobin, hemoglobin, and light chains from immunoglobulins can be filtered. However, these small proteins appear in the plasma only under pathologic conditions, for example, when a transfusion reaction causes the release of hemoglobin from damaged erythrocytes.

Because of their small molecular dimensions, solutes with molecular weights of less than 5000 Da are freely filtered; this includes all components of the plasma except proteins and those solutes that bind to plasma proteins. Because of the Gibbs-Donnan effects of the plasma proteins (see Chapter 3), the concentrations of cations in the glomerular ultrafiltrate will be 4-5% lower than in the plasma and the concentrations of anions will

FIGURE 6 Rise in the COP at the end of the glomerular capillaries with increasing filtration fraction. Because the glomerular filtrate is protein free, the COP of the plasma remaining in the glomerular capillary is increased by ultrafiltration. If the COP rises to the same level as the net hydrostatic pressure (Pc — Pb; see Eq. [3]) at some point along the length of the capillary, there is no filtration beyond that point. The graph assumes that the net hydrostatic pressure is 40 mm Hg and the COP of the plasma entering the glomerular capillary is 25 mm Hg (as in Fig. 3). For a filtration fraction greater than 0.37, the net filtration pressure would fall to zero before the end of the glomerular capillary.

Filtration Fraction (FF)

FIGURE 6 Rise in the COP at the end of the glomerular capillaries with increasing filtration fraction. Because the glomerular filtrate is protein free, the COP of the plasma remaining in the glomerular capillary is increased by ultrafiltration. If the COP rises to the same level as the net hydrostatic pressure (Pc — Pb; see Eq. [3]) at some point along the length of the capillary, there is no filtration beyond that point. The graph assumes that the net hydrostatic pressure is 40 mm Hg and the COP of the plasma entering the glomerular capillary is 25 mm Hg (as in Fig. 3). For a filtration fraction greater than 0.37, the net filtration pressure would fall to zero before the end of the glomerular capillary.

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