The impact of electric charge on the filtration of macromolecules in the glomerulus is illustrated in Fig. 33.15. Dextrans, which are exogenous polysaccharides with varying molecular weights, can have no charge, negative charge (polyanionic), or positive charge (polycationic). The filtration rate of dextrans decreases as their size (effective molecular radius) increases. Among molecules with the same molecular radius, cationic molecules are filtered more easily than anionic molecules. The reduced filtration rate for anionic molecules is attributed to the presence of negatively charged glycoproteins on the surface of the glomerular filtration barrier, which repel molecules with similar charges. This negative charge restricts the filtration of anionic proteins more than neutral and polyanionic proteins with a molecular radius between approximately 18 to 42 Å. For instance, serum albumin, an anionic protein with an effective molecular radius of 35.5 Å, is poorly filtered. The small amount of filtered albumin is usually reabsorbed by the proximal tubule, resulting in minimal albumin appearing in urine.

The significance of negative charges on the filtration barrier in limiting the filtration of plasma proteins is demonstrated in Figs. 33.15 and 33.16. If the negative charges are removed from the filtration barrier, proteins will only be filtered based on their effective molecular radius, as depicted in Fig. 33.16. Consequently, filtration of polyanionic proteins will exceed the filtration observed under normal conditions when the filtration barrier possesses anionic charges. In several glomerular diseases, immunological damage and inflammation cause a reduction in the negative charges on the filtration barrier. This leads to an increased filtration of anionic proteins with a molecular radius between approximately 18 and 42 Å. When the amount of filtered proteins surpasses the capacity of the proximal tubule to reabsorb and break them down, anionic proteins start to appear in urine (proteinuria), which serves as an indicator of kidney disease.

The dynamics of ultrafiltration in the glomerulus rely on the same forces as other capillary beds. The combination of hydraulic and oncotic pressures, known as the Starling forces, drives the movement of fluid from the glomerular capillaries into Bowman's space, as shown in Fig. 33.17. The hydraulic pressure inside the glomerular capillary (PGC) promotes the flow of fluid from the capillary into Bowman's space. The glomerular ultrafiltrate is typically devoid of proteins due to the scarcity of serum proteins smaller than 18 Å in radius that can be effectively filtered. Consequently, the reflection coefficient (σ) for proteins across the glomerular capillary is essentially 1, resulting in a near-zero oncotic pressure in Bowman's space (πBS). Hence, PGC is the primary force driving filtration. On the other hand, the hydraulic pressure in Bowman's space (PBS) and the oncotic pressure in the glomerular capillary (πGC) act in opposition to filtration.
Fig. 33.15 shows how electric charge affects filtration of macromolecules (e.g., dextrans) by the glomerulus. Dextrans are a family of exogenous polysaccharides manufactured in various molecular weights. They can be electrically neutral or have either negative (polyanionic) or positive (polycationic) charges. As the size (i.e., effective molecular radius) of a dextran molecule increases, the rate at which it is filtered decreases. For any given molecular radius, cationic molecules are more readily filtered than anionic molecules. The reduced filtration rate for anionic molecules is explained by the presence of negatively charged glycoproteins on the surface of all components of the glomerular filtration barrier. These charged glycoproteins repel similarly charged molecules. Because most plasma proteins are negatively charged, the negative charge on the filtration barrier restricts filtration of anionic proteins more than the filtration of neutral and polyanionic proteins with a molecular radius between approximately 18 to 42 Å. For example, serum albumin, an anionic protein that has an effective molecular radius of 35.5 Å, is poorly filtered. Because the small amount of filtered albumin is normally reabsorbed avidly by the proximal tubule, almost no albumin appears in urine.
The importance of the negative charges on the filtration barrier in restricting filtration of plasma proteins is shown in
Figs. 33.15 33.16 . Removal of the negative charges from the filtration barrier causes proteins to be filtered solely on the basis of their effective molecular radius (
Fig. 33.16 ). Hence at any molecular radius between approximately 18 and 42 Å, filtration of polyanionic proteins will exceed the filtration that prevails in the normal state (in which the filtration barrier has anionic charges). In a number of glomerular diseases the negative charges on the filtration barrier are reduced because of immunological damage and inflammation. As a result, filtration of anionic proteins between approximately 18 and 42 Å in radius is increased. When the filtered proteins exceed the ability of the proximal tubule to reabsorb and catabolize them, anionic proteins begin to appear in urine (proteinuria), which is a marker of kidney disease.
Dynamics of Ultrafiltration
The forces responsible for glomerular filtration of plasma are the same as those in other capillary beds. Ultrafiltration occurs because the Starling forces (i.e., hydraulic and oncotic pressures) combine to drive fluid from the lumen of glomerular capillaries across the filtration barrier and into Bowman’s space (
Fig. 33.17 ). The hydraulic pressure inside the glomerular capillary (PGC) is oriented to promote movement of fluid from the glomerular capillary into Bowman’s space. Because the glomerular ultrafiltrate is essentially protein free under normal conditions, owing in large part to the paucity of proteins in serum smaller than 18 Å in radius 1.0 0.8 0.6 0.4 0.2 0 • Fig. 33.16 Reduction of the negative charges on the glomerular wall results in filtration of proteins on the basis of size only. In this situation the relative filterability of proteins depends only on the molecular radius. Accordingly, excretion of polyanionic proteins (18–42 Å) in urine increases because more proteins of this size are filtered.
that can be effectively filtered, the reflection coefficient (σ) for proteins across the glomerular capillary is essentially 1. Thus the oncotic pressure in Bowman’s space (πBS) is near zero. Therefore PGC is the principal force favoring filtration. In contrast, the hydraulic pressure in Bowman’s space (PBS) and the oncotic pressure in the glomerular capillary (πGC) both oppose filtration.