44 Glomerular Filtration
High Pressure Bulk Filtration
The glomerular capillaries filter about one fifth of the plasma that flows through the kidneys into the renal tubules. Glomerular filtration works like any other filtration process. For example, a coffee filter prevents large coffee grounds from passing through it, while allowing the passage of water and small solutes such as flavor molecules and caffeine. In the same way, the glomerular filtration membrane blocks the passage of blood cells and proteins, while allowing water and other solutes from the blood to pass into the glomerular capsule. These substances are forced through the membrane by the higher hydrostatic pressure in the glomerular capillary than in the capsular space.
The glomerulus has the highest rate of filtration of any capillary bed in the body for a number of reasons. First, its filtration membrane has a very large surface area. In addition, the large fenestrations (capillary pores) in glomerular capillaries make them at least 50 times more permeable to water and solutes than other capillary beds. Finally, the blood pressure (hydrostatic pressure) in glomerular capillaries is about three times higher than in other capillary beds (around 55 mm Hg versus 18 mm Hg or less). This increased blood pressure is due to the relatively small pressure drop across the afferent arteriole leading into the glomerular capillary and the large pressure drop across the efferent arteriole, which leads out of the glomerular capillary.
Looking at filtrate production, the combined daily output of all other capillary beds in the body is about three to four liters (three to four quarts). In the kidneys, the daily output is 150 liters (158 quarts) in women and 180 liters (190 quarts) in men. Tubular reabsorption returns more than 99 percent of the glomerular filtrate (the fluid than enters the capsular space) to the bloodstream. This means than just one to two liters are eliminated in urine. The filtration fraction describes the portion of blood plasma in the afferent arterioles that ends up as glomerular filtrate. A typical filtration fraction ranges from 16 to 20 percent.
The Filtration Membrane
The porous filtration membrane separates the inside of the glomerular capsule from the blood. Water and solutes smaller than about three nanometers in diameter (e.g., glucose, amino acids, nitrogenous wastes) pass freely across the membrane between the blood and capsule. This means there are similar concentrations of these substances in both the blood and the glomerular filtrate. Some bigger molecules may find a way through the membrane, but molecules larger than five nanometers (large proteins and blood cells) do not usually get into the tubule.
The filtration membrane, three layers serve as barriers to larger molecules circulating in the blood. From most permissive to most restrictive, these layers are the glomerular capillary endothelium, the gel-like basal lamina basement membrane, and the podocyte-formed filtration slit. All plasma components (note plasma does not include blood cells) can pass through the fenestrations in the endothelium, but blood cells cannot. The smallest proteins and most other solutes can penetrate the basement membrane. This membrane is composed primarily of negatively charged glycoproteins that oppose other macromolecular anions and obstruct their entry into the tubule. In other words, the basement provides electrical selectivity to the filtration process. Most plasma proteins are too large to meet the size restrictions. Moreover, because most plasma proteins have a net negative charge, they are repelled by the negatively charged glycoproteins of the basement membrane. Any macromolecules that do manage to pass through the basement membrane face yet another barrier, thin membranes called slit diaphragms that span the filtration slits. Specialized cells in the glomerulus, known as mesangial cells, will destroy macromolecules that are trapped in the filtration membrane. Mesangial cells can also contract and alter the capillary surface area available for filtration.