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Urinary Filtration
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.
Net Filtration Pressure
Three forces affect glomerular filtration rate (volume filtered per unit
time); hydrostatic pressure of the blood in the glomerulus, hydrostatic
pressure of the fluid in the capsular space, and colloid osmotic force of
the blood in the glomerulus. Hydrostatic pressure within the glomerular
capillaries promotes or enhances filtration. The other two forces oppose
filtration and are: back pressure from hydrostatic pressure of the fluid
within the capsule and colloid osmotic pressure caused by the plasma
proteins within the glomerular capillaries. Subtracting the opposing
pressures from the promoting pressure yields the net filtration
pressure.
Hydrostatic Pressure – Resistance from fluid in the tubule – Osmotic
Collodial Pressure = Net Filtration Pressure
Net Filtration Pressure
Pressure
Description
Average
Glomerular
hydrostatic
Glomerular
pressure
Capsular
hydrostatic
Hydrostatic pressure applied 15
to the filtration membrane by mmHg
fluid in the capsular space and
renal tubule
Opposes
filtration
Blood colloid Results from proteins present 30
osmotic
in blood plasma
mmHg
Opposes
filtration
capillary
blood 55
Hg
Effect
mm Promotes
filtration
The glomerular hydrostatic pressure (i.e., the blood pressure in
glomerular capillaries) is the primary force responsible for pushing
water and solutes from blood across the filtration membrane. This
unusually high pressure (55 mm Hg) is opposed by two forces that resist
the influx of fluids: the capsular hydrostatic pressure exerted by
fluid already in the glomerular capsule and the blood colloid osmotic
pressure caused by proteins present in blood plasma (e.g., albumen,
fibrinogen). Net filtration pressure averages 10 mm Hg (i.e., 55 mmHg
−15 mmHg − 30 mmHg = 10 mmHg). This is in contrast to the 0.3mmHg net pressure found in most capillaries of the body.
The glomerular filtration rate (GFR) is the total amount of filtrate
formed by the two million renal corpuscles in the kidneys divided by
time. The average GFR is 125 milliliters (4 ounces) per minute, which
adds up to nearly 140 liters (50 gallons) a day. The kidneys must
maintain a relatively consistent GFR to prevent homeostatic imbalances
in body fluids. If the GFR is too high, needed substances will rush
through the renal tubules too quickly to be completely absorbed, and
they will be lost in urine. A GFR that is too low will allow waste products
to accumulate in the plasma, ultimately leading to illness and death if
not corrected.