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The rate at which plasma is filtered (measured in ml/min) is known as the glomerular
filtration rate (GFR). Filtration is a non-specific process of bulk flow:
water and small molecular weight substance move from the glomerular capillaries,
across the filtration membrane, and enter Bowman's space. The proportion of
plasma filtered, the filtration
fraction, is roughly 20%. Because filtration involves bulk flow, the
concentration of a substance in Bowman's space is the same as its concentration
in the plasma.
Why does filtration occur? Filtration occurs because of the high pressure in the
glomerular capillaries (PGC). The glomerular capillaries are unique
in that they lie between two arterioles, the afferent
arteriole and the efferent
arteriole. Because of the added resistance of the efferent arteriole, PGC is
higher than pressure in a typical capillary.
The figure illustrates the forces that determine the filtration rate. The
pressure in the glomerular capillaries, PGC,
favors filtration. Opposing filtration is the osmotic pressure due to the blood
proteins (?GC) and the hydrostatic pressure in Bowmans space (PBS).Thenet
filtration pressure is the sum of
these forces.
Measures of GFR
In a single nephron, the rate of filtration is a function of the net filtration
pressure, the permeability of the filtration membrane, and the surface area
available for filtration. The measured GFR reflects these factors, and of
course, the total number of
functioning nephrons. Average GFR is 125 ml/min for a healthy young man, or 110
ml/min for a healthy young woman. Chronic
renal insufficiency is defined
as a GFR of less than 60 ml/min. Renal insufficiency is associated with an
increase in risk in all the major outcomes of kidney disease, in particular, an increased
risk of death from cardiovascular disease.
GFR is directly measured by measuring the inulin
clearance. Inulin is a plant carbohydrate that is neither reabsorbed nor
secreted, thus the clearance of inulin (volume of blood per unit time from which
inulin is removed) is completely due to filtration (see lab). However, because
inulin must be infused, in practice it is simpler to gauge kidney function by
looking at an endogenous substance,
namelycreatinine, a metabolic breakdown product of skeletal muscle
creatine. The creatinine clearance can
be used to estimate the GFR. Alternatively, just the serum
creatinine (plasma concentration
of creatinine) may be measured to monitor kidney function.
In general, any factor that reduces the number of nephrons can over time reduce
the GFR. GFR normally declines with age, but this decline occurs much more
rapidly in individuals with chronic
kidney disease. For instance, proteinuria,
which is a common feature in various kidney diseases, leads to decreased GFR
because protein in the filtrate causes inflammation and scarring in the renal
tubules with subsequent nephron loss. Another way that GFR can decline in kidney
disease is through the loss of surface area available for filtration. In glomerulosclerosis (a
typical feature of diabetic
nephropathy) there is increased extracellular material in the glomerulus,
which decreases the surface area of the glomerular capillaries.
Regulation of GFR
One
would think that changes in the systemic blood pressure would cause changes in PGC and
thus, changes in the GFR. In healthy individuals, this does not occur because of renal
autoregulation. Renal autoregulation involves feedback mechanisms intrinsic
to the kidney that cause either
dilation or constriction in the afferent arteriole so as to counteract blood
pressure changes and keep a steady GFR. For instance, if the mean arterial
pressure increases, renal autoregulation causes the afferent arteriole to
constrict, preventing the pressure increase from being transmitted to the
glomerular capillaries, and keeping the GFR from increasing. As shown in the
graph, renal autoregulation normally operates to keep GFR steady over a wide
range of blood pressures. Renal autoregulation is disrupted in chronic
kidney disease.
If blood pressure drops too low due to excessive fluid loss, then the sympathetic
nervous system will override
renal autoregulation. Sympathetic nerves innervate the afferent arteriole,
causing smooth muscle contraction. The sequence of events is as follows: loss of
ECF volume (due to hemorrhage, diarrhea or dehydration) causes a drop in mean
arterial pressure (MAP). Decreased MAP activates arterial baroreceptors, which
leads to sympathetic nervous system activation, afferent arteriole constriction,
and decreased GFR.
Another effect of the sympathetic nervous system is to stimulate
renin secretion by the juxtaglomerular cells, activating the
renin-angiotensin-aldosterone system (RAAS). The RAAS increases extracellular
fluid volume by increasing sodium reabsorption.
Finally, the hormone atrial
natriuretic peptide (ANP) is a
factor that can increase GFR.
ANP is a hormone that is produced in the heart and whose secretion increases in
response to increased plasma volume. The effect of ANP is to promote natriuresis (increased
sodium excretion), in part through increased GFR, and in part through effects on
Na+ reabsorption.