Page 1028 - Fundamentals of anatomy physiology
P. 1028
Chapter 26 The Urinary System 1015
resistance is fairly low. However, at the glomerulus, blood leav- The Glomerular Filtration Rate
ing the glomerular capillaries flows into an efferent arteriole,
whose diameter is smaller than that of the afferent arteriole. For The glomerular filtration rate (GFR) is the amount of
this reason, the efferent arteriole offers considerable resistance. filtrate the kidneys produce each minute. Each kidney con-
Relatively high pressures are needed to force blood into it. As a tains about 6 m2—some 64 square feet—of filtration surface,
result, glomerular pressures are similar to those of small arter- and the GFR averages an astounding 125 mL per minute. This
ies. These pressures average about 50 mm Hg, instead of the means that nearly 10 percent of the fluid delivered to the kid-
35 mm Hg typical of peripheral capillaries. Glomerular hydro- neys by the renal arteries leaves the bloodstream and enters
static pressure (GHP), capsular hydrostatic pressure (CsHP), the capsular spaces.
blood colloid osmotic pressure (BCOP), and net filtration pres-
sure (NFP) are shown in Figure 26–10. A creatinine clearance test is often used to estimate the GFR.
Creatinine results from the breakdown of creatine phosphate in
Capsular hydrostatic pressure (CsHP) opposes glomerular muscle tissue and is normally eliminated in urine. Creatinine
hydrostatic pressure. This pressure results from the resistance enters the filtrate at the glomerulus and is not reabsorbed in
to flow along the nephron and the conducting system. (Before significant amounts. By monitoring the creatinine concentra-
additional filtrate can enter the capsule, some of the filtrate al- tions in blood and the amount excreted in urine in a 24-hour
ready present must be forced into the PCT.) The CsHP averages period, a clinician can easily estimate the GFR.
about 15 mm Hg.
Consider, for example, a person who eliminates 84 mg of
The net hydrostatic pressure (NHP) is the difference between creatinine each hour and has a plasma creatinine concentration
the glomerular hydrostatic pressure, which tends to push water of 1.4 mg/dL. The GFR is equal to the amount secreted divided
and solutes out of the bloodstream, and the capsular hydrostatic by the plasma concentration, so this person’s GFR is
pressure, which tends to push water and solutes into the blood-
18.44 mg > h = 60 dl>h = 100 ml>min
stream. We can calculate net hydrostatic pressure as follows: mg > dl
NHP = 1GHP - CsHP2 = 150 mm Hg - 15 mm Hg2 = 35 mTmheHGgFR is usually reported in milliliters per minute.
= 35 mm Hg The value 100 mL/min is only an approximation of the GFR.
The reason is that up to 15 percent of creatinine in the urine
Colloid Osmotic Pressure enters by means of active tubular secretion. When necessary, a
more accurate GFR can be obtained by using the complex carbo-
The colloid osmotic pressure of a solution is the osmotic pressure hydrate inulin. This compound is not metabolized in the body
resulting from suspended proteins. Under normal conditions, and is neither reabsorbed nor secreted by the kidney tubules.
very few plasma proteins enter the capsular space, so no oppos-
ing colloid osmotic pressure exists within the capsule. However, In the course of a single day, the glomeruli generate about
if the glomeruli are damaged by disease or injury, and plasma 180 liters (48 gal) of filtrate, approximately 70 times the total
proteins begin passing into the capsular space, a capsular colloid plasma volume. But as filtrate passes through the renal tubules,
osmotic pressure is created that promotes filtration and increases about 99 percent of it is reabsorbed. You should now appreciate
fluid losses in urine. the significance of tubular reabsorption!
The glomerular filtration rate depends on the net filtration
Net Filtration Pressure 26pressure across glomerular capillaries. Any factor that alters the
The net filtration pressure (NFP) at the glomerulus is the dif- net filtration pressure also alters the GFR and affects kidney
ference between the net hydrostatic pressure and the blood function. One of the most significant factors is a decrease in
colloid osmotic pressure acting across the glomerular capil- renal blood pressure. If blood pressure at the glomeruli de-
laries. Under normal circumstances, we can summarize this creases by 20 percent (from 50 mm Hg to 40 mm Hg), kidney
relationship as filtration ceases, because the net filtration pressure is 0 mm Hg.
For this reason, the kidneys are sensitive to changes in blood
NFP = NHP - BCOP pressure that have little or no effect on other organs. Hemor-
or rhaging, shock, and dehydration are relatively common clinical
conditions that can cause a dangerous decrease in the GFR and
NFP = 35 mm Hg - 25 mm Hg = 10 mm Hg
lead to acute renal failure (p. 1034).
This is the average pressure forcing water and dissolved ma-
terials out of the glomerular capillaries and into the capsular Control of the GFR
spaces (Figure 26–10b). Problems that affect the net filtration
pressure can seriously disrupt kidney function and cause a vari- Glomerular filtration is the vital first step for all other kidney
ety of clinical signs and symptoms. functions. If filtration does not take place, waste products are
