Page 1027 - Fundamentals of anatomy physiology

1014 Unit 5 Environmental Exchange Glomerulus Dense
Figure 26–10 Glomerular Filtration. Capillary layer
lumen
Efferent Filtration
arteriole Podocyte slit

Afferent
arteriole

Factors Controlling Glomerular Filtration Pedicels

Pore

The glomerular hydrostatic pressure (GHP) is the blood pressure in the glomerular capillaries. Capsular
This pressure tends to push water and solute molecules out of the plasma and into the filtrate. The space
GHP, which averages 50 mm Hg, is significantly higher than capillary pressures elsewhere in the
systemic circuit, because the efferent arteriole is smaller in diameter than the afferent arteriole. Filtration
membrane
Filtrate in The blood colloid osmotic pressure
capsular (BCOP) tends to draw water out of the a The glomerular filtration
filtrate and into the plasma; it thus opposes membrane
space filtration. Over the entire length of the
Plasma 50 10 glomerular capillary bed, the BCOP
proteins 25 mm averages about 25 mm Hg.

Solutes Hg The net filtration pressure (NFP) is the
net pressure acting across the glomerular
15 capillaries. It represents the sum of the
hydrostatic pressures and the colloid
26 The capsular colloid osmotic pressure osmotic pressures. Under normal
is usually zero because few, if any, plasma circumstances, the net filtration pressure is
proteins enter the capsular space. approximately 10 mm Hg. This is the
average pressure forcing water and
dissolved substances out of the glomerular
capillaries and into the capsular space.

Capsular hydrostatic pressure (CsHP)
opposes GHP. CsHP, which tends to push
water and solutes out of the filtrate and into
the plasma, results from the resistance of
filtrate already present in the nephron that
must be pushed toward the renal pelvis.
The difference between GHP and CsHP is
the net hydrostatic pressure (NHP).

b Net filtration pressure

normal circumstances only a few plasma proteins—such as Figures 21–10 and 21–11, pp. 765, 766, before you proceed.)
albumin molecules, with an average diameter of 7 nm—can The primary factor involved in glomerular filtration is basically
cross the filtration membrane and enter the capsular space. the same one that regulates fluid and solute movement across
However, plasma proteins are all that stay behind, so the filtrate capillaries throughout the body. This factor is the balance be-
contains dissolved ions and small organic molecules in roughly tween hydrostatic pressure, or fluid pressure, and colloid osmotic
the same concentrations as in plasma. pressure, or pressure due to materials in solution, on either side
of the capillary walls.
Filtration Pressures
Hydrostatic Pressure
We discussed the major forces that act across capillary walls
in Chapters 21 and 22. (You may find it helpful to review Blood pressure is low in typical systemic capillaries. The reason
is that capillary blood flows into the venous system, where