Biomedical Engineering Reference
In-Depth Information
Bowman's Capsule
Parietal epithelium
afferent
arteriole
Distal
Convoluted
Tu b u l e
Glomerulus
efferent
arteriole
Proximal
Convoluted
Tu b u l e
Capsular
Space
Visceral epithelium
FIGURE 14.16
The glomerulus inside Bowman's capsule.
The glomerulus contains very large pores, each 50 angstroms in diameter and 500 ang-
stroms long. These large pores allow a cumulative 125-150 ml/minute of filtered blood
through to the remainder of the nephron. These pores allow water, ions, and metabolic
waste products through, but they are too small to allow blood cells, proteins, and large
sugars to pass. Once the fluid passes through the glomerulus, it is then known as
filtrate
.
Thus, the term
) corresponds to the 125-150 ml/min traveling
through glomerular pores into the tubules of the nephron. The pressure gradient across the
glomerulus that causes the filtrate to pass through the glomerular pores is 50 mmHg with a
back pressure (osmotic pressure) of 25 mm Hg for a net 25 mm Hg. One can estimate the
number of glomeruli in both kidneys by using a model for steady flow through a tube:
Poiseuille flow. The Poiseuille flow equation, which will be described later, is
glomerular filtration rate
(
GFR
r
4
Q (GFR
Þ¼
n
p
D
P
=
8
m
L
where r is the pore radius,
m
is the fluid viscosity, L is the pore length, and n is the number
of total pores.
The pore radius is 25 angstroms (25
10
8
cm), the pressure gradient is 25 mm Hg
10
4
g/cm-sec
2
), the pore length is 500 angstroms (500
10
8
cm), and the filtrate
(3.33
viscosity is 0.012 g/cm-sec.
Obviously, one cannot survive if 125 ml/minute of fluid were to leave the body. Thus,
most of this fluid is reabsorbed back into the bloodstream, with the concentrated remainder
resulting in urine. Normally, up to 124 ml are reabsorbed, resulting in a urine output of
1 ml/min, which is stored in the bladder.
