787
CHAPTER TWENTY
Urinary System
FIGURE 20.16
Compared to most capillaries in the (
a
) systemic circulation, those in the (
b
) kidneys are highly specialized for F
ltration.
FIGURE 20.17
┬▒Glomerular┬▒F
ltration. (
a
) The F
rst step in urine formation is F
ltration of substances out of glomerular capillaries and into the
glomerular capsule. (
b
) Glomerular F
ltrate passes through the fenestrae of the capillary endothelium.
Arteriole
Venule
Venule
Afferent
arteriole
Glomerular
capillaries
Efferent
arteriole
Peritubular
capillaries
Filtered
fluid
Renal tubule
Tubular fluid
Tubular
secretion
Tubular
reabsorption
Urine
(a)
In most systemic capillaries, filtration predominates at the arteriolar end
and osmotic
reabsorption predominates at the venular end.
(b)
In the kidneys, the glomerular capillaries are specialized for filtration. The renal tubule is
specialized to control movements of substances back into the blood of the peritubular
capillaries (tubular reabsorption) or from the blood into the renal tubule (tubular secretion).
Net reabsorption
Blood
flow
Blood
flow
Net filtration
Interstitial fluid
Glomerular
filtration
Efferent
arteriole
Afferent
arteriole
Capillary
endothelium
Fenestrae
Glomerular
filtrate
Proximal
convoluted
tubule
(a)
(b)
Glomerular
capsule
Blood
flow
Podocyte
Podocyte
Glomerulus
previous page 817 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 819 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off