must undergo tubular secretion. Look back at our hypo-
thetical substance X (see Figure 14–7): X is ﬁ ltered, and all
the X that escapes ﬁ ltration is secreted; no X is reabsorbed.
Consequently, all the plasma that enters the kidney per unit
time is cleared of its X. Therefore, the clearance of X is a mea-
renal plasma ﬂ
A substance that is handled like
X is the organic anion para-amino-hippurate (PAH), which is
used for this purpose experimentally. (Like inulin, it must be
A similar logic leads to another important generaliza-
tion. When the clearance of a ﬁ lterable substance is less than
the GFR, that substance must undergo reabsorption.
Urine ﬂ ow through the ureters to the bladder is propelled by
contractions of the ureter wall smooth muscle. The urine is
stored in the bladder and intermittently ejected during urina-
The bladder is a balloon-like chamber with walls of
smooth muscle collectively termed the
contraction of the detrusor muscle squeezes on the urine in the
bladder lumen to produce urination. That part of the detru-
sor muscle at the base (or “neck”) of the bladder where the
urethra begins functions as the
internal urethral sphincter.
Just below the internal urethral sphincter, a ring of skeletal
muscle surrounds the urethra. This is the
the contraction of which can prevent urination
even when the detrusor muscle contracts strongly.
What factors inﬂ uence these bladder structures (
)? (1) The detrusor muscle is innervated by parasympa-
thetic neurons, which cause muscular contraction. Because of
the arrangement of the smooth muscle ﬁ bers, when the detru-
sor muscle is relaxed, the internal urethral sphincter is closed.
When the detrusor muscle contracts, changes in its shape pull
open the internal urethral sphincter. (2) In addition, the inter-
nal sphincter receives sympathetic innervation, which causes
contraction of the sphincter. (3) The external urethral sphinc-
ter, being skeletal muscle, is innervated by somatic motor neu-
rons that cause contraction.
While the bladder is ﬁ lling, there is little parasympathetic
input to the detrusor muscle but strong sympathetic input to
the internal urethral sphincter and strong input by the somatic
motor neurons to the external urethral sphincter. Therefore,
the detrusor muscle is relaxed, and the sphincters are closed.
What happens during micturition? As the bladder ﬁ lls
with urine, the pressure within it increases, which stimulates
stretch receptors in the bladder wall. The afferent ﬁ bers from
these receptors enter the spinal cord and stimulate the para-
sympathetic neurons, which then cause the detrusor muscle to
of inulin in
plasma = 4 mg/L
Rate of fluid
inulin in filtrate
Total inulin filtered
= 7.5 L/h
No reabsorption of inulin
No secretion of inulin
Total inulin excreted = 30
Example of renal handling of inulin, a substance that is ﬁ ltered by
the renal corpuscles but is neither reabsorbed nor secreted by the
tubule. Therefore, the mass of inulin excreted per unit time is equal
to the mass ﬁ ltered during the same time period. As explained in the
text, the clearance of inulin is equal to the glomerular ﬁ ltration rate.
Control of the bladder.