d. Many of the mediated transport systems exhibit transport
maximums. When the ﬁ ltered load of a substance exceeds the
transport maximum, large amounts may appear in the urine.
IV. Tubular secretion, like glomerular ﬁ ltration, is a pathway for
the entrance of a substance into the tubule.
The Concept of Renal Clearance
I. The clearance of any substance can be calculated by dividing
the mass of the substance excreted per unit time by the plasma
concentration of the substance.
II. GFR can be measured by means of the inulin clearance and
estimated by means of the creatinine clearance.
I. In the basic micturition reﬂ ex, bladder distension stimulates
stretch receptors that trigger spinal reﬂ exes; these reﬂ exes
lead to contraction of the detrusor muscle, mediated by
parasympathetic neurons, and relaxation of both the internal
and the external urethral sphincters, mediated by inhibition of
the neurons to these muscles.
II. Voluntary control is exerted via descending pathways to the
parasympathetic nerves supplying the detrusor muscle, the
sympathetic nerves supplying the internal urethral sphincter,
and the motor nerves supplying the external urethral sphincter.
Additional Clinical Examples
I. Incontinence is the involuntary release of urine that occurs
most commonly in the elderly (particularly women).
SECTION A KEY TERMS
SECTION A CLINICAL TERMS
collecting duct system
cortical collecting duct
creatinine clearance (
distal convoluted tubule
external urethral sphincter
ﬁ l t e r e d l o a d
glomerular ﬁ ltrate
glomerular ﬁ ltration
glomerular ﬁ ltration rate
internal urethral sphincter
juxtaglomerular (JG) cell
loop of Henle
medullary collecting duct
net glomerular ﬁ ltration
renal plasma ﬂ ow
transport maximum (
familial renal glucosuria
SECTION A REVIEW QUESTIONS
1. What are the functions of the kidneys?
2. What three hormones/factors do the kidneys secrete into the
3. Fluid ﬂ ows in sequence through what structures from
the glomerulus to the bladder? Blood ﬂ ows through what
structures from the renal artery to the renal vein?
4. What are the three basic renal processes that lead to the
formation of urine?
5. How does the composition of the glomerular ﬁ ltrate compare
with that of plasma?
6. Describe the forces that determine the magnitude of the GFR.
What is a normal value of GFR?
7. Contrast the mechanisms of reabsorption for glucose and urea.
Which one shows a
8. Diagram the sequence of events leading to micturition.
Regulation of Ion
and Water Balance
Total-Body Balance of Sodium
summarizes total-body water balance. These are
average values, that are subject to considerable normal varia-
tion. There are two sources of body water gain: (1) water pro-
duced from the oxidation of organic nutrients, and (2) water
ingested in liquids and food (a rare steak is approximately
70 percent water). There are four sites that lose water to the
external environment: skin, respiratory airways, gastrointesti-
nal tract, and urinary tract. Menstrual ﬂ ow constitutes a ﬁ
potential source of water loss in women.
The loss of water by evaporation from the skin and the
lining of the respiratory passageways is a continuous process. It
insensible water loss
because the person is unaware
of its occurrence. Additional water can be made available for