500
Chapter 14
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.
Micturition
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
afferent arteriole
487
ascending limb
487
basolateral membrane
495
bladder
487
Bowman’s capsule
487
Bowman’s space
487
clearance
497
collecting duct system
487
cortical (nephron)
489
cortical collecting duct
487
creatinine
486
creatinine clearance (
C
Cr
)
497
descending limb
487
detrusor muscle
498
distal convoluted tubule
487
efferent arteriole
487
external urethral sphincter
498
fi l t e r e d l o a d
494
glomerular capillaries
487
glomerular fi ltrate
489
glomerular fi ltration
489
glomerular fi ltration rate
(GFR)
493
glomerulus
487
gluconeogenesis
486
internal urethral sphincter
498
inulin
497
juxtaglomerular apparatus
(JGA)
489
juxtaglomerular (JG) cell
489
juxtamedullary (nephron)
489
loop of Henle
487
luminal membrane
495
macula densa
489
medullary collecting duct
487
mesangial cells
487
micturition
498
nephron
487
net glomerular fi ltration
pressure
493
peritubular capillary
488
podocyte
487
proximal tubule
487
renal
486
renal corpuscle
487
renal cortex
488
renal medulla
488
renal pelvis
488
renal plasma fl ow
498
transport maximum (
T
m
)
496
tubular reabsorption
489
tubular secretion
489
tubule
487
urea
486
ureter
487
urethra
487
uric acid
486
vasa recta
489
diabetes mellitus
496
diabetic nephropathy
496
familial renal glucosuria
497
glucosuria
496
incontinence
499
stress incontinence
499
urge incontinence
499
SECTION A REVIEW QUESTIONS
1. What are the functions of the kidneys?
2. What three hormones/factors do the kidneys secrete into the
blood?
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
T
m
?
8. Diagram the sequence of events leading to micturition.
SECTION B
Regulation of Ion
and Water Balance
Total-Body Balance of Sodium
and Water
Table 14–3
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
fth
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
is called
insensible water loss
because the person is unaware
of its occurrence. Additional water can be made available for
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