Chapter 25
The Urinary System
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25
obliquely through the posterior bladder wall. Tis arrangement
prevents backflow of urine because any increase in bladder
pressure compresses and closes the distal ends of the ureters.
Histologically, the ureter wall has three layers
(Figure 25.19)
.
From the inside out:
Te
mucosa
contains a transitional epithelium that is con-
tinuous with the mucosae of the kidney pelvis superiorly and
the bladder medially.
Te
muscularis
is composed chiefly of two smooth muscle
sheets: the internal longitudinal layer and the external cir-
cular layer. An additional smooth muscle layer, the external
longitudinal layer, appears in the lower third of the ureter.
Te
adventitia
covering the ureter’s external surface is typical
fibrous connective tissue.
Te ureter plays an active role in transporting urine. Incoming
urine distends the ureter and stimulates its muscularis to con-
tract, propelling urine into the bladder. (Urine does
not
reach the
bladder through gravity alone.) Te strength and frequency of
the peristaltic waves are adjusted to the rate of urine formation.
Both sympathetic and parasympathetic fibers innervate each ure-
ter, but neural control of peristalsis appears to be insignificant
compared to the way ureteral smooth muscle responds to stretch.
Homeostatic Imbalance
25.5
On occasion, calcium, magnesium, or uric acid salts in urine
may crystallize and precipitate in the renal pelvis, forming
re-
nal
calculi
(kal
9
ku-li;
calculus
5
little stone), or kidney stones.
pH
Urine is usually slightly acidic (around pH 6), but changes
in body metabolism or diet may cause the pH to vary from
about 4.5 to 8.0. A predominantly
acidic
diet that contains large
amounts of protein and whole wheat products produces acidic
urine. A vegetarian (
alkaline
) diet, prolonged vomiting, and
bacterial infection of the urinary tract all cause the urine to be-
come alkaline.
Specific Gravity
Te ratio of the mass of a substance to the
mass of an equal volume of distilled water is its
specific gravity
.
Because urine is water plus solutes, a given volume has a greater
mass than the same volume of distilled water. Te specific grav-
ity of distilled water is 1.0 and that of urine ranges from 1.001 to
1.035, depending on its solute concentration.
Chemical Composition
Water accounts for about 95% of urine volume; the remain-
ing 5% consists of solutes. Te largest component of urine by
weight, apart from water, is
urea
, which is derived from the
normal breakdown of amino acids. Other
nitrogenous wastes
in urine include
uric acid
(an end product of nucleic acid me-
tabolism) and
creatinine
(a metabolite of creatine phosphate,
which is found in large amounts in skeletal muscle tissue where
it stores energy to regenerate A±P).
Normal solute constituents of urine, in order of decreasing
concentration, are urea, Na
1
, K
1
, PO
4
3
2
, SO
4
2
2
, creatinine, and
uric acid. Much smaller but highly variable amounts of Ca
2
1
,
Mg
2
1
, and HCO
3
2
are also present.
Unusually high concentrations of any solute, or the presence
of abnormal substances such as blood proteins, WBCs (pus), or
bile pigments, may indicate pathology
(Table 25.2)
. (Normal
urine values are listed in Appendix F.)
Check Your Understanding
16.
What would you expect the normal clearance value for
amino acids to be? Explain.
17.
What are the three major nitrogenous wastes excreted in the
urine?
For answers, see Appendix H.
Urine Transport, Storage,
and Elimination
Te kidneys form urine continuously and the ureters transport
it to the bladder. It is usually stored in the bladder until its release
through the urethra is convenient, a process called
micturition
.
Ureters
Describe the general location, structure, and function of
the ureters.
Te
ureters
are slender tubes that convey urine from the kidneys
to the bladder (Figure 25.1 and
Figure 25.18
). Each ureter
begins at the level of L
2
as a continuation of the renal pel-
vis. From there, it descends behind the peritoneum and runs
Kidney
Ureter
Renal
pelvis
Urinary
bladder
Figure 25.18
Pyelogram.
This X-ray image was obtained using a
contrast medium to show the ureters, kidneys, and urinary bladder.
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