The Urinary System
Where the urethra leaves the bladder, it is surrounded by an
internal urethral sphincter, an involuntary smooth muscle
sphincter. Where it passes through the urogenital diaphragm,
the voluntary external urethral sphincter is formed by skeletal
In females the urethra is 3–4 cm long and conducts only urine. In
males it is 20 cm long and conducts both urine and semen.
Micturition is emptying of the bladder.
Accumulating urine stretches the bladder wall, which initiates
the micturition reﬂex. In infants, this is a simple spinal reﬂex:
Parasympathetic ﬁbers are excited (and sympathetic ﬁbers
inhibited), causing the detrusor to contract and the internal
urethral sphincter to open. In adults, pontine storage and
micturition centers can override this simple reﬂex.
Because the external sphincter is voluntarily controlled,
micturition can usually be delayed temporarily.
Developmental Aspects of the Urinary System
Tree sets of kidneys (pronephric, mesonephric, and
metanephric) develop from the intermediate mesoderm. Te
metanephros excretes urine by the third month of development.
Common congenital abnormalities are horseshoe kidney,
hypospadias, and polycystic kidney disease (PKD).
Te kidneys of newborns are less able to concentrate urine;
their bladder is small and voiding is frequent. Neuromuscular
maturation generally allows toilet training for micturition to
begin by 24 months of age.
Te most common urinary system problems in children and
young to middle-aged adults are bacterial infections.
With age, nephrons are lost, the ﬁltration rate decreases, and
tubule cells become less eﬃcient at concentrating urine.
Bladder capacity and tone decrease with age, leading to frequent
micturition and (oFen) incontinence. Urinary retention is a
common problem of elderly men.
Clinical Evaluation of Kidney Function
Renal clearance is the volume of plasma that is completely cleared
of a particular substance per minute. Studies of renal clearance
provide information about renal function or the course of renal
Renal failure has serious consequences: Te kidneys are unable
to concentrate urine, acid-base and electrolyte imbalances occur,
and nitrogenous wastes accumulate in the blood.
Urine is typically clear, yellow, aromatic, and slightly acidic. Its
speciﬁc gravity ranges from 1.001 to 1.035.
Urine is 95% water; solutes include nitrogenous wastes (urea, uric
acid, and creatinine) and various ions (always sodium, potassium,
sulfate, and phosphate).
Substances not normally found in urine include glucose, proteins,
erythrocytes, leukocytes, hemoglobin, and bile pigments.
Urine Transport, Storage, and Elimination
Te ureters are slender tubes running retroperitoneally from each
kidney to the bladder. Tey conduct urine by peristalsis from the
renal pelvis to the urinary bladder.
Te urinary bladder, which functions to store urine, is a distensible
muscular sac that lies posterior to the pubic symphysis. It has two
inlets (ureters) and one outlet (urethra) that outline the trigone. In
males, the prostate surrounds the bladder outlet.
Te bladder wall consists of a mucosa containing transitional
epithelium; a three-layered detrusor; and an adventitia.
Te urethra is a muscular tube that conveys urine from the
bladder to the body exterior.
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
Te lowest blood concentration of nitrogenous waste occurs in
inferior vena cava,
Te glomerular capillaries diﬀer from other capillary networks
in the body because they
have a larger area of anastomosis,
are derived from and drain into arterioles,
are not made of
are sites of ﬁltrate formation.
Damage to the renal medulla would interfere
the functioning of the
Which is reabsorbed by the proximal convoluted tubule cells?
all of the above.
Glucose is not normally found in the urine because it
pass through the walls of the glomerulus,
is kept in the blood
by colloid osmotic pressure,
is reabsorbed by the tubule cells,
is removed by the body cells before the blood reaches the
±iltration at the glomerulus is inversely related to
capsular hydrostatic pressure,
acidity of the urine.
of glucose and many other substances
is a ²
-limited active transport process,
of chloride is always
linked to the passive transport of Na
is the movement of
substances from the blood into the nephron,
occurs only in the proximal tubule.
If a freshly voided urine sample contains excessive amounts of
urochrome, it has
an ammonia-like odor,
a pH below
a dark yellow color,
a pH above normal.
Conditions such as diabetes mellitus and starvation are closely
Which of the following is/are true about ADH?
obligatory water reabsorption,
it is secreted in response to an
increase in extracellular ﬂuid osmolality,
it causes insertion of
aquaporins in the PC²,
it promotes Na