is made up of more than a million tiny filtration plants called nephrons. As the pelvis
leaves the renal sinus, it becomes smaller, and finally merges into the ureter. The
medulla is made up of a series of striated, conical masses called the renal pyramids.
The pyramids vary from eight to eighteen in each kidney, and they are situated with
their bases directed to the circumference of the kidney, and their tops toward the pelvis.
c. Ureters. The pelvis of the kidney is drained by a tube called the ureter, which
leaves the kidney at its hilus. The ureter drains the urine from the kidney to the urinary
d. Urinary Bladder. The urinary bladder is a muscular sac, which serves as a
reservoir for urine and is located in the pelvic part of the abdomen. Its size varies with
the amount of urine it contains. When empty, the bladder is shaped like a pear pointing
downward; when moderately full (about 0.5 liter), it assumes an oval form. The urine
enters from the two ureters at the back, and near the bottom of the bladder. The
bladder empties through an opening at its midline. These three openings bound an
area called the trigone. There are strong muscles in this central triangle of the bladder
wall, essential for proper voiding. The bladder is supplied with a number of arteries,
veins, and lymphatics, which are derived from abdominal blood vessels.
e. Urethra. The urethra drains the urine from the bladder to the outside. It
usually is about six to eight inches long in the male, extending from the bladder to the
end of the penis. In the female, the urethra is about 1 1/2 inches long. There are two
circular cutoff muscles, or sphincters that keep the urine from leaking. One is situated
around the neck of the bladder, and the other is around the membranous part of the
urethra. The urethra is composed of a mucous membrane that is supported by
submucous tissue, which connects it with the various structures through which it
222. MICROSCOPIC STRUCTURE
a. Nephrons. Nephrons (figure 211) are the functional units of the kidney.
Their main functions are to remove by filtration certain waste materials from the body,
and to reabsorb water freed of waste for use in the body. The two main divisions of the
nephron are the renal corpuscle and the tubular system.
b. Renal Corpuscle. The kidney contains large numbers of renal corpuscles.
There are approximately one million of these structures in each kidney. Blood is carried
to each corpuscle by an afferent arteriole that is a minute branch of the renal artery.
This afferent arteriole divides into approximately 50 capillaries that are bent into short
loops. This tangled mass of capillaries is referred to as the glomerulus. These
capillaries converge into another fine vessel known as the efferent arteriole that carries
the blood out of the glomerulus. This entire mass of capillaries, the glomerulus, is
covered by a membranous sac called Bowman's capsule (which empties into the renal
tubule), and the complete structure is known as a renal corpuscle (figure 212). It is
from blood within the capillaries of the glomerulus that water, salts, glucose, urea, uric
acid, creatinine, and other substances are filtered.