3-14. THE KIDNEYS
a. Location and Position. The kidneys are paired organs in the upper
posterior part of the abdominal cavity, one on either side of the vertebral column, behind
the peritoneum. The upper extremity, or pole, of each kidney is, approximately, at the
level of the upper border of the twelfth thoracic vertebra. The lower extremity, or pole, is
at the level of the third lumbar vertebra. Generally, the right kidney is slightly (about 1/2
inch) lower than the left. Since the kidneys are not rigidly fixed to the abdominal wall,
they move up and down during respiration and normally drop about
1-1/2 inches when the person changes from supine to the erect position.
(1) The kidneys have a characteristic bean-shaped form, their lateral
borders being convex and their medial borders concave. Their anterior and posterior
surfaces are slightly convex. Near the center of the concave medial border is a
depression (the hilum) that marks the entrance of the vessels and nerves and the
emergence of the ureters. Each kidney is about 11 centimeters in length, 5.5
centimeters in width, and 3.5 centimeters in thickness (in the average adult male).
Contiguous with the kidney tissue is a mass of fatty tissue, the adipose capsule, which
is comparatively radiolucent.
(2) On the longitudinal section, the kidney consists of a cortex or external
part, a medulla or internal portion, and the renal pelvis (figure 3-6).
(a) The cortical substance is reddish brown in color and is composed
of the glomeruli and Bowman's capsule, forming the nephrons (the functional units of
the kidney, figure 3-7). There are over 3/4 million of these units in each kidney. The
glomerulus is a tuft of capillaries surrounded by a capsule that acts as a filter to keep
the large molecules of proteins, etc., from entering the convoluted tubules, but allows
water, urea, salts, and excess sugars to enter. The efferent artery leaving the
glomerulus breaks up into a plexus surrounding the convoluted tubules. About 99
percent of the water is reabsorbed and the waste products enter the collecting tubes as
(b) The medullary substance consists of thousands of collecting tubes
that converge to form a series of conical masses, the renal pyramids. The bases of the
pyramids are directed toward the cortex of the kidney while the apices converge toward
(c) The hilum expands internally into a dome-shaped central cavity,
called the renal sinus. The calyces and the renal pelvis are situated within the sinus.
(d) The minor renal calyces unite to form two to five short channels, the
major renal calyces, and these, in turn, combine to form the renal pelvis. At the apex of
the renal pyramids, the urine is first emptied into the minor calyces; it then flows into the
major calyces and on into the renal pelvis.