C H A P T E R 5 0
Introduction to the kidneys and the urinary tract
795
T
he renal system is composed of the kidneys and the
structures of the urinary tract: the ureters, the urinary
bladder and the urethra. This system has four major
functions in the body:
• Maintaining the volume and composition of body
fluids within normal ranges, including the following
functions:
• clearing nitrogenous wastes from protein
metabolism
• maintaining acid–base balance and electrolyte levels
• excreting various drugs and drug metabolites
• Regulating vitamin D activation, which helps to
maintain and regulate calcium levels
• Regulating blood pressure through the renin–
angiotensin–aldosterone system
• Regulating red blood cell production through the
production and secretion of erythropoietin
THE KIDNEYS
The kidneys are two small organs that make up about
0.5% of total body weight but receive about 25% of
the cardiac output. Approximately 1600 L of blood
flows through these two small organs each day for
cleansing. Most of the fluid that is filtered out by the
kidneys is returned to the body, and the waste products
that remain are excreted in a relatively small amount of
water as urine.
Urinary:
Renal function
Structure
The kidneys are located under the ribs, for protection
from injury. They have three protective layers that
make up the renal capsule: a fibre layer, a perirenal, or
brown fat, layer and the renal parietal layer. The capsule
contains pain fibres, which are stimulated if the capsule
is stretched secondary to an inflammatory process.
The kidneys have three identifiable regions: the
outer cortex, the inner medulla and the renal pelvises.
The renal pelvises drain the urine into the ureters. The
ureters are muscular tubes that lead into the urinary
bladder, where urine is stored until it is excreted
(Figure 50.1).
Nephron
The functional unit of the kidneys is called the
nephron
. There are approximately 2.4 million nephrons
in an adult. All of the nephrons filter fluid and make
urine, but only the medullary nephrons can concen-
trate or dilute urine. It is estimated that only about
25% of the total number of nephrons are necessary to
maintain healthy renal function. That means that the
renal system is well protected from failure with a large
backup system. However, it also means that by the
time a person manifests signs and symptoms, suggest-
ing failure of the kidneys, extensive kidney damage has
already occurred.
The nephron is basically a tube that begins at
Bowman’s capsule and becomes the proximal and
then distal convoluted tubule (Figure 50.2). Bowman’s
capsule has a fenestrated or “window-like” epithelium
that works like a sieve or a strainer to allow fluid to flow
through but keep large components (e.g. proteins) from
entering. The tube exits the capsule curling around in
a section called the proximal convoluted tubule. From
there, it narrows to form the descending and ascending
loop of Henle. It widens as the distal convoluted tubule
and then flows into the collecting ducts, which meet at
the renal pelvises. Each section of the tubule functions in
a slightly different manner to maintain fluid and electro
lyte balance in the body.
Urinary bladder
Ureter
Fundus of bladder
Epithelial layer
Opening of ureter
Muscle layer
Neck of bladder
Urethral sphincter
Prostate gland
Urethra
FIGURE 50.1
The kidney and organs of the urinary tract.
Adrenal medulla
Adrenal cortex
Renal capsule
Renal cortex
Renal medulla
Ureter
Renal sinus
Renal pelvis
