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P A R T 9
Drugs acting on the renal system
a person develops renal failure and the production of
erythropoietin drops, the production of red blood cells
falls and the person becomes anaemic.
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The kidneys are two small, bean-shaped organs that
receive about 25% of the cardiac output.
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The nephron is the functional unit of the kidneys and
is involved in three processes: glomerular filtration,
tubular secretion and tubular reabsorption.
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The kidney plays a key role in regulating body fluid
volume and maintaining blood pressure, red blood
cell production, acid–base balance and electrolyte
stability.
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The renin–angiotensin–aldosterone system is
activated when blood flow to the nephron is
decreased and renin is released. The end result is
increased vasoconstriction and increased blood
pressure and sodium and water retention, which
increase blood volume and pressure.
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Red blood cell production is controlled by
erythropoietin released from the juxtaglomerular
apparatus when oxygen delivery to the nephron
is decreased. Erythropoietin stimulates the bone
marrow to produce red blood cells to increase
oxygen delivery to the nephrons.
THE URINARY TRACT
As noted previously, the urinary tract is composed of the
ureters, urinary bladder and urethra (see Figure 50.1).
Ureters
One ureter exits each kidney, draining the filtrate
from the collecting ducts. The ureters have a smooth
endothelial lining and circular muscular layers. Urine
entering the ureter stimulates a peristaltic wave that
pushes the urine down towards the urinary bladder.
Urinary bladder
The urinary bladder is a muscular pouch that stretches
and holds the urine until it is excreted from the body.
Urine is usually a slightly acidic fluid; this acidity helps
to maintain the normal transport systems and to destroy
bacteria that may enter the bladder. Control of bladder
emptying is learned control over the urethral sphincter;
once it is established, a functioning nervous system is
necessary to maintain control.
Urethra
In the female, the urethra is a very short tube that leads
from the bladder to an area populated by normal flora,
KEY POINTS
including
Escherichia coli
, which can cause frequent
bladder infections or cystitis. In the male, the urethra
is much longer and passes through the
prostate gland
,
a small gland that produces an alkaline fluid that is
important in maintaining the sperm and lubricating the
tract. Enlargement and infection in the prostate gland
are often problems in older men.
■■
The ureters, urinary bladder and urethra make up the
rest of the urinary tract.
■■
The shorter female urethra leads from the urinary
bladder to the outer body into an area rich in gram-
negative bacteria. Cystitis, or infection of the urinary
bladder, is a common problem for women.
■■
The longer male urethra passes through the prostate
gland, which may enlarge or become infected, a
problem often associated with advancing age.
CHAPTER SUMMARY
■■
The functional unit of the kidneys is called the
nephron; it is composed of Bowman’s capsule, the
proximal convoluted tubule, the loop of Henle, the
distal convoluted tubule and the collecting duct.
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The blood flow to the nephron is unique, allowing
autoregulation of blood flow through the glomerulus.
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Sodium levels are regulated throughout the tubule by
active and passive movement and are fine tuned by
the presence of aldosterone in the distal tubule.
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The countercurrent mechanism in the medullary
nephrons allows for the concentration or dilution
of urine under the influence of ADH secreted by the
hypothalamus.
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Potassium concentration is regulated throughout the
tubule, with aldosterone being the strongest influence
for potassium loss.
■■
The kidneys play a key role in the regulation of
calcium by activating vitamin D to allow GI calcium
reabsorption and by reabsorbing or excreting calcium
from the tubule under the influence of parathyroid
hormone.
■■
The kidneys influence blood pressure control,
releasing renin to activate the renin–angiotensin
system, which leads to increased blood pressure and
volume and a resultant increased blood flow to the
kidney. The balance of this reflex system can lead to
water retention or excretion and has an impact on
drug therapy that promotes water or sodium loss.
■■
The ureters, urinary bladder and urethra make up
the rest of the urinary tract. The longer male urethra
passes through the prostate gland, which may enlarge
or become infected, a problem often associated with
advancing age.
KEY POINTS
