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UNIT
Six
Alterations in the
Urinary System
22
CHAPTER
Control of Kidney Function
Tests of Renal Function
Urine Tests
Glomerular Filtration Rate
Blood Tests
Serum Creatinine
Blood Urea Nitrogen
Kidney Structure and Function
Gross Structure and Location
The Nephron
The Glomerulus
Tubular Components of the Nephron
Nephron Blood Supply
Urine Formation
Glomerular Filtration
Tubular Reabsorption and Secretion
Regulation of Renal Blood Flow
Neural and Humoral Control Mechanisms
Autoregulation
Elimination Functions of the Kidney
Renal Clearance
Regulation of Sodium and Potassium Elimination
Regulation of pH
pH-Dependent Elimination of Organic Ions
Uric Acid Elimination
Urea Elimination
Drug Elimination
Endocrine Functions of the Kidney
The Renin-Angiotensin-Aldosterone Mechanism
Erythropoietin
Vitamin D
■
It is no exaggeration to say that the composition of
the blood is determined not so much by what the
mouth takes in as by what the kidneys keep.
■
Homer Smith, From Fish to Philosopher
T
he kidneys are remarkable organs. Each is smaller than
a person’s fist, but in a single day the two organs process
approximately 1700 L of blood and combine its waste
products into approximately 1.5 L of urine. As part of their
function, the kidneys filter physiologically essential substan-
ces, such as sodium and potassium ions, from the blood and
selectively reabsorb those substances that are needed to main-
tain the normal composition of internal body fluids. Substan-
ces that are not needed for this purpose or are in excess pass
into the urine. In regulating the volume and composition of
body fluids, the kidneys perform excretory and endocrine func-
tions. The renin-angiotensin mechanism participates in the reg-
ulation of blood pressure and the maintenance of circulating
blood volume, and erythropoietin stimulates red blood cell
production.
401
402
Unit Six: Alterations in the Urinary System
topped by a region of cortex, forms a lobe of the kidney. The
apices of the pyramids form the papillae, which are perforated
by the openings of the collecting ducts. The renal pelvis is a
wide, funnel-shaped structure at the upper end of the ureter. It
is made up of the calices or cuplike structures that drain the
upper and lower halves of the kidney.
The kidney is ensheathed in a fibrous external capsule and
surrounded by a mass of fatty connective tissue, especially at its
ends and borders. The adipose tissue protects the kidney from
mechanical blows and assists, together with the attached blood
vessels and fascia, in holding the kidney in place. Although the
kidneys are relatively well protected, they may be bruised by
blows to the loin or by compression between the lower ribs and
the ilium. Because the kidneys are outside the peritoneal cav-
ity, injury and rupture do not produce the same threat of peri-
toneal involvement as rupture of organs such as the liver or
spleen.
Each kidney is supplied by a single renal artery that arises on
either side of the aorta. As the renal artery approaches the kid-
ney, it divides into five segmental arteries that enter the hilus of
the kidney. In the kidney, each segmental artery subdivides and
branches several times. The smallest branches, the intralobular
arteries, give rise to the afferent arterioles that supply the
glomeruli (Fig. 22-3).
KIDNEY STRUCTURE AND FUNCTION
Gross Structure and Location
The kidneys are paired, bean-shaped organs that lie outside the
peritoneal cavity in the back of the upper abdomen, one on
each side of the vertebral column at the level of the 12th tho-
racic to 3rd lumbar vertebrae (Fig. 22-1). The right kidney nor-
mally is situated lower than the left, presumably because of the
position of the liver. In the adult, each kidney is approximately
10 to 12 cm long, 5 to 6 cm wide, and 2.5 cm deep and weighs
approximately 113 to 170 g. The medial border of the kidney
is indented by a deep fissure called the
hilus
. It is here that
blood vessels and nerves enter and leave the kidney. The
ureters, which connect the kidneys with the bladder, also enter
the kidney at the hilus.
The kidney is a multilobular structure, composed of up to
18 lobes. Each lobule is composed of nephrons, which are the
functional units of the kidney. Each nephron has a glomerulus
that filters the blood and a system of tubular structures that se-
lectively reabsorb material from the filtrate back into the blood
and secrete materials from the blood into the filtrate as urine is
being formed.
On longitudinal section, a kidney can be divided into an
outer cortex and an inner medulla (Fig. 22-2). The cortex,
which is reddish-brown, contains the glomeruli and convo-
luted tubules of the nephron and blood vessels. The medulla
consists of light-colored, cone-shaped masses—the renal pyra-
mids—that are divided by the columns of the cortex (
i.e.
, col-
umns of Bertin) that extend into the medulla. Each pyramid,
The Nephron
Each kidney is composed of more than 1 million tiny, closely
packed functional units called
nephrons
. Each nephron con-
sists of a glomerulus, where blood is filtered, and a tubular
component. Here, water, electrolytes, and other substances
needed to maintain the constancy of the internal environment
are reabsorbed into the bloodstream while other unneeded
materials are secreted into the tubular filtrate for elimination
(see Fig. 22-4).
Diaphragm
T11
T12
Renal
artery
Adrenal
gland
The Glomerulus
The glomerulus consists of a compact tuft of capillaries en-
cased in a thin, double-walled capsule, called
Bowman’s cap-
sule
. Blood flows into the glomerular capillaries from the af-
ferent arteriole and flows out of the glomerular capillaries
into the efferent arteriole, which leads into the peritubular
capillaries. Fluid and particles from the blood are filtered
through the capillary membrane into a fluid-filled space in
Bowman’s capsule, called
Bowman’s space
. The portion of
the blood that is filtered into the capsule space is called
the
filtrate
. The mass of capillaries and its surrounding ep-
ithelial capsule are collectively referred to as the
renal corpus-
cle
(Fig. 22-5A). The glomerular capillary membrane is com-
posed of three layers: the capillary endothelial layer, the
basement membrane, and the single-celled capsular epithelial
layer (see Fig. 22-5B). The endothelial layer lines the glo-
merulus and interfaces with blood as it moves through the
capillary. This layer contains many small perforations, called
fenestrations
.
The epithelial layer that covers the glomerulus is continu-
ous with the epithelium that lines Bowman’s capsule. The
cells of the epithelial layer have unusual octopus-like struc-
tures that possess a large number of extensions, or
foot pro-
cesses
(
i.e.
, podocytes), which are embedded in the basement
membrane. These foot processes form
slit pores
through which
Renal
vein
Left kidney
Right
kidney
Aorta
Inferior
vena cava
Ureter
Bladder
Urethra
■
FIGURE 22-1
■
Kidneys, ureters, and bladder.
403
Chapter 22: Control of Kidney Function
Renal
cortex
Renal blood vessel
Renal medulla
Renal papillae
Renal column (Bertin)
Calyx (cut edge)
Renal pelvis
Renal artery
Calyx
Capsule
Ureter
■
FIGURE 22-2
■
Internal structure of the kidney.
the glomerular filtrate passes. The
basement membrane
consists
of a homogeneous acellular meshwork of collagen fibers, gly-
coproteins, and mucopolysaccharides (see Fig. 22-5C).
Because the endothelial and the epithelial layers of the
glomerular capillary have porous structures, the basement
membrane determines the permeability of the glomerular
capillary membrane. The spaces between the fibers that make
up the basement membrane represent the pores of a filter and
determine the size-dependent permeability barrier of the
glomerulus. The size of the pores in the basement membrane
normally prevents red blood cells and plasma proteins from
passing through the glomerular membrane into the filtrate.
There is evidence that the epithelium plays a major role in
producing the basement membrane components, and it is
probable that the epithelial cells are active in forming new
basement membrane material throughout life. Alterations in
the structure and function of the glomerular basement mem-
brane are responsible for the leakage of proteins and blood
cells into the filtrate that occurs in many forms of glomerular
disease.
Another important component of the glomerulus is the
mesangium
. In some areas, the capillary endothelium and the
basement membrane do not completely surround each cap-
illary. Instead, the mesangial cells, which lie between the
capillary tufts, provide support for the glomerulus in these
areas (see Fig. 22-5B). The mesangial cells produce an inter-
cellular substance similar to that of the basement membrane.
This substance covers the endothelial cells where they are not
covered by basement membrane. The mesangial cells possess
(or can develop) phagocytic properties and remove macro-
molecular materials that enter the intercapillary spaces. Mes-
angial cells also exhibit contractile properties in response to
neurohumoral substances and are thought to contribute to the
regulation of blood flow through the glomerulus. In normal
glomeruli, the mesangial area is narrow and contains only a
small number of cells. Mesangial hyperplasia and increased
mesangial matrix occur in a number of glomerular diseases.
Interlobular
artery
Intralobular
artery
Arcuate
artery
Tubular Components of the Nephron
The nephron tubule is divided into four segments: a highly
coiled segment called the
proximal convoluted tubule
, which
drains Bowman’s capsule; a thin, looped structure called the
loop of Henle
; a distal coiled portion called the
distal convoluted
tubule
; and the final segment called the
collecting tubule
, which
joins with several tubules to collect the filtrate (Fig. 22-4). The
filtrate passes through each of these segments before reaching
the pelvis of the kidney.
Nephrons can be roughly grouped into two categories.
Approximately 85% of the nephrons originate in the superficial
part of the cortex and are called
cortical nephrons
. They have
Interlobar
artery
Pyramid
Renal
artery
Ureter
■
FIGURE 22-3
■
Simplified illustration of the arterial supply
of the kidney. (Cormack D.H. [1987].
Ham’s histology
[9th ed.].
Philadelphia: J.B. Lippincott)
404
Unit Six: Alterations in the Urinary System
Proximal convoluted tube
Efferent arteriole
Bowman's capsule
Juxtaglomerular
apparatus
Glomerulus
Afferent arteriole
Interlobular artery
Interlobular vein
Distal convoluted tubule
Cortex
Medulla
Collecting tubule
Descending limb
Peritubular capillary
Ascending limb
Loop of Henle
To papilla
■
FIGURE 22-4
■
Nephron, showing the glomerular and tubular structures along with the blood supply.
short, thick loops of Henle that penetrate only a short distance
into the medulla. The remaining 15% are called
juxtamedullary
nephrons
. They originate deeper in the cortex and have longer
and thinner loops of Henle that penetrate the entire length of
the medulla. The juxtamedullary nephrons are largely con-
cerned with urine concentration.
The proximal tubule is a highly coiled structure that dips
toward the renal pelvis to become the descending limb of the
loop of Henle. The ascending loop of Henle returns to the re-
gion of the renal corpuscle, where it becomes the distal tubule.
The distal convoluted tubule, which begins at the juxtaglomer-
ular complex, is divided into two segments: the
diluting segment
and the
late distal tubule
. The late distal tubule fuses with the
collecting tubule. Like the distal tubule, the collecting duct is
divided into two segments: the
cortical collecting tubule
and the
inner medullary collecting tubule
.
Throughout its course, the tubule is composed of a single
layer of epithelial cells resting on a basement membrane. The
structure of the epithelial cells varies with tubular function.
KEY CONCEPTS
THE NEPHRON
The nephron, which contains a glomerulus and tu-
bular structures, is the functional unit of the kidney.
■
Each nephron is closely associated with two capillary
beds: the glomerulus, where water-soluble nutrients,
wastes, and other small particles are filtered from the
blood, and the peritubular capillaries that surround
the tubular structures.
■
Tubular structures process the glomerular (urine) fil-
trate, selectively reabsorbing substances from the
tubular fluid into the peritubular capillaries and se-
creting substances from the peritubular capillaries
into the urine filtrate.
■
405
Chapter 22: Control of Kidney Function
Nephron Blood Supply
The nephron is supplied by two capillary systems, the glomeru-
lus and the peritubular capillary network (Fig. 22-4). The glo-
merulus is a unique, high-pressure capillary filtration system
located between two arterioles—the afferent and the efferent
arterioles—that selectively dilate or constrict to regulate glo-
merular capillary pressure and consequently their filtration.
The peritubular capillary network is a low-pressure reabsorp-
tive system that originates from the efferent arteriole. These
capillaries surround all portions of the tubules, an arrangement
that permits rapid movement of solutes and water between the
fluid in the tubular lumen and the blood in the capillaries. The
medullary nephrons are supplied with two types of capillaries:
the peritubular capillaries, which are similar to those in the cor-
tex, and the vasa recta, which are long, straight capillaries. The
vasa recta accompany the long loops of Henle in the medullary
portion of the kidney to assist in exchange of substances flow-
ing in and out of that portion of the kidney and play an im-
portant role in concentrating the urine. The peritubular capil-
laries rejoin to form the venous channels by which blood
leaves the kidneys and empties into the inferior vena cava.
Although nearly all the blood flow to the kidneys passes
through the cortex, less than 10% is directed to the medulla
and only approximately 1% goes to the papillae. Under condi-
tions of decreased perfusion or increased sympathetic nervous
system stimulation, blood flow is redistributed away from the
cortex toward the medulla. This redistribution of blood flow
decreases glomerular filtration while maintaining the urine
concentrating ability of the peritubular capillaries, a factor that
is important during conditions such as shock.
Proximal
tubule
Efferent
arteriole
Bowman's
space
Afferent
arteriole
A
Mesangial
cell
Epithelial
podocytes
Basement
membrane
Glomerular
capsule
Bowman's
space
Endothelial
cell
Mesangial
matrix
B
Urine Formation
Urine formation involves the filtration of blood by the glo-
merulus to form an
ultrafiltrate of urine
and the tubular re-
absorption of electrolytes and nutrients needed to maintain the
constancy of the internal environment while eliminating waste
materials.
Bowman's
space
Glomerular Filtration
Urine formation begins with the filtration of essentially
protein-free plasma through the glomerular capillaries into
Bowman’s space. The movement of fluid through the glomeru-
lar capillaries is determined by the same factors (
i.e.
, capillary
filtration pressure, colloidal osmotic pressure, and capillary
permeability) that affect fluid movement through other capil-
laries in the body (see Chapter 6). The glomerular filtrate has a
chemical composition similar to plasma, but it contains almost
no proteins because large molecules do not readily cross the
glomerular wall. Approximately 125 mL of filtrate is formed
each minute. This is called the
glomerular filtration rate
(
GFR
).
This rate can vary from a few milliliters per minute to as high
as 200 mL/minute.
The location of the glomerulus between two arterioles al-
lows for maintenance of a high-pressure filtration system. The
capillary filtration pressure (approximately 60 mm Hg) in the
glomerulus is approximately two to three times higher than
that of other capillary beds in the body. The filtration pressure
and the GFR are regulated by the constriction and relaxation of
the afferent and efferent arterioles. Constriction of the efferent
arteriole increases resistance to outflow from the glomeruli and
Lumen of
capillary
Glomerular
basement
membrane
■
FIGURE 22-5
■
Renal corpuscle. (
A
) Structures of the glomeru-
lus. (
B
) Position of the mesangial cells in relation to the capillary
loops and Bowman’s capsule. (
C
) Cross-section of the glomeru-
lar membrane, showing the position of the endothelium, base-
ment membrane, and epithelial foot processes.
Epithelial foot
process
Endothelium
C
The cells of the proximal tubule have a fine villous structure
that increases the surface area for reabsorption; they also are
rich in mitochondria, which support active transport processes.
The epithelial layer of the thin segment of the loop of Henle
has few mitochondria, indicating minimal metabolic activity
and passive reabsorptive function.
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