LESSON 13
Topography and structure of
urinary system organs
ORGANIZATION OF THE URINARY SYSTEM
The urinary system (Figure 26-1a) includes the kidneys, ureters, urinary bladder, and urethra.
The excretory functions of the urinary system are performed by the two kidneys. These organs produce urine, a fluid containing water, ions, and small
soluble compounds. Urine leaving the kidneys travels along the paired ureters to the urinary bladder for temporary storage. Urine excretion, a process called urination, or micturition, occurs when the contraction of the muscular urinary bladder forces
urine through the urethra and out of the body.
The kidneys are located on either side of
the vertebral column between vertebrae T12 and L3. The left
kidney lies slightly superior to the right kidney (Figure 26-1a,b).
On gross dissection, the anterior surface
of the right kidney is covered by the liver, the right colic (hepatic) flexure
of the colon, and the duodenum. The anterior surface of the left kidney is
covered by the stomach, pancreas, jejunum, and left colic (splenic) flexure of
the colon. The superior surface of each kidney is capped by an adrenal gland
(Figures 26-1a,b and 26-2a
, b
). The kidneys and adrenal glands lie between the muscles of the dorsal
body wall and the parietal peritoneum in a retroperitoneal position (Figure
26-1c
).
The position of the kidneys in the
abdominal cavity is maintained by (1) the overlying peritoneum, (2) contact
with adjacent visceral organs, and (3) supporting connective tissues. Each
kidney is protected and stabilized by three concentric layers of connective
tissue (Figure 26-1c):
1.
The renal capsule is a
layer of collagen fibers that covers the outer surface of the entire organ.
This layer is also known as the fibrous tunic of the kidney.
2.
The adipose capsule, a
layer of adipose tissue, surrounds the renal capsule. This layer can be quite
thick, and on dissection it generally obscures the outline of the kidney.
3.
The renal fascia is a
dense outer layer. Collagen fibers extend outward from the renal capsule
through the adipose capsule to this layer. The renal fascia anchors the kidney
to surrounding structures. Posteriorly, the renal fascia fuses with the deep
fascia surrounding the muscles of the body wall. Anteriorly, the renal fascia
forms a thick fibrous layer that fuses with the peritoneum.
4.
In effect, each kidney hangs suspended by
collagen fibers from the renal fascia and packed in a soft cushion of adipose
tissue. This arrangement prevents the jolts and shocks of day-to-day existence
from disturbing normal kidney function. If the suspensory fibers break or
become detached, a slight bump or jar may displace the kidney and stress the
attached vessels and ureter. This condition, called a floating kidney,
can be especially dangerous, because the ureters or renal blood vessels may
become twisted or kinked during movement.
Superficial
Anatomy of the Kidneys
Each reddish brown kidney has the shape of
a kidney bean. A typical adult kidney (Figures 26-2a, b
and 26-3
) is about
Sectional
Anatomy of the Kidneys
The fibrous renal capsule has inner and
outer layers. In sectional view (Figure 26-3a
), the inner layer folds inward at the hilus and lines an internal
cavity, the renal sinus. Renal blood vessels and the ureter draining the kidney
pass through the hilus and branch within the renal sinus. A thickened, outer
layer of the capsule extends across the hilus and stabilizes the position of
these structures.
The renal cortex is the outer layer of the
kidney in contact with the capsule. The cortex is reddish brown and granular in texture. The renal medulla consists of
6 to 18 distinct conical or triangular structures called renal pyramids. The
base of each pyramid faces the cortex, and the tip of each pyramid, a region
known as the renal papilla, projects into the renal sinus. Each pyramid has a
series of fine grooves that converge at the papilla. Adjacent renal pyramids
are separated by bands of cortical tissue called renal columns, which extend
into the medulla. The columns have a distinctly granular texture, similar to
that of the cortex. A renal lobe consists of a renal pyramid, the overlying
area of renal cortex, and adjacent tissues of the renal columns.
Urine production occurs in the renal
lobes. Ducts within each renal papilla discharge urine into a cup-shaped drain
called a minor calyx. Four or five minor calyces merge to form a major calyx, and two or
three major calyces combine to form the renal pelvis, a large, funnel-shaped
chamber. The renal pelvis, which fills most of the renal sinus, is connected to
the ureter at the hilus of the kidney.
Urine production begins in microscopic
structures called nephrons in the cortex of each renal lobe. There are roughly 1.25 million
nephrons in each kidney, with a combined length of about
Each nephron consists of a renal
corpuscle and a renal tubule roughly ). The renal corpuscle is approximately 200 µm (
From the renal corpuscle, the filtrate
enters a long tubular passageway. The renal tubule has two convoluted (coiled
or twisted) segments--the proximal convoluted tubule (PCT) and the distal
convoluted tubule (DCT)--separated by a simple U-shaped tube, the loop
of Henle. The convoluted segments are in the cortex, and the loop extends
partially or completely into the medulla. For clarity, the nephron diagrammed
in Figure 26-4 has been shortened and straightened. The regions of the nephron vary in
their structural and functional characteristics. As it travels along the
tubule, the filtrate, now called tubular fluid, gradually changes in composition.
The changes that occur and the characteristics of the urine that results vary
with the activities under way in each segment of the nephron; Figure 26-4
provides an overview of the regional specializations.
Each nephron empties into the collecting
system. A connecting tubule carries the tubular fluid from the distal
convoluted tubule to a nearby collecting duct. The collecting duct,
which receives tubular fluid from many different nephrons, leaves the cortex
and descends into the medulla, carrying fluid to a papillary duct that
drains into a minor calyx.
The urine arriving at the renal pelvis is
very different from the filtrate produced at the renal corpuscle. Filtration is
a passive process that permits or prevents movement across a barrier solely on
the basis of solute size. A filter with pores large enough to permit the
passage of organic waste products is unable to prevent the passage of water,
ions, and other organic molecules, such as glucose, fatty acids, and amino
acids. These useful substances must be reclaimed and the waste products
excreted. The segments of the nephron distal to the renal corpuscle are
responsible for:
Additional water and salts will be removed
from the tubular fluid in the collecting system before the fluid is released
into the renal sinus as urine. Table 26-1
gives an overview of important information concerning the regions of the
nephron and collecting system.
Nephrons differ slightly in structure,
depending on their location. Roughly 85 percent of all nephrons are cortical
nephrons; they are located in the superficial cortex of the kidney. The
remaining 15 percent of nephrons, termed juxtamedullary nephrons (juxta,
near), are located closer to the medulla. Because they are more numerous than
juxtamedullary nephrons, cortical nephrons perform most of the reabsorptive and
secretory functions of the kidneys. However, the juxtamedullary nephrons are
responsible for the ability to produce a concentrated urine.
We shall now examine the structure of each
segment of a representative nephron.
The Renal
Corpuscle
The renal corpuscle (Figure 26-5a,b,c) has a diameter averaging 150-250 µm. It includes (1) the glomerular
capillary network and (2) a region known as Bowman's
capsule. Connected to the
initial segment of the renal tubule, Bowman's capsule forms the outer wall of
the renal corpuscle and covers the glomerular capillaries.
Bowman'S
Capsule
The glomerulus projects into Bowman's capsule much as the heart projects into
the pericardial cavity (Figure 26-5c). The outer wall of the capsule is lined by a simple squamous parietal
epithelium (capsular epithelium). This layer is continuous with the
visceral epithelium (glomerular epithelium) that covers the glomerular
capillaries. The visceral epithelium consists of large cells with complex
processes, or "feet," that wrap around the lamina densa, the
specialized basement membrane of the glomerular capillaries (Figure 26-5c
, e
). These unusual cells are called podocytes (podos, foot + -cyte, cell). The podocyte feet are known
as pedicels. Materials passing out of the blood at the glomerulus must be small
enough to pass between the narrow gaps, or filtration slits, between adjacent
pedicels. These slits are small enough to prevent the loss of all but the
smallest plasma proteins.
The capsular space separates the visceral
and parietal epithelia (Figures 26-4,and 26-5b,c
). The connection between the two epithelial layers lies at the vascular
pole of the renal corpuscle. At the vascular pole, blood flows into and out of
the glomerular capillaries. Blood arrives in an afferent
arteriole and departs in an efferent arteriole.
The Glomerular Capillaries
The glomerular capillaries (Figure 26-5c) are fenestrated capillaries whose endothelium contains
large-diameter pores. The openings are small enough to prevent the passage of
blood cells but too large to restrict the diffusion of dissolved or suspended
compounds, even those the size of plasma proteins.
The endothelial cells lining the
capillaries are surrounded by the lamina densa (Figure 26-5e). During filtration, the lamina densa restricts the passage of large
plasma proteins but permits the movement of smaller molecules, including
albumin, many organic nutrients, and ions. Unlike basement membranes elsewhere,
the lamina densa may encircle two or more capillaries. When it does, mesangial
cells are situated between the capillaries. Mesangial
cells have several important functions:
Together, the fenestrated endothelium, the
lamina densa, and the filtration slits form the filtration membrane.
During filtration, blood pressure forces water and small solutes across this
membrane and into the capsular space. The larger solutes, especially plasma
proteins, are excluded. Filtration at the renal corpuscle is both effective and
passive, but it has one major limitation: In addition to metabolic wastes and
excess ions, compounds such as glucose, free fatty acids, amino acids,
vitamins, and other solutes enter the capsular space. These potentially useful
materials are recaptured before the filtrate leaves the kidneys, with much of
the reabsorption occurring in the proximal convoluted tubule.
The
Proximal Convoluted Tubule
The entrance to the proximal
convoluted tubule (PCT) lies almost directly
opposite the vascular pole, at the tubular pole of the renal corpuscle (Figure
26-5c). The lining of the PCT consists of a simple cuboidal epithelium whose
exposed surfaces are blanketed with microvilli (Figure 26-4
). The cuboidal tubular cells actively absorb organic nutrients, ions,
and plasma proteins (if any) from the tubular fluid and release them into the
peritubular fluid, the interstitial fluid surrounding the renal tubule. As
these solutes are absorbed and transported, osmotic forces pull water across
the wall of the PCT and into the peritubular fluid. Although reabsorption is
the primary function of the PCT, the epithelial cells can also secrete
substances into the lumen.
The Loop
of Henle
The PCT makes an acute bend that turns the
renal tubule toward the renal medulla. This turn marks the start of the loop of Henle (Figures 26-4b and 26-5a
). The loop of Henle can be divided into a descending limb and an
ascending limb. Fluid in the descending limb travels toward the renal pelvis,
and that in the ascending limb travels toward the renal cortex. Each limb
contains a thick segment and a thin segment. (The terms thick and thin
refer to the height of the epithelium, not to the diameter of the lumen.)
The thick segments have a cuboidal
epithelium, whereas a thin squamous epithelium lines the thin segments (Figure
26-4). The thick descending limb has functions similar to those of the PCT.
The thick ascending limb pumps sodium and chloride ions out of the tubular
fluid. The effect of this pumping is most noticeable in the medulla, where the
long ascending limbs of juxtamedullary nephrons create unusually high solute
concentrations in the peritubular fluid.
The
Distal Convoluted Tubule
The thick ascending limb of the loop of
Henle ends where it forms a sharp angle near the vascular pole of the renal
corpuscle. The distal
convoluted tubule (DCT) begins there. The
initial portion of the DCT passes between the afferent and efferent arterioles
(Figure 26-5c).
In sectional view (Figure 26-4), the DCT differs from the PCT in that (1) the DCT has a smaller
diameter, (2) the epithelial cells of the DCT lack microvilli, and (3) the
boundaries between the epithelial cells in the DCT are distinct. The DCT is an important site for:
The
Juxtaglomerular Apparatus
The epithelial cells of the DCT near the vascular pole of the renal corpuscle
are taller than those elsewhere along the DCT, and their nuclei are clustered
together. This region, detailed in Figure 26-5c, is called the macula densa. The cells of the macula densa are closely associated with unusual
smooth muscle fibers in the wall of the afferent arteriole. These fibers are
known as juxtaglomerular
cells. Together, the macula densa and
juxtaglomerular cells form the juxtaglomerular
apparatus (JGA). The
juxtaglomerular apparatus is an endocrine structure that secretes the erythropoietin
and renin, as we described in Chapter 18.
The
Collecting System
The DCT, the last segment of the nephron,
opens into the collecting system. The collecting system consists of connecting
tubules, collecting ducts, and papillary ducts (Figure 26-4). Individual connecting tubules connect each nephron to a nearby
collecting duct (Figure 26-6
). Each collecting duct receives tubular fluid from many connecting tubules.
Several collecting ducts converge to empty into a larger papillary duct, which
in turn empties into a minor calyx. The epithelium lining the collecting system
begins with simple cuboidal cells in the connecting tubules and changes to a
columnar epithelium in the collecting and papillary ducts.
In addition to transporting tubular fluid
from the nephron to the renal pelvis, the collecting system adjusts its
composition and determines the final osmotic concentration and volume of the
urine.
The Blood
Supply to the Kidneys
Your kidneys receive 20-25 percent of your
total cardiac output. In normal individuals, about 1200 ml of blood flows
through the kidneys each minute. That is a phenomenal amount of blood for
organs with a combined weight of less than
Each kidney receives blood from a renal
artery that originates along the lateral surface of the abdominal aorta
near the level of the superior mesenteric artery (Figure 21-26). As it enters the renal sinus, the renal artery provides blood to the
segmental arteries (Figure 26-7a
). Segmental arteries further divide into a series of interlobar
arteries that radiate outward through the renal columns between the renal
pyramids. The interlobar arteries supply blood to the arcuate arteries, which
arch along the boundary between the cortex and medulla of the kidney. Each
arcuate artery gives rise to a number of interlobular arteries, which supply
parts of the adjacent renal lobe. Branching from each interlobular artery are
numerous afferent arterioles (Figure 26-7b
).
Blood reaches the vascular pole of each
glomerulus through an afferent arteriole and leaves in an efferent arteriole
(Figures 26-5c and 26-7c
). Blood travels from the efferent arteriole to form a capillary plexus,
a network of peritubular
capillaries that supplies the PCT and
DCT. The peritubular capillaries provide a route for the pickup or delivery of
substances that are reabsorbed or secreted by these portions of the nephron.
In juxtamedullary nephrons, the efferent
arterioles and peritubular capillaries are connected to a series of long,
slender capillaries that accompany the loops of Henle into the medulla (Figure
26-7d). These capillaries, known as the vasa recta (rectus, straight),
absorb and transport solutes and water reabsorbed into the medulla from tubular
fluid in the loops of Henle and collecting ducts. Under normal conditions, the
removal of solutes and water by the vasa recta balances the rates of solute and
water reabsorption in the medulla.
From the peritubular capillaries and vasa
recta, blood enters a network of venules and small veins that converge on the
interlobular veins. In a mirror image of the arterial distribution, the interlobular
veins deliver blood to arcuate veins, which empty into interlobar veins. The
interlobar veins drain into the segmental veins, which merge to form a renal
vein. Many of the blood vessels just described are visible in the corrosion
cast of the kidneys shown in Figure 26-8.
Innervation
of the Kidneys
The kidneys and ureters are innervated by
renal nerves. Most of the nerve fibers involved are sympathetic postganglionic
fibers from the superior mesenteric ganglion. A renal nerve enters each kidney
at the hilus and follows the tributaries of the renal arteries to reach
individual nephrons. The sympathetic innervation targets (1) the
juxtaglomerular apparatus, (2) the smooth muscles in the walls of the afferent
and efferent arterioles, and (3) mesangial cells. Known
functions of sympathetic innervation include the following:
CONCEPT CHECK QUESTIONS
1.
What portions of the
nephron are located in the renal cortex?
2.
Why don't plasma proteins
pass into the capsular space under normal circumstances?
3.
Damage to what part of
the nephron would interfere with the control of blood pressure?
URINE TRANSPORT, STORAGE, AND
ELIMINATION
Filtrate modification and urine production
end when the fluid enters the renal pelvis. The remaining parts of the urinary
system (the ureters, urinary bladder, and urethra) are
responsible for the transport, storage, and elimination of urine. A pyelogram
is an image of the urinary system, obtained by taking an X-ray of the kidneys
after a radiopaque compound has been administered. Such an image provides an
orientation to the relative sizes and positions of these organs. The sizes of
the minor and major calyces, the renal pelvis, the ureters, the urinary bladder,
and the proximal portion of the urethra are somewhat variable, because these
regions are lined by a transitional epithelium that can tolerate cycles
of distension and contraction without damage.
We shall now examine these components of
the urinary system.
The ureters are a pair of muscular tubes
that extend inferiorly from the kidneys for about ). The ureters extend inferiorly and medially, passing over the anterior
surfaces of the psoas major muscles (Figures 26-1c
and 26-2b
). The ureters are retroperitoneal and are firmly attached to the
posterior abdominal wall. The paths taken by the ureters in men and women are
different due to variations in the nature, size, and position of the
reproductive organs. As Figure 26-19a
shows, in males, the base of the urinary bladder lies between the
rectum and the pubic symphysis; in females, the base of the urinary bladder
sits inferior to the uterus and anterior to the vagina (Figure 26-19b
).
The ureters penetrate the posterior wall
of the urinary bladder without entering the peritoneal cavity. They pass
through the bladder wall at an oblique angle, and the ureteral openings are
slit-like rather than rounded (Figure 26-19c). This shape helps prevent the backflow of urine toward the ureter and
kidneys when the urinary bladder contracts.
Histology
of the Ureters
The wall of each ureter consists of three
layers: (1) an inner mucosa covered by a transitional epithelium, (2) a middle
muscular layer made up of longitudinal and circular bands of smooth muscle, and
(3) an outer connective tissue layer that is continuous with the fibrous renal
capsule and peritoneum. About every 30 seconds, a peristaltic contraction
begins at the renal pelvis and sweeps along the ureter, forcing urine toward
the urinary bladder.
The urinary bladder is a hollow, muscular
organ that functions as a temporary storage reservoir for urine. The dimensions
of the urinary bladder vary with the state of distension, but the full urinary
bladder can contain about a liter of urine.
The superior surfaces of the urinary
bladder are covered by a layer of peritoneum, and several peritoneal folds
assist in stabilizing its position (Figure 26-19c). The middle umbilical ligament extends from the anterior and superior
border toward the umbilicus (navel). The lateral umbilical ligaments pass along
the sides of the bladder and also reach the umbilicus. These fibrous cords
contain the vestiges of the two umbilical arteries, which supplied blood
to the placenta during embryonic and fetal development.
The urinary bladder's posterior, inferior, and anterior surfaces lie
outside the peritoneal cavity. In these areas, tough ligamentous bands anchor
the urinary bladder to the pelvic and pubic bones.
In sectional view, the mucosa lining the
urinary bladder is usually thrown into folds, or rugae, that disappear as the
bladder fills. The triangular area bounded by the ureteral openings and the
entrance to the urethra constitutes the trigone of the urinary bladder. The mucosa here is smooth and very thick. The
trigone acts as a funnel that channels urine into the urethra when the urinary
bladder contracts.
The urethral entrance lies at the apex of
the trigone, at the most inferior point in the urinary bladder. The region
surrounding the urethral opening, known as the neck of the urinary bladder,
contains a muscular internal urethral sphincter, or sphincter vesicae.
The smooth muscle fibers of the internal urethral sphincter provide involuntary
control over the discharge of urine from the urinary bladder. The urinary
bladder is innervated by postganglionic fibers from ganglia in the hypogastric
plexus and by parasympathetic fibers from intramural ganglia that are
controlled by branches of the pelvic nerves.
Histology
of the Urinary Bladder
The wall of the urinary bladder contains
mucosa, submucosa, and muscularis layers (Figure 26-20).The muscularis layer consists of inner and outer longitudinal smooth
muscle layers, with a circular layer sandwiched between. Collectively, these
layers form the powerful detrusor muscle of the urinary bladder. Contraction of this muscle compresses the
urinary bladder and expels its contents into the urethra.
The urethra extends from the neck of the
urinary bladder (Figure 26-19c) to the exterior. The female and male urethrae differ in length and in
function. In females, the urethra is very short, extending 3-
). The external urethral opening, or external urethral meatus, is
situated near the anterior wall of the vagina.
In males, the urethra extends from the
neck of the urinary bladder to the tip of the penis, a distance that may be 18-, c
): (1) the prostatic urethra, (2) the membranous urethra,
and (3) the penile urethra.
The prostatic urethra passes through the
center of the prostate gland (Figure 26-19c). The membranous urethra includes the short segment that penetrates the
urogenital diaphragm, the muscular floor of the pelvic cavity. The
penile urethra extends from the distal border of the urogenital diaphragm to
the external urethral meatus at the tip of the penis (Figure 26-19a
). We shall consider the functional differences among these regions in
Chapter 28.
In both genders, as the urethra passes
through the urogenital diaphragm, a circular band of skeletal muscle forms the
external urethral sphincter. This muscular band acts as a valve. The external
urethral sphincter is under voluntary control, via the perineal branch of the pudendal nerve. This sphincter has a resting muscle tone and must be voluntarily
relaxed to permit micturition.
Histology
of the Urethra
The urethral lining consists of a
stratified epithelium that varies from transitional at the neck of the urinary
bladder, through stratified columnar at the midpoint, to stratified squamous
near the external urethral meatus. The lamina propria is thick and elastic, and
the mucous membrane is thrown into longitudinal folds. Mucin-secreting cells
are located in the epithelial pockets. In males, the epithelial mucous glands
may form tubules that extend into the lamina propria. Connective tissues of the
lamina propria anchor the urethra to surrounding structures. In females, the
lamina propria contains an extensive network of veins, and the entire complex
is surrounded by concentric layers of smooth muscle.
The URINARY BLADDER lies in cavity of lesser pelvis
behind pubic symphysis. It has an apex, body and fundus,
which is directed down and posterior. Inferior part forms а neck, which
continues into urethra. Empty urinary bladder lies extraperitoneally. Full
bladder covered by peritoneum anteriorly, laterally and posteriorly -
mesoperitoneal position. Fundus of the bladder in male adjoins from below to
prostate gland, seminal vesicles and ampoule of ductus deferens, and behind -
to ampoule of rectum. In female urinary bladder behind adjoins to vagina and
uterus.
The
urinary bladder is a
musculomembranous sac which acts as a reservoir for the urine; and as its size,
position, and relations vary according to the amount of fluid it contains, it
is necessary to study it as it appears (a) when empty, and (b)
when distended.) In both conditions the position of the bladder varies
with the condition of the rectum, being pushed upward and forward when the
rectum is distended.
The Empty Bladder.—When hardened in
situ, the empty bladder has the form of a flattened tetrahedron, with its
vertex tilted forward. It presents a fundus, a vertex, a superior and an
inferior surface. The fundus is
triangular in shape, and is directed downward and backward toward the rectum,
from which it is separated by the rectovesical fascia, the vesiculæ
seminales, and the terminal portions of the ductus deferentes. The vertex is directed forward toward the
upper part of the symphysis pubis, and from it the middle umbilical ligament is
continued upward on the back of the anterior abdominal wall to the umbilicus.
The peritoneum is carried by it from the vertex of the bladder on to the
abdominal wall to form the middle umbilical fold. The superior surface is triangular, bounded on either side by a
lateral border which separates it from the inferior surface, and behind by a
posterior border, represented by a line joining the two ureters, which
intervenes between it and the fundus. The lateral borders extend from the
ureters to the vertex, and from them the peritoneum is carried to the walls of
the pelvis. On either side of the bladder the peritoneum shows a depression,
named the paravesical fossa. The
superior surface is directed upward, is covered by peritoneum, and is in
relation with the sigmoid colon and some of the coils of the small intestine.
When the bladder is empty and firmly contracted, this surface is convex and
the lateral and posterior borders are rounded; whereas if the bladder be
relaxed it is concave, and the interior of the viscus, as seen in a median
sagittal section, presents the appearance of a V-shaped slit with a shorter
posterior and a longer anterior limb—the apex of the V corresponding with the
internal orifice of the urethra. The inferior
surface is directed downward and is uncovered by peritoneum. It may be divided
into a posterior or prostatic area and two infero-lateral surfaces. The
prostatic area is somewhat triangular: it rests upon and is in direct
continuity with the base of the prostate; and from it the urethra emerges. The
infero-lateral portions of the inferior surface are directed downward and
lateralward: in front, they are separated from the symphysis pubis by a mass of
fatty tissue which is named the retropubic
pad; behind, they are in contact with the fascia which covers the
Levatores ani and Obturatores interni.
Median
sagitta section of male pelvis.
1, corpus
cavernosum 2, corpus spongiosum (bulb of the penis) 3, ramus ischium 4,
ischiocavernosus m. 5, anal canal 6, sphincter ani externus m. 7, gluteus
maximus m.
When
the bladder is empty it is placed entirely within the pelvis, below the level
of the obliterated hypogastric arteries, and below the level of those portions
of the ductus deferentes which are in contact with the lateral wall of the
pelvis; after they cross the ureters the ductus deferentes come into contact
with the fundus of the bladder. As the viscus fills, its fundus, being more or
less fixed, is only slightly depressed; while its superior surface gradually
rises into the abdominal cavity, carrying with it its peritoneal covering, and
at the same time rounding off the posterior and lateral borders.
When the bladder is moderately full it contains about
Male
pelvic organs seen from right side. Bladder and rectum distended; relations of
peritoneum to the bladder and rectum shown in blue. The arrow points to the
rectovesical pouch.
The Bladder in the Child—In the
newborn child the internal urethral orifice is at the level of the upper border
of the symphysis pubis; the bladder therefore lies relatively at a much higher
level in the infant than in the adult. Its anterior surface “is in contact with
about the lower two-thirds of that part of the
abdominal wall which lies between the symphysis pubis and the umbilicus”. Its
fundus is clothed with peritoneum as far as the level of the internal orifice
of the urethra. Although the bladder of the infant is usually described as an
abdominal organ, Symington has pointed out that only about one-half of it lies
above the plane of the superior aperture of the pelvis. Disse maintains that
the internal urethral orifice sinks rapidly during the first years, and then
more slowly until the ninth year, after which it remains sta when it again
slowly descends and reaches its adult position.
Sagittal section through the pelvis of a newly born male child.
1,
rectus abdominis m. 2, bladder 3, pubis 4, ischium 5, testis 6, corpus
cavernosum
1,
rectus abdominis m. 2, symphysis pubis 3, corpus cavernosum 4, corpus
spongiosum 5, prostate 6, bladder 7,seminal vesicle 8, rectum 9, sacrum
Sagittal section through the pelvis of a newly born female child.
The Female Bladder—In the
female, the bladder is in relation behind with the uterus and the upper part of
the vagina. It is separated from the anterior surface of the body of the uterus
by the vesicouterine excavation, but below the level of
this excavation it is connected to the front of the cervix uteri and the upper
part of the anterior wall of the vagina by areolar tissue. When the bladder is
empty the uterus rests upon its superior surface. The female bladder is said by
some to be more capacious than that of the male, but probably the opposite is
the case.
Median
sagittal section of female pelvis.
Ligaments.—The bladder is connected to
the pelvic wall by the fascia endopelvina. In front this fascial attachment is
strengthened by a few muscular fibers, the Pubovesicales, which extend from the back of the pubic bones to
the front of the bladder; behind, other muscular fibers run from the fundus of
the bladder to the sides of the rectum, in the sacrogenital folds, and
constitute the Rectovesicales.
The
vertex of the bladder is joined to the umbilicus by the remains of the urachus
which forms the middle umbilical
ligament, a fibromuscular cord, broad at its attachment to the bladder
but narrowing as it ascends.
From
the superior surface of the bladder the peritoneum is carried off in a series
of folds which are sometimes termed the false
ligaments of the bladder. Anteriorly there are three folds: the middle umbilical fold on the middle
umbilical ligament, and two lateral
umbilical folds on the obliterated hypogastric arteries. The reflections
of the peritoneum on to the side walls of the pelvis form the lateral false
ligaments, while the sacrogenital folds constitute posterior false ligaments.
Interior of the Bladder—The mucous
membrane lining the bladder is, over the greater part of the viscus, loosely
attached to the muscular coat, and appears wrinkled or folded when the bladder
is contracted: in the distended condition of the bladder the folds are effaced.
Over a small triangular area, termed the trigonum vesicæ, immediately above and behind the internal
orifice of the urethra, the mucous membrane is firmly bound to the muscular
coat, and is always smooth. The anterior angle of the
trigonum vesicæ is formed by the internal orifice of the urethra: its
postero-lateral angles by the orifices of the ureters. Stretching behind the
latter openings is a slightly curved ridge, the torus uretericus, forming the base of the trigone and produced by
an underlying bundle of non-striped muscular fibers. The lateral parts of this
ridge extend beyond the openings of the ureters, and are named the plicæ uretericæ; they are
produced by the terminal portions of the ureters as they traverse obliquely the
bladder wall. When the bladder is illuminated the torus uretericus appears as a
pale band and forms an important guide during the operation of introducing a
catheter into the ureter.
The interior of bladder.
The
orifices of the ureters are
placed at the postero-lateral angles of the trigonum vesicæ, and are
usually slit-like in form. In the contracted bladder they are about
The
internal urethral orifice is
placed at the apex of the trigonum vesicæ, in the most dependent part of
the bladder, and is usually somewhat crescentic in form; the mucous membrane
immediately behind it presents a slight elevation, the uvula vesicæ, caused by the middle lobe of the prostate.
Structure—The bladder is composed of
the four coats: serous, muscular,
submucous, and mucous coats.
The
serous coat (tunica serosa)
is a partial one, and is derived from the peritoneum. It invests the superior
surface and the upper parts of the lateral surfaces, and is reflected from
these on to the abdominal and pelvic walls.
The
muscular coat (tunica
muscularis) consists of three layers of unstriped muscular fibers: an
external layer, composed of fibers having for the most part a longitudinal
arrangement; a middle layer, in which the fibers are arranged, more or less, in
a circular manner; and an internal layer, in which the fibers have a general
longitudinal arrangement.
The
fibers of the external layer arise from the posterior surface of the
body of the pubis in both sexes (musculi pubovesicales), and in the male
from the adjacent part of the prostate and its capsule.
They pass, in a more or less longitudinal manner, up the inferior surface of
the bladder, over its vertex, and then descend along its fundus to become
attached to the prostate in the male, and to the front of the vagina in the
female. At the sides of the bladder the fibers are arranged obliquely and
intersect one another. This layer has been named the Detrusor urinæ muscle.
The
fibers of the middle circular layer are very thinly and irregularly
scattered on the body of the organ, and, although to some extent placed
transversely to the long axis of the bladder, are for the most part arranged
obliquely. Toward the lower part of the bladder, around the internal urethral
orifice, they are disposed in a thick circular layer, forming the Sphincter vesicæ, which is
continuous with the muscular fibers of the prostate.
The
internal longitudinal layer is thin, and its fasciculi have a reticular
arrangement, but with a tendency to assume for the most part a longitudinal
direction. Two bands of oblique fibers, originating behind the orifices of the
ureters, converge to the back part of the prostate, and are inserted by means
of a fibrous process, into the middle lobe of that organ. They are the muscles of the ureters, described by
Sir C. Bell, who supposed that during the contraction of the bladder they serve
to retain the oblique direction of the ureters, and so prevent the reflux of
the urine into them.
The
submucous coat (tela
submucosa) consists of a layer of areolar tissue, connecting together the
muscular and mucous coats, and intimately united to the latter.
Vertical section of bladder wall.
The
mucous coat (tunica mucosa)
is thin, smooth, and of a pale rose color. It is continuous above through the
ureters with the lining membrane of the renal tubules, and below with that of
the urethra. The loose texture of the submucous layer allows the mucous coat to
be thrown into folds or rugæ when the bladder is empty. Over the
trigonum vesicæ the mucous membrane is closely attached to the muscular
coat, and is not thrown into folds, but is smooth and flat. The epithelium
covering it is of the transitional variety, consisting of a superficial layer
of polyhedral flattened cells, each with one, two, or three nuclei; beneath
these is a stratum of large club-shaped cells, with their narrow extremities
directed downward and wedged in between smaller spindle-shaped cells,
containing oval nuclei. The epithelium varies according as the bladder is
distended or contracted. In the former condition the superficial cells are
flattened and those of the other layers are shortened; in the latter they
present the appearance described above. There are no true glands in the mucous
membrane of the bladder, though certain mucous follicles which exist,
especially near the neck of the bladder, have been regarded as such.
Vessels and Nerves.—The arteries supplying the bladder are the
superior, middle, and inferior vesical, derived from the anterior trunk of the
hypogastric. The obturator and inferior gluteal arteries also supply small
visceral branches to the bladder, and in the female additional branches are
derived from the uterine and vaginal arteries.
The
veins form a complicated plexus
on the inferior surface, and fundus near the prostate, and end in the
hypogastric veins.
The
nerves of the bladder are (1)
fine medullated fibers from the third and fourth sacral nerves, and (2)
non-medullated fibers from the hypogastric plexus. They are connected with
ganglia in the outer and submucous coats and are finally distributed, all as
non-medullated fibers, to the muscular layer and epithelial lining of the
viscus.
Abnormalities.—A defect of
development, in which the bladder is implicated, is known under the name of extroversion
of the bladder. In this condition the lower part of the abdominal wall and
the anterior wall of the bladder are wanting, so that the fundus of the bladder
presents on the abdominal surface, and is pushed forward by the pressure of the
viscera within the abdomen, forming a red vascular tumor on which the openings
of the ureters are visible. The penis, except the glans, is rudimentary and is
cleft on its dorsal surface, exposing the floor of the urethra, a condition
known as epispadias. The pelvic bones are also arrested in
development.
Wall of urinary bladder is formed by mucous membrane and well
developed submucous stratum, thanks it mucous membrane forms the numerous
folds. Between orifices of ureters and internal urethral ostium
submucous base absent, so there are no folds here. This place called as
triangle of bladder. It is limited above interureteric fold of mucous membrane.
Middle membrane of urinary bladder is a muscular membrane, where muscles
are arranged in three layers: internal and external longitudinal and middle -
circular. The muscular layers form in area of the body muscle-detrussor
of bladder, and a circular layer most developed in area of internal urethral
ostium, forms an internal urethral muscle-sphincter (involuntary).
Two
Kidneys are pair parenchymatic organs, which positioned in
abdominal cavity behind peritoneum (retroperitoneal position) in right and left
lumbar regions. Kidney is projected on front abdominal wall in
epigastric, lateral and umbilical regions. Right kidney extends from Th 12
vertebra till L 3 lumbar vertebra, left one - from Th 11 vertebra till L 2
lumbar vertebra.
Posterior abdominal wall, after removal
of the peritoneum, showing kidneys, suprarenal capsules, and great vessels.
Posterior surface of each kidney in superior part
adjoins to diaphragm, and in middle and inferior - to muscular bed, which is
formed by muscle: psoas major, quadratus lumborum and transverse abdominis. To anterior
surface of left kidney adrenal gland adjoins above, to superolateral
part - spleen, to middle portion - stomach and pancreas, inferiorly - medially
is loops of small intestine, and superolaterally - colon. To anterior
surface of right kidney suprarenal gland adjoins above, to middle part -
liver, to medial margin - duodenum, to inferiomedial - loops of small intestine
and to inferiolateral - large intestine.
Vertical section of kidney.
Each kidney has superior extremity and inferior extremity,
anterior surface and posterior surface, medial margin
(concave) and lateral margin (convex). On medial margin are situated the
renal hilus, where artery, nerves enter, and vein, lymphatic and renal
pelvis exit. The renal hilus gets into kidneys, forming a renal
sinus, filled by adipose tissue, also major renal calices and
minor renal calices and initial part of renal pelvis are present there.
To parenchyma of the kidney a fibrous capsule adjoins. Outside
from last a fatty capsule is situated, which noticeable better near
posterior surface of kidney. More outer from adipose capsule renal fascia
disposed, which consists of anterior sheet and posterior sheet.
They fused together by superior edges and laterally. From renal fascia stratums
of connective tissue draw to fibrous capsule kidney, which fix a kidney. Peritoneum
adjoins to anterior sheet of renal fascia. Kidneys are fixed by abdominal
pressure, renal fascia, muscular bed, renal vessels and nerves, which form a
renal leg.
Sagittal section through
posterior abdominal wall, showing the relations of the capsule of the kidney.
Renal parenchyma consists of cortex (superficially) and medulla
(deep location). In medulla they distinguish 7-10 renal pyramids, each
from which has a base of renal pyramids and a top (apex). Last
terminates in renal papilla where cribriform area disposed. The
stratums of cortical matter, which form the renal columns, lie between
pyramids. Cortical matter consists of convoluted part, between
which the stratums of medulla are
contained. They have a name medullar rays (radiata part). Each
renal pyramid forms renal lobe, and one convoluted part and one radita
part form renal lobule in cortex. From top of renal pyramid urine gets
into minor renal calices (7-
BLOOD SUPPLYING of KIDNEYS. Kidney supplied by renal
artery, which ramifies in hilus area into anterior branch and posterior
branch. Last divide by segmental arteries, and segmental branches -
into interlobar arteries, which ramify on border of cortex and medulla
into arcuate arteries. Arcuate arteries give off the radial cortical (interlobular)
arteries in cortical matter. They give beginning for numerous of afferent
vasa, which disintegrate into arterial
capillaries and form a renal glomerulus. From renal glomerulus moves
away efferent vasa, which disintegrates into secondary arterial capillaries, that enshrouds the tubules of
nephron. Such system of blood supplying, when arterial vessels have double
disintegration into cappillaries called as
renal miracle arterial rete. Venous
capillaries form in cortical matter stellate venullae, which fall
into arcuate veins. Arcuate veins continue into interlobar veins,
last form a renal vein, which empties in inferior vena cava.
FORMINg and transportation of URINE within the KIDNEY. Primary
urine arises by filtration blood plasma in nephron capsule, which envelops
each renal glomerulus. Capsule of renal glomerulus together with glomerulus
form a renal corpuscle, which is situated in convoluted part of
cortex. Proximal canalicule of nephron passes from renal corpuscle,
which continues into nephron loop (ansa of Henle). Last continues
into distal part of nephron canalicule which
falling into collecting duct. All of above counted urinary
tubules braid by thick net of secondary arterial capillaries and by
reabsorbtion secondary urine here is formed. The elements, where urine is
formed, compose function and structural kidney unit – nephron:
After nephron urine streams into straight colligens (collecting)
tubules, which terminate by pappillar foramens on top of renal
pyramid. Last open on cribriform area into minor renal calices.
From small renal calices urine flows into major renal calices, which
join together and form a renal pelvis, last continues into ureter.
The URETERS are
pair organ length 25-З0 cm, which lies retroperitoneally. Ureter has
abdominal part, pelvic part and intramural part. Last lies in the
wall of urinary bladder and opens on its fundus by foramen. Ureters wall
consists of external membrane, muscular membrane and mucous membrane. Muscular
membrane has external circular and internal longitudinal layers.
Ureter has follow narrow places:
• at transition of renal pelvis into ureter;
• at transition of abdominal part into pelvic part;
• at
transition of ureters into urinary bladder.
The Micturition Reflex and Urination
Urine reaches the urinary bladder by the
peristaltic contractions of the ureters. The process of urination is
coordinated by the micturition reflex. Components of this reflex are diagrammed
in Figure 26-21.
Stretch receptors in the wall of the
urinary bladder are stimulated as the bladder fills with urine. Afferent fibers
in the pelvic nerves carry the impulses generated to the sacral spinal cord.
Their increased level of activity (1) facilitates parasympathetic motor neurons
in the sacral spinal cord and (2) stimulates interneurons that relay sensations
to the thalamus and on to the cerebral cortex. As a result, you become
consciously aware of the fluid pressure in your urinary bladder.
The urge to urinate generally appears when
the bladder contains about 200 ml of urine. The micturition reflex begins to
function when the stretch receptors have provided adequate stimulation to
parasympathetic preganglionic motor neurons. Action potentials carried by
efferent fibers within the pelvic nerves then stimulate ganglionic neurons in
the wall of the urinary bladder. These neurons in turn stimulate sustained
contraction of the detrusor muscle.
This contraction elevates fluid pressures
in the urinary bladder, but urine ejection does not occur unless both the
internal and external urethral sphincters are relaxed. Relaxation of the
external urethral sphincter occurs under voluntary control. When the external
urethral sphincter relaxes, so does the internal sphincter. If the external
urethral sphincter does not relax, the internal sphincter remains closed and the
urinary bladder gradually relaxes.
A further increase in bladder volume
begins the cycle again, usually within an hour. Each increase in urinary volume
leads to an increase in stretch receptor stimulation that makes the sensation
more acute. Once the volume of the urinary bladder exceeds 500 ml, the
micturition reflex may generate enough pressure to force open the internal
urethral sphincter. This opening leads to a reflexive relaxation of the
external sphincter, and urination occurs despite voluntary opposition or
potential inconvenience. At the end of a normal micturition, less than 10 ml of
urine remains in the bladder.
Infants lack voluntary control over
urination, because the necessary corticospinal connections have yet to be
established. Toilet training before age 2 typically involves training the
parent to anticipate the timing of the reflex rather than training the child to
exert conscious control.