Quick Re-cap from previous pages:
- The urinary bladder stores urine prior to its elimination from
the body (functions
of the urinary system).
At micturation/urination, the bladder
expels urine into the urethra, leading to the exterior
of the body. The bladder is
a musculomembranous sac located on the floor of the pelvic
cavity, anterior to the uterus and upper vagina (in females).
- Outer surfaces of the Bladder:
The upper and side surfaces of the bladder are covered by
peritoneum (also called
"serosa"). This serous membrane of the abdominal cavity
consists of mesthelium and elastic fibrous connective tissue. "Visceral
peritoneum" covers the
bladder and other abdominal organs, while "parietal peritoneum" lines
the abdomen walls.
- Ureters:
The ureters deliver urine to the bladder from the kidneys (one
ureter from each kidney - see components
of human urinary system). The ureters pass through the posterior surface
of the bladder at the Ureter Orifices (shown above).
Urine drains through the ureters directly into the bladder as there are
no sphincter muscles or valves at the ureter orifices.
- Structure of Bladder (Detail): The
bladder itself consists of 4 layers:- (1) Serous -
this outer layer being a partial layer derived from
the peritoneum, (2) Muscular -
the detrusor muscle of the urinary
bladder wall, which consists of 3 layers incl. both longitudinal
and circularly arranged muscle
fibres, (3) Sub-mucous - a
thin layer of areolar tissue loosely connecting the
muscular layer with the mucous layer, and (4) Mucous -
the innermost layer of the wall of the urinary bladder loosely
attached to the (strong and substantial) muscular
layer. The mucosa falls into many folds known as rugae when
the bladder is empty or near empty.
- The features observable on the inside
of the bladder are the ureter
orifices, the trigone, and the internal
orifice
of the urethra.
- The trigone is a smooth triangular
region between the openings of the two ureters and the urethra
and never presents
any rugae even when the bladder is empty - because this area
is more tightly bound to its outer layer of bladder tissue.
- Exit from Bladder: When urine is released from the bladder is flows out via the neck of
the bladder (in the trigone region).
The internal urethral sphincter is a sphincter (circular)
muscle located at the neck of the bladder and formed from a thickening of
the detrusor
muscle. It closes the urethra when the bladder has emptied.
- More detail
about the above is included on the page about the bladder.
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At about 8-9 inches (200 mm) long, the adult
male urethra is longer than the adult female
urethra (of approx. 1.5 inches 35
mm). It has three portions (the prostatic urethra,
the membranous
urethra, and the spongy urethra -
see below) and extends from the neck of the bladder (shown in the
diagram
above) to
the meatus
urinarius at
the end of the male penis/urethra.
The general shape of the urethra (the tube itself) varies along
it's length: The prostatic section
of the male urethra is somewhat arched. Except during passage
of urine or semen through the urethra, it is a transverse slit
along it's length, the upper
and lower surfaces
of the transverse slit being in contact when fluid is not passing
along the urethra. However, at exit from the body at the meatus
urinarius (external
orifice of urethra) the slit is vertical.
The three regions/sections of the male urethra are:
- Prostatic Urethra
The prostatic urethra begins
at the neck of the bladder and includes
all of the section that passes through the prostrate
gland. It is the widest and most dilatable
part of the male urethral canal.
- Membranous Urethra
The membranous urethra is
the shortest and narrowest part of the male urethra. This section
measures approx. 0.5 - 0.75 inches (12 - 19 mm) in length and
is the section of the urethra that passes through the male urogenital
diaphragm.
The external urethral sphincter (muscle)
is located in the urogenital diaphragm (as for the female urethra). This
muscle is referred to as the "compressor urethrae muscle"
in some older textbooks.
The
passage of urine along the urethra through the urogenital diaphragm
is controlled by the external urethral sphincter,
which is a circular muscle under voluntary control (that
is, it is innervated by the somatic nervous system, SNS).
See
the page about micturation for more about control of these structures
by the nervous system.
- Spongy Urethra
The spongy urethra is the longest of the three
sections. It is approx. 6 inches (150 mm) in length and is contained
in the corpus spongiosum that extends from the end of the
membranous portion, passes
through the penis, and terminates at the external
orifice of the urethra - which is the point
at which the urine leaves the body.
Structure of the Male Urethra
The structure of the urethra (tube)
itself is a continuous mucous
membrane supported by submucous tissue connecting
it to the other structures through which it passes.
- The mucous coat is continuous with the mucous
membrane of the bladder, ureters and kidney. In the membranous
and spongy sections (2. and 3. above), the mucous membrane is
arranged in longitudinal folds when the tube is empty.
- The submucous tissue consists of a vascular
(i.e. containing many blood vessels) erectile layer surrounded
by a layer
of smooth (involuntary) muscle
fibres. These muscle fibres are arranged in a circular
configuration that separates the mucous membrane and submucous
tissue from
the surrounding structure - which is the tissue of the
corpus spongiosum
(labeled simply "penis" in the diagram above).
Unlike the female urethra, the male urethra has a reproductive
function in addition to it's urinary function - it conveys semen
out of the body at ejaculation. For further information about this
function red the section about the male reproductive system.
For further details about the structures
described here see Gray's
Anatomy. |
