Urination anatomy Class lecture Notes Anatomy Sweet Urination anatomy

Urination (the low down!)
Basic Anatomy





Urogenital diaphragm

The bladder serves to store urine until an appropriate (or not so appropriate) time for urination. It is located in
the pelvic cavity, posterior (deep) to the pubic symphysis, and below the parietal peritoneum. The size and
shape of the urinary bladder varies with the amount of urine it contains and with the pressure it receives from
surrounding organs (hello uterus!). Look at the bladder and the location of the female uterus (see your
worksheet). Imagine the pressure on the bladder with a baby growing in the uterus!
The inner lining of the urinary bladder is a mucosa with the lining epithelium being transitional epithelium that
is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae.
(similar to stomach rugae). The rugae, along with the lining transitional epithelium, allow the bladder to expand
as it fills.
The second layer in the walls is the submucosa, which supports the mucous membrane. It is composed of
connective tissue with elastic fibers.
The next layer is the muscularis, which is composed of smooth muscle. The smooth muscle fibers are

interwoven in all directions and, collectively, these are called the detrusor muscle. Contraction of this muscle
expels urine from the bladder.
Urine is delivered to the bladder from the kidneys via the ureters, which enter the posterior bladder wall
obliquely, with their openings at the trigone. The trigone is a triangular area of thickened smooth muscle on the
posterior wall of the bladder formed by three openings in the floor of the bladder. The two holes where the
ureters enter the bladder, form the base of the trigone.


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This oblique orientation of the ureter’s entry into the bladder ensures that the ureters will squeeze shut when
the detrusor muscle contracts during urination. This prevents reflux of urine back towards the kidneys.
The third opening at the apex of the trigone, is the opening into the urethra. A band of the detrusor muscle
encircles this 3rd opening to form the internal urethral sphincter. This is located in an area called the “neck of
the bladder”. The urethra transports the urine collected in the bladder and sends it to the outside world!
REMEMBER- the internal urethral sphincter surrounds the beginning of the urethra where it leaves the urinary
bladder. The internal sphincter is smooth (involuntary) muscle. The second sphinter, the external urethral
sphincter, is skeletal (voluntary) muscle and encircles the urethra where it goes thru the pelvic floor
(urogenital diaphragm. These two sphincters control the flow of urine through the urethra. Be sure you can
see all these structures on your worksheets and classroom models!

In females, the urethra is quite short (about 1.5 inches) and opens to the outside just anterior to the opening for
the vagina.
In males, the urethra is longer (about 7-8 inches long) and transports both urine and semen. The male urethra
is divided into 3 segments: 1. the prostatic urethra (which travels thru the prostate gland) 2. the membranous
urethra (at the level of the urogenital diaphragm). The prostatic urethral and membranous urethra are together
about 1.5 inches long. The third segment of the urethra: 3. the penile (AKA spongy) urethra. This portion of
the urethra extends the entire length of the penis (so the length is variable), and opens to the outside at the tip of
the penis.

Innervation (i.e. – the nervous system control)



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The bladder is controlled by the central nervous system. The figure above depicts the innervation of the
different muscles in the bladder. The smooth muscles (the detrusor muscle of the bladder and the internal
urethral sphincter) are controlled by the two different divisions of the autonomic nervous
system. Parasympathetic inputs stimulate contraction of the detrusor muscle. Sympathetic inputs stimulate
contraction of the internal urethral sphincter “tighten up”. Somatic efferent motor neurons stimulate contraction

of the external urethral sphincter, which is skeletal muscle.
The walls of the bladder stretch as the bladder expands during bladder filling. This is sensed by bladder afferent
neurons whose sensory dendrites are located in the bladder wall. Bladder afferents project to the spinal cord and
to various regions in the brain that are responsible for coordinating the efferent output to the bladder and
urethral sphincters.
Sympathetic
Parasympathetic
(Internal sphincter)
(Detrusor muscle)
smooth muscle



somatic efferent (external sphincter)
skeletal muscle

urine storage
Inhibited
(during bladder filling) (D.M. relaxed)

Stimulated
Stimulated (ext. Sphincter contracts)
(Int. sphincter contracts)


urination

Inhibited
(int. sphincter relaxes)

Stimulated
(D.M. contracts)

Inhibited (ext. sphincter relaxes)

There are two modes of operation for the bladder, which are outlined in the table above: either urine storage, or
urination. During the time that the bladder is filling, when urine is stored in the bladder, the parasympathetic
input to the detrusor muscle is inhbited, and there is activation of the sympathetic and somatic efferent
pathways, causing the two urethral sphincters to contract. The afferent (sensory) information from the bladder
provides a sense of bladder fullness and the urge to urinate. The parasympathetic efferents become active,

causing contraction of the detrusor muscle. At the same time, there is coordinated inhibition of the sympathetic
efferents i.e. “relaxation ” of internal urethral sphincter and the somatic efferents of the external urethral
sphincters also relax. BUT…remember the external sphincter (skeletal muscle) is controlled by you (for the
most part). But one can only “hold” it for so long …indeed.
What are “kegel” exercises? What are they strengthening? How may kagels help with bladder leakage (ie
incontinence)?




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