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Urination (the low down!)
Basic Anatomy
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.
2 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.
3 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.
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)?
(Int. sphincter contracts) Stimulated (ext. Sphincter contracts)
urination
Stimulated (D.M. contracts)
Inhibited
