The first thing to realize is that a man's prostate is surrounded by a capsule, and this capsule plays an important part in the development of prostatism. As a man ages, his prostate increases in size. However, the capsule prevents the prostate from growing outward. Therefore, when the prostate tissue expands, it expands inward, causing a blockage to the flow of urine. If the prostate can be thought of as a doughnut, then the doughnut grows by growing inward, making the hole in the middle of the doughnut smaller and smaller.
The prostate also contains smooth muscle cells. These smooth muscle cells are controlled by a specific part of the nervous system, called the adrenergic nervous system. The resting tone of the smooth muscle cells, as well as the active or changing tone of the smooth muscle cells, plays a role in bladder outlet obstruction. Increased stimulation of the smooth muscle cells causes increased resistance to the flow of urine from the bladder, worsening a man's voiding problems.
The next thing to consider is the bladder's response to obstruction. This response is an adaptive response, and it is responsible for many of the symptoms of BPH.
One change that occurs with bladder obstruction is thickening of the bladder wall. This bladder wall thickening is initially due to enlargement of bladder smooth muscle. It is as if the bladder is adapting and trying to maintain flow across an obstructing prostate.
However, this leads to bladder instability, which is commonly known as an overactive bladder. This causes urgency, the need to get to a bathroom immediately, and urgency incontinence, the involuntary loss of urine when a man cannot get to the bathroom fast enough.
Outflow obstruction causes difficulty in initiating urination and decrease in caliber and force of stream.
After the smooth muscle mass of the bladder wall increases, further bladder wall thickening is due to an increase in collagen, which is the body's repair tissue. Collagen does not stretch easily, so as the bladder fills with urine, it cannot expand well. The pressure within the bladder increases fairly quickly, and this causes the symptoms of nocturia (waking from sleep to urinate), frequency, and urgency. The increased amount of collagen in the bladder wall also prevents the bladder from contracting normally. The bladder cannot contract long enough or forcefully enough to empty itself completely of urine. Now, when a man leaves the bathroom after voiding, his bladder is still partially filled with urine. He has a feeling of incomplete emptying. It will not take much time before the bladder fills a little more with urine and the man has to return to the bathroom to void. This is known as nocturia (frequent nighttime urinations) and frequency (frequent daytime urinations).
Incomplete emptying of bladder results in frequency, nocturia, and dribbling at the end of urination.
There are other factors which come into play. Aging has an effect on the bladder. As a man ages, the bladder becomes a little more unstable, or overactive, and yet contracts with a little less force. Behavior influences voiding symptoms. An excessive fluid intake, including diuretics such as coffee and tea, may overwhelm the bladder, causing a further increase in nocturia and freqency. Disease which affect the nervous system, such as Parkinson's disease, multiple sclerosis, and diabetes mellitus, make all voiding symptoms worse. Prior radiation therapy to the pelvis for the treatment of various cancers leads to fibrosis of the bladder wall, causing nocturia, frequency, urgency, and urgency incontinence.