When a man urinates, his brain signals the bladder wall to squeeze while simultaneously relaxing two ring-shaped muscles that normally hold urine in. This coordination pushes urine from the bladder through a roughly 8-inch-long tube called the urethra, which runs through the prostate gland and down the length of the penis. The whole process depends on a precise back-and-forth between the nervous system, the bladder muscle, and several valves along the way.
The Anatomy Involved
The male urinary tract starts at the kidneys, which filter blood and produce urine. That urine travels down two tubes called ureters into the bladder, a hollow, muscular organ that sits in the lower abdomen. The bladder wall is made of a specialized muscle called the detrusor, which can stretch to hold urine and contract to push it out.
At the base of the bladder, the urethra begins its long path out of the body. The male urethra is about 20 centimeters (roughly 7 to 8 inches) long, compared to just 3 to 4 centimeters in women. This difference matters: the urethra doesn’t just pass straight down. It first travels through the prostate gland, a walnut-sized organ that wraps around the tube just below the bladder. From there, it continues through the pelvic floor muscles and runs the full length of the penis to the opening at the tip.
Two sphincters act as gatekeepers. The internal sphincter sits where the bladder meets the urethra and operates automatically. The external sphincter, located a bit further down near the pelvic floor, is under voluntary control. This is the muscle you consciously tighten when you’re holding it in.
How the Brain Decides When to Go
The bladder doesn’t just empty whenever it’s full. A region deep in the brainstem called the pontine micturition center (often shortened to PMC) acts as the master switch between “storage mode” and “voiding mode.” During storage, higher brain centers actively suppress the PMC by sending inhibitory signals. This suppression is what lets you feel the urge to urinate but delay acting on it until you find a bathroom.
When you decide the time is right, your brain lifts that suppression. The PMC activates and triggers two things almost simultaneously: it fires up the parasympathetic nerves that make the bladder wall contract, and it shuts down the nerve signals that keep the sphincters clenched. The external sphincter relaxes first, and a few seconds later, the bladder wall begins to squeeze. The result is a coordinated push of urine out through the urethra.
What Happens Inside the Body
As the bladder fills, stretch receptors in its wall send increasingly urgent signals up the spinal cord and to the brain. Once you give the go-ahead, the sequence unfolds quickly. Parasympathetic nerve fibers stimulate the detrusor muscle to contract, building pressure inside the bladder. At the same time, those parasympathetic signals cause the internal sphincter to relax. The external sphincter, controlled by a separate set of nerves in the lower spinal cord, also releases its grip.
With both sphincters open and the bladder wall squeezing, urine flows down through the prostate section of the urethra, past the pelvic floor, and out through the penis. The smooth muscle lining the urethra also relaxes during this phase, helped by the release of nitric oxide, a chemical signal that widens the tube. Once the bladder is empty, the detrusor relaxes, the sphincters close again, and the system returns to storage mode.
Normal Flow Rate and What Affects It
In healthy men between the ages of 16 and 50, urine comes out at a peak rate of about 22 to 23 milliliters per second, with an average flow closer to 13 milliliters per second. After age 50, those numbers typically drop. Peak flow slows to around 17 milliliters per second, and average flow falls to roughly 9 milliliters per second.
Several factors drive this age-related slowdown. The prostate naturally grows larger over time, and because the urethra passes directly through it, an enlarged prostate can squeeze the tube and restrict flow. The bladder muscle also becomes less efficient at contracting with age. Men who notice a weak stream, difficulty starting, or a feeling that the bladder hasn’t fully emptied are often experiencing the effects of prostate growth narrowing the urethral channel.
The Role of the Pelvic Floor
The pelvic floor is a hammock of muscles that stretches across the bottom of the pelvis, supporting the bladder and other organs. In men, these muscles do double duty during urination. They help maintain continence by keeping the external sphincter contracted during the storage phase, and they relax in coordination with the bladder contraction when it’s time to void.
You can feel these muscles working if you try to stop your urine stream midflow or clench as if preventing yourself from passing gas. Strengthening them through pelvic floor exercises can help with urinary control, particularly after prostate surgery or in cases of age-related leakage.
Standing vs. Sitting
Most men urinate standing up, but the position you choose has measurable effects, at least for some men. A meta-analysis comparing standing and sitting positions found that in healthy men, there was no meaningful difference in flow rate, voiding time, or the amount of urine left in the bladder afterward.
For men with an enlarged prostate, however, sitting made a noticeable difference. Sitting reduced the amount of urine remaining in the bladder after voiding by about 25 milliliters compared to standing. Flow rate also trended higher in the sitting position, though the difference was modest. The likely explanation is that sitting allows the pelvic floor and abdominal muscles to relax more completely, reducing the resistance the urine has to push through. For men who struggle with urinary symptoms, sitting may help the bladder empty more thoroughly.