Photo: Brett Jordan

What triggers the urination reflex?

When the bladder is full of urine, stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle that surrounds the bladder contracts. The internal urethral sphincter relaxes, allowing for urine to pass out of the bladder into the urethra.

visiblebody.com - Urine Storage and Elimination - Visible Body

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Urine produced in the kidneys travels down the ureters into the urinary bladder. The bladder expands like an elastic sac to hold more urine. As it reaches capacity, the process of micturition, or urination, begins. Involuntary muscle movements send signals to the nervous system, putting the decision to urinate under conscious control.

1. The Two Urethral Sphincters Maintain Urinary Continence

The internal urethral sphincter and the external urethral sphincter both provide muscle control for the flow of urine. The internal sphincter is involuntary. It surrounds the opening of the bladder to the urethra and relaxes to allow urine to pass. The external sphincter is voluntary. It surrounds the urethra outside the bladder and must be relaxed for urination to occur.

2. The Bladder Expands As It Fills with Urine

The bladder is shaped like a pyramid when empty. It becomes more oval as it fills with urine and expands. A smooth muscle called the detrusor surrounds the bladder, and folds called rugae line the interior wall. These structures give the bladder elasticity and allow it to expand. The floor of the bladder includes a funnel-like region called the trigone, formed by the two ureteral orifices and the internal urethral sphincter. Urine flows into the bladder via the ureteral orifices and out through the internal sphincter.

3. Muscles of Micturition: The Detrusor and Urethral Sphincters

Micturition, or urination, is the act of emptying the bladder. When the bladder is full of urine, stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle that surrounds the bladder contracts. The internal urethral sphincter relaxes, allowing for urine to pass out of the bladder into the urethra. Both of these reactions are involuntary. The external urethral sphincter is voluntary. It must be relaxed for urine to flow through the urethra and outside the body.

4. How Full Does the Bladder Get Before We Have to Urinate?

The bladder expands as urine flows in from the ureters, but there is a limit to the volume it can contain. At about 200 ml of urine, the detrusor muscle begins to contract and the internal urethral sphincter muscle begins to relax. This sends signals through the nervous system and creates the “urge” to urinate. If this urge is ignored, continence may be threatened. At about 500 ml, detrusor muscle contractions begin to force open the internal urethral sphincter. Unless the external urethral sphincter is powerful enough to prevent it, micturition (urination) will occur involuntarily.

5. Bladder Stretch Triggers the Nervous System to Initiate and Control Micturition

Smooth muscle stretch initiates the micturition reflex by activating stretch receptors in the bladder wall. This autonomic reflex causes the detrusor muscle to contract and the internal urethral sphincter muscle to relax, allowing urine to flow into the urethra. The stretch receptors also send a message to the thalamus and the cerebral cortex, giving voluntary control over the external urethral sphincter. We usually gain this control of urination between the ages of 2 and 3, as our brains develop.

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How many people have pee shyness?

As many as 20 million Americans have this problem. It's known as shy or bashful bladder syndrome. Your doctor may call it by its official name, paruresis. Without treatment, it can affect your personal, social, and professional life.

webmd.com - Shy Bladder (Paruresis): Causes and Treatments - WebMD

When you’re on the go and you have to go, a public restroom is where you head. For millions, though, that’s not an option. It’s a place where peeing seems physically impossible. That’s because being near other people causes your sphincter muscles to lock up. Those muscles control the flow of urine from your bladder. Once they freeze up, you simply can’t pee. It can happen in a public restroom, a bathroom in someone else’s home, and even in your own place if other folks are nearby. As many as 20 million Americans have this problem. It’s known as shy or bashful bladder syndrome. Your doctor may call it by its official name, paruresis. Without treatment, it can affect your personal, social, and professional life. You might find it hard to be away from home for more than a short time. You could end up avoiding parties, sporting events, and dates. It might even limit your choice of jobs. But you can get it under control with therapy.

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